HYPERSONIC TURBULENT BOUNDARY LAYERS WITH INTERACTIONS

Abstract

The Mach 10 Hollow-Cylinder Flare (HCF) and the Mach 6 BoLT-II hypersonic configurations were simulated and studied. Wall-Resolved Large Eddy Simulations (WRLES) with a high-bandwidth resolving WENO method and explicit third order Runge-Kutta time advancement was used to obtain detailed solutions of the flowfields, to include mean data, first and second order turbulent statistics and spectral content. The two configurations each presented a different set of flow physics which were interrogated using a variety of processing techniques. Investigation of the low frequency dynamics in the corner shock-separation of the HCF revealed pairs of Görtler-type centrifugal instabilities across the span generated by the concavity of the separation streamline. This study provided strong qualitative and statistical evidence that the low frequency pulsations of the separation bubble and shockwave are driven by the Görtler-type centrifugal instabilities, which are in turn generated by streamline distortion caused by the separation bubble, thus creating a low-frequency feedback loop. That is, the unsteady shock-separation dynamics, which are also responsible for the large pressure and heat flux fluctuations, are driven by instabilities intrinsic to the corner interaction itself. This study provided new and important insight into the behavior of the turbulent velocity and thermodynamic correlations in the separation zone and post-shock nonequilibrium flow of the HCF. The nonequilibrium flowfield exhibits significant compression and mean strain with strong amplification of the Reynolds stresses, turbulent kinetic energy and their transport budgets, viz, production, viscous diffusion, turbulent diffusion, pressure diffusion, viscous dissipation, pressure strain and turbulent mass flux. The amplifications far exceeded that of supersonic and lower speed shock-boundary layer interaction flows, and often departed from canonical trends. Classical single-point closures used in lower-order RANS modeling of the Reynolds stress transport budgets, the turbulent kinetic energy transport budgets and turbulent heat flux were also evaluated a priori by comparing to the WRLES solution. While the models performed very well in the upstream equilibrium boundary layer, many performed poorly in the downstream post-shock nonequilibrium region, with few exceptions. The results herein have important implications in wall-bounded turbulence modeling which has typically been based on canonical non-hypersonic equilibrium flows, and to a lesser extent simple nonequilibrium flows. A single flight condition during the descent phase of the BoLT-II hypersonic research vehicle was simulated using WRLES. For the unperturbed adiabatic wall case, flowfield solutions of the turbulent environment over the vehicle show multiple and distinct large-scale vortical instabilities on top of what is typically seen in broad-frequency turbulence. The downstream boundary layer was marked by high vorticity, temperature and skin friction. Wall normal extractions in key regions of the downstream boundary layer revealed broad-frequency turbulence as well as low frequency content in the energy spectra. An examination of the boundary layer velocity profile showed a significant departure of the transformed velocity from classical turbulence similarity scaling. For the isothermal wall case, the inlet was perturbed according to acoustic wave theory to simulate turbulence in the freestream environment. The simulations predicted precipitous rises in the downstream wall shear stress and wall heat flux mainly in the central and outboard regions, indicative of transition-to-turbulence with a two-dimensional transition front. Except for some locations across the span, the predictions of wall shear stress and wall heat flux were generally lower than that reported from the inflight sensors. However, it was noted that the particular flight condition simulated in this study fell within a very unstable flight regime where the flow flip-flops multiple times between laminar and turbulent states producing inordinately large fluctuations in the wall heat flux as indicated by flight sensor data.