Effects of Cavities and Protuberances on Transition over Hypersonic Vehicles

Abstract

Surface protuberances and cavities on a hypersonic vehicle are known to cause several aerodynamic or aerothermodynamic issues. Most important of all, premature transition due to these surface irregularities can lead to a significant rise in surface heating. To help understand laminar-turbulent transition induced by protuberances or cavities on a Crew Exploration Vehicle (CEV) surface, high-fidelity numerical simulations are carried out for both types of trips on a CEV wind tunnel model. Due to the large bluntness, these surface irregularities reside in an accelerating subsonic boundary layer. For the Mach 6 wind tunnel conditions with a roughness Reynolds number Re kk of 800, it was found that a protuberance with a height to boundary layer thickness ratio of 0.73 leads to strong wake instability and spontaneous vortex shedding, while a cavity with identical geometry only causes a rather weak flow unsteadiness. The same cavity with a larger Reynolds number also leads to similar spontaneous vortex shedding and wake instability. The wake development and the formation of hairpin vortices for both protuberance and cavity were found to be qualitatively similar to that observed for an isolated hemisphere submerged in a subsonic, low speed flat-plate boundary layer. However, the shed vortices and their accompanying instability waves were found to be slightly stabilized downstream by the accelerating boundary layer along the CEV surface. Despite this stabilizing influence, it was found that the wake instability spreads substantially in both wall-normal and azimuthal directions as the flow is evolving towards a transitional state. Similarities and differences between the wake instability behind a protuberance and a cavity are investigated. Computations for the Mach 6 boundary layer over a slender cylindrical roughness element with a height to the boundary layer thickness of about 1.1 also shows spontaneous vortex shedding and strong wake instability. Comparisons of detailed flow structures associated with protuberances at subsonic and supersonic edge Mach numbers indicate distinctively different instability mechanisms. Nomenclature

Investigation on the wake flow instability behind isolated roughness elements on the forebody of a blunt generic reentry capsule

“…For the far wake region (s/d > 4), the authors report that the leading mode is damped in the FPG case, whereas in the ZPG and APG cases, the amplitude remains at a nearly constant level before being slightly damped [30]. A stabilizing in §uence of the FPG is also mentioned in [9]. Figure 15 shows the maximum N -factor, N max , computed downstream of the roughness at its corresponding frequency.…”

“…Thereby, the local growth rates will be accumulated to an N -factor using Eq. (9). Figure 14 shows the N -factor envelope curves vs. the dimensionless surface arc length for modes S1 and A1.…”

“…Transition to turbulence was induced by a trip array of three diamond-shaped discrete roughness elements on the leeward side, but not on the windside even for the largest (k/δ ≈ 1) element at the highest wind tunnel Reynolds number. Chang et al [9] numerically investigated the e¨ect of surface irregularities in the form of a protuberance and cavity on the CEV surface by means of direct numerical simulation (DNS). The authors showed that the investigated cylindrical protrusion element is more e¨ective to trigger transition than a cavity at identical roughness Reynolds number Re kk and Reynolds number values.…”

Hypersonic Boundary-Layer Instabilities due to Near-Critical Roughness

“…Thanks in large part to advancements in computational methods, researchers are recently beginning to characterize the mechanisms responsible for transition due to roughness at hypersonic speeds [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, even direct numerical simulation must construct an appropriate grid and make assumptions regarding the input disturbance field when simulating these flows.…”

Wake Instabilities behind Discrete Roughness Elements in High Speed Boundary Layers

“…Strong tonal perturbations amplifying within the near-wake region were also detected via measurements of surface pressure fluctuations in quiet flow experiments 6 at M ≈ 6 and recent numerical simulations of that experiment. 7 Subsonic boundary layers with roughness elements are also known to support absolute instabilities, [8][9][10] but those appear to be associated with the separation region immediately behind the roughness element rather than ahead of it. Roughness induced transition is currently an active area of research and the reader is referred to Refs.…”