Abstract
In this work, we experimentally investigated the impact of surface roughness on drag reduction as well as the plastron stability of superhydrophobic surfaces (SHSs) in turbulent flows. A series of SHSs were fabricated by spraying hydrophobic nanoparticles on sandpapers. By changing the grit size of sandpapers from 240 to 1500, the root mean square roughness height (k(rms)) of the SHSs varied from 4 to 14 mu m. The experiments were performed in a turbulent channel flow facility, where the mean flow speed (U-m) varied from 0.5 to 4.4 m/s, and the Reynolds number (Re-m) based on U-m and channel height changed from 3400 to 26 400. The drag reduction by SHSs was measured based on pressure drops in the fully developed flow region. The plastron status and gas fraction (phi(g)) were simultaneously monitored by reflected-light microscopy. Our results showed a strong correlation between drag reduction and k(rms)(+) = k(rms)/delta(v), where delta(v) is the viscous length scale. For k(rms)(+) < 1, drag reduction was independent of k(rms)(+). A maximum 47% drag reduction was observed. For 1 < k(rms)(+) < 2, less drag reduction was observed due to the roughness effect. And for k(rms)(+) > 2, the SHSs caused an increase in drag. Furthermore, we found that surface roughness influenced the trend of plastron depletion in turbulent flows. As increasing Re-m, phi(g) reduced gradually for SHSs with large k(rms), but reduced rapidly and maintained as a constant for SHSs with small k(rms). Finally, we found that as increasing Re-m, the slip length of SHS reduced, although phi(g) was nearly a constant.