Evolutions of hairpin vortexes over a superhydrophobic surface in turbulent boundary layer flow

Abstract

Turbulent flows over a superhydrophobic surface and a smooth surface have been measured and studied by particle image velocimetry technology at Reθ = 990. The Reynolds shear stress distributions over the two surfaces are significantly different. Specifically, for the superhydrophobic surface, the Reynolds shear stress is suppressed in the near-wall region (y/δ < 0.3, δ is the boundary layer thickness) and increases in the outer region (0.3 < y/δ < 0.5), which forms a second peak of the Reynolds shear stress curve. Evolutions of hairpin vortexes are analyzed to interpret differences in the Reynolds shear stress, based on some comparisons in the low-speed streaks and Q2/Q4 (ejection/sweep) events. The results show that, in the near wall region, the turbulent coherent structures (low-speed streaks and hairpin vortex) over the superhydrophobic surface are more stable and flat, due to the suppression in the strength and the lifting effect of the hairpin vortex. In the outer region, the superhydrophobic surface lifts the hairpin vortex away from the wall with a value of 0.14δ in our experiment, which makes the Q4 events occur further from the wall and contribute less to skin friction.