Mechanism of control of the near-wall turbulence using a micro-cavity array

Coherent structures in a turbulent boundary layer have been shown to have an influence on the skin-friction drag acting on surfaces beneath the boundary layer. The use of micro-cavities on a flat surface has recently shown the potential to passively control a turbulent boundary layer by attenuating the sweep events. Previous experiments have determined the design parameters of the cavity array for the optimal boundary-layer control by reducing the sweep events. However, investigating the flow physics behind the interaction of the boundary-layer flow with the cavities is challenging. High near-wall velocity gradients and very small scales and sizes of the cavity holes limit the experiments from investigating the flow characteristics very close to the wall and inside the holes. Therefore, in the present work, direct numerical simulations have been utilized to model the boundary layer flow over a flat surface with a micro-cavity array in order to understand the flow interactions. Detection of coherent structures in the boundary layer shows a reduction in the number of events over the cavity array. Reynolds stresses have been analyzed to determine the effect of micro-cavities. The reduction in the Reynolds shear stress results in a lower skin-friction drag. The flow fluctuations through the holes in the streamwise sequence have been found to be highly correlated using cross correlation. These flow fluctuations interact with the boundary layer to suppress the coherent structures. Overall, the use of the micro-cavity array has resulted in a reduced wall shear stress and approximately 5.6% lower local skin-friction drag.