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Abstract

We review discuss derivation and then write down equations of weak nonlinear wavefront and shock front.

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Notes

  1. 1.

    Generally, in any book on wave propagation we use an intuitive idea of a wave and wavefront. A mathematical definition is available in Sect. 3.2.1 of [42], according to which a wave is an approximate concept in high-frequency approximation—see the next section. This precise definition was first given in 1977 [36].

  2. 2.

    Note that only \(d{-1}\) components of unit normal \(\varvec{n}\) are independent.

  3. 3.

    Unlike a wavefront, which is an approximate concept in high-frequency approximation, a shock in a solution of a system of conservation laws is an exact phenomenon and high-frequency approximation is exactly satisfied.

  4. 4.

    We shall explain later the reason of using the word ‘sufficient’ for just one more unknown.

  5. 5.

    In this section, for an element the arc length \(d\ell \) of a linear ray, we have \(d\ell = a_0 dt\). A proof of the result \(\frac{1}{\mathcal {A}} \frac{d \mathcal {A}}{d \ell } = - \frac{1}{2}{\varOmega }\) for a general ray system is given in Sect. 7.1.

  6. 6.

    \(\tilde{w}\) in [32, 42] is non-dimensional.

  7. 7.

    The term on the right-hand side of the second equation in (5.11) is missing in all references quoted in Remark 5.1.1. This term is necessary in order that the rays given by (5.11) satisfy Fermat’s principle of stationary time of transit (see [42], Sect. 3.2.7).

  8. 8.

    Note that the symbols \(\Omega _{t}\) and \({\varOmega }\) represent respectively a surface in \(I\!\!R^{d}\) (here d =3) and mean curvature of \(\Omega _t\).

  9. 9.

    This theorem is reproduced from Sect. 3 of [45] with some changes. The author thanks Indian Academy Sciences for kindly giving permission for reproduction.

  10. 10.

    See (5.50) below, \(\mu _1\) has dimension of \(\frac{a_0}{L}\) and \(\mu _2\) has dimension of \(\frac{a^2_0}{L^2}\).

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Correspondence to Phoolan Prasad .

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Prasad, P. (2017). Equations of Nonlinear Wavefront and Shock Front. In: Propagation of Multidimensional Nonlinear Waves and Kinematical Conservation Laws. Infosys Science Foundation Series(). Springer, Singapore. https://doi.org/10.1007/978-981-10-7581-0_5

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