Experimental investigation of the effects of superhydrophobic surface treatment on the flow in the near wake of a sphere at Reynolds number 7780

Superhydrophobic surfaces introduce partial slip at the surface submerged in water, reducing viscous drag. While the drag reduction of superhydrophobic surfaces is well understood in laminar flow, the complexity of turbulent flows requires further investigation to understand how these highly dynamic flows interact with surface superhydrophobicity. Additionally, the efficacy of a superhydrophobic surface is highly dependent on retaining the plastron, the air layer that generates the partial slip required for viscous drag reduction. This paper presents an experimental investigation of the effects of superhydrophobic surface treatment on the wake of a sphere, with the plastron maintained throughout the experiments using an active air supply. The first- and second-order flow statistics are compared between spheres with and without surface treatment, as are the wake characteristics. Proper orthogonal decomposition is performed on the velocity fluctuations in the near wake to compare the turbulent structures and the effect of superhydrophobicity thereon. The addition of superhydrophobic surface treatment is shown to have a substantial effect on the mean flow in the wake of the sphere as well as the structure of the turbulent fluctuations. The behavior of the flow in the wake of the superhydrophobic sphere is consistent with the fluctuating plastron observed in previous studies.