Drag reduction induced by the addition of a multi-cavity at the base of a bluff body

We present a three-dimensional numerical study on the drag reduction of a D-shaped body, of chord length L, height H and spanwise width W, with H / W ⪯¡ 1 , aligned with a turbulent incompressible free-stream of velocity U ∞ , density ρ and viscosity μ. In particular, an extensive parametric study is performed numerically using the IDDES turbulent model, at a Reynolds number, Re = ρ U ∞ H / μ = 20 000 , to analyze the effect on the drag coefficient, CD, of adding a single or a multi-cavity of variable depth, h, at the base of the body. It has been observed that, in the range 0 ≤ h / H ≤ 0.3 , the average drag decreases monotonically reaching an asymptotic value in both cases. However, shorter cavity depths are necessary to reach the same reduction in the case of the body with a multi-cavity. On the other hand, the temporal evolution of the drag shows a considerable reduction of the flow randomness with the addition of a multi-cavity, which translates into a much less chaotic and quasi-two dimensional wake.