Hypersonic viscous flow over large roughness elements

Abstract

Viscous flow over discrete or distributed surface roughness has great implications for hypersonic flight due to aerothermodynamic considerations related to laminar-turbulent transition. Current prediction capability is greatly hampered by the limited knowledge base for such flows. To help fill that gap, numerical computations are used to investigate the intricate flow physics involved.An unstructured mesh, compressible Navier-Stokes code based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for two roughness shapes investigated in wind tunnel experiments at NASA Langley Research Center. It was found through 2D parametric study that at subcritical Reynolds numbers of the boundary layers, absolute instability resulting in vortex shedding downstream, is likely to weaken at supersonic free-stream conditions. On the other hand, convective instability may be the dominant mechanism for supersonic boundary layers. Three-dimensional calculations for a rectangular or cylindrical roughness element at post-shock Mach numbers of 4.1 and 6.5 also confirm that no self-sustained vortex generation is present.

Effects of Cavities and Protuberances on Transition over Hypersonic Vehicles

“…3,5 In this paper, we focus on the effects of protuberances or cavities on large blunt bodies, in particular, NASA's Project Orion crew entry vehicle configuration. The CEV model as shown in Fig.…”

“…[1][2][3][4][5] It is well-known that the presence of protuberances or cavities in hypersonic boundary layers can lead to premature laminar turbulent transition in the wake region. Early transition to turbulence causes a rapid rise in surface heating, which in turn may impact the aerodynamic and aerothermodynamic performance of the vehicle.…”

“…It has been shown, on the other hand, that the wake becomes strongly unsteady when the roughness height is substantially larger than the boundary layer thickness. 3 When the roughness height exceeds the boundary-layer thickness, instability waves begin to develop in the wake region, causing the wake streak and horse-shoe vortices to oscillate downstream. At even larger heights, the instability waves may eventually lead to vortex breakdown.…”

IR thermography investigation on roughness induced transition in high-speed flows

“…For the flow around an hemispherical bump placed in an incoming laminar boundary layer at Mach number ranging from 3.37, 5.26 and 8. 23 Iyer and Mahesh [21] stated that upstream of the roughness the vortex system can be formed by 4 or 6 vortices depending on the Reynolds number, the Mach number and * ⁄ ratio where * is the displacement thickness at the roughness location. They found good agreement with the subsonic experiments carried out by Beker [22] for incompressible flow past a cylinder placed on a flat plate.…”

“…This paper is only the last of series of paper in which several configurations have been analysed in order to investigate several transitional scenarios. For example, Chang and Choudari [23] simulated the flow around a rectangular and cylindrical element at Mach number equal to 4.5 and 6.5 concluding that for these Mach numbers no self-sustaining vortex-generation was present and that convective instability might be the dominant. Groskpof et al [24] presented similar conclusion performing a bi-global stability analysis of the region behind a 3D pizza box shaped roughness.…”