DNS of flow over realistically rough superhydrophobic surfaces

Direct numerical simulations are performed for two wall-bounded flow configurations: laminar Couette flow at Re= 740 and turbulent channel flow at Reτ= 180, where τ is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air-water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The laminar Couette flow is studied with varying interface heights h to understand the effect on slip and drag reduction (DR). A power law linear regression fit is used to obtain an effective slip as a function of gas fraction For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall. The fully wetted roughness increases the peak value in turbulent intensities, whereas the presence of the interface suppresses them, as evident from vorticity and turbulent kinetic energy. Overall, there exists a competing effect between the interface and the asperities, where the interface suppresses turbulence whereas the asperities enhance them.