Free-stream fluctuations of a hypersonic conventional wind-tunnel using static and stagnation pressure sensors
Sylvain Morilhat, Mathieu Lugrin, Jean-Pierre Tobeli, Nicolas Severac, Jean-Marc Luyssen, Reynald Bur
ONERA DAAA, 8 rue des Vertugadins, 92190 Meudon, France,
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Keywords : Blow-down wind-tunnel, Turbulence level, Pressurefluctuations
Hypersonic wind tunnels cannot reproduce all the features of atmospheric flight of a hypersonic vehicle, such as Mach number, Reynolds number, enthalpy, test duration, size and roughness of the vehicle, etc. However, there are facilities capable of replicating a fraction of these features. By utilizing a combination of different facilities, it becomes possible to study each phenomenon occurring during atmospheric re-entry individually.
One of the most challenging aspects to replicate in a wind tunnel is the presence of upstream uctuations. While the atmosphere can be considered relatively calm compared to hypersonic flight conditions, the same cannot be said for the flow from a wind-tunnel nozzle. There are “quiet” wind tunnels  that exhibit signifcantly lower levels of upstream fluctuations compared to conventional wind tunnels. However, it is important to note that they still produce higher noise levels than what is experienced during atmospheric flight.
The influence of noise level will be particularly noticeable in the study of the laminar-to-turbulent transition. The higher the noise level, the earlier the transition will occur compared to actual flight conditions.Two wind tunnels that are identical in terms of stagnation conditions, nozzle shape and model will yield different transition Reynolds numbers if their upstream fluctuations are different. The comparison with high- fidelity simulations will be more relevant if the same perturbations encountered in the wind tunnel are injected upstream [3, 1].
Upstream fluctuations in incompressible wind tunnels are typically quantifed using a quantity called turbulence intensity, which is based on the variance of velocity fluctuations. Turbulence intensity alone is insuffcient to quantify the disturbances in a hypersonic wind tunnel. It is necessary to consider also fluctuations in pressure and density. Furthermore, the distribution in terms of spatial and temporal spectral content must be known to fully characterize the disturbances.
For a disturbance to lead to transition, it is necessary for it to be present in both the spectrum of upstream fluctuations and within the amplifcation spectrum of the boundary layer. This is why modelling upstream fluctuations as white noise can potentially bias CFD restitutions.This paper presents measurements aiming to quantify pressure fluctuations in a hypersonic wind tunnel at Mach 5 and 6, continuing work initiated by Lugrin .
The experimental campaign took place in the cold hypersonic blow-down wind-tunnel R2Ch at ONERA-Meudon center. Pressure fluctuations were measured using Kulite sensors for low frequencies and PCB sensors for high frequencies. A Kulite sensor was mounted at the stagnation point of a Pitot tube, while a Kulite sensor and a PCB sensor were mounted on the generatrix of a cone. The cone’s angle was low and its tip was as sharp as possible in order to minimize disturbances of the probe to the measured flow. Figure 1 shows the instrumentation used, located just after the exit plane of the wind tunnel nozzle.
Figure 1: Sharp cone model equipped with Kulite XCQ93 and PCB 132B39 sensors and Pitot tube equipped with Kulite XCQ93
The experimental campaign consisted of 18 runs with variations in Mach and Reynolds numbers and stagnation conditions. Pressure fluctuations outside the wind tunnel jet were also studied, as well as the influence of the grids placed in the settling chamber.
The power spectral densities of the different sensors were studied based on the measurements. An example is shown in Figure 2. Special attention was given to examining the frequency correlation between the sensors and establishing a connection between the measurements obtained using the Kulite sensors mounted on the cone and the Pitot tube.
Figure 2: Mach 5 series, PSD of Pitot tube’s Kulite. Unitary upstream Reynolds number (106/m)
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 Mathieu Lugrin. On the use of random inflow disturbances to simulate transitional hypersonic wind tunnel experiments. In HISST 2022, 2022.
 Steven P Schneider. Development of hypersonic quiet tunnels. Journal of Spacecraft and Rockets, 45(4):641 664, 2008.