The Effect of a Sustained Plastron on the Wake of a Sphere with Superhydrophobic Treatment

Abstract: Superhydrophobic surfaces, inspired by the self-cleaning leaves of plants, generate a partial slip at a surface by trapping a layer of air, called the plastron, in the surface roughness, which has the potential to reduce skin friction drag. While superhydrophobic drag reduction in laminar flow is relatively well understood, the complexity of turbulent flows presents additional challenges in drag reduction using superhydrophobic surface treatments. A major challenge in gathering data during experimental investigations of these surfaces in turbulent flows is the tendency for the plastron to detach during experiments, due to the high velocity and pressure fluctuations. We measure the flow in the near wake of a sphere treated with a superhydrophobic surface treatment using cross-correlation digital particle image velocimetry, while actively maintaining the plastron by supplying air through pores in the surface of the sphere via the mounting system. Comparing the characteristics of the wakes of a smooth untreated sphere to that of the superhydrophobic surface treatment, we show that the addition of superhydrophobic surface treatment reduces the size of the mean recirculation region, increases the streamwise turbulence, and increases the local wake Reynolds number in the near wake of the sphere. Keywords: Flow over spheres, Wakes, Superhydrophobic surfaces