UCLA

Recent developments of superhydrophobic surfaces (SHSs) have attracted much attention because of the possibility of achieving substantial skin-friction drag reduction at high Reynolds number turbulent flows. An SHS, consisting of a hydrophobic surface combined with micro- or nano-scaled topological features, can yield an effective slip length on the order of several hundred microns. In this numerical study, direct numerical simulations of turbulent channel flows and turbulent boundary layers (TBLs) developing over SHSs were performed. An SHS was modeled through the shear-free boundary condition, assuming the sustainable gas-liquid interface remained as a flat surface. For the considered Reynolds number ranges and SHS geometries, it was found that the effective slip length normalized by viscous wall units was the key parameter and the effective slip length should be on the order of the buffer layer in order to have the maximum benefit of drag reduction. The effective surface slip length can be interpreted as a depth of influence into which SHSs affect the flow in the wall-normal direction. This result demonstrates that an SHS achieves its drag reduction by affecting the turbulence structures within the buffer layer of wall-bounded turbulent flow. It was also found that the width of an SHS, relative to the spanwise width of near-wall turbulence structures, was also a key parameter to the total amount of drag reduction. Significant suppression of near-wall turbulence structures were observed, which resulted in large skin-friction drag reduction due to the lack of the shear over SHSs. A comparison between TBLs and turbulent channel flows over SHSs were also examined. In contrast to fully developed turbulent channel flows, the effective slip velocity and hence the effective slip length varied in the streamwise direction of TBL, implying that total drag reduction of TBL would depend on the streamwise length of a given SHS. The present numerical study was compared with recent experimental results and showed good agreement. In addition to flow and SHS geometry conditions, the streamwise length of SHSs was also a key factor to understand the underlying physics of wall-bounded shear flows. Finally, it was found that the amount of drag reduction was theoretically estimated as a function of the effective slip length normalized by viscous wall units.