Numerical Computations of Hypersonic Boundary-Layer over Surface Irregularities

Abstract

Surface irregularities such as protuberances inside a hypersonic boundary layer may lead to premature transition on the vehicle surface. Early transition in turn causes large localized surface heating that could damage the thermal protection system. Experimental measurements as well as numerical computations aimed at building a knowledge base for transition Reynolds numbers with respect to different protuberance sizes and locations have been actively pursued in recent years. This paper computationally investigates the unsteady wake development behind large isolated cylindrical roughness elements and the scaled wind-tunnel model of the trip used in a recent flight measurement during the reentry of space shuttle Discovery. An unstructured mesh, compressible flow solver based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for the flow past a roughness element under several wind-tunnel conditions. For a cylindrical roughness element with a height to the boundary-layer thickness ratio from 0.8 to 2.5, the wake flow is characterized by a mushroom-shaped centerline streak and horse-shoe vortices. While time-accurate solutions converged to a steady-state for a ratio of 0.8, strong flow unsteadiness is present for a ratio of 1.3 and 2.5. Instability waves marked by distinct disturbance frequencies were found in the latter two cases. Both the centerline streak and the horse-shoe vortices become unstable downstream. The oscillatory vortices eventually reach an early breakdown stage for the largest roughness element. Spectral analyses in conjunction with the computed root mean square variations suggest that the source of the unsteadiness and instability waves in the wake region may be traced back to possible absolute instability in the front-side separation region.

High-Speed PLIF Imaging of Hypersonic Transition over Discrete Cylindrical Roughness

“…Analogous streaks have been noted in recent computations modeling select flow conditions from the present experiment. 15,16 High amplitude stationary streaks of this type can have strong temperature gradients (see for example, Fig. 11 of Ref.…”

“…11 of Ref. 15) and, furthermore, can transport unseeded fluid towards the wall, adjacent to the seeded fluid. Both of these effects (temperature and NO density gradients) contribute to the strong contrast in the fluorescence intensity in the image.…”

Wake Instabilities behind Discrete Roughness Elements in High Speed Boundary Layers

“…We note that the possibility of self-sustained unsteadiness in the immediate vicinity of the roughness element cannot be ruled out entirely. 5 However, the main focus herein is to compare the stability characteristics in the mean wake flow and, since the L2 norms of the residual levels had dropped to O(10 -11 ) or less when they leveled off, it was assumed that the wake flow computed via local time stepping would provide a meaningful basic state for the analysis of wake instabilities. Future computations are planned to verify this assumption as well as to confirm the accuracy of the IBM in the Mach 6 cases.…”

“…The flow behind an isolated roughness element was further investigated by Chang et al, 5 who addressed the potential onset of spontaneous unsteadiness behind a roughness element of height that is close to the boundary layer thickness. It was shown that the wake flow spontaneously became unsteady at sufficiently large roughness heights, exhibiting a complex interplay between the unsteadiness in the separation region immediately upstream of the roughness element and the disturbance motion farther downstream within the wake.…”

The Hythirm Project: Flight Thermography of the Space Shuttle During Hypersonic Re-Entry

“…The global HYTHIRM and the companion discrete BLT Flight Experiment measurements have fostered a diverse set of complementary work highlighting a variety of advancements in both experimental technique 27,28 and numerical modeling 29,[32][33] . In terms of CFD, turbulence models play a crucial role in the simulation of complex flows where separation, shock/boundary layer interaction, and flow reattachment are present.…”