Three-dimensional mean flow field around two cubes in tandem

Abstract

The mean flow field around two surface-mounted cubes in tandem was investigated through large-eddy simulations at a Reynolds number of Re $=1\times 10^4$ and with a turbulent boundary layer of thickness $\delta /D=0.8$ at the location of the upstream cube. Center-to-center spacing ratios of $L/D=2$, 2.5 and 4 were considered to describe the intermittent reattachment, cavity-locked and synchronized shedding regimes, respectively. The mean flow features were related to the near-surface flow field of the cubes and their drag and normal force coefficients. Although an arch vortex was always present behind the upstream cube, the flow in the gap changed significantly depending on the different flow regimes, affecting the shape, size and strength of the arch vortex. The flow field surrounding the upstream cube did not change significantly with $L/D$, presenting a similar near-surface flow field to an isolated cube. The near-surface flow for the downstream cube changed from a reattachment pattern for $L/D=2$, to impingement for $L/D=2.5$, to flow separation from the leading edges and the appearance of a second horseshoe vortex for $L/D=4$. The arch vortex of the downstream cube was uniquely shaped and similar for all $L/D$, due to flow separation from the cube’s rear edges. Base-like streamwise vorticity regions were present in the downstream cube’s wake, which suggest they may be the time-averaged signature of the vortices shed from the cube.