1 Introduction

1.1 The Compressible Primitive Equations

The general hydrodynamic and thermodynamic equations (see, e.g., [36]) with Coriolis force and gravity are used to model the motion and state of the atmosphere, which is a specific compressible fluid. However, such equations are extremely complicated and prohibitively expensive computationally. However, since the vertical scale of the atmosphere is significantly smaller than the planetary horizontal scale, the authors in [17] take advantage, as it is commonly done in planetary scale geophysical models, of the smallness of this aspect ratio between these two orthogonal directions to formally derive the compressible primitive equations (CPE) from the compressible Navier–Stokes equations. Specifically, in the CPE the vertical component of the momentum in the compressible Navier–Stokes equations is replaced by the hydrostatic balance equation (1.1)\(_{3}\), below, which is also known as the quasi-static equilibrium equation. It turns out that the hydrostatic approximation equation is accurate enough for practical applications and has become a fundamental equation in atmospheric science. It is the starting point of many large scale models in the theoretical investigations and practical weather predictions (see, e.g., [35]). This has also been observed by meteorologists (see, e.g., [39, 44]). In fact, such an approximation is reliable and useful in the sense that the balance of gravity and pressure dominates the dynamic in the vertical direction and that the vertical velocity is usually hard to observe in reality. In many simplified models, it is assumed that the atmosphere is under adiabatic process and therefore the entropy remains unchanged along the particle path. In particular, if the entropy is constant in the spatial variables initially, it remains so in later time. On the other hand, instead of the molecular viscosity, eddy viscosity is used to model the statistical effect of turbulent motion in the atmosphere. The observations above and more perceptions from the meteorological point of view can be found in [39, Chapter 4]. Therefore, under the above assumptions, one can write down the isentropic compressible primitive equations as in (1.1), below. Moreover, we also study the problem by further neglecting the gravity in (1.2), below. We remark here that, although it does not cause any additional difficulty, we have omitted the Coriolis force in this work for the convenience of presentation. That is, the local well-posedness theorems still work for systems (1.1) and (1.2) with the Coriolis force.

The first mathematical treatment of the compressible primitive equations (CPE) can be tracked back to Lions, Temam and Wang [35]. Actually, the authors formulated the compressible primitive equations in the pressure coordinates (p-coordinates) and show that in the new coordinate system, the equations are in the form of classical primitive equations (called primitive equations, or PE hereafter) with the incompressibility condition. In yet another work [34], the authors modeled the nearly incompressible ocean by the PE. It is formulated as the hydrostatic approximation of the Boussinesq equations. The authors show the existence of global weak solutions and therefore indirectly study the CPE (see, e.g., [32, 33] for additional work by the authors). Notably, the PE have been the subject of intensive mathematical research. For instance, Guillén-González, Masmoudi and Rodríguez-Bellido in [21] study the local existence of strong solutions and global existence of strong solutions for small initial data to the PE. In [47] the authors address the global existence of strong solutions to PE in a domain with small depth for restricted large initial data depending on the depth. In [38], the authors study the Sobolev and Gevrey regularity of the solutions to PE. The first breakthrough concerning the global well-posedness of PE is obtained by Cao and Titi in [8], in which the authors show the existence of unique global strong solutions (see, also, [9, 22,23,24,25, 27,28,29, 46] and the references therein for related study). On the other hand, with partial anisotropic diffusion and viscosity, Cao, Li and Titi in [3,4,5,6,7, 10] establish the global well-posedness of strong solutions to PE. For the inviscid primitive equations, or hydrostatic incompressible Euler equations, in [1, 26, 37], the authors show the short time existence of solutions in the analytic function space and in \(H^s \) space. More recently, the authors in [2, 45] construct finite-time blowup for the inviscid PE in the absence of rotation. Also, in [20], the authors establish the Gevrey regularity of hydrostatic Navier–Stokes equations with only vertical viscosity.

Despite the fruitful study of the primitive equations, it still remains interesting to study the compressible equations. On the one hand, it is a more direct model to study the atmosphere and perform practical weather predictions. On the other hand, the former deviation of the PE from the CPE in the p-coordinates did not treat the corresponding derivation of the boundary conditions. In fact, due to the change of pressure on the boundary, the appropriate studying domain for the PE should be evolving together with the flows in order to recover the solutions to the CPE. Thus, even though the formulation of the PE significantly simplifies the equations of the CPE, the boundary conditions are more complicated than before in order to study the motion of the atmosphere. We believe that this might be one of the reasons that is responsible for the not-completely successful prediction of the weather by using the PE.

Recently, Gatapov, Kazhikhov, Ersoy, Ngom construct a global weak solution to some variant of two-dimentional compressible primitive equations in [16, 19]. Meanwhile, Ersoy, Ngom, Sy, Tang, Gao study the stability of weak solutions to the CPE in [17, 41] in the sense that a sequence of weak solutions satisfying some entropy conditions contains a subsequence converging to another weak solution. In recent work, we show the existence of such weak solutions in [31]. See also [43].

In this and subsequent works, we aim to address several problems concerning the compressible primitive equations. In this work, we start by studying the local well-posedness of strong solutions to the CPE. That is, we will establish the local strong solutions to (1.1) and (1.2), below, in the domain \( \Omega = \Omega _h \times (0,1) \), with \( \Omega _h = {\mathbb {T}}^2 = [0,1]^2 \subset {\mathbb {R}}^2 \) being the fundamental periodic domain. In comparison with the compressible Navier–Stokes equations [18], the absence of evolutionary equations for the vertical velocity (vertical momentum) causes the main difficulty. This is the same difficulty as in the case of the PE. In fact, the procedure of recovering the vertical velocity is a classical one in the modeling of the atmosphere [39, Chapter 5]. This is done with the help of the hydrostatic equation, which causes the stratification of density profiles in the CPE. On the one hand, in (1.1), as one will see later, the hydrostatic equation implies that if there is vacuum in the physical domain \( \Omega \), the sound speed will be at most 1/2-Hölder continuous. Thus the \( H^2 \) estimate of the density is not available in the presence of vacuum. However in (1.2), such an obstacle no longer exists. For this reason, the local well-posedness established in this work doesn’t allow vacuum in the presence of gravity, but vacuum is allowed in the case without gravity. On the other hand, the hydrostatic equation does have some benefits. Indeed, such a relation yields that the density admits a stratified profile along the vertical direction. This fact will help us recover the vertical velocity from the continuity equation (see (1.8) and (1.13), below).

In this work, we will first reformulate the compressible primitive equations (1.1), (1.2) by making use of the stratified density profile. Then we will study the local well-posedness of the reformulated systems under the assumption that there is no vacuum initially. This is done via a fixed point argument. Next, in order to obtain the existence of strong solutions to (1.2) with non-negative density, we establish some uniform estimates independent of the lower bound of the density. We point out that in comparison to the compressible Navier–Stokes equations (see, e.g., [12,13,14,15]), we will require \( H^2 \) estimate of \( \rho ^{1/2} \) in order to derive the above mentioned uniform estimates. Such estimates are not available in the case with gravity (1.1). To this end, continuity arguments are used to establish the solutions with vacuum. We also study the continuous dependence on the initial data and the uniqueness of the strong solutions.

Through out this work, we will use \( {x} := (x,y,z)^\top , {x}_h := (x,y)^\top \) to represent the coordinates in \( \Omega \) and \( \Omega _h \), respectively. In addition, we will use the following notations to denote the differential operators in the horizontal direction:

$$\begin{aligned}&\nabla _h:= ( \partial _x, \partial _y)^\top ,~ \partial _h \in \lbrace \partial _x, \partial _y \rbrace ,\\&\mathrm {div}_h\,:= \nabla _h\cdot , \Delta _h:= \mathrm {div}_h\,\nabla _h. \end{aligned}$$

The isentropic compressible primitive equations with gravity are governed by the system

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\rho + \mathrm {div}_h\,(\rho v) + \partial _z(\rho w) = 0 &{} \text {in} ~ \Omega , \\ \partial _t(\rho v) + \mathrm {div}_h\,(\rho v \otimes v) + \partial _z(\rho w v) + \nabla _hP = \mu \Delta _hv + \mu \partial _{zz} v \\ ~~~~ ~~~~ ~~~~ + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _zP - \rho g = 0 &{} \text {in} ~ \Omega , \end{array}\right. } \end{aligned}$$
(1.1)

with \( P := \rho ^\gamma \). We will study in this work only the case when \( \gamma = 2 \) in (1.1) for the sake of simplifying our presentation. For general \( \gamma > 1 \), we refer to Remark 1, below.

On the other hand, the isentropic compressible primitive equations without gravity are governed by the system

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\rho + \mathrm {div}_h\,(\rho v) + \partial _z(\rho w) = 0 &{} \text {in} ~ \Omega , \\ \partial _t(\rho v) + \mathrm {div}_h\,(\rho v \otimes v) + \partial _z(\rho w v) + \nabla _hP = \mu \Delta _hv + \mu \partial _{zz} v \\ ~~~~ ~~~~ ~~~~ + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _zP = 0 &{} \text {in} ~ \Omega , \end{array}\right. } \end{aligned}$$
(1.2)

with \( P := \rho ^\gamma \) and \( \gamma > 1 \).

In the above systems, (1.1) and (1.2), the viscosity coefficients \( \mu , \lambda \) are assumed to be strictly positive. Also, (1.1) and (1.2) are supplemented with the following boundary conditions:

$$\begin{aligned} w = 0, ~ \partial _zv = 0 ~~~~ \text {on} ~~ \Omega _h \times \lbrace 0,1 \rbrace . \end{aligned}$$
(1.3)

The rest of this paper will be organized as follows: in section 1.2, we present a reformulation of (1.1) and (1.2) by making use of the stratified density profiles. Also, we present the formula for recovering the vertical velocity and the main theorems of this work. After listing some useful inequalities and notations, we study in section 2 the existence theory. Next, in section 3 we show the continuous dependence on the initial data and the uniqueness of strong solutions.

1.2 Reformulation, Analysis and Main Theorems

In this section, we will reformulate (1.1) and (1.2) and point out how to recover the vertical velocity in terms of the density and the horizontal velocity.

1.2.1 The Case with Gravity and \( \gamma = 2 \)

We first consider (1.1). From (1.1)\(_{3}\), one has

$$\begin{aligned} \rho ^{\gamma -1}({x},t)&= \dfrac{\gamma -1}{\gamma }gz + \rho ^{\gamma -1}({x}_h,0,t). \end{aligned}$$

Denote by \( \xi = \xi ({x}_h,t) : = \rho ^{\gamma -1}({x}_h,0,t) \). The continuity equation (1.1)\(_{1}\) implies

$$\begin{aligned} \partial _t\xi + v \cdot \nabla _h\xi + (\gamma - 1)\left( \xi + \dfrac{\gamma -1}{\gamma }gz\right) (\mathrm {div}_h\,v + \partial _zw) + \dfrac{\gamma -1}{\gamma } g w = 0.\nonumber \\ \end{aligned}$$
(1.4)

In particular, since \( \gamma = 2 \), we have \( \rho ({x},t) = \xi ({x}_h,t) + \dfrac{1}{2} gz \) and

(1.1) can be written as

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\xi + v \cdot \nabla _h\xi + \left( \xi + \dfrac{1}{2}gz\right) \mathrm {div}_h\,v + \partial _z\left( \xi w + \dfrac{1}{2}g z w \right) = 0 &{} \text {in} ~ \Omega ,\\ \left( \xi + \dfrac{1}{2}gz\right) ( \partial _tv + v \cdot \nabla _hv + w \partial _zv) + (2\xi +gz) \nabla _h\xi \\ ~~~~ ~~~~ ~~~~ = \mu \Delta _hv + \mu \partial _{zz} v + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _z\xi = 0 &{} \text {in} ~ \Omega . \end{array}\right. } \end{aligned}$$
(1.5)

Hereafter, we denote, for any \( f :\Omega \mapsto {\mathbb {R}} \),

$$\begin{aligned} {\overline{f}}: = \int _0^1 f \,\hbox {d}z , ~ {\widetilde{f}} := f - {\overline{f}}. \end{aligned}$$
(1.6)

Then averaging over the vertical variable in (1.5)\(_{1}\) yields, thanks to (1.3),

$$\begin{aligned} \partial _t\xi + {\overline{v}} \cdot \nabla _h\xi + \xi \overline{\mathrm {div}_h\,v} + \dfrac{1}{2} g \overline{z \mathrm {div}_h\,v} = 0. \end{aligned}$$
(1.7)

Then comparing (1.7) with (1.5)\(_{1}\) implies

$$\begin{aligned} \partial _z(\rho w) = \partial _z\left( \xi w + \dfrac{1}{2}gz w\right) = - {\widetilde{v}}\cdot \nabla _h\xi - \xi \mathrm {div}_h\,{\widetilde{v}} - \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v} ~~ \text {in}~ \Omega . \end{aligned}$$

Therefore, the vertical velocity w is determined, thanks to the boundary condition (1.3), by the relation

$$\begin{aligned} \rho w = \bigl (\xi + \dfrac{1}{2}gz\bigr ) w = - \int _0^z \bigl ( \mathrm {div}_h\,(\xi {\widetilde{v}}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v} \bigr ) \,\hbox {d}z. \end{aligned}$$
(1.8)

System (1.5) is complemented with the initial data

$$\begin{aligned} (\xi , v)|_{t=0} = (\xi _0, v_0), \end{aligned}$$
(1.9)

with \( \xi _0, v_0 \in H^2(\Omega ) \). Also the following compatible conditions are imposed:

$$\begin{aligned} \begin{aligned}&\rho _0 = \xi _0 + \dfrac{1}{2} gz \geqq {\underline{\rho }} > 0 ~ \text {in} ~ \Omega , ~ \text {and} ~ \partial _zv_0|_{z=0,1} = 0,\\&\mu \Delta _hv_0 + \mu \partial _{zz} v_0 + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v_0 - (2\xi _0 + gz) \nabla _h\xi _0 \\&- \rho _0 v_0 \cdot \nabla _hv_0 - \rho _0 w_0 \partial _zv_0 =: \rho _0 V_1, ~~~~ \text {with} ~ V_1 \in L^{2}(\Omega ),\\&\text {and} ~~ \rho _0 w_0 = - \int _0^z \bigl ( \mathrm {div}_h\,(\xi _0 \widetilde{v_0}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v_0} \bigr ) \,\hbox {d}z. \end{aligned} \end{aligned}$$
(1.10)

Also, we will denote the bounds

$$\begin{aligned} \bigl \Vert \xi _0 \bigr \Vert _{H^{2}}^2 \leqq B_{g,1}, ~~ \bigl \Vert v_0 \bigr \Vert _{H^{2}}^2 + \bigl \Vert V_1 \bigr \Vert _{L^{2}}^2 \leqq B_{g,2}. \end{aligned}$$
(1.11)

Theorem 1

Suppose the initial data \( (\rho _0, v_0) = (\xi _0 + \frac{1}{2}gz, v_0) \) satisfy (1.11) and the compatible conditions (1.10). Then there is a unique strong solution \( (\rho ,v) \) to system (1.1), with the boundary condition (1.3), in \( \Omega \times (0,T) \), for some positive constant \( T = T(B_{g,1},B_{g,2},{{\underline{\rho }}}) > 0 \). Also, the solution satisfies

$$\begin{aligned}&\rho \in L^\infty (0,T;H^2(\Omega )), \partial _t\rho \in L^\infty (0,T;H^1(\Omega )), \\&v \in L^\infty (0,T;H^2(\Omega ))\cap L^2(0,T;H^3(\Omega )), \\&\partial _tv \in L^\infty (0,T;L^2(\Omega ))\cap L^2(0,T;H^1(\Omega )). \end{aligned}$$

Furthermore, for some positive constant \( {\mathcal {C}}(B_{g,1},B_{g,2},{{\underline{\rho }}}) \),

$$\begin{aligned} \inf _{({x},t)\in \Omega \times (0,T)} \rho ({x},t )\geqq & {} \dfrac{1}{2} {\underline{\rho }} > 0, \\&\sup _{0\leqq t\leqq T} \bigl ( \bigl \Vert \rho (t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _t\rho (t) \bigr \Vert _{H^{1}}^2 + \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv(t) \bigr \Vert _{L^{2}}^2 \bigr ) \\&+ \int _0^T \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert \partial _tv(t) \bigr \Vert _{H^{1}}^2 \bigr ) \,\hbox {d}t \leqq {\mathcal {C}}(B_{g,1},B_{g,2},{{\underline{\rho }}}). \end{aligned}$$

Moreover, for any two solutions \( (\rho _i, v_i), i = 1,2 \) with initial data \( (\rho _{i,0},v_{i,0}) , i = 1,2 \) satisfying the conditions mentioned above, we have the following inequality

$$\begin{aligned} \begin{aligned}&\bigl \Vert \rho _1 - \rho _2 \bigr \Vert _{L^\infty (0,T;L^2(\Omega ))} + \bigl \Vert v_1- v_2 \bigr \Vert _{L^\infty (0,T;L^2(\Omega ))}\\&\qquad + \bigl \Vert \nabla (v_1 - v_2) \bigr \Vert _{L^2(0,T;L^2(\Omega ))} \leqq C_{\mu , \lambda , B_{g,1}, B_{g,2},{{\underline{\rho }}}, T} \\&\qquad \times \bigl (\bigl \Vert \rho _{1,0}-\rho _{2,0} \bigr \Vert _{L^2(\Omega ))} + \bigl \Vert v_{1,0}-v_{2,0} \bigr \Vert _{L^2(\Omega ))}\bigr ), \end{aligned} \end{aligned}$$

for some positive constant \( C_{\mu , \lambda , B_{g,1}, B_{g,2},{{\underline{\rho }}}, T} \).

Remark 1

For general \( \gamma > 1 \), after multiplying (1.4) with

$$\begin{aligned} \biggl ( \dfrac{\xi +\frac{\gamma -1}{\gamma }gz}{\xi } \biggr )^{\frac{2-\gamma }{\gamma -1}} \end{aligned}$$

and averaging the resultant in the z-variable, one obtains

$$\begin{aligned}&\dfrac{\gamma }{g} \bigl ( (\xi + \dfrac{\gamma -1}{\gamma } g)^{\frac{1}{\gamma -1}} \xi ^{\frac{\gamma -2}{\gamma -1}} - \xi \bigr ) \partial _t\xi + \xi ^{\frac{\gamma -2}{\gamma -1}} \overline{ \biggl ( {\xi +\frac{\gamma -1}{\gamma }gz} \biggr )^{\frac{2-\gamma }{\gamma -1}} v}\cdot \nabla _h\xi \\&\qquad + (\gamma -1) \xi ^{\frac{\gamma -2}{\gamma -1}} \overline{ \biggl ( {\xi +\dfrac{\gamma -1}{\gamma }gz} \biggr )^{\frac{1}{\gamma -1}} \mathrm {div}_h\,v} = 0 ~~ \text {in} ~ \Omega _h. \end{aligned}$$

Consequently, by eliminating \( \partial _t\xi \) from the above equation and (1.4), it follows that

$$\begin{aligned}&(\gamma -1) \partial _z\bigl ( \bigl (\xi +\dfrac{\gamma -1}{\gamma } gz \bigr )^{\frac{1}{\gamma -1}} w \bigr ) = - (\gamma -1) \bigl ( \xi + \dfrac{\gamma -1}{\gamma } gz \bigr )^{\frac{1}{\gamma -1}} \mathrm {div}_h\,v \\&\qquad - \bigl (\xi +\dfrac{\gamma -1}{\gamma }\bigr )^{\frac{2-\gamma }{\gamma -1}} v \cdot \nabla _h\xi +\dfrac{ g \xi ^{\frac{\gamma -2}{\gamma -1}} \bigl (\xi +\dfrac{\gamma -1}{\gamma }gz\bigr )^{\frac{2-\gamma }{\gamma -1}} }{\gamma \bigl ( \bigl (\xi + \dfrac{\gamma -1}{\gamma } g\bigr )^{\frac{1}{\gamma -1}} \xi ^{\frac{\gamma -2}{\gamma -1}} - \xi \bigr ) } \\&\qquad \times \biggl ( \overline{ \biggl ( {\xi +\frac{\gamma -1}{\gamma }gz} \biggr )^{\frac{2-\gamma }{\gamma -1}} v}\cdot \nabla _h\xi + (\gamma -1) \overline{ \biggl ( {\xi +\dfrac{\gamma -1}{\gamma }gz} \biggr )^{\frac{1}{\gamma -1}} \mathrm {div}_h\,v} \biggr ). \end{aligned}$$

Therefore, from above, and as in the case when \( \gamma =2 \), the vertical velocity w can be represented in the form

$$\begin{aligned} w = \int _0^z H(\mathrm {div}_h\,v, v, \nabla _h\xi , \xi ) \,\hbox {d}z', \end{aligned}$$

similarly to (1.8), for an explicit function \(H(\cdot ) \). Notably, the arguments and proofs, below, apply equally, and similar conclusion of Theorem 1 also holds for \( \gamma > 1 \).

1.2.2 The Case Without Gravity and \( \gamma > 1 \)

Concerning system (1.2), since (1.2)\(_{3}\) already yields the independence of the density of the vertical variable, after taking the vertical average of (1.2)\(_{1}\), as before, one has

$$\begin{aligned} \partial _t\rho + \mathrm {div}_h\,(\rho {{\overline{v}}}) = 0. \end{aligned}$$
(1.12)

Comparing (1.12) with (1.2)\(_{1}\) yields, thanks to the boundary condition (1.3), that the vertical velocity w is determined by the relation

$$\begin{aligned} \rho w = - \int _0^z \mathrm {div}_h\,(\rho {{\widetilde{v}}} )\,\hbox {d}z. \end{aligned}$$
(1.13)

In particular, by denoting \( \sigma := \rho ^{1/2} \), from (1.12) and (1.13), one has either \( \sigma = 0 \) or

$$\begin{aligned}&\partial _t\sigma + {\overline{v}} \cdot \nabla _h\sigma + \frac{1}{2} \sigma \overline{\mathrm {div}_h\,v} = 0, \end{aligned}$$
(1.14)
$$\begin{aligned}&\sigma w = - \int _0^z \bigl ( \sigma {\widetilde{\mathrm {div}_h\,v}} + 2 {\widetilde{v}} \cdot \nabla _h\sigma \bigr ) \,\hbox {d}z. \end{aligned}$$
(1.15)

In fact, for \( (\sigma , v) \) regular enough, (1.14), (1.15) hold regardless of whether \( \sigma = 0 \) or not. See also the justification in the beginning of section 3.2.

System (1.2) is complemented with the initial data

$$\begin{aligned} (\rho , v)|_{t=0} = (\rho _0, v_0), ~~ \text {or equivalently} ~ (\sigma , v)|_{t=0} = (\sigma _0, v_0), \end{aligned}$$
(1.16)

with \( \sigma _0 = \rho _0^{1/2}, v_0 \in H^2(\Omega ) \), and the initial total mass and physical energy satisfy

$$\begin{aligned} \begin{aligned}&0< \int _{\Omega }\rho _0 \,\hbox {d}{x}= \int _{\Omega }\sigma _0^2 \,\hbox {d}{x}= M< \infty , \\&0< \int _{\Omega }\rho _0 \bigl | v_0 \bigr |^{2} \,\hbox {d}{x}+ \dfrac{1}{\gamma -1} \int _{\Omega }\rho _0^\gamma \,\hbox {d}{x}= \int _{\Omega }\sigma _0^2 \bigl | v_0 \bigr |^{2} \,\hbox {d}{x}\\&\qquad + \dfrac{1}{\gamma -1} \int _{\Omega }\sigma _0^{2\gamma } \,\hbox {d}{x}= E_0 < \infty . \end{aligned} \end{aligned}$$
(1.17)

Also the following compatible conditions are imposed:

$$\begin{aligned} \begin{aligned}&\rho _0 \geqq 0, ~~ \partial _zv_0|_{z=0,1} = 0, \\&\mu \Delta _hv_0 + \mu \partial _{zz} v_0 + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v_0 - \nabla _h\rho _0^\gamma - \rho _0 v_0 \cdot \nabla _hv_0 \\&- \rho _0 w_0 \partial _zv_0 =: \rho _0^{1/2} h_1, ~~~~ \text {with} ~ h_1 \in L^{2}(\Omega ),\\&\text {and} ~~ \rho _0 w_0 = - \int _0^z \mathrm {div}_h\,(\rho _0 \widetilde{v_0}) \,\hbox {d}z. \end{aligned} \end{aligned}$$
(1.18)

Also, we will denote the bounds

$$\begin{aligned} \bigl \Vert \sigma _0 \bigr \Vert _{H^{2}}^2 = \bigl \Vert \rho _0^{1/2} \bigr \Vert _{H^{2}}^2 \leqq B_1, ~~ \bigl \Vert v_0 \bigr \Vert _{H^{2}}^2 + \bigl \Vert h_1 \bigr \Vert _{L^{2}}^2 \leqq B_2. \end{aligned}$$
(1.19)

Moreover, if \( \rho = \sigma ^2 > 0 \), (1.2) can be written as

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\sigma + v \cdot \nabla _h\sigma + w \partial _z\sigma + \dfrac{1}{2} \sigma (\mathrm {div}_h\,v + \partial _zw) = 0 &{} \text {in} ~ \Omega ,\\ \sigma ^2 (\partial _tv + v \cdot \nabla _hv + w \partial _zv) + \nabla _h\sigma ^{2\gamma } \\ ~~~~ ~~~~ = \mu \Delta _hv + \mu \partial _{zz} v + (\mu + \lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _z\sigma = 0 &{} \text {in} ~ \Omega . \end{array}\right. } \end{aligned}$$
(1.20)

Theorem 2

Suppose the initial data \( (\rho _0,v_0) = (\sigma _0^2, v_0) \) satisfy (1.17), (1.19) and the compatible conditions (1.18). Then there is a unique strong solution \( (\rho , v) \) to system (1.2), with the boundary condition (1.3), in \( \Omega \times (0,T^*) \), for some positive constant \( T^* = T^*(B_1,B_2) > 0 \). Also, the solution satisfies

$$\begin{aligned}&\rho ^{1/2} \in L^\infty (0,T^*;H^2(\Omega )), ~~ \partial _t\rho ^{1/2} \in L^\infty (0,T^*; H^1(\Omega )), \\&v \in L^\infty (0,T^*;H^2(\Omega ))\cap L^2(0,T^*;H^3(\Omega )), ~~ \partial _tv \in L^2(0,T^*;H^1(\Omega ))\\&\rho ^{1/2} \partial _tv \in L^\infty (0,T^*;L^2(\Omega )). \end{aligned}$$

Furthermore, for some positive constant \( {\mathcal {C}}(B_1,B_2) \),

$$\begin{aligned}&\inf _{({x},t) \in \Omega \times (0,T^*)} \rho ({x},t) \geqq 0,\\&\qquad \sup _{0\leqq t\leqq T^*} \bigl ( \bigl \Vert \rho ^{1/2}(t) \bigr \Vert _{H^{2}}^2+\bigl \Vert \partial _t\rho ^{1/2}(t) \bigr \Vert _{H^{1}}^2 + \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert (\rho ^{1/2} v_{t})(t) \bigr \Vert _{L^{2}}^2 \bigr ) \\&\qquad + \int _0^{T^*} \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_{t}(t) \bigr \Vert _{H^{1}}^2 \bigr ) \,\hbox {d}t \leqq {\mathcal {C}}(B_1,B_2). \end{aligned}$$

Moreover, for any two strong solutions \( (\rho _i, v_i), i = 1,2 \), with initial data \( (\rho _{i,0},v_{i,0}) , i = 1,2 \), satisfying the conditions mentioned above, we have the inequality

$$\begin{aligned} \begin{aligned}&\bigl \Vert \rho _1^{1/2} - \rho _2^{1/2} \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))} + \bigl \Vert \rho _1^{1/2}(v_1- v_2) \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))} \\&\qquad + \bigl \Vert \rho _2^{1/2}(v_1- v_2) \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))} + \bigl \Vert v_1 - v_2 \bigr \Vert _{L^2(0,T^*;L^2(\Omega ))} \\&\qquad + \bigl \Vert \nabla (v_1 - v_2) \bigr \Vert _{L^2(0,T^*;L^2(\Omega ))}\\&\quad \leqq C_{\mu , \lambda , B_{1}, B_{2}, T^*}\bigl ( \bigl \Vert \rho _{1,0}^{1/2}-\rho _{2,0}^{1/2} \bigr \Vert _{L^2(\Omega ))} + \bigl \Vert v_{1,0}-v_{2,0} \bigr \Vert _{L^2(\Omega ))}\bigr ) \end{aligned} \end{aligned}$$

for some positive constant \( C_{\mu , \lambda , B_{1}, B_{2}, T^*} \).

1.3 Preliminaries

We will use \( \bigl | \cdot \bigr |_{}, \bigl \Vert \cdot \bigr \Vert _{} \) to denote norms in \( \Omega _h \subset {\mathbb {R}}^2 \) and \( \Omega \subset {\mathbb {R}}^3 \), respectively. After applying Ladyzhenskaya’s and Agmon’s inequalities in \( \Omega _h \) and \( \Omega \), directly we have

$$\begin{aligned} \begin{aligned}&\bigl | f \bigr |_{L^{4}} \leqq C \bigl | f \bigr |_{L^{2}}^{1/2} \bigl | f \bigr |_{H^{1}}^{1/2}, ~ \bigl | f \bigr |_{L^{\infty }} \leqq C \bigl | f \bigr |_{L^{2}}^{1/2} \bigl | f \bigr |_{H^{2}}^{1/2}, \\&\bigl \Vert f \bigr \Vert _{L^{3}} \leqq C \bigl \Vert f \bigr \Vert _{L^{2}}^{1/2} \bigl \Vert f \bigr \Vert _{H^{1}}^{1/2} \end{aligned} \end{aligned}$$
(1.21)

for any function f with bounded right-hand sides. Also, \( \bigl | {{\overline{f}}} \bigr |_{L^{p}}, \bigl \Vert {{\widetilde{f}}} \bigr \Vert _{L^{p}} \leqq C \bigl \Vert f \bigr \Vert _{L^{p}} \), for every \( p \geqq 1 \). Considering any quantities AB, we use the notation \( A \lesssim B \) to denote \( A \leqq CB \) for some generic positive constant C, which may be different from line to line. In what follows \( \delta , \omega > 0 \) are arbitrary constants which will be chosen later in the relevant paragraphs to be adequately small. \( C_q \) represents a positive constant depending on the quantity q. We will also need the following classical inequality:

Lemma 1

Let \( 2\leqq p \leqq 6 \), and \( \rho \geqq 0 \) such that \( 0< \int _{\Omega }\rho \,\hbox {d}{x}= M <\infty \), and \( \int _{\Omega }\rho ^\gamma \,\hbox {d}{x}\leqq E_0 \), for some \( \gamma \in (1, \infty ) \). Then one has

$$\begin{aligned} \bigl \Vert f \bigr \Vert _{L^{p}} \leqq C \bigl \Vert \nabla f \bigr \Vert _{L^{2}} + C \bigl \Vert \rho ^{1/2} f \bigr \Vert _{L^{2}} \end{aligned}$$
(1.22)

for some constant \( C = C(M, E_0) \), provided the right-hand side is finite.

Proof

This is standard. See, e.g., [18, Lemma 3.2]. \(\quad \square \)

2 Associated Linear Systems and Existence Theory

In this section, we will establish the local existence theory of (1.1) and (1.2). To do this, we will first study the local existence of solutions to (1.5) and (1.20) via the Schauder–Tchonoff fixed point theorem capitalizing on some a priori estimates. In fact, under the assumption that

$$\begin{aligned} \rho _0 = {\left\{ \begin{array}{ll} \xi _0 + \dfrac{1}{2} gz &{}\text {in the case with gravity}\\ (\sigma _0)^2 &{} \text {in the case without gravity} \end{array}\right. }> {\underline{\rho }} > 0, \end{aligned}$$
(2.1)

we will first introduce linear systems and the function spaces \( {\mathfrak {Y}} \) associated with (1.5) and (1.20) with some given input states \( (\xi ^o,v^o) \) and \((\sigma ^o,v^o)\), respectively, in section 2.1 and 2.2. Here, \( {\mathfrak {Y}} \) are compactly embedded in some corresponding spaces \( {\mathfrak {V}} \). Also, we will show that the maps \( {\mathcal {T}}: {\mathfrak {X}} \mapsto {\mathfrak {X}} \), for some convex bounded subsets \( {\mathfrak {X}} \) of \( {\mathfrak {Y}} \), given by

$$\begin{aligned} (\xi ^o, v^o) \leadsto (\xi ,v) ~~~~&\text {in the case with gravity, and}\\ (\sigma ^o, v^o) \leadsto (\sigma ,v) ~~~~&\text {in the case without gravity,} \end{aligned}$$

are well-defined; observing that \( {\mathfrak {X}} \) are convex subsets of \({\mathfrak {Y}} \) and hence compact in \( {\mathfrak {V}} \). We will use the same notations \( {\mathfrak {X}}, {\mathfrak {Y}}, {\mathfrak {V}}, {\mathcal {T}} \) to denote the convex bounded sets, the compact function spaces, the embedded function spaces and the constructed maps in both cases. We summarize the relevant regularity estimates in section 2.3 and show that the Schauder-Tchonoff fixed point theorem will yield the existence of solutions to (1.5) and (1.20) in the corresponding set. Recall that the Schauder-Tchonoff fixed point theorem states that for a Banach space V with a convex compact subset \( X \subset V \), if \(F : X \mapsto X\) is continuous, then F has at least one fixed point in X. In our case, we will take \( X = {\mathfrak {X}} \) and \( V = {\mathfrak {V}} := \lbrace (\xi ,v)| \xi , v \in L^\infty (0,T;L^2(\Omega )), \nabla v\in L^2(0,T;L^2(\Omega )) \rbrace \) in the case with gravity, or \( V= {\mathfrak {V}} := \lbrace (\sigma ,v)| \sigma , v \in L^\infty (0,T;L^2(\Omega )), \nabla v\in L^2(0,T;L^2(\Omega )) \rbrace \) in the case without gravity, with the corresponding norms.

We will only sketch the key steps in this paper. For more detailed calculation, we refer to our preprint [30].

2.1 The Case with Gravity and \( \gamma = 2 \)

2.1.1 Associated Linear Inhomogeneous System

Consider a finite positive time T, which will be determined later. Let \( {\mathfrak {Y}} = {\mathfrak {Y}}_T \) be the function space defined by

$$\begin{aligned} \begin{aligned} {\mathfrak {Y}} = {\mathfrak {Y}}_T: =&\lbrace (\xi , v)| \xi \in L^\infty (0,T;H^2(\Omega )), \partial _t\xi \in L^\infty (0,T;H^1(\Omega )), \\&v \in L^\infty (0,T;H^2(\Omega ))\cap L^2(0,T;H^3(\Omega )), \\&\partial _tv \in L^\infty (0,T;L^2(\Omega ))\cap L^2(0,T;H^1(\Omega )) \rbrace , \end{aligned} \end{aligned}$$
(2.2)

with the norm

$$\begin{aligned} \bigl \Vert (\xi ,v) \bigr \Vert _{{\mathfrak {Y}}} :=&\bigl \Vert \xi \bigr \Vert _{L^\infty (0,T;H^{2}(\Omega ))} + \bigl \Vert \partial _t\xi \bigr \Vert _{L^\infty (0,T;H^{1}(\Omega ))} + \bigl \Vert v \bigr \Vert _{L^\infty (0,T;H^{2}(\Omega ))}\\&+ \bigl \Vert v \bigr \Vert _{L^2(0,T;H^{3}(\Omega ))} + \bigl \Vert \partial _tv \bigr \Vert _{L^\infty (0,T;L^{2}(\Omega ))} + \bigl \Vert \partial _tv \bigr \Vert _{L^2(0,T;H^{1}(\Omega ))}. \end{aligned}$$

Notice that, thanks to the Aubin compactness theorem (see, e.g., [42, Theorem 2.1] and [11, 40]), every bounded subset of \( {\mathfrak {Y}} \) is a compact subset of the space

$$\begin{aligned} {\mathfrak {V}} = {\mathfrak {V}}_T := \lbrace (\xi ,v)| \xi , v \in L^\infty (0,T;L^2(\Omega )), \nabla v\in L^2(0,T;L^2(\Omega )) \rbrace , \end{aligned}$$
(2.3)

with the norm

$$\begin{aligned} \begin{aligned} \bigl \Vert (\xi ,v) \bigr \Vert _{{\mathfrak {V}}} :=&\bigl \Vert \xi \bigr \Vert _{L^\infty (0,T;L^{2}(\Omega ))} + \bigl \Vert v \bigr \Vert _{L^\infty (0,T;L^{2}(\Omega ))} \\&+ \bigl \Vert v \bigr \Vert _{L^2(0,T;H^{1}(\Omega ))}. \end{aligned} \end{aligned}$$
(2.4)

Let \( {\mathfrak {X}} = {\mathfrak {X}}_{T} \) be a bounded subset of \( {\mathfrak {Y}} \) defined by

$$\begin{aligned} \begin{aligned} {\mathfrak {X}} =&{\mathfrak {X}}_T:= \bigl \lbrace (\xi , v)\in {\mathfrak {Y}}| (\xi , v)|_{t=0} = (\xi _0,v_0), \partial _zv|_{z=0,1} = 0, \partial _z\xi = 0, \\&\xi + \dfrac{1}{2}gz \geqq \dfrac{1}{2} {\underline{\rho }} > 0,\sup _{0\leqq t\leqq T} \bigl \Vert \xi (t) \bigr \Vert _{H^{2}}^2 \leqq 2 M_0, \sup _{0\leqq t\leqq T} \bigl \Vert \partial _t\xi (t) \bigr \Vert _{H^{1}}^2 \leqq C_2, \\&\sup _{0\leqq t\leqq T} \lbrace \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv(t) \bigr \Vert _{L^{2}}^2 \rbrace + \int _0^T \biggl ( \bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 \\&+ \bigl \Vert \partial _tv(t) \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t \leqq C_1 M_1 \bigr \rbrace , \end{aligned} \end{aligned}$$
(2.5)

where \( M_0, M_1 \) are the bounds of initial data in (2.9) and \(C_1 = C_1(M_0,\mu ,\lambda ,{{\underline{\rho }}})\), \( C_2 = C_2(M_0,C_1M_1) \) are given below in (2.34), (2.17), respectively. Notice, for \( (\xi , v) \in {\mathfrak {X}} \),

$$\begin{aligned} \int _0^1\bigl ( \mathrm {div}_h\,(\xi {\widetilde{v}}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v} \bigr ) \,\hbox {d}z = 0. \end{aligned}$$

Let \( (\xi ^o,v^o) \in {\mathfrak {X}} \). The following inhomogeneous linear system is inferred from (1.5) using \((\xi ^o,v^o) \) as an input:

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\xi + \overline{v^o} \cdot \nabla _h\xi + \xi \overline{\mathrm {div}_h\,v^o} + \dfrac{g}{2} \overline{z \mathrm {div}_h\,v^o} = 0 &{} \text {in} ~ \Omega ,\\ (\xi ^o + \dfrac{1}{2}gz) ( \partial _tv + v^o \cdot \nabla _hv^o + w^o \partial _zv^o) + (2\xi ^o +gz) \nabla _h\xi ^o \\ ~~~~ ~~~~ ~~~~ = \mu \Delta _hv + \mu \partial _{zz} v + (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _z\xi = 0 &{} \text {in} ~ \Omega . \end{array}\right. } \end{aligned}$$
(2.6)

Here \( w^o \) is given by (1.8) with \( (\xi ^o, v^o) \) instead of \( (\xi , v) \), i.e.,

$$\begin{aligned} \rho ^o w^o = (\xi ^o + \dfrac{1}{2}gz) w^o := - \int _0^z \bigl ( \mathrm {div}_h\,(\xi ^o \widetilde{v^o}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v^o} \bigr ) \,\hbox {d}z. \end{aligned}$$
(2.7)

Notice that (2.6)\(_{1}\) is inferred from (1.7). For details, see the deviation from (1.5) to (1.8). Hereafter, denote by \( \rho ^o := \xi ^o + \dfrac{1}{2}gz \) . The initial and boundary conditions for the linear system (2.6) are given by

$$\begin{aligned} (\xi ,v)|_{t=0} = (\xi _0,v_0),~ \partial _zv|_{z=0,1} = 0. \end{aligned}$$
(2.8)

The compatible conditions in (1.10) are still imposed and we require

$$\begin{aligned} \bigl \Vert \xi _0 \bigr \Vert _{H^{2}}^2 \leqq M_0, \bigl \Vert v_0 \bigr \Vert _{H^{2}}^2 + \bigl \Vert V_1 \bigr \Vert _{L^{2}}^2 \leqq M_1. \end{aligned}$$
(2.9)

Recall that \( V_1 \) is given in (1.10), essentially, \( V_1 = v_t|_{t=0} \).

Then the map \( {\mathcal {T}} \), in this case, is defined as

$$\begin{aligned} {\mathcal {T}}: (\xi ^o,v^o) \leadsto (\xi , v), \end{aligned}$$
(2.10)

where \( (\xi , v) \) is the unique solution to the linear system (2.6) with \( (\xi ^o, v^o) \in {\mathfrak {X}} \). We claim that \( {\mathcal {T}} \) is a well defined map from \( {\mathfrak {X}} \) to \( {\mathfrak {X}} \), which is the consequence of the following two propositions:

Proposition 1

For given \( (\xi ^o,v^o) \in C^\infty ({\overline{\Omega }}\times [0,T]) \cap {\mathfrak {X}}_T \), there is a unique strong solution \( (\xi , v) \in {\mathfrak {Y}}_T \) of system (2.6) with the initial and boundary conditions (2.8).

Suppose, in addition, that \( \xi _0, v_0 \in H^3(\Omega ) \). One will have the following regularity of the unique solution \( (\xi , v) \) of system (2.6):

$$\begin{aligned} \begin{aligned}&\xi \in L^\infty (0,T;H^3(\Omega )), ~ \partial _t\xi \in L^\infty (0,T; H^2(\Omega )), \\&v \in L^\infty (0,T;H^3(\Omega ))\cap L^2 (0,T;H^4(\Omega )), \\&\partial _tv \in L^\infty (0,T;H^1(\Omega )) \cap L^2(0,T;H^2(\Omega )). \end{aligned} \end{aligned}$$
(2.11)

Proposition 2

Consider the initial data with the bounds \( M_0, M_1 \) in (2.9) and \( (\xi ^o, v^o) \in {\mathfrak {X}} = {\mathfrak {X}}_T \). There is a \( T_g = T_g(M_0,M_1,\mu ,\lambda ,{{\underline{\rho }}}) > 0 \) sufficiently small such that for any \( T \in (0, T_g) \), there exists a unique solution to (2.6). Moreover, the solution belongs to \( {\mathfrak {X}} = {\mathfrak {X}}_T \). Therefore, for any such T, the map \( {\mathcal {T}} \) in (2.10) is a well defined map from \( {\mathfrak {X}} \) into \( {\mathfrak {X}} \).

We omit the proof of Proposition 1, and refer to [30] for the details. The existence of solutions in Proposition 2 follows from Proposition 1 and a standard approximating argument. We only show the required a priori estimates in the rest of this subsection, which are sufficient to establish Proposition 2. See Propositions 3 and 4, below.

2.1.2 A Priori Estimates for the Inhomogeneous Linear System

Hereafter, we assume that the solution \( (\xi , v) \) to the linear system (2.6) is smooth enough so that the following estimates are rigorous.

We start by establishing some estimates for the solutions of (2.6)\(_{1}\). In particular, we will establish the following:

Proposition 3

There exists a \( T'= T'(M_0,C_1 M_1,{{\underline{\rho }}}) > 0 \) sufficiently small such that for any \( T \in (0,T'] \), the solution \( \xi \) to (2.6)\(_{1}\) satisfies that

$$\begin{aligned} \xi + \dfrac{1}{2} gz \geqq \dfrac{1}{2} {\underline{\rho }} ; \sup _{0\leqq t\leqq T} \bigl \Vert \xi (t) \bigr \Vert _{H^{2}}^2 \leqq 2 M_0 ; \sup _{0\leqq t\leqq T} \bigl \Vert \partial _t\xi (t) \bigr \Vert _{H^{1}}^2 \leqq C_2 , \end{aligned}$$

where \( M_0 \) is as in (2.9).

The lower bound for \( \xi \)

In order to derive the lower bound of \( \xi \), we employ the following Stampaccia-like argument. Let \( M = M(t) > 0 \) be a nonnegative integrable function to be determined later. Consider \( \eta = \eta (x,y,t) := \xi - {{\underline{\rho }}} + \int _0^t M(s) \,\hbox {d}s \). Then according to (2.6)\(_{1}\), \( \eta \) satisfies the equation

$$\begin{aligned}&\partial _t\eta + \overline{v^o} \cdot \nabla _h\eta + \eta \overline{\mathrm {div}_h\,v^o} = -({{\underline{\rho }}} - \int _0^t M(s) \,\hbox {d}s ) \overline{\mathrm {div}_h\,v^o} \\&\qquad - \dfrac{g}{2} \overline{z \mathrm {div}_h\,v^o} + M(t). \end{aligned}$$

Let

$$\begin{aligned} \mathbb {1}_{\lbrace \eta< 0 \rbrace } = {\left\{ \begin{array}{ll} 1 &{} \text {whenever} ~ \lbrace \eta < 0 \rbrace ,\\ 0 &{} \text {otherwise}, \end{array}\right. } \end{aligned}$$

and denote by \( \eta _- := - \eta \mathbb {1}_{\lbrace \eta < 0 \rbrace } \geqq 0 \). Observe that since \( \xi \in H^1(\Omega \times [0,T]) \), so is \( \eta _- \). Thus, one has

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} \int _{\Omega _h}\eta _- \,\hbox {d}{x}_h= \int _{\lbrace \eta < 0\rbrace }\biggl ( ({{\underline{\rho }}} - \int _0^t M(s)\,\hbox {d}s) \overline{\mathrm {div}_h\,v^o} + \dfrac{g}{2} \overline{z \mathrm {div}_h\,v^o} - M(t) \biggr ) \,\hbox {d}{x}_h. \end{aligned}$$

Now, let \( 0< M(t) := C \max \lbrace \bigl | \overline{\mathrm {div}_h\,v^o} \bigr |_{L^{\infty }}, \bigl | \overline{z \mathrm {div}_h\,v^o} \bigr |_{L^{\infty }} \rbrace \leqq C \bigl \Vert v^o \bigr \Vert _{H^{3}} < \infty , a.e.,\), for some constant \( C>0 \). Then the integrand on the right-hand side of the above equation satisfies

$$\begin{aligned}&\biggl ({{\underline{\rho }}} - \int _0^t M(s)\,\hbox {d}s\biggr ) \overline{\mathrm {div}_h\,v^o} + \dfrac{g}{2} \overline{z \mathrm {div}_h\,v^o} - M(t) \\&\quad \leqq \dfrac{1}{C} \biggl ( {{\underline{\rho }}} + C \int _0^T \bigl \Vert v^o \bigr \Vert _{H^{3}}(s) \,\hbox {d}s + \dfrac{g}{2} \biggr )M(t) - M(t) < 0, \end{aligned}$$

provided C is large enough and T is small enough such that

$$\begin{aligned} \dfrac{1}{C} \bigl ( {{\underline{\rho }}} + 1 + \dfrac{g}{2} \bigr )&< 1, ~ \text {and} \\ C \int _0^T \bigl \Vert v^o \bigr \Vert _{H^{3}}(s) \,\hbox {d}s&\leqq C T^{1/2} \bigl ( \int _0^T \bigl \Vert v^o \bigr \Vert _{H^{3}}^2(s)\,\hbox {d}s\bigr )^{1/2} < 1 . \end{aligned}$$

Therefore, we have

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} \int _{\Omega _h}\eta _- \,\hbox {d}{x}_h\leqq 0 ~~ a.e., \end{aligned}$$

which, after integrating over \( [0,t_0] \) for any \( t_0 \in [0,T] \), thanks to the fact \( \eta _-(0) \equiv 0 \), yields

$$\begin{aligned} \int _{\Omega _h}\eta _-(t_0) \,\hbox {d}{x}_h\leqq 0. \end{aligned}$$
(2.12)

Hence \( \eta _- = 0 ~ \text {in} ~ \Omega _h \times [0,T] \). That is, \( \eta (t) = \xi (t) - {{\underline{\rho }}} + \int _0^t M(s) \,\hbox {d}s \geqq 0 \) and

$$\begin{aligned} \begin{aligned} \xi (t) + \dfrac{1}{2} gz&> \xi (t) \geqq {{\underline{\rho }}} - C \int _0^T \bigl \Vert v^o \bigr \Vert _{H^{3}}(s) \,\hbox {d}s \\&\geqq {{\underline{\rho }}} - C C_1^{1/2} M_1^{1/2} T^{1/2} \geqq \dfrac{1}{2} {{\underline{\rho }}} \end{aligned} \end{aligned}$$
(2.13)

for \( t \in [0,T], ~ T \leqq T_1 \), with \( T_1= T_1(C_1M_1, {{\underline{\rho }}}) \) sufficiently small.

The \( H^2(\Omega ) \) norm for \(\xi \)

Applying the standard \( H^2 \) estimate of linear transport equations to (2.6)\(_{1}\) yields, since \( \bigl | \xi \bigr |_{H^{2}} = \bigl \Vert \xi \bigr \Vert _{H^{2}} \),

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} \bigl \Vert \xi \bigr \Vert _{H^{2}}^2 \leqq C \bigl \Vert v^o \bigr \Vert _{H^{3}}\bigl \Vert \xi \bigr \Vert _{H^{2}}^2 + C \bigl \Vert v^o \bigr \Vert _{H^{3}}\bigl \Vert \xi \bigr \Vert _{H^{2}}. \end{aligned}$$

Thanks to the Grönwall and the Hölder inequalities, one has

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T}\bigl \Vert \xi (t) \bigr \Vert _{H^{2}}^2 \leqq \dfrac{1}{4} \sup _{0\leqq t\leqq T} \bigl \Vert \xi \bigr \Vert _{H^{2}}^2 + C^2 C_1M_1 e^{2C C_1^{1/2}M_1^{1/2}T^{1/2}}T \\&\qquad + e^{C C_1^{1/2}M_1^{1/2}T^{1/2}} M_0. \end{aligned} \end{aligned}$$
(2.14)

Then for \( T \in (0, T_2] \) with \( T_2(M_0,C_1M_1) \) sufficiently small, (2.14) yields

$$\begin{aligned} \sup _{0\leqq t\leqq T} \bigl \Vert \xi (t) \bigr \Vert _{H^{2}}^2 \leqq 2 M_0. \end{aligned}$$
(2.15)

The \( H^1(\Omega ) \) norm for \( \partial _t\xi \) Using equation (2.6)\(_{1}\), \( \partial _t\xi \) can be represented in terms of the spatial derivatives of \( \xi , v^o \). Then applying the Hölder and the Sobolev embedding inequalities implies

$$\begin{aligned} \bigl \Vert \partial _t\xi \bigr \Vert _{H^{1}}^2 \leqq C \bigl \Vert v^o \bigr \Vert _{H^{2}}^2\bigl \Vert \xi \bigr \Vert _{H^{2}}^2 + C \bigl \Vert v^o \bigr \Vert _{H^{2}}^2 \leqq C_2, \end{aligned}$$
(2.16)

where \( C_2 = C_2(M_0,C_1M_1) \) is given by

$$\begin{aligned} C (1 + 2M_0)C_1M_1 =: C_2. \end{aligned}$$
(2.17)

Proof of Proposition 3

By choosing \( T' = \min \lbrace T_1,T_2 \rbrace \), the proof of Proposition 3 follows from (2.13), (2.15) and (2.16). \(\quad \square \)

Next, we show

Proposition 4

There exists a \( T''=T''(M_0,M_1,C_1,C_2,{{\underline{\rho }}}) \in (0,\infty ) \), sufficiently small, such that for every \( T \in (0, T''] \), the solution v to (2.6)\(_{2}\) satisfies

$$\begin{aligned} \sup _{0\leqq t\leqq T} (\bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{L^{2}}^2 ) + \int _0^T \biggl ( \bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{H^{1}}^2\biggr ) \,\hbox {d}t \leqq C_1M_1. \end{aligned}$$

Horizontal spatial derivative estimates for v

Applying \( \partial _{hh} \) to (2.6)\(_{2}\) will yield

$$\begin{aligned} \begin{aligned}&\rho ^o \partial _t\partial _{hh} v - \mu \Delta _h\partial _{hh} v - \mu \partial _{zz} \partial _{hh} v - (\mu + \lambda ) \nabla _h\mathrm {div}_h\,\partial _{hh} v \\&\quad = - 2 \partial _h \rho ^o \partial _t\partial _h v - \partial _{hh} \rho ^o \partial _tv - \partial _{hh} (\rho ^o v^o \cdot \nabla _hv^o) - \partial _{hh} (\rho ^o w^o \partial _zv^o) \\&\qquad - \partial _{hh}((2\xi ^o + gz) \nabla _h\xi ^o), \end{aligned}\nonumber \\ \end{aligned}$$
(2.18)

where \( \rho ^o = \xi ^o + \frac{1}{2} gz \). After taking the inner product of (2.18) with \( \partial _{hh} v \) and integrating by parts, one has

$$\begin{aligned}&\dfrac{\text {d}}{\text {d}t} \biggl \lbrace \dfrac{1}{2} \int _{\Omega }\rho ^o \bigl | \partial _{hh}v \bigr |^{2} \,\hbox {d}{x}\biggr \rbrace + \int _{\Omega }\biggl ( \mu \bigl | \nabla _h\partial _{hh}v \bigr |^{2} + \mu \bigl | \partial _{hhz} v \bigr |^{2} + (\mu +\lambda ) \nonumber \\&\qquad \times \bigl | \mathrm {div}_h\,\partial _{hh} v \bigr |^{2} \biggr ) \,\hbox {d}{x}= - \int _{\Omega }\biggl ( 2 \partial _h \rho ^o \partial _t\partial _h v \cdot \partial _{hh} v + \partial _{hh} \rho ^o \partial _tv \cdot \partial _{hh} v \biggr ) \,\hbox {d}{x}\nonumber \\&\qquad + \dfrac{1}{2} \int _{\Omega }\partial _t\rho ^o \bigl | \partial _{hh}v^o \bigr |^{2} \,\hbox {d}{x}+ \int _{\Omega }\partial _h ( \rho ^o v^o \cdot \nabla _hv^o) \cdot \partial _{hhh} v \,\hbox {d}{x}\nonumber \\&\qquad + \int _{\Omega }\partial _h(\rho ^o w^o \partial _zv^o) \cdot \partial _{hhh} v \,\hbox {d}{x}+ \int _{\Omega }\partial _h((2\xi ^o + gz) \nabla _h\xi ^o) \cdot \partial _{hhh} v \,\hbox {d}{x}\nonumber \\&\quad =: \sum _{i=1}^{5}I_{i}. \end{aligned}$$
(2.19)

While we only will omit the detailed estimates, which are standard, we list below the estimates for the \( I_i\) terms (see [30] for details). We will use the fact \( \bigl \Vert \rho ^o \bigr \Vert _{H^{2}}^2 \leqq C \bigl \Vert \xi ^o \bigr \Vert _{H^{2}}^2 + C g^2 \leqq C M_0 + C \), \( \bigl \Vert \partial _t\rho ^o \bigr \Vert _{L^{2}}^2 = \bigl \Vert \partial _t\xi ^o \bigr \Vert _{L^{2}}^2 \leqq C_2\). Also, hereafter the estimates hold for every \( \delta , \omega >0 \) which will be chosen later to be adequately small. Correspondingly, \( C_{\delta }, C_{\omega }, C_{\delta ,\omega } \) are some positive constants depending on \( \delta , \omega \):

$$\begin{aligned}&I_1 \lesssim \delta \bigl \Vert \partial _{hh}v \bigr \Vert _{H^{1}}^2 + \omega \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 + C_{\delta ,\omega } (M_0^2 + 1) (\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv \bigr \Vert _{L^{2}}^2). \\&I_2 \lesssim \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + C_\omega C_2^2 C_1M_1. \\&I_3 \lesssim \delta \bigl \Vert \partial _{hh} v \bigr \Vert _{H^{1}}^2 + C_\delta (M_0 + 1) C_1^2 M_1^2. \\&I_5 \lesssim \delta \bigl \Vert \partial _{hh} v \bigr \Vert _{H^{1}}^2 + C_\delta ( M_0^2+ 1). \end{aligned}$$

In order to estimate \( I_4 \), we shall plug in (2.7). One has

$$\begin{aligned}&I_4 = \int _{\Omega }\partial _h (\rho ^o w^o) \partial _zv^o \cdot \partial _{hhh} v\,\hbox {d}{x}+ \int _{\Omega }\rho ^o w^o \partial _{hz} v^o \cdot \partial _{hhh} v\,\hbox {d}{x}\\&\quad = - \int _0^1 \int _{\Omega _h}\biggl [\int _0^z \bigl [\partial _h\mathrm {div}_h\,(\xi ^o \widetilde{v^o}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,\partial _h v^o} \bigr ]\,\hbox {d}z' \partial _zv^o \cdot \partial _{hhh} v \biggr ]\,\hbox {d}{x}_h\,\hbox {d}z \\&\qquad - \int _0^1 \int _{\Omega _h}\biggl [\int _0^z \bigl [\mathrm {div}_h\,(\xi ^o \widetilde{v^o}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v^o} \bigr ]\,\hbox {d}z' \partial _{hz} v^o \cdot \partial _{hhh} v \biggr ]\,\hbox {d}{x}_h\,\hbox {d}z \\&\quad = :I_4' + I_4''. \end{aligned}$$

Then applying the Minkowski and the Sobolev embedding inequalities yields

$$\begin{aligned}&I_4'' = - \int _0^1 \int _0^z \int _{\Omega _h}\bigl [\mathrm {div}_h\,(\xi ^o \widetilde{v^o}) + \dfrac{g}{2} \widetilde{z \mathrm {div}_h\,v^o} \bigr ]{({x}_h,z',t)} \\&\qquad \times \bigl [\partial _{hz} v^o \cdot \partial _{hhh} v \bigr ]({x}_h,z,t) \,\hbox {d}{x}_h\,\hbox {d}z' \,\hbox {d}z \\&\quad \lesssim \int _0^1 \biggl ( \bigl | \nabla _h\xi ^o \bigr |_{L^{4}} \bigl | \widetilde{v^o} \bigr |_{L^{\infty }} + \bigl | \xi ^o \bigr |_{L^{\infty }} \bigl | \widetilde{\nabla _hv^o} \bigr |_{L^{4}} + \bigl | \widetilde{\nabla _hv^o} \bigr |_{L^{4}} \biggr ) \,\hbox {d}z'\\&\qquad \times \int _0^1 \bigl | \partial _{hz} v^o \bigr |_{L^{4}} \bigl | \partial _{hhh} v \bigr |_{L^{2}} \,\hbox {d}z \lesssim (\bigl \Vert \xi ^o \bigr \Vert _{H^{2}} + 1)\bigl \Vert v^o \bigr \Vert _{H^{2}}^{3/2}\bigl \Vert v^o \bigr \Vert _{H^{3}}^{1/2}\\&\qquad \times \bigl \Vert \partial _{hh} v \bigr \Vert _{H^{1}} \lesssim \delta \bigl \Vert \partial _{hh} v \bigr \Vert _{H^{1}}^2 +\omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + C_{\delta ,\omega } ( M_0^2 + 1) C_1^3M_1^{3}, \end{aligned}$$

and, similarly,

$$\begin{aligned} I_4' \lesssim \delta \bigl \Vert \partial _{hh}v \bigr \Vert _{H^{1}}^2 + \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + C_{\delta ,\omega } (M_0^2 + 1) C_1^3M_1^3, \end{aligned}$$

where we have employed (1.21). Summing up the above inequalities, with \( \delta \) small enough, yields the following estimate

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} \bigl \Vert \sqrt{\rho ^o}\partial _{hh} v \bigr \Vert _{L^{2}}^2 + c_{\mu , \lambda } \bigl \Vert \partial _{hh} v \bigr \Vert _{H^{1}}^2 \lesssim \omega ( \bigl \Vert \partial _tv \bigr \Vert _{H^{1}}^2 + \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 ) \\&\qquad + C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2) (\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv \bigr \Vert _{L^{2}}^2 + 1). \end{aligned} \end{aligned}$$

Hereafter, \( {\mathcal {H}} \) will be used to denote a polynomial quantity of its arguments (i.e., the norms of the initial data and \(\xi ^o, v^o\)) which may be different from line to line. Also \( c_{\mu ,\lambda }, C_\omega \) denote positive constants depending on \( \mu , \lambda \) and \( \omega \), respectively. Similar arguments also hold for the lower order derivatives. Then after suitable choice of \( \omega \), one has

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t}\bigl ( \bigl \Vert \sqrt{\rho ^o}v \bigr \Vert _{L^{2}}^2 +\bigl \Vert \sqrt{\rho ^o}\nabla _hv \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sqrt{\rho ^o}\nabla _h^2 v \bigr \Vert _{L^{2}}^2 \bigr ) + c_{\mu , \lambda }\bigl ( \bigl \Vert v \bigr \Vert _{H^{1}}^2 \\&\qquad + \bigl \Vert \nabla _hv \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2 \bigr ) \leqq \omega \bigl ( \bigl \Vert \partial _tv \bigr \Vert _{H^{1}}^2 + \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 \bigr ) \\&\qquad + C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2) \bigl (\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv \bigr \Vert _{L^{2}}^2 + 1\bigr ). \end{aligned} \end{aligned}$$
(2.20)

Time derivative estimates for v

Applying \( \partial _t\) to (2.6)\(_{2}\) yields

$$\begin{aligned} \begin{aligned}&\rho ^o \partial _t v_t - \mu \Delta _hv_t - \mu \partial _{zz} v_t - (\mu + \lambda ) \nabla _h\mathrm {div}_h\,v_t = - \partial _t\rho ^o \partial _tv \\&\qquad - \partial _t(\rho ^o v^o \cdot \nabla _hv^o) - \partial _t(\rho ^o w^o \partial _zv^o) - \partial _t((2\xi ^o + gz) \nabla _h\xi ^o ). \end{aligned} \end{aligned}$$
(2.21)

Consequently, one has

$$\begin{aligned}&\dfrac{\text {d}}{\text {d}t} \dfrac{1}{2} \int _{\Omega }\rho ^o \bigl | \partial _tv \bigr |^{2} \,\hbox {d}{x}+ \int _{\Omega }\biggl ( \mu \bigl | \nabla _hv_t \bigr |^{2} + \mu \bigl | \partial _zv_t \bigr |^{2} + (\mu +\lambda ) \bigl | \mathrm {div}_h\,v_t \bigr |^{2} \biggr ) \,\hbox {d}{x}{\nonumber } \\&\quad = - \dfrac{1}{2} \int _{\Omega }\partial _t\rho ^o \bigl | v_t \bigr |^{2} \,\hbox {d}{x}- \int _{\Omega }\partial _t(\rho ^o v^o \cdot \nabla _hv^o) \cdot \partial _tv \,\hbox {d}{x}\nonumber \\&\qquad - \int _{\Omega }\partial _t(\rho ^o w^o \partial _zv^o) \cdot \partial _tv\,\hbox {d}{x}- \int _{\Omega }\partial _t((2\xi ^o + gz) \nabla _h\xi ^o ) \cdot \partial _tv \,\hbox {d}{x}\nonumber \\&\quad =: \sum _{i=6}^{9}I_i. \end{aligned}$$
(2.22)

Then applying similar estimates as before to the right-hand side of (2.22) (see [30] for details) and making use of the fact that \( w^o \) is given by (2.7), one has

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} \bigl \Vert \sqrt{\rho ^o}v_t \bigr \Vert _{L^{2}}^2 + c_{\mu ,\lambda } \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \leqq \omega \bigl ( \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + \bigl \Vert \partial _tv^o \bigr \Vert _{H^{1}}^2 \bigr ) \\&\qquad + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2) \bigl ( \bigl \Vert \partial _tv \bigr \Vert _{L^{2}}^2 + 1 \bigr ). \end{aligned} \end{aligned}$$
(2.23)

Vertical derivative estimates for v

Taking the inner product of (2.6)\(_{2}\) with \( v_t \) and applying similar estimates as before to the resultant implies

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} \bigl ( \mu \bigl \Vert \nabla _hv \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _zv \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2 \bigr ) + \bigl \Vert \sqrt{\rho ^o}v_t \bigr \Vert _{L^{2}}^2 \\&\quad \leqq {\mathcal {H}}(M_0,C_1M_1,C_2,{{\underline{\rho }}}). \end{aligned}\qquad \end{aligned}$$
(2.24)

On the other hand, (2.6)\(_{2}\) can be written as

$$\begin{aligned} \begin{aligned}&\mu \partial _{zz} v - \rho ^o \partial _tv = - \mu \Delta _hv - (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v + \rho ^o v^o \cdot \nabla _hv^o \\&\quad + \rho ^o w^o \partial _zv^o + (2\xi ^o + gz) \nabla _h\xi ^o. \end{aligned} \end{aligned}$$
(2.25)

Taking the inner product of (2.25) with \( \partial _t\partial _{zz}v \) and applying similar estimates as before will yield,

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} ( \mu \bigl \Vert \nabla _h\partial _z v \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _{zz} v \bigr \Vert _{L^{2}}^2 + (\mu +\lambda )\bigl \Vert \mathrm {div}_h\,\partial _z v \bigr \Vert _{L^{2}}^2 ) \\&\qquad + c_{\mu ,\lambda } \bigl \Vert \sqrt{\rho ^o}\partial _zv_t \bigr \Vert _{L^{2}}^2 \leqq \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 \\&\qquad + C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2,{{\underline{\rho }}}) (\bigl \Vert v_t \bigr \Vert _{L^{2}}^2 +1). \end{aligned} \end{aligned}$$
(2.26)

Next, applying \( \partial \in \lbrace \partial _x, \partial _y, \partial _z \rbrace \) to (2.25) yields

$$\begin{aligned} \begin{aligned}&\mu \partial \partial _{zz} v - \rho ^o \partial v_t = \partial \rho ^o \partial _tv - \mu \Delta _h\partial v - (\mu + \lambda ) \nabla _h\mathrm {div}_h\,\partial v \\&\qquad + \partial (\rho ^o v^o \cdot \nabla _hv^o) + \partial (\rho ^o w^o \partial _zv^o) + \partial ((2\xi ^o + gz) \nabla _h\xi ^o) . \end{aligned} \end{aligned}$$
(2.27)

This implies

$$\begin{aligned}&\mu \bigl \Vert \partial \partial _{zz} v \bigr \Vert _{L^{2}} \lesssim \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}} + (M_0^{1/2} + 1) (\bigl \Vert v_t \bigr \Vert _{H^{1}} + M_0^{1/2} + C_1 M_1 )\\&\qquad + \bigl \Vert \partial (\rho ^o w^o) \partial _zv^o \bigr \Vert _{L^{2}} + \bigl \Vert \rho ^o w^o \partial \partial _zv^o \bigr \Vert _{L^{2}}. \end{aligned}$$

Also, by employing the Minkowski’s inequality, one obtains

$$\begin{aligned}&\bigl \Vert \partial _h (\rho ^o w^o) \partial _zv^o \bigr \Vert _{L^{2}}^2 \lesssim \int _0^1 \bigl | \partial _h(\rho ^o w^o) \bigr |_{L^{2}}^2 \bigl | \partial _zv^o \bigr |_{L^{\infty }}^2 \,\hbox {d}z \\&\quad \lesssim \biggl ( \int _0^1 \bigl ( \bigl | \nabla _h^2 (\xi ^o \widetilde{v^o}) \bigr |_{L^{2}} + \bigl | \widetilde{\nabla _h^2 v^o} \bigr |_{L^{2}} \bigr ) \,\hbox {d}z' \biggr )^2 \times \int _0^1 \bigl | \partial _zv^o \bigr |_{H^{1}}\bigl | \partial _zv^o \bigr |_{H^{2}}\,\hbox {d}z \\&\quad \lesssim (\bigl \Vert \xi ^o \bigr \Vert _{H^{2}}^2 + 1) \bigl \Vert v^o \bigr \Vert _{H^{2}}^3 \bigl \Vert v^o \bigr \Vert _{H^{3}} \lesssim \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + C_\omega (M_0^2 + 1) C_1^3 M_1^3, \end{aligned}$$

and, similarly,

$$\begin{aligned}&\bigl \Vert \rho ^o w^o \partial \partial _zv^o \bigr \Vert _{L^{2}}^2 \lesssim \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + C_\omega (M_0^2 + 1) C_1^3 M_1^3, \\&\bigl \Vert \partial _z(\rho ^o w^o) \partial _zv^o \bigr \Vert _{L^{2}}^2 \lesssim ( M_0 + 1) C_1^2M_1^2. \end{aligned}$$

Therefore, we have

$$\begin{aligned} \begin{aligned}&\bigl \Vert \partial _{zz} v \bigr \Vert _{H^{1}}^2 \leqq C\bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2 + C ( M_0 + 1) \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 + \omega \bigl \Vert v^o \bigr \Vert _{H^{3}}^2\\&\quad + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2). \end{aligned} \end{aligned}$$
(2.28)

Proof of Proposition 4

From (2.20), (2.23), (2.24) and (2.26), there is a constant \( c_{\mu , \lambda , {{\underline{\rho }}}} \) such that

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} {\mathcal {E}}_g(t) + c_{\mu ,\lambda ,{{\underline{\rho }}}}\bigl ( \bigl \Vert v \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _hv \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \bigr ) \\&\quad \leqq \omega \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 + \omega \bigl ( \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t^o \bigr \Vert _{H^{1}}^2 \bigr )\\&\qquad + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}} ) ( {\mathcal {E}}_g(t) + 1), \end{aligned} \end{aligned}$$
(2.29)

where

$$\begin{aligned} \begin{aligned}&{\mathcal {E}}_g(t) : = \bigl \Vert \sqrt{\rho ^o} v \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sqrt{\rho ^o} \nabla _hv \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sqrt{\rho ^o} \nabla _h^2 v \bigr \Vert _{L^{2}}^2 \\&\qquad + \bigl \Vert \sqrt{\rho ^o} v_t \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \nabla v \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2 \\&\qquad + \mu \bigl \Vert \nabla \partial _{z} v \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,\partial _{z}v \bigr \Vert _{L^{2}}^2. \end{aligned} \end{aligned}$$
(2.30)

Notice that for some positive constants \( C_{i,\mu , \lambda , {{\underline{\rho }}}, M_0}, i = 1,2 \), depending on \( \mu , \lambda , {{\underline{\rho }}}, M_0 \), we have

$$\begin{aligned} C_{1,\mu , \lambda , {{\underline{\rho }}},M_0} (\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t \bigr \Vert _{L^{2}}^2) \leqq {\mathcal {E}}_g(t) \leqq C_{2,\mu , \lambda , {{\underline{\rho }}},M_0} (\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t \bigr \Vert _{L^{2}}^2).\nonumber \\ \end{aligned}$$
(2.31)

For \( 0 < \omega \leqq \frac{c_{\mu ,\lambda ,{{\underline{\rho }}}}}{2}\), one infers from (2.29), that

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} {\mathcal {E}}_g(t) \leqq \omega ( \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t^o \bigr \Vert _{H^{1}}^2 ) + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}} ) ( {\mathcal {E}}_g(t) + 1 ). \end{aligned}$$

Therefore, applying the Grönwall’s inequality yields

$$\begin{aligned}&\sup _{0\leqq t\leqq T} {\mathcal {E}}_g(t) \leqq e^{C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}}) T } \biggl ( {\mathcal {E}}_g(0) + \omega \int _0^T ( \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t^o \bigr \Vert _{H^{1}}^2) \,\hbox {d}t \\&\qquad + \int _0^T C_\omega {\mathcal {H}}( M_0,C_1M_1,C_2,{{\underline{\rho }}})\,\hbox {d}t \biggr ) \leqq e^{C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}}) T} \biggl ( C_{2,\mu , \lambda , {{\underline{\rho }}},M_0} M_1 \\&\qquad + \omega C_1M_1 + C_\omega {\mathcal {H}}( M_0,C_1M_1,C_2,{{\underline{\rho }}}) T \biggr ), ~~ \text {where} \omega \text {is as above}. \end{aligned}$$

Now, we integrate with respect to the time variable inequality (2.29). It follows, since \( 0< \omega < \frac{c_{\mu ,\lambda ,{{\underline{\rho }}}}}{2} \), that

$$\begin{aligned}&\dfrac{c_{\mu ,\lambda ,{{\underline{\rho }}}}}{2} \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _hv \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2\biggr ) \,\hbox {d}t \leqq {\mathcal {E}}_g(0) + {\mathcal {E}}_g(t) \\&\qquad + \omega \int _0^T \biggl ( \bigl \Vert v^o \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t^o \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}} ) \int _0^T \bigl ( {\mathcal {E}}_g(t) + 1 \bigr ) \,\hbox {d}t \\&\quad \leqq \biggl (2 + TC_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}} )\biggr ) e^{C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}}) T} \\&\qquad \times \biggl ( C_{2,\mu , \lambda , {{\underline{\rho }}},M_0} M_1 + \omega C_1M_1 + C_\omega {\mathcal {H}}( M_0,C_1M_1,C_2,{{\underline{\rho }}}) T \biggr ). \end{aligned}$$

Additionally, from (2.28), we have

$$\begin{aligned}&\int _0^T \bigl \Vert \partial _{zz}v \bigr \Vert _{H^{1}}^2 \,\hbox {d}t \leqq C \int _0^T \biggl ( \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2 + (M_0+1) \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t \\&\qquad + \omega C_1M_1 + C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2) T. \end{aligned}$$

Therefore, we conclude that

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T} (\bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{L^{2}}^2 ) + \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2\biggr ) \,\hbox {d}t \\&\quad \leqq C_{1,\mu \lambda ,{{\underline{\rho }}}, M_0}^{-1} \sup _{0\leqq t\leqq T} {\mathcal {E}}_g(t) + \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _hv \bigr \Vert _{H^{1}}^2 + \bigl \Vert \nabla _h^2 v \bigr \Vert _{H^{1}}^2\\&\qquad + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t + \int _0^T \bigl \Vert \partial _{zz}v \bigr \Vert _{H^{1}}^2 \leqq (M_0+1) \biggl (C_{3,\mu ,\lambda ,{{\underline{\rho }}},M_0} \\&\qquad + C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}} )T \biggr ) \times e^{C_\omega {\mathcal {H}} (M_0,C_1M_1,C_2,{{\underline{\rho }}}) T}\biggl ( C_{2,\mu , \lambda , {{\underline{\rho }}},M_0} M_1 \\&\qquad + \omega C_1M_1 + C_\omega {\mathcal {H}}( M_0,C_1M_1,C_2,{{\underline{\rho }}}) T \biggr ) + \omega C_1M_1 \\&\qquad + C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2,{{\underline{\rho }}} )T, \end{aligned} \end{aligned}$$
(2.32)

for some positive constant \( C_{3,\mu ,\lambda ,{{\underline{\rho }}},M_0} \), depending on \( \mu , \lambda , {{\underline{\rho }}},M_0 \). Now fix \( \omega = \frac{1}{2} \min \lbrace \frac{c_{\mu ,\lambda ,{{\underline{\rho }}}}}{2}, \frac{1}{C_1} \rbrace \) and let \( T \in (0,T''] \), where \( T'' = T''(M_0,M_1,C_1,C_2,{{\underline{\rho }}}) \) is small enough and satisfying

$$\begin{aligned} C_\omega {\mathcal {H}}(M_0,C_1M_1,C_2,{{\underline{\rho }}} )T'' \leqq \min \lbrace 1,M_1\rbrace . \end{aligned}$$

Then (2.32) yields

$$\begin{aligned} \sup _{0\leqq t\leqq T} (\bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{L^{2}}^2 ) + \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \biggr )\,\hbox {d}t \leqq C_1M_1,\nonumber \\ \end{aligned}$$
(2.33)

where \( C_1 \) is given by

$$\begin{aligned} C_1:=(M_0+1)( C_{3,\mu ,\lambda ,{{\underline{\rho }}},M_0}e + e) (C_{2,\mu ,\lambda ,{{\underline{\rho }}},M_0} +2) + 2. \end{aligned}$$
(2.34)

This concludes the proof. \(\quad \square \)

2.2 The Case Without Gravity and \(\gamma > 1\)

Consider a finite positive time T, which will be determined later. Let \( {\mathfrak {Y}} = {\mathfrak {Y}}_T \) be the function space defined by

$$\begin{aligned} \begin{aligned} {\mathfrak {Y}} = {\mathfrak {Y}}_T :=&\lbrace (\sigma , v) | \sigma \in L^\infty (0,T;H^2(\Omega )), \partial _t\sigma \in L^\infty (0,T;H^1(\Omega )), \\&v \in L^\infty (0,T; H^2(\Omega )) \cap L^2(0,T;H^3(\Omega )), \\&\partial _tv \in L^\infty (0,T;L^2(\Omega )) L^2(0,T;H^1(\Omega )) \rbrace . \end{aligned} \end{aligned}$$
(2.35)

Notice that, thanks to Aubin compactness theorem (see [42, Theorem 2.1] and [11, 40]), every bounded subset of \( {\mathfrak {Y}} \) is a compact subset of the space

$$\begin{aligned} {\mathfrak {V}} = {\mathfrak {V}}_T := \lbrace (\sigma ,v)| \sigma , v \in L^\infty (0,T;L^2(\Omega )), \nabla v\in L^2(0,T;L^2(\Omega )) \rbrace .\nonumber \\ \end{aligned}$$
(2.36)

Let \( {\mathfrak {X}} = {\mathfrak {X}}_T \) be a bounded subset of \( {\mathfrak {Y}} \) defined by

$$\begin{aligned} \begin{aligned} {\mathfrak {X}} =&{\mathfrak {X}}_T := \biggl \lbrace (\sigma , v) \in {\mathfrak {Y}} | (\sigma , v)|_{t=0} = (\sigma _0, v_0), \partial _zv|_{z=0,1} = 0, \partial _z\sigma = 0, \\&\sigma ^2 \geqq \frac{1}{2} {\underline{\rho }} > 0, \sup _{0\leqq t\leqq T} \bigl \Vert \sigma (t) \bigr \Vert _{H^{2}}^2 \leqq 2M_0, \sup _{0\leqq t\leqq T} \bigl \Vert \partial _t\sigma (t) \bigr \Vert _{H^{1}}^2 \leqq C_2,\\&\sup _{0\leqq t\leqq T} \biggl ( \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{L^{2}}^2 \biggr ) + \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert \partial _tv \bigr \Vert _{H^{1}}^2 \biggr )\,\hbox {d}t\\&\leqq C_1M_1 \biggr \rbrace , \end{aligned}\nonumber \\ \end{aligned}$$
(2.37)

where \({{\underline{\rho }}}\) is the positive lower bound of initial density profile as in (2.1), for some positive constants \(C_1 = C_1(M_0,\mu ,\lambda ,{{\underline{\rho }}})\), \( C_2 = C_2(M_0,C_1M_1)\). Notice, for \( (\sigma , v) \in {\mathfrak {X}} \), \( \int _0^1 \mathrm {div}_h\,(\sigma ^2 {\widetilde{v}}) \,\hbox {d}z =0. \) Let \( (\sigma ^o, v^o) \in {\mathfrak {X}} \). The linear system inspired by (1.20) with \( (\sigma ^o, v^o ) \) as an input is given by

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\sigma + \overline{v^o} \cdot \nabla _h\sigma + \dfrac{1}{2} \sigma \mathrm {div}_h\,\overline{v^o} = 0 &{} \text {in} ~ \Omega ,\\ \rho ^o \partial _tv + \rho ^o v^o \cdot \nabla _hv^o + \sigma ^o \sigma ^o w^o \partial _zv^o + 2\gamma (\sigma ^o)^{2\gamma -1}\nabla _h\sigma ^o \\ ~~~~ ~~~~ = \mu \Delta _hv + \mu \partial _{zz} v + (\mu + \lambda ) \nabla _h\mathrm {div}_h\,v &{} \text {in} ~ \Omega ,\\ \partial _z\sigma = 0 &{} \text {in} ~ \Omega , \end{array}\right. } \end{aligned}$$
(2.38)

where \( \rho ^o = (\sigma ^o)^2 \) and \( w^o \) is determined, as in (1.15), by

$$\begin{aligned} \sigma ^o w^o := - \int _0^z \biggl ( \sigma ^o \mathrm {div}_h\,{\widetilde{v^o}} + 2 \widetilde{v^o} \cdot \nabla _h\sigma ^o \biggr ) \,\hbox {d}z. \end{aligned}$$
(2.39)

The initial and boundary conditions for system (2.38) are given by

$$\begin{aligned} (\sigma , v)|_{t=0} = (\sigma _0, v_0) = (\rho _0^{1/2}, v_0), ~ \partial _zv|_{z=0,1} = 0. \end{aligned}$$
(2.40)

Here, in addition to the compatible conditions in (1.18), we require \( \rho _0 \geqq {{\underline{\rho }}} > 0 \) , for some positive constant \( {{\underline{\rho }}} \) as in (2.1). Also, we denote by \( V_1 : = h_1 / \rho _0^{1/2} \). Recall that \( h_1 \) is given in (1.18). Then \( V_1 \in L^2 (\Omega ) \) and we require \( \bigl \Vert \sigma _0 \bigr \Vert _{H^{2}}^2 \leqq M_0, \bigl \Vert v_0 \bigr \Vert _{H^{2}}^2 + \bigl \Vert V_1 \bigr \Vert _{L^{2}}^2 \leqq M_1 \). Essentially \( V_1 = v_t|_{t=0} \).

Then the map \( {\mathcal {T}} \), in the case without gravity, is defined as

$$\begin{aligned} {\mathcal {T}}:(\sigma ^o, v^o) \leadsto (\sigma ,v), \end{aligned}$$
(2.41)

where \( (\sigma , v) \) is the unique solution to (2.38) for given \( (\sigma ^o, v^o) \in {\mathfrak {X}} \).

Proposition 5

There is a \( T_v = T_v(M_0,M_1,\mu ,\lambda ,{{\underline{\rho }}}) >0 \), sufficiently small, such that for every \( T \in (0, T_v] \), there is a unique solution \( ( \sigma , v) \) to (2.38) in the set \( {\mathfrak {X}} = {\mathfrak {X}}_T \). Therefore, for such T, the map \( {\mathcal {T}} \) defined in (2.41) is a well defined map from \( {\mathfrak {X}} \) into \( {\mathfrak {X}} \).

The proof is similar as Proposition 1 and Proposition 2 in section 2.1 and therefore is omitted.

2.3 Existence Theory

In this subsection, we will establish the existence of solutions for (1.1) and (1.2) for given corresponding initial data and boundary conditions.

2.3.1 The Case With Gravity and \( \gamma = 2 \), but Without Vacuum

We will apply the Schauder-Tchonoff fixed point theorem to establish the existence of strong solutions to (1.1). With Proposition 2, it is sufficient to verify that \( {\mathcal {T}} \), defined by (2.10), is continuous in \( {\mathfrak {V}} = {\mathfrak {V}}_T \) given in (2.3), where the norm is given by

figure a

In order to show this, let \( M_0 = B_{g,1} , M_1 = B_{g,2} \) in \( {\mathfrak {X}}_T \) and \( T \in (0, T_g] \), with \( T_g \) given in Proposition 2. Here \( B_{g,1}, B_{g,2} \) are given in (1.11). We denote \( (\xi _1^o,v_1^o), (\xi _2^o,v_2^o) \in {\mathfrak {X}}_T \) and

$$\begin{aligned} (\xi _1,v_1) = {\mathcal {T}} (\xi _1^o,v_1^o), (\xi _2,v_2) = {\mathcal {T}} (\xi _1^o,v_1^o). \end{aligned}$$

Denote by \( \xi _{12} : = \xi _1 - \xi _2, v_{12} := v_1 - v_2, \xi _{12}^o : = \xi _1^o - \xi _2^o, v^o_{12} := v_1^o - v_2^o \). Then \( (\xi _{12}, v_{12})|_{t=0} = 0 \). By taking the differences of the equations satisfied by \( (\xi _i, v_i), i = 1,2 \), we have

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\xi _{12} + \overline{v_1^o} \cdot \nabla _h\xi _{12} + \xi _{12} \overline{\mathrm {div}_h\,v_1^o} + \overline{v_{12}^o} \cdot \nabla _h\xi _2 + \xi _2 \overline{\mathrm {div}_h\,v^o_{12}}\\ ~~~~ ~~~~ + \dfrac{g}{2} \overline{z \mathrm {div}_h\,v_{12}^o} = 0, \\ \rho _1^o \partial _tv_{12} - \mu \Delta _hv_{12} - \mu \partial _{zz} v_{12} - (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v_{12} = - \xi _{12}^o \partial _tv_2 \\ ~~~~ ~~~~ - \nabla _h(\xi _{12}^o(\rho _1^o + \rho _2^o)) - \xi _{12}^o v_1^o \cdot \nabla _hv_1^o - \rho _2^o v_{12}^o \cdot \nabla _hv_1^o \\ ~~~~ ~~~~- \rho _2^o v_2^o \cdot \nabla _hv_{12}^o - (\rho _1^o w_1^o - \rho _2^o w_2^o) \partial _zv_1^o - \rho _2^o w_2^o \partial _zv_{12}^o. \end{array}\right. } \end{aligned}$$
(2.42)

Now we perform standard \( L^2 \) estimates for (2.42). Take the \( L^2 \)-inner product of (2.42)\(_{1}\) with \( 2 \xi _{12} \) and take the \( L^2 \)-inner product of (2.42)\(_{2}\) with \( 2 v_{12} \). Then applying similar estimates as before yields, together with the Grönwall inequality, that

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T} (\bigl \Vert \xi _{12}(t) \bigr \Vert _{L^{2}}^2 + \bigl \Vert v_{12}(t) \bigr \Vert _{L^{2}}^2 ) + \int _0^T \bigl \Vert \nabla v_{12}(t) \bigr \Vert _{L^{2}}^2 \,\hbox {d}t \\&\quad \leqq C_{M_0,C_1M_1,C_2,{{\underline{\rho }}}}\biggl ( \sup _{0<t<T} (\bigl \Vert \xi _{12}^o \bigr \Vert _{L^{2}}^2 + \bigl \Vert v_{12}^o \bigr \Vert _{L^{2}}^2 ) + \int _0^T \bigl \Vert \nabla v_{12}^o \bigr \Vert _{L^{2}}^2 \,\hbox {d}t\biggr ). \end{aligned}\nonumber \\ \end{aligned}$$
(2.43)

This implies the continuity of \( {\mathcal {T}} \) in \({\mathfrak {V}}\). Therefore, after applying the fixed point theorem mentioned before, we have the following:

Proposition 6

Consider

$$\begin{aligned} (\rho _0,v_0) = (\xi _0 + \dfrac{1}{2}gz, v_0), \end{aligned}$$

given in (1.9) satisfying (1.10) and (1.11). There is a positive constant T depending on the initial data such that there is a strong solution \( (\rho , v) = (\xi + \frac{1}{2}gz, v) \) to (1.1) (or equivalently (1.5)) with the boundary conditions (1.3) and with \( (\xi , v) \in {\mathfrak {X}}_T \).

2.3.2 The Case With Vacuum and \( \gamma > 1 \), but Without Gravity

When \( \rho _0 \geqq {\underline{\rho }} > 0 \), the existence of strong solutions to (1.2) follows from the estimates in section 2.2 and similar arguments to those in section 2.3.1. In fact, taking \( M_0 = B_1, M_1 = B_2 + {{\underline{\rho }}}^{-1} B_2 \), we have the following:

Proposition 7

Suppose that (1.17), (1.18), (1.19) hold for the given initial data (1.16) with \( \rho _0 \geqq {{\underline{\rho }}} > 0 \). Then there is a positive constant T, depending on the initial data and \( {{\underline{\rho }}} \), such that there exists a strong solution \((\rho , v) = (\sigma ^2, v) \) to (1.2) (or equivalently (1.20)) satisfying the boundary conditions (1.3) and that \( (\sigma ,v) \in {\mathfrak {X}}_T \).

In the following, we shall present some estimates independent of \({\underline{\rho }} \) and show that for a given non-negative initial density \( \rho _0 \geqq 0 \), there are strong solutions to equations (1.2). We will use here the notation \( \sigma ^2 = \rho \) and the alternative form of equations (1.20), as well as (1.2). Meanwhile, let us assume that

$$\begin{aligned} \begin{aligned} \bigl \Vert \sigma _0 \bigr \Vert _{H^{2}} = \bigl \Vert \rho _0^{1/2} \bigr \Vert _{H^{2}} \leqq K_1,\\ \bigl \Vert v_0 \bigr \Vert _{H^{2}} + \bigl \Vert h_1 \bigr \Vert _{L^{2}} \leqq K_2, \end{aligned} \end{aligned}$$
(2.44)

for given \( K_1, K_2 > 0 \). Recall essentially \( h_1 = (\sigma v_t)|_{t=0} \) from (1.18). Also, taking inner product of (1.2)\(_{2}\) with v yields, the conservation of physical energy

$$\begin{aligned} \begin{aligned}&\dfrac{1}{2}\int _{\Omega }\rho \bigl | v \bigr |^{2} \,\hbox {d}{x}+ \dfrac{1}{\gamma -1} \int _{\Omega }\rho ^\gamma \,\hbox {d}{x}+ \int _0^T \int _{\Omega }\biggl ( \mu \bigl | \nabla v \bigr |^{2} + (\mu +\lambda ) \bigl | \mathrm {div}_h\,v \bigr |^{2} \biggr ) \,\hbox {d}{x}\\&\quad = \dfrac{1}{2}\int _{\Omega }\rho _0 \bigl | v_0 \bigr |^{2} \,\hbox {d}{x}+ \dfrac{1}{\gamma -1} \int _{\Omega }\rho _0^\gamma \,\hbox {d}{x}< \infty , \end{aligned} \end{aligned}$$
(2.45)

where (1.2)\(_{1}\) is also applied. Also, integrating (1.2)\(_{1}\) over \( \Omega \times (0,T) \) yields the conservation of total mass

$$\begin{aligned} 0< \int _{\Omega }\rho \,\hbox {d}{x}= \int _{\Omega }\rho _0 \,\hbox {d}{x}= M < \infty . \end{aligned}$$
(2.46)

These facts are important when applying (1.22) in what follows.

A priori assumptions

Let \( (\sigma , v) \) be the solution to (1.20) given in Proposition 7. We assume first, for some constants \( C_d \geqq K_2^2 \), \( T_d \) (may depend on \( {\underline{\rho }} \)),

$$\begin{aligned} \sup _{0\leqq t\leqq T_d} \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \sigma v_t(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^{T_d}\biggl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t < C_d.\nonumber \\ \end{aligned}$$
(2.47)

In what follows, we will derive some a priori estimates independent of \( {\underline{\rho }} \). Also, we set \( T \in (0, T_d] \) to be determined later. We emphasize that the smallness of T in what follows is independent of \( {{\underline{\rho }}} \).

\( {\underline{\rho }} \)-independent lower bound: non-negativity of \( \rho \)

We will use the same Stampaccia-like argument as before to derive the lower bound of \( \rho \). Consider

$$\begin{aligned} \eta = \eta (x,y,t) := \dfrac{\rho }{\inf _{{x}_h \in \Omega _h} \rho _0({x}_h)} - 1 + \int _0^t 2 \bigl | \mathrm {div}_h\,{{\overline{v}}}(s) \bigr |_{L^{\infty }} \,\hbox {d}s, ~ \text {for} ~ t \in [0,T_d]. \end{aligned}$$

Due to (1.12), \( \eta \) satisfies

$$\begin{aligned}&\partial _t\eta + {{\overline{v}}} \cdot \nabla _h\eta + \eta \mathrm {div}_h\,{\overline{v}} = \bigl ( \int _0^t 2 \bigl | \mathrm {div}_h\,{{\overline{v}}}(s) \bigr |_{L^{\infty }}\,\hbox {d}s - 1 \bigr ) \times \mathrm {div}_h\,{\overline{v}} \\&\quad + 2 \bigl | \mathrm {div}_h\,{{\overline{v}}}(s) \bigr |_{L^{\infty }} \geqq - 2 \bigl | \mathrm {div}_h\,{{\overline{v}}} \bigr |^{} + 2 \bigl | \mathrm {div}_h\,{{\overline{v}}} \bigr |_{L^{\infty }} \geqq 0, \end{aligned}$$

for every \( t \in [0,T] \) with \( T \in (0,T_1] \), and \( T_1 \) sufficiently small such that

$$\begin{aligned} 2 \int _0^t \bigl | \mathrm {div}_h\,{\overline{v}}(s) \bigr |_{L^{\infty }}\,\hbox {d}s&\leqq 2 C \int _0^t \bigl \Vert v(s) \bigr \Vert _{H^{3}}\,\hbox {d}s \\&\quad \leqq 2 C T^{1/2} \bigl (\int _0^t \bigl \Vert v(s) \bigr \Vert _{H^{3}}^2 \,\hbox {d}s \bigr )^{1/2} \leqq 2 C C_d^{1/2} T_1^{1/2} \leqq \dfrac{1}{2}. \end{aligned}$$

Denote by \( \eta _-: = - \eta \mathbb {1}_{\lbrace \eta < 0 \rbrace } \geqq 0 \). Then multiplying the above equation with \( - \mathbb {1}_{\lbrace \eta < 0 \rbrace } \) and integrating the resultant in the spatial variable yield

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} \int _{\Omega _h}\eta _- \,\hbox {d}{x}_h\leqq 0. \end{aligned}$$

Hence, \( \eta _- = 0 \) in \( \Omega _h \times (0,T] \), since \( \eta _-(0) \equiv 0 \). Therefore, \( \eta \geqq 0 \) and

$$\begin{aligned} \begin{aligned}&\rho = \inf _{{x}_h\in \Omega _h} \rho _0 ({x}_h) \times \bigl ( \eta + 1 - \int _0^t 2 \bigl | \mathrm {div}_h\,{\overline{v}}(s) \bigr |_{L^{\infty }} \,\hbox {d}s \bigr ) \\&\quad \geqq \inf _{{x}_h\in \Omega _h} \rho _0({x}_h) \times \bigl ( 0 + 1 - \dfrac{1}{2} \bigr ) = \dfrac{1}{2} \inf _{{x}_h\in \Omega _h} \rho _0({x}_h). \end{aligned} \end{aligned}$$
(2.48)

\({\underline{\rho }} \)-independent estimate: \( H^2(\Omega ) \) for \( \sigma = \rho ^{1/2} \)

After performing standard \( H^2 \) estimate of (1.14) and applying the Grönwall inequality to the result, one has

$$\begin{aligned} \sup _{0\leqq t\leqq T} \bigl \Vert \sigma (t) \bigr \Vert _{H^{2}}^2 \leqq e^{C \int _0^T \bigl \Vert v \bigr \Vert _{H^{3}} \,\hbox {d}t } \bigl \Vert \sigma _0 \bigr \Vert _{H^{2}}^2 \leqq e^{CC_d^{1/2}T^{1/2}} K_1^2 < 2 K_1^2,\qquad \end{aligned}$$
(2.49)

for all \( T \in (0,T_2] \), provided \( T_2 \) is sufficiently small.

\( {\underline{\rho }} \)-independent estimate: \( H^1(\Omega ) \) for \( \sigma _t \)

It directly follows from (1.14) that

$$\begin{aligned} \bigl \Vert \partial _t\sigma (t) \bigr \Vert _{H^{1}} \leqq C \bigl \Vert v(t) \bigr \Vert _{H^{2}}\bigl \Vert \sigma (t) \bigr \Vert _{H^{2}} \leqq \sqrt{2} C C_d^{1/2} K_1 =: K_3', \end{aligned}$$
(2.50)

for all \( t \in [0,T] \).

\( {\underline{\rho }} \)-independent estimate: \( L^2(\Omega ) \) for \( v_t \)

Taking the time derivative of (1.20)\(_{2}\) yields

$$\begin{aligned} \begin{aligned}&\sigma ^2 \partial _tv_t - \mu \Delta _hv_t - \mu \partial _{zz} v_t - (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v_t =- 2 \sigma \partial _t\sigma \partial _tv \\&\qquad - \partial _t(\sigma ^2 v \cdot \nabla _hv) - \partial _t(\sigma ^2 w \partial _zv) - \partial _t\nabla _h\sigma ^{2\gamma }. \end{aligned} \end{aligned}$$
(2.51)

Taking the \(L^2\)-inner product of (2.51) with \( \partial _tv \) gives

$$\begin{aligned}&\dfrac{1}{2} \dfrac{\text {d}}{\text {d}t} \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \nabla _hv_t \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _zv_t \bigr \Vert _{L^{2}}^2+ (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,v_t \bigr \Vert _{L^{2}}^2 \nonumber \\&\quad = - \int _{\Omega }\sigma \partial _t\sigma \bigl | v_t \bigr |^{2}\,\hbox {d}{x}- \int _{\Omega }\partial _t(\sigma ^2 v \cdot \nabla _hv) \cdot v_t \,\hbox {d}{x}\nonumber \\&\qquad - \int _{\Omega }\partial _t(\sigma ^2 w \partial _zv) \cdot v_t \,\hbox {d}{x}+ \int _{\Omega }\partial _t\sigma ^{2\gamma } \mathrm {div}_h\,v_t \,\hbox {d}{x}=: \sum _{i=1}^{4} L_i. \end{aligned}$$
(2.52)

Then one has the following estimates to the terms in the right-hand side of (2.52) (see [30] for details):

$$\begin{aligned}&L_1 \lesssim \bigl \Vert \partial _t\sigma \bigr \Vert _{L^{2}} \bigl \Vert \sigma v_t \bigr \Vert _{L^{3}} \bigl \Vert v_t \bigr \Vert _{L^{6}} \lesssim \bigl \Vert \partial _t\sigma \bigr \Vert _{L^{2}}\bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^{1/2} \bigl \Vert \sigma \bigr \Vert _{L^{\infty }}^{1/2}\bigl \Vert v_t \bigr \Vert _{L^{6}}^{3/2} \\&\quad \lesssim \delta \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + C_\delta C_d (K_1^2 K_3'^4 + 1) . \\&L_2 \lesssim \bigl \Vert \partial _t\sigma \bigr \Vert _{L^{2}} \bigl \Vert v \bigr \Vert _{L^{\infty }} \bigl \Vert \nabla _hv \bigr \Vert _{L^{6}} \bigl \Vert \sigma v_t \bigr \Vert _{L^{3}} + \bigl \Vert \sigma v_t \bigr \Vert _{L^{3}} \bigl \Vert \nabla _hv \bigr \Vert _{L^{6}} \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}\\&\qquad + \bigl \Vert \sigma \bigr \Vert _{L^{\infty }}\bigl \Vert v \bigr \Vert _{L^{\infty }} \bigl \Vert \nabla _hv_t \bigr \Vert _{L^{2}} \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}} \lesssim \delta \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 \\&\qquad + C_\delta ( C_d + 1 )( K_1^2 C_d + K_1 K_3' C_d^2 + 1) . \\&L_4 \lesssim \bigl \Vert \sigma \bigr \Vert _{L^{\infty }}^{2\gamma -1} \bigl \Vert \partial _t\sigma \bigr \Vert _{L^{2}}\bigl \Vert \nabla v_t \bigr \Vert _{L^{2}} \lesssim \delta \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + C_\delta K_1^{4\gamma -2}K_3'^2. \end{aligned}$$

We have applied above the Hölder inequality and (1.22). Notice that \( \sigma = \rho ^{1/2} \) and that the conservations of energy and mass (2.45), (2.46) hold. In order to estimate \( L_3 \) term, we substitute (1.13) and integrate by parts. Then

$$\begin{aligned}&L_3 = - \int _{\Omega }\partial _t(\rho w) \partial _zv \cdot v_t \,\hbox {d}{x}- \int _{\Omega }\rho w \partial _zv_t \cdot v_t \,\hbox {d}{x}\\&\quad = - \int _0^1 \int _{\Omega _h}\biggl [\biggl ( \int _0^z (\sigma ^2 {\widetilde{v}})_t \,\hbox {d}z' \biggr ) \cdot \nabla _h(\partial _zv \cdot v_t) \biggr ]\,\hbox {d}{x}_h\,\hbox {d}z \\&\qquad + \int _0^1 \int _{\Omega _h}\biggl [\biggl ( \int _0^z \mathrm {div}_h\,(\sigma ^2 {\widetilde{v}}) \,\hbox {d}z' \biggr ) \times \bigl ( \partial _zv_t \cdot v_t \bigr ) \biggr ]\ \,\hbox {d}{x}_h\,\hbox {d}z =: L_3' + L_3''. \end{aligned}$$

Now we use (1.21), the Minkowski and Hölder inequalities,

$$\begin{aligned}&L_3' = - \int _0^1 \int _{\Omega _h}\biggl [\biggl ( \int _0^z \bigl ( \sigma \widetilde{v_t} + 2 \sigma _t {\widetilde{v}} \bigr ) \,\hbox {d}z' \biggr ) \cdot (\nabla _h\partial _zv \cdot \sigma v_t + \sigma \nabla _hv_t \cdot \partial _zv ) \biggr ]\,\hbox {d}{x}_h\,\hbox {d}z \\&\quad \lesssim \int _0^1 \bigl ( \bigl | \sigma \widetilde{v_t} \bigr |_{L^{2}} + \bigl | \sigma _t \bigr |_{L^{2}}\bigl | {{\widetilde{v}}} \bigr |_{L^{\infty }}\biggr ) \,\hbox {d}z' \times \int _0^1 \biggl ( \bigl | \sigma \bigr |_{L^{\infty }} \bigl | \nabla _h\partial _zv \bigr |_{L^{4}} \bigl | v_t \bigr |_{L^{4}} \\&\qquad + \bigl | \sigma \bigr |_{L^{\infty }} \bigl | \nabla _hv_t \bigr |_{L^{2}} \bigl | \partial _zv \bigr |_{L^{\infty }} \biggr ) \,\hbox {d}z \lesssim \int _0^1 \biggl ( \bigl | \sigma \widetilde{v_t} \bigr |_{L^{2}} + \bigl | \sigma _t \bigr |_{L^{2}} \bigl | {{\widetilde{v}}} \bigr |_{H^{2}} \biggr ) \,\hbox {d}z' \\&\qquad \times \int _0^1 \bigl | \sigma \bigr |_{H^{2}} \bigl | \partial _zv \bigr |_{H^{1}}^{1/2} \bigl | \partial _zv \bigr |_{H^{2}}^{1/2} \bigl | v_t \bigr |_{H^{1}} \,\hbox {d}z \lesssim \bigl \Vert \sigma \bigr \Vert _{H^{2}}(\bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}\\&\qquad + \bigl \Vert \sigma _t \bigr \Vert _{L^{2}} \bigl \Vert v \bigr \Vert _{H^{2}}) \bigl \Vert v \bigr \Vert _{H^{2}}^{1/2} \bigl \Vert v \bigr \Vert _{H^{3}}^{1/2} ( \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}} + \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}} )\\&\quad \lesssim \delta \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + \omega \bigl \Vert v \bigr \Vert _{H^{3}}^2 + C_{\delta ,\omega } C_d ( K_1^4(K_3'^4 + 1) C_d^2 + 1), \end{aligned}$$

and, similarly,

$$\begin{aligned}&L_3'' \lesssim \delta \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + C_\delta (K_1^{10} C_d^{4} + K_1^{10/3} C_d^2). \end{aligned}$$

After summing the above inequalities, (2.52) implies

$$\begin{aligned} \dfrac{\text {d}}{\text {d}t} \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 + c_{\mu ,\lambda } \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 \leqq \omega \bigl \Vert v \bigr \Vert _{H^{3}}^2 + C_\omega {\mathcal {H}}(K_1,K_3',C_d), \end{aligned}$$
(2.53)

where, as before, \( {\mathcal {H}} \) denotes a polynomial quantity of its arguments.

\( {\underline{\rho }} \)-independent estimate: spatial derivatives of v

Now we are able to derive the estimates on the spatial derivatives of v. Standard \( L^2 \) estimate of (1.20)\(_{2}\) yields that

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} \bigl \Vert \sigma v \bigr \Vert _{L^{2}}^2 + c_{\mu ,\lambda } \bigl \Vert \nabla v \bigr \Vert _{L^{2}}^2 \leqq C \bigl \Vert \sigma ^{2\gamma } \bigr \Vert _{L^{2}}^2 \leqq C {\mathcal {H}}(K_1). \end{aligned} \end{aligned}$$
(2.54)

Furthermore, taking the \( L^2 \)-inner product of (1.20)\(_{2}\) with \( v_t \) yields, after applying similar estimates as before,

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} (\mu \bigl \Vert \nabla _hv \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _zv \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2 ) + \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 \\&\quad \leqq {\mathcal {H}}(K_1,C_d). \end{aligned} \end{aligned}$$
(2.55)

Next we estimate the second order spatial derivatives. Taking the \( L^2 \)-inner product of (1.20)\(_{2}\) with \( \partial _{zz} v_t \) yields

$$\begin{aligned}&\dfrac{1}{2} \dfrac{\text {d}}{\text {d}t} ( \mu \bigl \Vert \nabla _h\partial _z v \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _{zz}v \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,\partial _z v \bigr \Vert _{L^{2}}^2 ) + \bigl \Vert \sigma \partial _z v_t \bigr \Vert _{L^{2}}^2 \nonumber \\&\quad = - \int _{\Omega }\partial _z( \sigma ^2 v \cdot \nabla _hv) \cdot \partial _zv_t \,\hbox {d}{x}- \int _{\Omega }\partial _z( \sigma ^2 w \partial _zv) \cdot \partial _zv_t \,\hbox {d}{x}=: L_8 + L_9. \end{aligned}$$
(2.56)

At the same time, taking the \( L^2 \)-inner product of (1.20)\(_{2}\) with \( \Delta _hv_t \) yields

$$\begin{aligned}&\dfrac{1}{2} \dfrac{\text {d}}{\text {d}t} ( \mu \bigl \Vert \nabla _h^2 v \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \nabla _h\partial _zv \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \nabla _h\mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2) \nonumber \\&\qquad + \bigl \Vert \sigma \nabla _hv_t \bigr \Vert _{L^{2}}^2 = - 2 \int _{\Omega }\bigl ( \sigma \nabla _h\sigma \cdot \nabla _hv_t \bigr ) \cdot v_t \,\hbox {d}{x}\nonumber \\&\qquad - \int _{\Omega }\nabla _h(\sigma ^2 v \cdot \nabla _hv) : \nabla _hv_t \,\hbox {d}{x}- \int _{\Omega }\nabla _h(\sigma ^2 w \partial _zv) : \nabla _hv_t \,\hbox {d}{x}\nonumber \\&\qquad - \int _{\Omega }\nabla _h^2 \sigma ^{2\gamma } : \nabla _hv_t\,\hbox {d}{x}=: \sum _{i=10}^{13} L_{i}. \end{aligned}$$
(2.57)

Applying similar estimates as before to the right-hand sides of (2.56) and (2.57) yields, after summing up the results,

$$\begin{aligned} \begin{aligned}&\dfrac{\text {d}}{\text {d}t} (\mu \bigl \Vert \nabla _h^2 v \bigr \Vert _{L^{2}}^2 + 2\mu \bigl \Vert \nabla _h\partial _zv \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _{zz} v \bigr \Vert _{L^{2}}^2\\&\qquad + (\mu + \lambda ) \bigl \Vert \nabla \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2) + \bigl \Vert \sigma \nabla v_t \bigr \Vert _{L^{2}}^2 \\&\quad \leqq \omega (\bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + \bigl \Vert v \bigr \Vert _{H^{3}}^2) + C_\omega {\mathcal {H}}(K_1,C_d). \end{aligned} \end{aligned}$$
(2.58)

Finally, we provide estimates for the third spatial derivative of v. Applying \( \partial \in \lbrace \partial _x, \partial _y,\partial _z \rbrace \) to (1.20)\(_{2}\) yields

$$\begin{aligned} \begin{aligned}&\mu \Delta _h\partial v + \mu \partial _{zz} \partial v + (\mu + \lambda ) \nabla _h\mathrm {div}_h\,\partial v = \partial (\sigma ^2 v_t) + \partial (\sigma ^2 v\cdot \nabla _hv) \\&\quad + \partial (\sigma ^2 w \partial _zv) + \partial \nabla _h\sigma ^{2\gamma }. \end{aligned} \end{aligned}$$
(2.59)

Taking the \( L^2 \)-inner product of (2.59) with \( \Delta _h\partial v \), for \( \partial \in \lbrace \partial _h, \partial _z \rbrace \), and integrating by parts imply, after applying the Cauchy-Schwarz inequality,

$$\begin{aligned}&\bigl \Vert \nabla ^3 v \bigr \Vert _{L^{2}}^2 \lesssim \bigl \Vert \nabla (\sigma ^2 v_t) \bigr \Vert _{L^{2}}^2 + \bigl \Vert \nabla (\sigma ^2 v\cdot \nabla _hv) \bigr \Vert _{L^{2}}^2 + \bigl \Vert \nabla (\sigma ^2 w \partial _zv) \bigr \Vert _{L^{2}}^2 \nonumber \\&\quad + \bigl \Vert \nabla _h^2 \sigma ^{2\gamma } \bigr \Vert _{L^{2}}^2. \end{aligned}$$
(2.60)

Then, similarly to (2.28), noticing the fact that w are given by (1.15), (2.60) yields

$$\begin{aligned} \bigl \Vert v \bigr \Vert _{H^{3}}^2 \leqq \bigl \Vert \nabla ^3 v \bigr \Vert _{L^{2}}^2 + C_d \leqq C K_1^4 \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + {\mathcal {H}}(K_1,C_d), \end{aligned}$$
(2.61)

where \( C_d \) is as in (2.47)

We summarize the estimates obtained, so far, in this section in the following:

Proposition 8

Consider the solution \((\sigma ,v) = (\rho ^{1/2},v) \) to (1.2) with the bound (2.47) and initial data satisfying (1.17), (2.44). There is a positive constant \( T^*=T^*(C_d,K_1,K_2) \), sufficiently small, such that \( (\sigma ,v) \) admits the following bounds, for \( T = \min \lbrace T^*, T_d \rbrace \),

$$\begin{aligned} \inf _{({x},t)\in \Omega \times [0,T]}\rho ({x},t) \geqq \dfrac{1}{2} \inf _{{x}\in \Omega }\rho _0 > 0 , ~ \sup _{0\leqq t\leqq T} \bigl \Vert \sigma (t) \bigr \Vert _{H^{2}} \leqq 2 K_1, \\ \sup _{0\leqq t\leqq T} \bigl \Vert \partial _t\sigma (t) \bigr \Vert _{H^{1}} \leqq K_3, \\ \sup _{0\leqq t\leqq T} \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert (\sigma v_t)(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^T \biggl ( \bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t(t) \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t \leqq K_4^2, \end{aligned}$$

where \( K_4 = \sqrt{2C_{\mu ,\lambda }} K_2, K_3 = CK_1K_4 \) are given in (2.63) and (2.64). Notably, the bounds in these estimates depend only on the initial bounds \( K_1, K_2 \) and do not depend on the lower bound of density. Also, the smallness of \( T^* \) does not depend on \( {{\underline{\rho }}} \), even though \( T_d \) may depend on \( {{\underline{\rho }}} \).

Proof

Denote by

$$\begin{aligned} \begin{aligned}&{\mathcal {E}}(t) := \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sigma v \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \nabla v \bigr \Vert _{L^{2}}^2 + (\mu +\lambda ) \bigl \Vert \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2 \\&\quad + \mu \bigl \Vert \nabla _h^2 v \bigr \Vert _{L^{2}}^2 + 2 \mu \bigl \Vert \nabla _h\partial _zv \bigr \Vert _{L^{2}}^2 + \mu \bigl \Vert \partial _{zz} v \bigr \Vert _{L^{2}}^2 \\&\quad + (\mu +\lambda )\bigl \Vert \nabla \mathrm {div}_h\,v \bigr \Vert _{L^{2}}^2. \end{aligned} \end{aligned}$$
(2.62)

Then from (2.53), (2.54), (2.55) and (2.58), we have

$$\begin{aligned}&\dfrac{\text {d}}{\text {d}t} {\mathcal {E}}(t) + c_{\mu ,\lambda } \bigl ( \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + \bigl \Vert \nabla v \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sigma \nabla v_t \bigr \Vert _{L^{2}}^2 \bigr )\\&\quad \leqq \omega \bigl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 \bigr ) + C_\omega {\mathcal {H}} (K_1,K_3',C_d). \end{aligned}$$

Then integrating the above inequality yields, for \( T \in (0, T_d] \), where \( T_d \) is as in (2.47),

$$\begin{aligned}&\sup _{0\leqq t\leqq T} {\mathcal {E}}(t) + c_{\mu ,\lambda } \int _0^T \biggl ( \bigl \Vert \nabla v_t \bigr \Vert _{L^{2}}^2 + \bigl \Vert \nabla v \bigr \Vert _{L^{2}}^2 + \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2 \\&\quad + \bigl \Vert \sigma \nabla v_t \bigr \Vert _{L^{2}}^2 \biggr ) \,\hbox {d}t \leqq {\mathcal {E}}(0) + \omega C_d + C_\omega T {\mathcal {H}}(K_1,K_3',C_d). \end{aligned}$$

Then together with (2.61), we have, after choosing \( \omega \) small enough and then T sufficiently small,

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T} \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \sigma v_t(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^T \biggl ( \bigl \Vert v \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_t \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t \\&\quad \leqq C_{\mu ,\lambda }K_2^2 + \omega C_d C_{\mu ,\lambda }(K_1^4+1) + C_\omega T {\mathcal {H}}(K_1,K_3',C_d) \\&\quad \leqq 2 C_{\mu ,\lambda } K_2^2 =: K_4^2, \end{aligned} \end{aligned}$$
(2.63)

where we have employed inequality (1.22) and the fact for some positive constant \( C_{\mu ,\lambda } > 0 \) we have

$$\begin{aligned} C^{-1}_{\mu ,\lambda }(\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2) \leqq {\mathcal {E}}(t) \leqq C_{\mu ,\lambda }(\bigl \Vert v \bigr \Vert _{H^{2}}^2 + \bigl \Vert \sigma v_t \bigr \Vert _{L^{2}}^2). \end{aligned}$$

Then plugging in (2.63) back into (2.50) implies

$$\begin{aligned} \bigl \Vert \partial _t\sigma \bigr \Vert _{H^{1}} \leqq C K_4 K_1 =: K_3. \end{aligned}$$
(2.64)

Thus the conclusion is drawn from (2.48), (2.49), (2.63) and (2.64). \(\quad \square \)

Existence of strong solutions with vacuum but no gravity and \( \gamma > 1 \)

Now we are in the place to remove the strict positivity (of the initial density profile) assumption in Proposition 7. In order to do so, we introduce a sequence of approximating initial data \( (\rho _{0,n}, v_{0,n} ) \) satisfying, in addition to (1.17), (1.18), (1.19),

$$\begin{aligned} \rho _{0,n} \geqq \dfrac{1}{n} > 0, \end{aligned}$$

such that

$$\begin{aligned} \rho _{0,n}^{1/2} \rightarrow \rho _0^{1/2}, v_{0,n} \rightarrow v_0 \end{aligned}$$

in \( H^2(\Omega ) \), as \( n \rightarrow \infty \), where \( (\rho _0, v_0 ) \) (or equivalently \( (\sigma _0, v_0) \)) is given in (1.16) satisfying (1.18) and (1.19).

We require that the initial physical energy and total mass given in (1.17) with \( \rho _0, v_0 \) replaced by \( \rho _{0,n}, v_{0,n} \) satisfy

$$\begin{aligned}&0< \int _{\Omega }\rho _{0,n} \,\hbox {d}{x}= M< \infty , \\&0< \int _{\Omega }\rho _{0,n} \bigl | v_{0,n} \bigr |^{2} \,\hbox {d}{x}+ \dfrac{1}{\gamma -1} \int _{\Omega }\rho _{0,n}^\gamma \,\hbox {d}{x}\leqq E_0 + 1 < \infty , \end{aligned}$$

uniformly in n, so that when we apply inequality (1.22), the constant in the inequality is independent of n.

Now we apply Proposition 7 with the initial data \( (\rho _{0,n},v_{0,n}) \). Indeed, consider \( M_0 = B_1 \), \( M_1 = B_2 + n B_2 \). Then Proposition 7 guarantees that there is a \( T_1 = T_1(n,B_1,B_2) \) such that (1.2) admits a strong solution \( (\rho _n, v_n) = (\sigma _n^2 , v_n) \) satisfying

$$\begin{aligned}&\sup _{0\leqq t\leqq T_1} \bigl \Vert \sigma _{n}(t) \bigr \Vert _{H^{2}}^2 \leqq 2B_1, ~~ \sup _{0\leqq t\leqq T_1} \bigl \Vert \partial _t\sigma _n(t) \bigr \Vert _{H^{1}}^2 \leqq {\mathcal {C}}_1(B_1,B_2,n), \\&\sup _{0\leqq t\leqq T_1} \bigl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \partial _tv_{n}(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^{T_1}\bigl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert \partial _tv_n(t) \bigr \Vert _{H^{1}}^2 \bigr ) \,\hbox {d}t \\&\quad \leqq {\mathcal {C}}_2(B_1,B_2,n), ~~ \text {and} ~ \rho _n = \sigma _n^2 \geqq \dfrac{1}{2n} . \end{aligned}$$

Sequently, we apply Proposition 8 with \( K_1 = B_1^{1/2}, K_2 = 2B_2^{1/2} \), \( C_d = (1 + 2 n) {\mathcal {C}}_2(B_1,B_2,n) \) and \( T_d = T_1 \). It yields that there is a \( T_2 = T_2(B_1,B_2,n) \leqq T_1 \) such that the following bounds are satisfied

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T_2} \bigl \Vert \sigma _n(t) \bigr \Vert _{H^{2}}\leqq 2 B_1^{1/2}, ~~ \sup _{0\leqq t\leqq T_2} \bigl \Vert \partial _t\sigma _n(t) \bigr \Vert _{H^{1}} \leqq {\mathcal {C}}_3 (B_1,B_2),\\&\sup _{0\leqq t\leqq T_2} \bigl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert (\sigma _n v_{n,t})(t) \bigr \Vert _{L^{2}}^2 \bigr ) \\&\quad + \int _0^{T_2} \bigl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_{n,t}(t) \bigr \Vert _{H^{1}}^2 \bigr ) \,\hbox {d}t \leqq {\mathcal {C}}_{4}(B_1,B_2), \\&\text {and} ~ \inf _{({x},t) \in \Omega \times [0,T_2]} \rho _n \geqq \dfrac{1}{2n}. \end{aligned} \end{aligned}$$
(2.65)

Next, let \( (\sigma _n,v_n)|_{t=T_2} \) as a new initial data for (1.2). The same arguments as above yield the bound (2.65) with lower bound of \( \rho _n \) replaced by \( \frac{1}{4n} \), \( B_1 \) replaced by \( 4 B_1 \) and \( B_2 \) replaced by \( {\mathcal {C}}_4(B_1,B_2) \). That is, for some \( \delta T =\delta T(B_1,B_2,n) > 0 \),

$$\begin{aligned}&\inf _{({x},t) \in \Omega \times (T_2,T_2+\delta T)} \rho _n \geqq \dfrac{1}{4n}, ~ \sup _{T_2<t<T_2+\delta T} \bigl \Vert \sigma _n(t) \bigr \Vert _{H^{2}}\leqq 4 B_1^{1/2},\\&\sup _{T_2<t<T_2+\delta T} \bigl \Vert \partial _t\sigma _n(t) \bigr \Vert _{H^{1}} \leqq {\mathcal {C}}_3 (4B_1,{\mathcal {C}}_{4}(B_1,B_2)),\\&\sup _{T_2<t<T_2+\delta T} \bigl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert \sigma _n v_{n,t}(t) \bigr \Vert _{L^{2}}^2 \bigr ) \\&\qquad + \int _{T_2}^{T_2+\delta T} \biggl ( \bigl \Vert v_n(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_{n,t}(t) \bigr \Vert _{H^{1}}^2 \biggr ) \,\hbox {d}t \\&\quad \leqq {\mathcal {C}}_{4}(4B_1,{\mathcal {C}}_{4}(B_1,B_2)). \end{aligned}$$

Now we apply Proposition 8 with \( T_d = T_2 + \delta T \) and \( C_d = {\mathcal {C}}_{4}(4B_1,{\mathcal {C}}_{4}(B_1,B_2)) \) in the time interval \( (0,T_d) \). This will yield that there is a \( T^* = T^*(B_1,B_2) \) and \( T_3 := \min \lbrace T^*, T_2 + \delta T \rbrace \), the bounds in (2.65) hold with \( T_2 \) replaced by \( T_3 \).

If \( T_3 = T^* \), we have got an existence time independent of n and this finishes the job. Otherwise, let \( (\sigma _n,v_n) |_{t= T_3} \) as a new initial data and repeat the arguments above to get the bounds in (2.65) with \( T_2 \) replaced by \( T_4:= \min \lbrace T^*, T_3 + \delta T\rbrace = \min \lbrace T^*, T_2 + 2 \delta T\rbrace \). Keep repeating this process, one will eventually get that there is a \( m \in {\mathbb {Z}}^+ \) sufficiently large that \( T_m := \min \lbrace T^*, T_2 + (m-2) \delta T \rbrace = T^* \).

Therefore, we have got a sequence of approximating solutions \( (\rho _n, v_n) = (\sigma _n^2, v_n) \) with a uniform existence time \( T^* \) independent of n for the approximating initial data \( (\rho _{0,n}, v_{0,n}) \) constructed above. In particular, \( (\sigma _n, v_n) \) satisfies the bounds in (2.65) with \( T_2 \) replaced by \( T^* \). Thus by taking \( n\rightarrow \infty \), it is straightforward to check that we have got a strong solution \( (\rho , v) = (\sigma ^2, v) \) to (1.2). In fact, we have the following:

Proposition 9

Consider the initial data \( (\rho _0, v_0) \) (or equivalently \( (\sigma _0, v_0) \)) given in (1.16) satisfying (1.17), (1.18) and (1.19). There is a constant \( T^* > 0 \) such that there exists a solution \( (\rho , v) = (\sigma ^{2}, v) \) to equation (1.2) satisfying

$$\begin{aligned} \begin{aligned}&\sigma \in L^\infty (0,T^*;H^2(\Omega )), ~~ \partial _t\sigma \in L^\infty (0,T^*; H^1(\Omega )), \\&v \in L^\infty (0,T^*;H^2(\Omega ))\cap L^2(0,T^*;H^3(\Omega )), ~~ \partial _tv \in L^2(0,T^*;H^1(\Omega ))\\&\sigma \partial _tv \in L^\infty (0,T^*;L^2(\Omega )), \end{aligned}\nonumber \\ \end{aligned}$$
(2.66)

and

$$\begin{aligned} \begin{aligned}&\sup _{0\leqq t\leqq T^*} \bigl \Vert \sigma (t) \bigr \Vert _{H^{2}}\leqq 2 B_1^{1/2}, ~~ \sup _{0\leqq t\leqq T^*} \bigl \Vert \partial _t\sigma (t) \bigr \Vert _{H^{1}} \leqq {\mathcal {C}}_3 (B_1,B_2),\\&\sup _{0\leqq t\leqq T^*} \bigl ( \bigl \Vert v(t) \bigr \Vert _{H^{2}}^2 + \bigl \Vert (\sigma v_{t})(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^{T^*} \bigl (\bigl \Vert v(t) \bigr \Vert _{H^{3}}^2 + \bigl \Vert v_{t}(t) \bigr \Vert _{H^{1}}^2 \bigr ) \,\hbox {d}t \\&\quad \leqq {\mathcal {C}}_{4}(B_1,B_2), ~~ \text {and} ~ \inf _{({x},t) \in \Omega \times [0,T^*]} \rho \geqq 0, \end{aligned}\nonumber \\ \end{aligned}$$
(2.67)

for some constant \( {\mathcal {C}}_3 = {\mathcal {C}}_3(B_1,B_2), {\mathcal {C}}_4 = {\mathcal {C}}_4(B_1,B_2) \).

3 Continuous Dependence on Initial Data and Uniqueness

In this section, we will show the continuous dependence of the solutions of (1.1) and (1.2) on the initial data. This will also imply the uniqueness of strong solutions constructed in Proposition 6 and Proposition 9.

3.1 The Case With Gravity and \( \gamma = 2 \), but Without Vacuum

Consider two sets of initial data \( (\rho _{i,0},v_{i,0}) = (\xi _{i,0}+\frac{1}{2}gz ,v_{i,0}), i = 1,2, \) in (1.9) for (1.1) satisfying (1.10), (1.11). Denote \( (\rho _i,v_i) = (\xi _i+\frac{1}{2} gz, v_i), i = 1,2, \) as the corresponding strong solutions constructed in Proposition 6 in the interval [0, T] for some \( T > 0 \). Then we have \((\xi _i, v_i) \in {\mathfrak {X}}_T, i=1,2 \). Throughout this section we will denote the constant \( C > 0 \) which may be different from line to line and depends on \( \mu , \lambda , B_{g,1}, B_{g,2},{{\underline{\rho }}}, T \). Also, we will use the notations

$$\begin{aligned}&\xi _{12} := \xi _1 - \xi _2, ~ v_{12} := v_1- v_2, \\&\xi _{12,0} := \xi _{1,0} - \xi _{2,0}, ~ v_{12,0} := v_{1,0}- v_{2,0}. \end{aligned}$$

Taking the difference of the equations satisfied by \( (\xi _i, v_i), i=1,2 \), as in (2.42), then \( (\xi _{12}, v_{12}) \) satisfies

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\xi _{12} + \overline{v_1} \cdot \nabla _h\xi _{12} + \xi _{12} \overline{\mathrm {div}_h\,v_1} + \overline{v_{12}} \cdot \nabla _h\xi _2 + \xi _2 \overline{\mathrm {div}_h\,v_{12}}\\ ~~~~ ~~~~ + \dfrac{g}{2} \overline{z \mathrm {div}_h\,v_{12}} = 0, \\ \rho _1 \partial _tv_{12} - \mu \Delta _hv_{12} - \mu \partial _{zz} v_{12} - (\mu +\lambda ) \nabla _h\mathrm {div}_h\,v_{12} = - \xi _{12} \partial _tv_2 \\ ~~~~ ~~~~ - \nabla _h(\xi _{12}(\rho _1 + \rho _2)) - \xi _{12} v_1 \cdot \nabla _hv_1 - \rho _2 v_{12} \cdot \nabla _hv_1 \\ ~~~~ ~~~~- \rho _2 v_2 \cdot \nabla _hv_{12} - (\rho _1 w_1 - \rho _2 w_2) \partial _zv_1 - \rho _2 w_2 \partial _zv_{12}. \end{array}\right. } \end{aligned}$$

Then after applying standard \( L^2 \) estimates to the above system and applying the Grönwall’s inequality to the resultant, one can show the following:

Proposition 10

Given two sets of initial data \( (\rho _{i,0},v_{i,0}) = (\xi _{i,0}+\frac{1}{2}gz ,v_{i,0}), i = 1,2 \), satisfying (1.10) and (1.11), the corresponding strong solutions \( (\rho _i,v_i) = (\xi _i+\frac{1}{2} gz, v_i), i = 1,2 \), of (1.1) constructed in Proposition 6 in the interval [0, T] , for some \( T > 0 \), satisfy

$$\begin{aligned} \begin{aligned}&\bigl \Vert \rho _1 - \rho _2 \bigr \Vert _{L^\infty (0,T;L^2(\Omega ))} + \bigl \Vert v_1- v_2 \bigr \Vert _{L^\infty (0,T;L^2(\Omega ))}\\&\quad + \bigl \Vert \nabla (v_1 - v_2) \bigr \Vert _{L^2(0,T;L^2(\Omega ))} \leqq C_{\mu , \lambda , B_{g,1}, B_{g,2},{{\underline{\rho }}}, T} \\&\quad \times (\bigl \Vert \rho _{1,0}-\rho _{2,0} \bigr \Vert _{L^2(\Omega ))} + \bigl \Vert v_{1,0}-v_{2,0} \bigr \Vert _{L^2(\Omega ))}). \end{aligned} \end{aligned}$$

In particular, if \( \rho _{1,0} = \rho _{2,0}, v_{1,0} = v_{2,0} \), we have \( \rho _1 =\rho _2, v_1 = v_2 \) in [0, T] .

3.2 The Case With Vacuum and \( \gamma > 1 \), but Without Gravity

First, we claim that any solution \( (\rho , v) = (\sigma ^2, v) \) to (1.2) satisfying (2.66) with the bounds in (2.67) will also satisfy the following equations:

figure b

To show this claim, we first consider the non-degenerate variable \( \rho + \varepsilon = \sigma ^2 + \varepsilon \), for some constant \( \varepsilon > 0 \). From (1.12), one has

$$\begin{aligned} \partial _t(\rho + \varepsilon ) + {\overline{v}} \cdot \nabla _h(\rho + \varepsilon ) + (\rho + \varepsilon ) \overline{\mathrm {div}_h\,v} - \varepsilon \overline{\mathrm {div}_h\,v} = 0. \end{aligned}$$

Then after dividing \( (\rho + \varepsilon )^{1/2} \), one has

$$\begin{aligned} \begin{aligned}&2 \partial _t(\rho + \varepsilon )^{1/2} + 2 {\overline{v}} \cdot \nabla _h(\rho + \varepsilon )^{1/2} + (\rho + \varepsilon )^{1/2} \overline{\mathrm {div}_h\,v} \\&\quad - \dfrac{\varepsilon }{(\rho + \varepsilon )^{1/2}} \overline{\mathrm {div}_h\,v} = 0. \end{aligned} \end{aligned}$$
(3.1)

Now it is easy to verify that (3.1) will converge to 1.2’\(_{1}\) in the sense of distribution, as \( \varepsilon \rightarrow 0 \). On the other hand, from (1.13), one has

$$\begin{aligned} \sigma ^2 w = - \sigma \int _0^z \bigl ( \sigma \widetilde{\mathrm {div}_h\,v} + 2 {\widetilde{v}} \cdot \nabla _h\sigma \bigr ) \,\hbox {d}z. \end{aligned}$$

We define

$$\begin{aligned} \sigma w_\sigma : = - \int _0^z \sigma \widetilde{\mathrm {div}_h\,v} + 2 {\widetilde{v}} \cdot \nabla _h\sigma \,\hbox {d}z. \end{aligned}$$

Then \( \sigma \sigma w_\sigma = \rho w \) and we will use hereafter the notation \( \sigma w = \sigma w_\sigma \). As before it is easy to verify that 1.2’\(_{3}\) is equivalent to 1.2’\(_{2}\) in the sense of distribution. Summing up the facts above, we have shown that the solutions to (1.2) satisfying the (2.66) regularity with the bounds in (2.67) are also solutions to 1.2’.

Consider two sets of initial data \( ( \rho _{i,0},v_{i,0}) = (\sigma _{i,0}^2, v_{i,0}), i = 1,2 \), in (1.16) for (1.2) satisfying (1.18) and (1.19). Denote \( (\rho _i,v_i) = (\sigma _i^2, v_i), i = 1,2 \), as the corresponding strong solutions constructed in Proposition 9 in the interval \( [0,T^*] \), for some \( T^* > 0 \). Then we have \( (\sigma _i, v_i), i=1,2 \), satisfying the bounds in (2.67). Also \( (\sigma _i, v_i), i=1,2 \), are solutions to 1.2’. Throughout this section, we will denote the constant \( C > 0 \) which may be different from line to line and depends on \( \mu , \lambda , B_1,B_2, T^* \). Also, we will use the notations

$$\begin{aligned}&\sigma _{12} := \sigma _1 - \sigma _2, ~ v_{12} := v_1- v_2, \\&\sigma _{12,0} := \sigma _{1,0} - \sigma _{2,0}, ~ v_{12,0} := v_{1,0}- v_{2,0}. \end{aligned}$$

Taking the difference of the equations satisfied by \( (\sigma _i, v_i), i=1,2 \), we have

$$\begin{aligned} {\left\{ \begin{array}{ll} \partial _t\sigma _{12} + \overline{v_1} \cdot \nabla _h\sigma _{12} + \dfrac{1}{2} \sigma _{12} \overline{\mathrm {div}_h\,v_1} + \overline{v_{12}} \cdot \nabla _h\sigma _2 \\ ~~~~ ~~~~ + \dfrac{1}{2} \sigma _2 \overline{\mathrm {div}_h\,v_{12}} = 0,\\ \sigma _{1}^2 \partial _tv_{12} - \mu \Delta _hv_{12} - \mu \partial _{zz} v_{12} - (\mu +\lambda )\nabla _h\mathrm {div}_h\,v_{12} \\ ~~~~ = - \sigma _{12} ( \sigma _{1}+\sigma _2)\partial _tv_{2} - \nabla _h\bigl ( \sigma _{12}\dfrac{\sigma _1^{2\gamma } - \sigma _2^{2\gamma }}{\sigma _1 - \sigma _2}\bigr ) \\ ~~~~ ~~~~ - \sigma _{12}(\sigma _1 + \sigma _2) v_2 \cdot \nabla _hv_2 - \sigma _1^2 v_{12} \cdot \nabla _hv_2\\ ~~~~ ~~~~ - \sigma _1^2 v_1 \cdot \nabla _hv_{12} - \sigma _{12} \sigma _2 w_2 \partial _zv_2 -\sigma _1 (\sigma _1 w_1 - \sigma _2 w_2) \partial _zv_2 \\ ~~~~ ~~~~ - \sigma _1 \sigma _1 w_1 \partial _zv_{12},\\ \sigma _i w_i = - \int _0^z \bigl ( \sigma _i \widetilde{\mathrm {div}_h\,v_i} + 2 \widetilde{v_i} \cdot \nabla _h\sigma _i \bigr ) \,\hbox {d}z, ~~~~~ i = 1,2. \end{array}\right. } \end{aligned}$$
(3.2)

Then as before, one can derive

$$\begin{aligned}&\sup _{0\leqq t\leqq T^*} \bigl ( \bigl \Vert \sigma _{12}(t) \bigr \Vert _{L^{2}}^2 + C_{\mu ,\lambda }' \bigl \Vert (\sigma _1v_{12})(t) \bigr \Vert _{L^{2}}^2 \bigr ) + \int _0^{T^*}\bigl \Vert \nabla v_{12} \bigr \Vert _{L^{2}}^2 \,\hbox {d}t \\&\quad \leqq C \bigl ( \bigl \Vert \sigma _{12,0} \bigr \Vert _{L^{2}}^2 + C_{\mu ,\lambda }' \bigl \Vert \sigma _{1,0}v_{12,0} \bigr \Vert _{L^{2}}^2 \bigr ) \leqq C \bigl ( \bigl \Vert \sigma _{12,0} \bigr \Vert _{L^{2}}^2 + \bigl \Vert v_{12,0} \bigr \Vert _{L^{2}}^2 \bigr ). \end{aligned}$$

Therefore, after employing (1.22) and noticing the fact that we can interchange \( (\sigma _1, v_1), (\sigma _2,v_2) \) in the previous arguments, we will have the following:

Proposition 11

Given two sets of initial data \( (\rho _{i,0},v_{i,0}) = (\sigma _{i,0}^2 ,v_{i,0}), i = 1,2 \), for (1.2) satisfying (1.17), (1.18) and (1.19), the corresponding strong solutions \( (\rho _i,v_i) = (\sigma _i^2, v_i), i = 1,2 \), constructed in Proposition 9 in the interval \([0,T^*] \), for some \( T^* > 0 \), satisfy

$$\begin{aligned} \begin{aligned}&\bigl \Vert \sigma _1 - \sigma _2 \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))} + \bigl \Vert \sigma _1(v_1- v_2) \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))}\\&\qquad + \bigl \Vert \sigma _2(v_1-v_2) \bigr \Vert _{L^\infty (0,T^*;L^2(\Omega ))} + \bigl \Vert v_1 - v_2 \bigr \Vert _{L^2(0,T^*;L^2(\Omega ))} \\&\qquad + \bigl \Vert \nabla (v_1 - v_2) \bigr \Vert _{L^2(0,T^*;L^2(\Omega ))}\\&\quad \leqq C_{\mu , \lambda , B_{1}, B_{2}, T^*}\Big (\bigl \Vert \sigma _{1,0}-\sigma _{2,0} \bigr \Vert _{L^2(\Omega ))} + \bigl \Vert v_{1,0}-v_{2,0} \bigr \Vert _{L^2(\Omega ))}\Big ). \end{aligned} \end{aligned}$$

In particular, if \( \rho _{1,0} = \rho _{2,0}, v_{1,0} = v_{2,0} \), we have \( \rho _1 =\rho _2, v_1 = v_2 \) in \( [0,T^*] \).

3.3 Proofs of the Main Theorems

Theorem 1 follows from Proposition 6 and Proposition 10. Theorem 2 follows from Proposition 9 and Proposition 11.