The flow over a surface mounted rib turbulator under single-mode and dual-mode excitation

The characteristics of the large-scale structures in the separated shear layer past a surface-mounted square rib is experimentally examined using laser Doppler velocimetry and hot wire anemometry. The ratio of the rib height to channel height set during the experiment are 0.052, 0.104, 0.156 and 0.260 and the Reynolds number are 8200, 9800, 11400, 13000 and 14600. The behavior of the shear layer to controlled perturbation is explored. The controlled perturbation studies include excitation with both single tone and two harmonically related tones. The effects of the phase relation between the subharmonic and the fundamental on the shear layer growth rate are studied. The fundamentally excited shear layer offers maximum growth among all single frequency excitation. The two frequency excitation has comparable shear layer growth as the fundamental excitation; its effectiveness being weakly dependent on the phase difference and the lower phase difference i.e. 0° and 90° is the most effective. The appropriate length scale for characterizing the large-scale structures is the momentum thickness of the shear layer at the downstream edge of the rib and the corresponding non-dimensional frequency is equal to 0.013. This is in contrast to a rib in the free stream, where the rib height is the correct scaling parameter.