Numerical investigation of the turbulent boundary layer over dimpled surfaces

Abstract

Dimples are shallow, indented surfaces that attempt to reduce drag in turbulent boundary layer flows. However, the underlying effect of the dimples on the drag is not entirely understood. This thesis sets out to expand our understanding; a numerical investigation of turbulent boundary layer flow over dimpled surfaces is conducted. This research aims to verify the drag results from recent experimental studies and investigate the possible drag-reducing mechanism of a dimpled plate. The research considers shallow, rounded-edge dimples with a staggered layout. Implicit large eddy simulation (ILES) is carried out with the cell spacing close to direct numerical simulations (DNS). Simulation outputs conclude that the dimple plate causes a total drag increase of approximately 1% compared to a smooth plate. This value confirms the results of Spalart et al. and recent wind tunnel measurements within the Aerodynamics group. The turbulent coherent structures are further investigated by performing hole-filtering sampling to the quadrant events. Results suggest that the dimple plate induces a more intense turbulent activity in the buffer layer. The increased occurrence contributes to a higher Reynolds shear stress. The development of quadrant events is further analysed using a Variable-Interval Time-Averaging (VITA) technique. It reveals that the averaged quadrant event development between two plates is almost the same. However, a more extended sweep development is found in the wake region. Given such a mild even evolution, the resulting Reynolds shear stress generation remains the same. Lastly, the coherent structure response is linked to the skin friction response through the Fukagata-Iwamoto-Kasagi (FIK) identity. The resulting decomposition using the FIK identity reveals that dimples contribute to higher total drag due to increased Reynolds shear stress. On the other hand, the observed skin friction reduction seems relative to the mean flow convection.