Drag reduction in turbulent boundary layers using superhydrophobic surfaces*

We studied the use of superhydrophobic (SH) surfaces for the frictional drag reduction in turbulent boundary layers. First, SH surfaces were manufactured and characterized in laboratory. Then, the reduction of frictional drag over these SH surfaces in turbulent boundary layers at the Reynolds number ranging from 4.0 × 10⁵ to 10 × 10⁵ was measured in a water channel using a dedicated, high-resolution force measurement system. The near-wall velocity field was measured with a particle image velocimetry system. Results revealed that, regardless of the Reynold number, the SH surface can achieve significant drag reduction if a homogeneous air film is able to form. However, we also observed that, as time advanced, air bubbles started to form and accumulate on the trailing edge of the SH surface and gradually broke the uniformity of the air film. When the inhomogeneous gas film formed, the SH surface exhibited less drag reduction and even drag increase. This evolution of air firm has been systematically studied in our experiments. In addition, high-fidelity computational fluid dynamics (CFD) simulations were also conducted to provide more detailed information on the frictional drag experienced by the SH surface.