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

An experimental investigation aimed at detecting the critical roughness Reynolds number (Re-kk,Re-tr) both with and without Tollmien-Schlichting (T-S) waves is described in this paper. As a novel technique to examine Re-kk,Re-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 Re-kk,Re-tr without T-S waves confirmed previous results and showed a strong correlation between the roughness aspect ratio and root Re-kk,Re-tr. Controlling the roughness height while keeping the free-stream velocity fixed revealed noteworthy hysteresis for Re-kk,Re-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.
Part of proceedings: ISBN 978-3-030-67902-6, ISBN 978-3-030-67901-9QC 20211112