Air Layer on Superhydrophobic Surface for Frictional Drag Reduction

Abstract

We examined the feasibility of combining a superhydrophobic surface (SHS) and air layer drag reduction (ALDR) to achieve the frictional drag reduction (DR) shown achievable with traditional ALDR, but at a reduced gas flux to increase the achievable net energy savings. The effect of a commercial SHS coating on the gas flux required to maintain a stable air layer (AL) for DR was investigated and compared with that of a painted non-SHS at Reynolds numbers up to 5.1 X 106. Quantitative electrical impedance measurements and more qualitative image analysis were used to characterize surface coverage and to determine whether a stable AL was formed and maintained over the length of the model. Analysis of video and still images for both the SHS and painted surface gives clear indications that the SHS is able to maintain AL consistency at significantly lower gas flux than required on the non-SHS painted surface. Hydrophobicity of the surfaces was characterized through droplet contact angle measurements, and roughness of all the flow surfaces was measured. The results from these preliminary experiments seem to indicate that for conditions explored (up to Rex = 5.1 X 106), there is a significant decrease in the amount of gas required to establish a uniform AL (and hence presumably achieve ALDR) on the SHS when compared with a hydraulically smooth painted non-SHS.