Comparisons of flow structure above dimpled surfaces with different dimple depths in a channel

Flow structural characteristics over dimple surfaces located on one wall of a rectangular channel with three different dimple depths (δ∕D=0.1, 0.2, and 0.3) are studied experimentally. Reynolds number based on channel height ReH ranges from 2100 to 20 000, and the ratio of channel height to dimple print diameter H∕D is 1.0. Presented are instantaneous flow visualization images, spectra of longitudinal velocity fluctuations, vortex pair frequency information, and time-averaged surveys and profiles of different quantities. Regardless of dimple depth, primary vortex pairs are periodically ejected from the central parts of each dimple and exist in conjunction with edge vortex pairs present near the spanwise edges of staggered dimples. As dimple depth increases, larger deficits of total pressure and streamwise velocity are present, along with higher magnitudes of time-averaged streamwise vorticity, vortex circulation, and longitudinal Reynolds normal stress. Bigger and stronger vortices with increased turbulence transport capabilities are thus produced by deeper dimples. Ensemble-averaged power spectral density profiles show that primary vortex pair ejection frequencies range from 7to9Hz, and edge vortex pair oscillation frequencies range from 5to7Hz, with similar distributions as the Reynolds number varies, regardless of dimple depth.