Over the past decades, many attempts have been devoted to the control of the skin friction induced by the development of a boundary layer over a moving body, mainly with the purpose of drag mitigation. As far as naval applications are concerned, one of the most effective techniques is the creation of an air layer or air pockets between the solid surface and water [1] [2]. Among these techniques, the use of super-hydrophobic surfaces (SHS) in the framework of flow control has been recently initiated, but their potential is still far from being fully exploited [3]. The physicochemical properties of SHS enable them to entrap a gas layer (plastron) in their roughness, restricting thereby the solid/liquid contact area. This feature leads to liquid repellency, the so-called “Lotus effect” [4]. Recent studies have evidenced the remarkable ability of SHS to reduce viscous drag in pressure-driven flows [5] [6] [7]. Very few works have been dedicated to the effect of SHS on bluff body wakes [8] [9]. In this work, we investigate the effect of a super-hydrophobic coating on the drag of a sphere over a wide range of Reynolds numbers (up to 5*10^4, based on sphere diameter and terminal velocity).

This work reports an experimental study of the free fall of spheres in water at rest. Spheres with diameters ranging from 5 to 25 mm are investigated. Emphasis is given to the effect of surface properties on the onset of instabilities in the wake. In that purpose, experiments performed with super-hydrophobic coated spheres are compared to smooth spheres used as reference. A high-speed digital camera is used to track the three-dimensional trajectory of the sphere. Our results evidence that the critical Reynolds number at which transition occurs is strongly affected by the SHS. This is related to the perturbations induced by the motion of the air plastron over the solid surface. This transition change has a direct impact on drag. A comprehensive discussion of the SHS manufacturing procedure, the experimental setup and the results will be presented in the full paper.

References

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