Effects of Roughness on Hypersonic Boundary-Layer Transition

Abstract

The effect of roughness on hypersonic boundary-layer transition has been studied for three primary purposes: to trip a laminar layer to turbulence, to determine whether naturally occurring roughness is expected to cause early transition, and to determine the largest allowable roughness that will not affect the location of transition. Roughness is often divided into two classes: isolated roughness, in which each protuberance can be considered separately, and distributed roughness similar to sandpaper, in which the roughness elements are many and are not considered separately. The effects of roughness on hypersonic transition are reviewed, considering the physics of the process, known parametric effects, some of the common correlations, and a few case studies. The three or more modes by which roughness can affect transition are outlined. At hypersonic edge Mach nunbers, it requires very large roughness heights to affect transition. Various correlations are often used to estimate the effect of roughness; several of these are described, although none provide good agreement with all the data.

Reattachment heating upstream of short compression ramps in hypersonic flow

“…6), the heat transfer ahead of the obstacle increases gradually and reaches levels well over an order of magnitude higher than the undisturbed turbulent reference level q u,T and even two orders of magnitude higher than the laminar level q u,L (note the θ = 45 • cases are presented again to highlight the order of magnitude increase). As a result, particularly drastic oscillations are measured just ahead of high-deflection ramps under laminar incoming flow conditions, which even exceed slightly the respective fully turbulent counterparts given the transitional state of the flow (Reshotko 2008;Schneider 2008). The increasing magnitude of the heat transfer augmentation in the area surrounding the obstacles is further evidenced in the q/q u,T and σ q /q contours in Fig.…”

Thermal Management at Hypersonic Leading Edges

“…Surface ________________________________________________________________________ roughness can "trip" laminar boundary layer flow on the vehicle skin, generating an increase in heat flux from turbulent flow. While many factors influence the tripping point, including roughness height, boundary-layer thickness, and Reynolds number, surface elements down to several thousandths of an inch can be problematic for hypersonic vehicle skins [25].…”

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

“…In this paper, 3D roughness elements placed in an incoming laminar boundary layer are experimentally investigated. The presence of such elements, that can appear in form of steps, joints or patterns left on a metal surface by ablation of the thermal protection system (TPS), can accelerate the laminarturbulent transition process enhancing the thermal loads and the skin friction coefficient [1]. The thermal loads are mostly due to the convective heat transfer, which depends on several parameters: re-entry trajectory, vehicle configuration and flow conditions [2].…”

“…Van Driest and McCauley [6] defined a roughness element critical when it starts effecting on the transition location while they named effective a roughness able to move transition as close as possible to it. Although the definition of critical and effective roughness is rather clear, the identification of the onset of the transition location is quite ambiguous [1]. Moreover, the dependence of the laminar-turbulent transition by several parameters, such as Reynolds number and Mach number [6], size and geometry of the roughness, surface polishing, surface temperature and wind tunnel noise [7], make the comprehension of the physical mechanisms that lead to transition [8] very hard.…”

“…Therefore, these parameters are often not well measured and do not exist any ground facility able to reproduce completely the flight environments: high Mach number, high Reynolds number, low noise and high enthalpy. Even if a general mechanism is not recognized, Schneider [1] reported three ways by which roughness can effect transition. In the first scenario the roughness generates a wake with streamwise vorticity and a possibly unstable shear layer.…”