Laminar-turbulent boundary-layer transition over a rough rotating disk

Boundary-layer transition over a disk spinning under water is investigated. Transitional Reynolds numbers, Re_c, and associated boundary-layer velocity profiles are determined from flow-visualizations and hot-film measurements, respectively. The value of Re_c and the velocity profiles are studied as a function of the disk's surface roughness. It is found that transition over rough disks occurs in a similar fashion to that over smooth disks, i.e., abruptly and axisymmetrically at well-defined radii. Wall roughness has little effect on Re_c until a threshold relative roughness is reached. Above the threshold Re_c decreases sharply. The decrease is consistent with the drop one expects for our flow for the absolute instability discovered by Lingwood [J. Fluid Mech. 299, 17 (1995); 314, 373 (1996); 331, 405 (1997)]. This indicates that the Lingwood absolute instability may continue to play a major role in the transition process even for large relative roughness.