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Lock‐on phenomena in cavity flows

Abstract This experimental investigation reveals the lock‐on phenomena of cavity flows under periodic excitations. All the experiments were conducted in a recirculating water channel. The LDV system and the laser‐light sheet technique are employed to perform the quantitative velocity measurements and the qualitative flow visualization, respectively. The Reynolds number, based on the momentum thickness at the upstream edge of the cavity, is kept around 152. An oscillating flat plate, situated above the cavity, generates the periodic perturbations. Depending upon the excitation amplitudes, several lock‐on regions of the cavity shear layer are found when the excitation frequency ratio varies from 0.28 to 2.0. Also, the frequency bandwidth of each lock‐on region becomes wider as the excitation amplitude increases. As the excitation amplitude increases beyond a threshold value (0.78 mm), the number of lock‐on regions increases. Moreover, when the excitation amplitude exceeds 0.78 mm, the lock‐on regions centered at 0.7 fo and fo merge into each other. Here fo is the natural instability of the cavity shear layer. Unlike that of the oscillating cylinder, the side‐wise expansion of each lock‐on region is relatively mild. Slimmer lock‐on regions may be caused by the robustness of the self‐excited oscillation within the cavity.