Boundary layer control using a dynamic free-slip surface *

Recently we discovered that an oscillating free-slip surface attached to the solid surface in a turbulent boundary layer can reduce the wall shear stress for more than 40% (Wang & Gharib 2020). Evidence suggests that the significant drag reduction effect is linked to the streaming motion (e.g. a laminar jet) induced by the dynamic oscillation (Wang & Gharib 2021). To better understand the physical mechanism of the drag reduction effect, in this work, we investigate the dynamic interaction of a bounded free-slip surface with a laminar boundary layer. The velocity/vorticity field suggests that the near-wall transverse vorticity is gradually lifted up by the accumulative effect of the dynamic oscillation. In addition, well-organized counter-rotating streamwise vortices are created and sustained in the downstream regions. The lift-up of transverse vorticity and the formation of patterned streamwise vortices can explain the strong drag reduction effect of the dynamic free-slip surface.