Flow visualization over drag reducing dimpled surfaces in turbulent boundary layers using Particle Image Velocimetry

Although various experimental studies have confirmed a potential drag reduction of dimpled surfaces in turbulent boundary layers, the working mechanism behind the effect remains largely unresolved. The goal of this experimental study is to reveal the flow structures that could explain this drag reduction. To this end, flow visualizations over drag reducing dimpled surfaces are performed, using planar and stereoscopic Particle Image Velocimetry (PIV). The PIV measurements show that there is no significant vortex generation in the present dimples, but that instead a converger-diffuser type of flow occurs. It can be therefore concluded that it is not the generation of vortices which causes the drag reduction, in contrast to what has been proposed in literature. Based on the present measurements, a new drag reducing mechanism is proposed: the interaction between dimples causes alternating spanwise excitations of the near-wall flow which interacts with the turbulent coherent structures and leads to a reduction of the turbulent drag