Isolated Roughness and Tollmien-Schlichting Waves in Boundary-Layer Transition

Abstract

An experimental investigation aimed at detecting the critical roughness Reynolds number (Rekk, tr ) both with and without Tollmien-Schlichting (T-S) waves is described in this paper. As a novel technique to examine Rekk, tr systematically, we employed isolated, cylindrical roughnesses with automatically adjustable height of micro-meter precision. The experiment was performed using hot-wire anemometry in flat-plate boundary layers developing under close to zero-pressure-gradient conditions in a low-turbulence level wind tunnel. The present data for Rekk, tr without T-S waves confirmed previous results and showed a strong correlation between the roughness aspect ratio and Rekk,tr. Controlling the roughness height while keeping the free-stream velocity fixed revealed noteworthy hysteresis for Rekk, tr. As expected the critical roughness Reynolds number decreased with the presence of T-S waves. The necessary T-S wave amplitude needed for transition became smaller with increasing the roughness height with a sudden drop when approaching the critical roughness height without T-S waves.