Mean wake evolution behind low aspect-ratio wall-mounted finite prisms

Abstract

The evolution in the mean wake of wall-mounted prisms with varying depth-ratio (length-to-width) between 0.016 and 4 is examined numerically over a range of Reynolds numbers (Re = 50 - 500). The aspect-ratio (height-to-width) of the prism is limited to 1. This study aims to ascertain the mechanism of the evolution of mean wake topology due to changing depth-ratio. The mean wake topology is classified into Dipole, Multipole, and Quadrupole types as functions of Reynolds number and depth-ratio. The threshold depth-ratio for the wake evolution changes with Reynolds number. The Multipole-type wake appears as an evolutionary or intermediate wake pattern between Dipole and Quadrupole-type wakes. Increasing the depth-ratio leads to the enhancement of the downwash flow, resulting in the mean wake evolving from Quadrupole to Dipole wake. This suggests that the downwash flow dominates in dictating the wake topology downstream of the prism. Enhancing the downwash flow (with increase in depth-ratio) results in the reattachment of the flow on the prism surfaces, which further results in the suppression of velocity gradients and streamwise momentum transfer downstream of the prism trailing edge. This suppression of gradients and momentum leads to the deterioration of the base vortex, which evolves the wake from Quadrupole (at a small depth-ratio) to Dipole-type (at a large depth-ratio).