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.