(136c) Biomimetic Surfaces for Drag Reduction in Turbulent Flow

A significant amount of fuel consumed by marine vehicles is expended in overcoming frictional drag. Any significant reduction in this frictional drag would result in noteworthy economic savings and reduced environmental impact. Superhydrophobic surfaces (SHSs) have shown promise in reducing drag in small-scale applications and/or in laminar flow conditions. The efficacy of these surfaces in reducing drag in turbulent flow, however, is not well characterized. Large, scalable SHSs in turbulent flow have shown drag increase, no change, or 10% to 30% drag reduction under certain conditions. In this work we discuss the design of mechanically durable, large-scale SHSs that cause significant drag reduction (> 50% reduction) in fully-developed turbulent flow. A large-scale flow facility was constructed specially to evaluate SHSs in fully-developed turbulence. We also discuss how the conventional methods of evaluating SHSs at ambient pressure cannot predict their drag reduction under turbulent flow conditions. Instead, we propose a new characterization parameter, contact angle hysteresis at higher pressure, which aids in the rational design of friction-reducing SHSs. Finally, we relate the drag reduction observed on our SHSs to the viscous length scales of the flow. Overall, we find that both the contact angle hysteresis at higher pressure, and the non-dimensionalized surface roughness, must be minimized to achieve meaningful turbulent drag reduction. In contrast, we show that even SHSs that are considered hydrodynamically smooth can cause significant drag increase if designed incorrectly.