Wall-Cooling Effect on Boundary-Layer Instability Growth and Transition on a Sharp Cone in Mach-6 Flow

Date of Award

12-1-2023

Degree Name

M.S. in Aerospace Engineering

Department

Department of Mechanical and Aerospace Engineering

Advisor/Chair

Advisor: Carson Running

Abstract

High-speed schlieren was used to determine the boundary-layer transition location on a highly cooled 7° sharp cone at zero incidence in the Air Force Research Laboratory's Mach-6 Ludwieg Tube. Additionally, linear array focused laser differential interferometry (LA-FLDI) was used to determine the second-mode disturbance frequency and amplitude. All experiments were conducted with a freestream unit Reynolds number of 15 million per meter. The surface of the cone was cooled with liquid nitrogen (LN2) achieving average cone surface to boundary-layer edge temperature ratios (T_w/T_e) within the range of 1.34 to 4.74. For a highly cooled cone (T_w/T_e < 3), the transition front shifted downstream from a transition onset Reynolds number of 3.5x10^6 to 5.5x10^6 as T_w/T_e was gradually reduced. For a mildly cooled cone (T_w/T_e > 3), boundary-layer transition location is scattered within the range of 2.7 < (Re_x_TR)x10^6 < 3.7. The difference in transition onset between a fully-cooled cone and a room-temperature cone was approximately 150 mm. A small study compared multiple flow diagnostics to loosely bound the error in transition location as determined by schlieren. The LA-FLDI system confirmed the presence of spectral amplitude peaks at frequencies characteristic of the second-mode instability mechanism. The most-amplified disturbance frequencies for the uncooled and fully-cooled surface conditions at similar locations with respect to the transition location were within <10%. Additionally, disturbance amplitudes for both wall-temperature conditions were within a decade at similar locations with respect to transition. Continuous wavelet transforms were performed give further insight into cases where weak frequency content was observed in the power spectral density plots when strong content was expected. Finally, a hypothesis, based on (1) differences in initial second-mode amplitude and (2) fast- versus slow-mode unstable branches, is proposed that could explain different transition behavior documented in the literature.

Keywords

Hypersonic boundary-layer transition, Cooled wall, Second mode, Mach 6, Sharp cone

Rights Statement

Copyright © 2023, author.

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