FLOW PAST DIMPLED SURFACES

Abstract

Shallow axisymmetric circular dimples are studied in a turbulent channel using pressure measurements, single element hot-wire velocimetry and Detached Eddy Simulations. The results show a drag reduction of up to 3%, and suggest that the mechanism of drag reduction is similar to that observed with spanwise wall motions and transverse wall jets over flat plates. The dimples introduce streamwise vorticity, resulting in spanwise flow near the wall which suppresses the normal energy cascade to the smaller scales and lead to turbulent skin friction reduction. Increasing dimple depth increases the streamwise vorticity, but can cause flow separation, increasing form drag. The flow separation is also dependent on the Reynolds number, and can shrink with increasing Reynolds number. The total drag depends on the relative dominance between the drag reducing streamwise vorticity and the drag increasing flow separation. Understanding this flow physics aids in optimizing the dimple geometry for maximum drag reduction.