Wake structures behind a rotor with superhydrophobic-coated blades at low Reynolds number

We experimentally investigate the flow structures generated by a rotor with the superhydrophobic coating applied on the blade surface in static water. Considered Reynolds number based on the rotating velocity and blade tip to tip distance is 96 000, and spray coating of hydrophobic nanoparticles is used to produce a superhydrophobic surface. We focus on the changes in both instantaneous and time-/ ensemble-averaged flows measured with a stereoscopic particle image velocimetry. The vortical structures behind a rotor is characterized by the periodic shedding of hub and tip vortices, whose interactions induce a cone-shaped low-speed region where higher velocities are induced over it. These are closely connected to the spatial distribution of velocity fluctuation. With superhydrophobic surface, the organized formation of vortical structures is disturbed due to the slip on the blade surface, that is, the accumulation of vorticity on the surface is delayed or not strong. Thus the conical region shrinks toward the rotation axis, and the vortex strength is reduced. As a result, about 20% reduction in the turbulent kinetic energy is achieved in the wake, followed by smaller decrease (similar to 6%) in the streamwise momentum flux. Also, it is found that superhydrophobic surface on the pressure side is more effective, in terms of turbulence reduction. This is the first study to investigate the effect of superhydrophobic surface on the flow around a rotating body, and we think the results will be useful to extend the application of superhydrophobic surface.