An experimental investigation into the flow mechanics of dimpled surfaces in turbulent boundary layers

Although various experimental studies have confirmed the potential drag reducing effect of dimpled surfaces in a turbulent boundary layer, the working mechanism remains largely unresolved. An experimental investigation has been performed with the objective to strengthen the understanding of this aerodynamic surface and its interaction with the turbulent boundary layer. Direct force measurements were combined with Particle Image Velocimetry (PIV) and Particle Image Surface Flow Visualization (PISFV). The direct force measurements reveal that the drag reduction is highly sensitive to flow conditions: a finding with significant implications for further research as well as for potential applications. Furthermore, the PIV and PISFV measurements reveal a spanwise oscillation of the flow near the surface due to the interaction of individual dimple flow topologies, which are of the converger-diffuser type. The measurement of this oscillation provides evidence for a novel drag reduction theory: the interaction between dimples causes alternating spanwise excitations of the near-wall flow which interacts with the turbulent coherent structures which leads to a reduction of the turbulent drag.