Modification of a turbulent boundary layer by circular cavities

It is shown how well-chosen perforations in a wall flow can locally reduce skin friction drag by modifying the generation of bursts in the boundary layer. For this purpose, a detailed hot wire boundary layer experimental investigation of the flow past a perforated plate, complemented with large eddy simulations, is carried out and compared to the smooth case. The perforated plate is obtained with an array of flush-mounted circular cavities. These cavities are disposed in a periodic staggered arrangement. For the three tested flow velocities, the momentum thickness-based Reynolds number varies from $Reθ=1830$ to 3380 and the cavity diameter and spacing in wall units, respectively, from $d+=130$ to 250 and $L+=587$ to 1075, the latter being identical in both spanwise and streamwise directions. The mean velocity profiles evidence a thickening of the viscous sublayer and a decrease in the friction velocity as compared to the smooth wall case. The application of the Variable Interval Time Averaging technique highlights an upward shift of the bursts from the wall and an attenuation of the average burst intensity and duration. Spanwise measurements evidence an overall bursts attenuation despite the lack of spanwise uniformity. The three-dimensional (3D) mean flow topology arising from the large eddy simulations provides evidence for the qualitative similarities between the current setup and the spanwise wall oscillations.