Transitional and Turbulent Boundary Layers on a Cold Flat Plate in Hypersonic Flow

Abstract

SummaryAn experimental investigation of natural transition and turbulent boundary layers on a cold flat plate in hypersonic flow was carried out in the Imperial College Gun Tunnel. Studies were also made of the effects of both two- and three-dimensional roughness elements including vortex generators.Results indicate that the Reynolds number based on the end of transition consists of two separate parts. These are the Reynolds numbers based respectively on the position of the apparent origin of the transition region behaviour, Ri, and on the length of the transition region measured from this origin. Ri is shown to be independent of unit Reynolds number for a sharp leading-edge model, and by correlations of other published data to vary little with Mach number. This origin is perhaps the most relevant point when comparisons are made between experiment and classical stability theory. The variation of the heat transfer rate during the transition region is compared with findings at low speed and shown to be similar to that expected from the turbulent spot concept of Emmons. The measured variation of the length of the transition region (x) from this origin gives an explanation for the unit Reynolds number effect on transition, and the rapid increase of transition Reynolds number with Mach number in hypersonic flow. The effect of bluntness on these factors is discussed.The virtual origin of the turbulent boundary layer is shown to lie close to the measured beginning of transition. At these high Mach number and low wall temperature conditions experimental values of heat transfer and values of skin friction coefficient derived from the estimated momentum thickness growth in the turbulent region are found to be appreciably higher than predictions by present theories.Roughness trips were found to bring the whole transition region forward. Only small differences in the measured boundary layer thickness, shape and heat transfer rates in the turbulent region (referenced to the estimated virtual origin in each case) were found after natural and forced transition. The transition point locations on a flat plate with either spherical roughness elements or vortex generators were found to agree with values predicted by the method of Potter and Whitfield as modified by Luxton to account for the effects of heat transfer.