Drag reduction using a multi-cavity at the afterbody

We present a numerical study on the drag reduction of a two-dimensional bluff body with a blunt trailing edge, which has a chord length $L$, body height $H$ and spanwise width $W$, being $H/W\ll 1$, aligned with a turbulent incompressible free-stream of velocity $U_{\infty}$, density $\rho$ and viscosity $\mu$. In particular, an extensive parametric study is performed numerically using the IDDES turbulent model, at a Reynolds number, $Re=\rho U_{\infty}H/\mu=20000$, to analyze the effect on the drag coefficient $C_D$ of both a single-cavity as a multi-cavity of variable depth $h$ at the base of the body. It is observed within the range, $0\le h/H \le 0.2$, that $C_D$ decreases monotonically reaching an asymptotic value in both cases. In turn, shorter cavity depths are necessary to reach the same drag reduction with a multi-cavity than with a single-cavity. On the other hand, the temporal evolution of the drag coefficient shows a lower standard deviation with a multi-cavity than with a single-cavity, which is manifested in the flow as a wake with a lower level of disorder.