Experiments on roughness effects on boundary- layer transition up toMach 16

Abstract : Free-stream Mach number 14 to 16 experiments in a high Reynolds number hotshot-type wind tunnel with a sharp 9-deg cone model under relatively cold-wall conditions are presented that reveal the expected strong influence of Mach number and a unit Reynolds number effect similar to that obtained in conventional wind tunnels. Comparison of cone and hollow cylinder (i.e., equivalent to a flat plate) transition Reynolds numbers from several wind tunnels and ranges fails to reveal a conclusive picture concerning the relationship of cone and flat-plate transition Reynolds numbers. The effectiveness of spherical roughness in promoting premature boundary-layer transition is shown to decrease exponentially with increasing Mach number. The results compare favorably with the hypersonic extension proposed by Potter and Whitfield for their original correlation. It is shown that care must be exercised in selecting the physical roughness size at hypersonic speeds because of possible flow field distortion.