Cavity as a Passive Device for the Skin-Friction Drag Reduction

A possibility of reducing the skin-friction drag in a turbulent boundary layer with an open cavity is investigated in this study. Large eddy simulations are performed for turbulent boundary layer flows over four different cavity configurations at Re = u_∞ θ /ν =1520, where u_∞ is the free-stream velocity and θ is the momentum thickness. The cavities considered are combinations of two different depths (D/θ =1.46 and 3.44) and two different lengths (L/D=1 and 2). As the cavity length increases at a fixed cavity depth, friction drag decreases but pressure drag increases due to the recirculating bubble inside the cavity. On the other hand, pressure drag significantly increases with increasing cavity depth, because the size of the recirculating bubble is proportional to the cavity depth. Therefore, total-drag increase or decrease mainly depends on the depth of the cavity: total drags decrease for the cases of D/θ =1.46, while they increase for the cases of D/θ =3.44. The turbulence intensities above the cavities are larger than those above the flat plate due to momentum transfer between the flow inside the cavity and the flow in the boundary layer, and they become larger as the cavity length increases. The velocity and pressure fluctuations are maximum near the edge of the forward-facing wall of the cavity.