Experimental Characterization of Skin-Friction Drag Reduction on Superhydrophobic Surfaces in High-Reynolds Number Flows

Abstract

American marine vessels used more than seven billion gallons of fossil fuels in 2012, and approximately 60 to 70 percent of this fuel is expended to overcome resistance due to skin-friction. Hence, considerable efforts have been devoted to reduce skin-friction using both passive and active methods of skin-friction reduction. The efficacy and practicality of these skin-friction reduction methods has often been limited, restricting their use; therefore, the search continues for effective and economically viable means of friction drag reductions.

Superhydrophobic surfaces (SHSs) have been shown to reduce skin-friction for laminar boundary layers. It is our goal to investigate their efficacy for turbulent flows. A SHS is characterized as a low surface energy material that, when in contact with a water drop maintains high contact angle and low resistance to rolling. These attributes are ascribed to the difference in interfacial energy between the water and solid surface of the SHSs and the solid-surface interface of the SHS - a base of micro- and nano-scale features, which act to trap pockets of air in the surface. These air pockets result in a heterogeneous three-phase interface that may reduce the wetted area between the water and solid surface, and consequently, have the potential to alter the no-slip boundary condition at the water and air interface, resulting in a reduced local shear stress. Therefore, SHSs have the potential to provide a passive and potentially more attractive alternative to the traditional means of active drag reduction.

In this dissertation, we experimentally examined the viability of SHSs for skin-friction drag reduction for turbulent boundary layers (TBL). To do so, we developed composite, sprayable SHSs that can be designed and applied to areas significantly greater than one square meter. These SHSs are mechanically robust and can withstand the extreme shear and pressure fluctuations experienced beneath turbulent boundary layers. The SHSs were spray applied to a test panel and placed in a specially designed fully-developed, turbulent flow facility designed by the author. The coefficient of friction was inferred using the streamwise pressure drop along the SHSs and velocity measurements of the mean flow through the channel. The experimental test data showed more than 50% sustained skin-friction drag savings for height-based Reynolds numbers ranging from $10,000 le Rey_H le 40,000$ and friction Reynolds number ranging from $300 le Rey_tau le 1,000$. Measurements of near-zero pressure-gradient TBL flow over similar SHSs were also conducted at a U.S. Naval Academy flow facility. The TBL experiments were conducted with a free-stream speed of 1.26 m/s, corresponding to a $Rey_tau$ of 1,600. Near-wall velocity indicated that greater than 10% reduction in the total stress at the wall could be achieved.

The skin-friction measurements were coupled with a topological evaluation of the SHSs to develop insights regarding the surface characteristics relevant to the skin-friction in turbulent flows. The results showed that the skin-friction in TBLs produced by SHSs is highly dependent on the surface's roughness characteristics and resistance to wetting. More specifically, the essential SHSs characteristics were their non-dimensional roughness $k^+$, the presence of large, unwanted asperity features, and the areal wetted fraction $phi_s$ the surface experienced at the mean pressures of the flow. These findings resulted in the development of a scaling model, along with design guidance that can be implemented for a given Reynolds number.