Variable Density Turbulence Tunnel Facility

The Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization in G\"ottingen, Germany produces very high turbulence levels at moderate flow velocities, low power consumption and adjustable kinematic viscosity between $10^{-4} m^2/s$ and $10^{-7} m^2/s$. The Reynolds number can be varied by changing the pressure or flow rate of the gas or by using different non-flammable gases including air. The highest kinematic viscosities, and hence lowest Reynolds numbers, are reached with air or nitrogen at 0.1 bar. To reach the highest Reynolds numbers the tunnel is pressurized to 15 bar with the dense gas sulfur hexafluoride (SF$_6$). Turbulence is generated at the upstream ends of two measurement sections with grids, and the evolution of this turbulence is observed as it moves down the length of the sections. We describe the instrumentation presently in operation, which consists of the tunnel itself, classical grid turbulence generators, and state-of-the-art nano-fabricated hot-wire anemometers provided by Princeton University [Vallikivi et al. (2011) Exp. Fluids 51, 1521]. We report measurements of the characteristic scales of the flow and of turbulent spectra up to Taylor Reynolds number $R_\lambda \approx 1600$, higher than any other grid-turbulence experiment. We also describe instrumentation under development, which includes an active grid and a Lagrangian particle tracking system that moves down the length of the tunnel with the mean flow. In this configuration, the properties of the turbulence are adjustable and its structure is resolvable up to $R_\lambda \approx 8000$.


I. INTRODUCTION
A. On the need for the VDTT Turbulence plays a decisive role in the universe. Its influence extends broadly throughout nature and technology. 2 For example, turbulence controls the spread of trace gases, 3,4 pollutants, 5 and particulate matter 6 in the atmosphere and oceans, the mixing of fuel and air in engines, 7 the generation of energy-draining wakes behind airplanes and cars, 8 and even the evolution of planets, stars and the universe as a whole. 9,10 What underlies all turbulent motion is the balance between the inertia of the fluid and the pressure and friction forces that the fluid exerts on itself. In almost every practical setting this balance is complicated by additional effects, such as buoyancy-driven convection where temperature gradients drive the flow, 11 rotation-induced Coriolis accelerations in oceanic, atmospheric flows on earth or other planets, 12 electromagnetic forces in conducting fluids like those that make up the sun, 13 nonlinear stresses in non-Newtonian fluids like blood, 14 or changes in material properties in flames. 15 If we want to discover something generic about turbulence, something that is essential wherever turbulence is fundamental, we may limit our inquiries to its most essential ingredients: inertia, pressure, and friction. Such turbulence can be created by mechanically stirring a liquid or gas. In this spirit of interest in universality, we may also wish to exclude the influences of the geometry of the flow. We then want to study a flow that minimizes the effect of the boundaries of its container, and does not exhibit a preferred orientation. 16 Such a flow is called statistically homogeneous and isotropic, 17 and a close approximation of it can be realized in a wind tunnel by disturbing a uniform free-stream flow with a mesh or grid. 2,18 When turbulence is well-developed, it comprises large sweeping motions that contain most of the mechanical energy of the flow, and relatively small and sharp gradients that dissipate most of this energy. 19 The Reynolds number, Re = ρu L/µ, gauges the separation between these large and small scales. In other words, high Reynolds numbers mean large scale separations. Here, ρ and µ are the density and dynamic viscosity of the fluid, respectively, u is the amplitude of its velocity fluctuations, and L a characteristic scale over which motion is correlated. For typical values in the atmosphere, for example, the Reynolds number may The VDTT is a wind tunnel around which pressurized gases circulate in an upright, closed loop. At the upstream ends of two test sections in the VDTT, the free stream is disturbed mechanically. The resulting turbulence evolves down the length of the tunnel without the middle region being substantially influenced by the walls of the tunnel. The test sections are long enough (8 m) that the turbulence evolves through at least one eddy turnover time, L/u ≈ 1 s, during its passage down the tunnel. That is, the turbulence can be observed over the time it takes energy to cascade from the large scales all the way down to the dissipative ones.
The width of the cross section of the tunnel constrains the characteristic scale L of the flow, and for fixed L higher Reynolds numbers lead to ever smaller scales of motion. In the VDTT, the cross section is wide enough (1.5 m) that even at the highest Reynolds numbers, the smallest scales of motions are neither too small nor too fast to be resolved by existing state-of-the-art instrumentation. We note that scales of a modest size are also desirable if one wants to use optical techniques to resolve all scales of the flow. With current technologies large optical components, like lenses, mirrors and detectors, are difficult to produce.
The VDTT was designed around a Lagrangian particle tracking system (LPT). This requirement implies a limit on the mean flow rate U . The VDTT's maximum flow speed of U = 5 m/s is sufficiently small that we can accelerate the LPT system to match the flow speed at the upstream end of a test section, move it at mean-flow speed down the tunnel, and stop it at the downstream end. We can then follow the evolution of the flow in the frame of the flow itself, by imaging the motions of particles carried by the flow.
Eulerian measurements are not excluded, and the data we present here were acquired with both traditional hot wires and the new NSTAP probes. 20 These were the constraints on the design of the VDTT.
The history of using pressurized gases goes back almost 100 years. We provide in the remainder of this introduction a short historical review of predecessors to the VDTT. That is, of wind tunnels in which the density of the working fluid could be varied. There are several other types of wind tunnels and a great variety of turbulence facilities in general that we do not review, and which are worthy of an article in and of themselves. We continue in Section II with a description of the apparatus and its technical details. In section III, we describe velocity measurements and their statistics, and we conclude with an outlook in Section IV.

B. Historical Review of Variable Density Facilities
Variable density wind tunnels have been an important tool for aeronautical research and development for almost 100 years. Since both the density and the speed of the wind in such tunnels could be adjusted independently, both the Reynolds number and the Mach number of the flows could be set independently. This made it possible with small-scale models to observe the aerodynamics of full-scale airplanes under well-defined laboratory conditions. Before the advent of computers, these tunnels provided the only way to test design ideas. We emphasize that even with today's computers, wind tunnel tests remain essential in the development of airplanes. In the following, we give the maximum Reynolds numbers attainable in the various facilities in terms of its mean flow speed and 10% of its Recirculating tunnels such as the VDT are called Göttingen-type tunnels. 22 The test section of the VDT was housed in a 10.2 m long, 4.6 m wide cylindrical pressure vessel that could withstand pressures up to 21 atm. 23 The tunnel was used to test airfoils and model airplanes in states corresponding to various atmospheric conditions. The original tunnel was made of wood, and was destroyed by a fire in 1927. It also faced serious difficulties with vibration and flow quality, so that the tunnel was redesigned as a more rigid and fire-proof structure when it was rebuilt in 1930. 24 The VDT produced a huge amount of airfoil data, reaching Reynolds numbers of about Re W T = 5.4·10 6 . But high free-stream turbulence intensities led to inaccurate measurements of drag. To overcome this problem, the "Langley two-dimensional low-turbulence pressure tunnel" (LTPT) was designed and completed in 1938, also at the Langley Research Center. 25 The LTPT was large, at 44.5 m long and 17.7 m wide. By design, the airfoils in this tunnel could span the whole test section, effectively reducing by one the dimension of the flow.
The tunnel ran at pressures up to 10 atm and at Reynolds numbers of up to about Re W T = 6.1 · 10 6 , A low turbulence level of less than 1% was achieved through a contraction with a large area ratio in combination with a series of wire screens. This tunnel was operational for many years and underwent massive modifications in the early 1980's. 26   In the late 1990's, the Princeton/DARPA-ONR SuperPipe Facility was built at the Princeton Gas Dynamics Lab Facilities. It was a 34 m long and 1.5 m wide pipe flow facility with a 12.9 cm test section diameter. It circulated air pressurized to between 1 and 220 atm, 38 so that Reynolds numbers reached Re W T = 2.3 · 10 6 . This highly pressurized air made it possible to uncover the scaling laws of pipe flows at very high Reynolds numbers,

II. CONSTRUCTION DETAILS
The facility at the Max Planck Institute in Göttingen comprises a pressure vessel, the associated gas handling apparatus, safety systems, grids to generate turbulence, and turbulence measurement systems. We describe these systems in some detail in the following sections. We also describe some of the measures undertaken to control the homogeneity of the flow in the measurement sections in Sec. II J.

A. Pressure Vessel
As can be seen in Figs can be removed with a movable frame (Fig. 6). To ensure precise docking and undocking of the flanges between the elbows and the straight sections, the last few centimeters of the movement are controlled by four hydraulic cylinders (Fig. 7).

B. Working Fluids
The wind tunnel can be filled and operated with any non-corrosive gas. We have so far used air, nitrogen and SF 6 , but other gases are possible. At room temperature and pressure, SF 6 has a density of ρ = 6.52 kg/m 3 and a kinematic viscosity of ν = 2.32 × 10 −6 m 2 /s. is an inert gas, but decomposes above 1200 • C, 42 though this temperature can be lower on catalytic surfaces, which include certain metals.

C. Gas Handling System
The facility includes an automated gas handling system (Fig. 8). The system supplies the wind tunnel with SF 6 and other gases including dried air. The SF 6 is stored in the liquid phase in tanks (Fig. 9). The system both evacuates and pressurizes the wind tunnel. It also gasifies, liquefies, and cleans the SF 6 .
The typical cycle to prepare the wind tunnel for a run starts by closing the vessel to evacuate it. The evacuation is performed to minimize the amount of residual air in the tunnel. Once a pressure of 1 mbar has been reached, the tunnel is filled with the desired gas up to 15 bar. The system can maintain a given pressure during measurements. When SF 6 is used, and after the run is complete, the system pumps the gas back to the storage tanks, and reduces the pressure in the vessel to 1 mbar. The vessel is then filled again with air to

D. Safety System
All components of the facility have appropriate safety systems. The safety equipment includes over-pressure valves and burst plates, which prevent pressures so high that they could damage parts of the system. To avoid contamination of the laboratory space with SF 6 , all safety valves open into a pipe that ends outside the building. In addition, many components of the system have sensors that detect malfunctions and can shut down the entire system. For example, there is a sensor that shuts the motor down if the temperature inside the wind tunnel exceeds 40 ℃.
The pressure vessel itself has a safety release system consisting of a burst plate and a safety valve (Fig. 10) that limits the operational pressure to 15 bar. At this pressure the burst plate first breaks without releasing any SF 6 through the safety valve. Next, a gauge that monitors the pressure in the space between the burst plate and the safety value indicates an overpressure. Only if the pressure increases beyond 19 bar does the safety valve release the gas from the pressure vessel. The wind tunnel itself is certified up to 20 bar, and the operational pressure can be increased up to that value, provided the safety release system is redesigned in such a way that evacuation of the gas is guaranteed even when there is energy being injected into the system by, for example, the motor. There is a similar safety release system on the filter bypass (described in the next section), which opens at 19. During maintenance or installation of experimental equipment the safety of persons working in the wind tunnel must be guaranteed. Therefore, after an experiment and after the pressure vessel is again filled with air, the manholes are opened and the gas handling system is decoupled from the pressure vessel by double block-and-bleed valves (Fig. 11). These are two valves in series blocking the gas supply pipe from the pressure vessel, and one valve opening the space between the two valves to release to the environment any gas that leaks.

E. Filter Bypass
The wind tunnel is equipped with a bypass to clean the gas in the pressure vessel. A pump draws gas from one elbow (see Fig. 2), pushes it through a class F9 filter to remove particles from the gas, and returns it to the other elbow. The F9 filter efficiency is 98% for 1 micron particles. The bypass itself is a 19 m long tube ( Fig. 12) with an inner diameter of 250 mm.
The flow rate through the bypass is up to 400 m 3 /h. To avoid leaks the pump is entirely encapsulated, its connection to an external motor made through a magnetic coupling. To compensate for mechanical stresses caused by differences in the expansions of the bypass and the main body of the pressure vessel, the filter bypass includes two flexible couplers (Fig. 13).
The expansion is due to temperature or pressure fluctuations. To prevent pressure-driven flow through the filter bypass during measurements, the bypass can be closed with any of three valves along its length. Since the gas in the bypass can be enclosed by any pair of valves, an additional safety release consisting of a burst plate and a safety valve with pressure monitoring was necessary here.

F. Fan
The pump that recirculates the gas in the wind tunnel is a 210 kW electric motor coupled to a fan with 20 blades. There is a stator with 17 blades, and the annular passage through The motor is housed to prevent particles added to the flow from causing damage to the bearings. The motor is water cooled, with 40 kW cooling power available from the main cooling system of the facility. The cooling water for the motor is driven by a dedicated pump to ensure the minimum required flow rate necessary to cool the motor. If there were a leak in the cooling system, SF 6 could leak into it. Should this happen, the pressure increase in the system would be detected and valves in the cooling lines would close automatically.
Although it is housed, the motor is exposed to the full range of pressures in the tunnel between vacuum and 20 bar. For this reason, the motor coils were insulated by a specially made bubble-free coating. If bubbles were present, they could expand or contract under variations in pressure and damage the insulation.
The motor temperature and vibration are monitored. The motor controller shuts the motor down when any problems are detected. All connections to the motor are made through leak-tight feedthroughs. temperature gradients over the tunnel's cross-section, the cooling water flows through the two heat exchangers in the same rotational orientation but the two heat-exchangers are rotated by 180°with respect to each other.

G. Heat Exchanger
To achieve maximum efficiency, the heat exchanger is connected directly to the cooling water system of the building; there is no additional heat exchanger. The two registers of the heat exchanger receive the same constant flow rate, which is driven by a common pump.
The temperature controller holds constant the fluid temperature to set points between 20 ℃ and 35 ℃. This is realized by mixing cold water from the building with hot water from the heat exchanger return flow. Since this causes a varying flow rate in the support line from the building, an additional pump on the building side in combination with an excess flow valve realizes a constant flow rate on the building side.
Since the cooling water is at the nominal pressure of the building water supply, there is a pressure difference across the surfaces of the heat exchanger. The heat exchanger is designed to sustain the full pressure difference between the cooling water and the pressure inside the wind tunnel (between vacuum and maximum 20 bar absolute). The cooling water flows to the heat exchanger through feedthroughs that on one hand are flexible enough to accommodate deformations due to temperature and pressure variations, and on the other hand are stiff enough to withstand the full pressure difference between the pressure of the cooling water system inside and the wind tunnel pressure on the other side.
To prevent SF 6 from penetrating the cooling water system of the building in the case of a leak in the heat exchanger, the cooling water supply pipes include two valves that close at a pressure of 4 bar over the nominal. An additional four valves (see Fig. 16) close at an over-pressure of 6 bar. In the case of a leak, the pressure in the building system first increases, which causes the two valves in the supply pipes to close and isolate the building side of the cooling system. Then the pressure continues to increase until the four valves at the wind tunnel close in order to enclose the SF 6 in the wind tunnel. In the case of a leak of water into the tunnel, a conductive humidity sensor placed below the heat exchanger initiates an alarm and shuts down both the wind tunnel fan and the cooling system.

H. Measurement Sections
The long straight sections of the pressure vessel contain the measurement sections. The wind flows through interior ducts, or wind tunnels, as seen in cross section in Fig. 17. The installed. Up to 60 signal wires can pass through each 3/4 " NPT tap (see Fig. 18).
Optical access to high-pressure devices is generally difficult to implement. The VDTT has two borosilicate glass windows with an open diameter of 50 mm with a direct view into the wind tunnel (see Fig. 19). The outer diameter of the glass is 107 mm, with thickness 20 mm. Aside from these two windows, optical access also is possible through optical fibers, which pass through feedthroughs. A 150 W Nd:YAG laser has been coupled into such a fiber for illumination of the test section for optical particle tracking.

K. Grids
We introduce here an active grid that we designed for the VDTT, and which is under development. We also describe the classical grids that we employed to produce the turbulence we characterize in the next section.
Active grids were developed as a way to generate in wind tunnels high-Reynolds number flows with convenient properties. 46 Active grids work by stirring the flow with rotating paddles, rather than disturbing it through the wakes of stationary bars, as in a classical grid. Modern active grids generate not only high-Reynolds number flows, but also flows with tailored properties. 47 Such control is desirable where turbulence with certain statistical properties is needed, as is the case when the atmospheric boundary layer needs to be synthesized to observe its effect on wind turbines, 48 or where the effects of variation of the large-scale properties of the turbulence on the small-scale dynamics need to be understood. 49 Since the large-scales are created by the geometry of the apparatus in an experiment, one The grid is shown here installed in the Prandtl tunnel, which is itself shown in Fig. 22 below.
advantage of the active grid is that its geometry is variable and can be adjusted during its operation. In this way, the response of the turbulence to changes in the properties at the large scales can be measured.
Our active grid advances the state of the art because there are many more degrees of freedom in the motions of its paddles than in previous grids. There are 129 degrees of freedom, whereas others had about 20. This gives an unprecedented level of control over the turbulence generated by the grid. Each degree of freedom corresponds to a single diamondshaped paddle, the collection of which tile the cross section of the tunnel, as seen in Fig. 21.
Each paddle has its own computer-controlled servomotor that adjusts the angle of the paddle As seen in Fig. 22, the test section is 10 m long, with a cross section identical to the one in the VDTT. The maximum flow speed in the Prandtl tunnel is approximately 11 m/s. The outcome of these tests will be presented in a separate paper.
To make the measurements presented in this paper, we installed classical grid turbulence

L. Traverses
Within the VDTT, measuring instruments such as cameras will move down the length of the tunnel at the mean speed of the flow. This way, it is possible to follow the motions of particles within the flow, rather than sampling them as they sweep past a fixed position. That is, one can view turbulence from the Lagrangian, rather than Eulerian, perspective.
The basic idea is that cameras take movies of particles that are suspended in the flow. the velocity of the platform.
As in most wind tunnels, measurement equipment can otherwise be mounted anywhere along the measurement sections. Devices can be mounted directly to the walls, or on linear traverses.
The probes for the test measurements presented below were mounted on a 2D traverse, manufactured by Isel, to make possible measurements at different fixed positions in the cross-section of the tunnel, and at a single distance from the grid (Fig. 24). The traverse the part be about ten times smaller than distance of the part from the probe. This was to minimize the flow distortion caused by the probe supports, while still providing the stability needed to minimize probe vibration. For some of the measurements, one probe was held at a fixed height, so that the distance between it and the probes moving up and down on the vertical traverse could be varied. We did this to measure correlations between velocities separated across the width of the tunnel.

M. Measurement Systems
The arsenal of diagnostic equipment familiar in fluid mechanics can be used in the VDTT.
We have designs to incorporate hot wire anemometry, cold wire thermometry, laser Doppler velocimetry, acoustic velocimetry, particle sizing, dynamic pressure measurement, Particle Image Velocimetry (PIV), and Lagrangian Particle Tracking (LPT). The latter option will be implemented on the linear traverse mentioned above.
The system employed to acquire the data in this paper was a Dantec StreamLine hot wire anemometry system. We used two kinds of Dantec wires, with 2.5 µm diameter and 450 µm length, and with 5 µm diameter and 1 mm length. We also used the new NSTAP probes developed at Princeton, 20 which were either 30 µm or 60 µm long. The probes were calibrated in situ against Prandtl (or Pitot-static) tubes while varying the fan speed. We also used X-wires with 2.5 µm wires to measure the Reynolds stresses. The angular responses of the X-wires were calibrated with the Dantec calibrator using air at standard temperature and pressure. The signals were filtered at 30 kHz and sampled at 60 kHz with a digital acquisition card.

III. TEST EXPERIMENTS
We measured the characteristics of the turbulence at a fixed distance, 7.1 m, from the 186.6 mm classical grid, and on a array of 150 points covering a 60 by 60 cm square centered in the cross section of the tunnel. The measurements were made in both air and SF 6 at different pressures between 1 bar and 15 bar. The profiles of the flow were measured by Dantec hot wires, and the spectra by NSTAPs. Table I summarizes the properties of the turbulence. These properties were calculated as follows. The energy dissipation rate was extracted from the third order structure functions using the 4/5 ths law, D LLL (r) ≈ − 4 5 r in the inertial range, so that our practical definition of it was = max(− 5 4 D LLL (r)/r). Here D LLL (r) = δu 3 (x, r) x is the third moment of the longitudinal velocity differences, · x is the average over x, δu(x, r) = u(x + r) − u(x) are the velocity differences, u, x, and r are parallel, and we used Taylor's hypothesis to extract x and r from the time series of each probe. We are aware that this measure of the dissipation rate is smaller than the actual dissipation rate at low Reynolds numbers where TABLE I. The flow parameters for the passive grid experiments. P is the pressure of the gas in the tunnel, ρ and ν are the density and viscosity of the gas, respectively, U is the mean speed of the flow, u /U is the turbulence intensity, L is the streamwise longitudinal integral scale, is the dissipation rate per unit mass, η and τ η are the Kolmogorov length and time scales, respectively, λ is the Taylor scale, and R λ is the Taylor  the inertial range is not well-developed. The measure becomes more accurate as the Reynolds number increases. 55 This has the effect, among others, of slightly inflating the value of the lower Reynolds numbers. The amplitude of the velocity fluctuations is u = u 2 1/2 x . The integral scale, L, is the integral of the longitudinal correlation functions, L = u −2 ∞ 0 u(x + r)u(x) x dr, and we used exponential extrapolations to extend the integrals to infinity. 56 The Taylor scale was evaluated through the isotropic relation, λ = (15ν u 2 x / ) 1/2 . Since the turbulence was approximately isotropic at all scales, 56 the Reynolds number is given by R λ = u λ/ν. The Kolmogorov scales are given by η = (ν 3 / ) 1/4 , and τ η = ν/ , as usual.
Observe in Table I that  at which dissipation occurs, a point that we revisit below.
In the sections that follow we describe the statistics of the flow at positions midway between the side walls, and at various distances, z/H, from the floor of the tunnel, where z is the height of the probe above the floor and H is the height of the tunnel. Horizontal cuts, made across the width of the tunnel, were at least as good, or better, than the vertical cuts we present, in the sense that the deviations from homogeneity across the width were smaller. We then discuss the properties of the spectrum of the velocity fluctuations. Some analysis of these data, namely of conditional structure functions, has already been published in Blum et al. 49 The anisotropy in the fluctuations, u /v , where u was in the streamwise direction and v was in the spanwise direction, was between 1 and 1.1, so that the fluctuations were slightly stronger in the streamwise direction, as has been observed in grid turbulence before. 18 The turbulence intensity increased with Reynolds numbers for reasons that we do not know, though the same phenomenon was observed also in the HDG at the DLR in Göttingen (described above). We speculate that the turbulence decays more slowly at high Reynolds numbers, and are now performing experiments to test this idea.

D. Reynolds-stress Anisotropy
To quantify the the Reynolds-stress anisotropy, we plot the two invariants of the anisotropic Reynolds stress tensor u i u j − 1 3 u k u k δ ij on the so-called Lumley triangle. 57,58 We assume that the two transverse components are statistically the same, i.e., v = w , and that vw = 0. We compare the Reynolds-stress anisotropy of the turbulence in the VDTT with other laboratory flows, including: the von Kármán swirling flow between counter-rotating disks ("the French washing machine"), 59 the Lagrangian Exploration Module (LEM), 60 and the "soccer-ball". 61 The data for the French washing machine and the LEM were obtained using three-dimensional particle tracking, 51 while the data for the soccer-ball were measured with laser-Doppler velocimetry and similar assumptions on the two transverse velocity components were used to construct the Reynolds stress tensor. As shown in Fig. 26, the turbulence produced in the VDTT was closer to isotropic than all other flows. The degree of isotropy increased with Reynolds number.  Table I. The collapse of the spectra at large scales supports the view that the dynamics at these scales are Reynolds number independent. In other words, they are controlled by the mean flow speed and the geometry of the grid, and are not strongly influenced by the material properties of the gas. This shows that our device works in the sense that we control the large scales and modulate the small scales by changing the pressure. at the same moderate Reynolds number (it is the 4 bar data in Table I). The key point is that the shapes of the two spectra are nearly identical, down to the scale of the Dantec probe, l trad. . This finding builds confidence in the NSTAP data. The deviation of their ratio from one, visible in the inset at the largest scales, is probably due to the usual problems with convergence at these scales, which are much longer than the integral scale. At scales smaller than the Dantec probe, or at large k 1 l trad. , the ratio falls off. This roll-off indicates that the Dantec probe lost sensitivity relative to the NSTAP for k 1 l trad. > 1. In addition to this spatial filtering effect, there are hints that temporal filtering of the hot-wire system can influence the measurement at high frequencies. 62 Most recent studies in pipe flows show that the frequency response of different anemometer/probe combinations can vary substantially. 63 This ongoing work will need careful consideration. Figure 29 shows the same spectra as in Fig. 27, but in the Kolmogorov variables. The inertial range is the approximate plateau in the curves. With increasing Reynolds number, the so-called bottleneck, or the bump on the right side of the inertial range, grows more pronounced. However, it seems to lose prominence at the highest Reynolds numbers. The overall slope of the spectra in the inertial range grows more shallow with increasing Reynolds number, as has been observed before. 64  of arbitrary amplitude, and it follows the data reasonably well. This illustrates one of the chief advantages of the tunnel, which is that scale separation can be achieved by changing the pressure in the tunnel, and so by modulating the small scales alone. In this way, we separate the influence of scale separation on the small-scale dynamics from the influence of changes in the large-scale structure of the flow.

IV. CONCLUSION AND OUTLOOK
The new high-pressure, high-turbulence wind tunnel in Göttingen, the VDTT, makes experimental measurement of the structure and dynamics of nearly homogeneous and shearless turbulence possible at higher Reynolds numbers than before in its configuration at the time of writing. With passive grids we reach Taylor Reynolds numbers of 1600, whereas comparable studies reach about 870 (with an active grid). 65 To characterize the quality of  Fig. 27, the data correspond to those summarized in Table I.
The spectra are shifted vertically by 0.2 in order to set them apart, with the lowest Reynolds number data at the bottom. The extension of the inertial range is visible, as is the flattening of the spectra with increasing Reynolds number.  the flow, hot-wire measurements were made behind a classical grid turbulence generator.
Using tools that we describe in the paper, we will in the future reach even higher Reynolds numbers and introduce new Lagrangian measurement techniques.
With the addition of the active grid in the VDTT, we expect to reach Reynolds numbers before attainable only in the atmospheric boundary layer. 66 That is, we will produce steady homogeneous and isotropic conditions, whereas existing data were acquired in unsteady inhomogeneous and anisotropic flows. As can be seen in Table II, very high Reynolds numbers up to R λ at least 4200 will be possible with the active grid. These estimate is based on our initial experience with the new active grid in an open-circuit air tunnel, which we will publish separately, and where measurements were made at the downstream end of the tunnel. The Reynolds number will be higher at the upstream ends of the test sections, though the decay of Reynolds number with distance from the grid is typically slow. 67 Further optimization of the active grid may yield yet higher Reynolds numbers, as has been observed with other active grids. 68 In this way, we may reach Reynolds numbers up to 8000.
With the addition of the linear motor and camera system, the Lagrangian properties of the turbulence in the VDTT will become accessible. The Lagrangian approach coupled with conventional Eulerian measurements under well-controlled conditions will provide a new perspective on fundamental turbulence questions. With its special properties the VDTT will make possible experiments in a well-understood and well-controlled flow at the highest turbulence levels yet possible in the laboratory, and with measurable spatial and temporal scales of motion. Therefore, it will make it possible to address problems important to environmental, atmospheric, and ocean sciences, to engineering and astrophysics.

ACKNOWLEDGMENTS
The project was initiated by Eberhard Bodenschatz, who led the project over the last ten Measurements with adequate resolution were made possible through collaboration with Margit Vallikivi, Marcus Hultmark, and Lex Smits at Princeton University. They manufactured the NSTAPs that we used, and have been a vital resource in their proper use. We look forward to further fruitful collaboration with them, as we continue to explore turbulence with the VDTT.