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

•We analyze the turbulent flow around a D-shaped body.•We compare the effects of adding single and multi-cavities at the base of the body.•The average drag decreases reaching an asymptotic value with the cavity depth.•Shorter cavity depths reach the same drag reduction in the case of the multi-cavity.•The wake becomes less chaotic and quasi-two dimensional with the multi-cavity.

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⪯¡1H/W⪯¡1, aligned with a turbulent incompressible free-stream of velocity U∞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=ρU∞H/μ=20000, 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≤0.30≤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.