Air Layer on Superhydrophobic Surface for Frictional Drag Reduction

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.

The characteristics and mechanism of microbubble drag reduction on the axisymmetric body

To investigate the characteristics and mechanism of microbubble drag reduction on the axisymmetric body, both experiments and the numerical simulation of microbubble drag reduction have been conducted in this paper. Based on the experiments, the morphology of microbubble flow and the characteristics of microbubble drag reduction were analyzed. The size distribution of microbubbles and the influence of microbubble size on the drag reduction efficiency were quantitatively investigated. Based on the numerical simulation, the influences of microbubbles on the turbulence intensity and vorticity were analyzed to investigate the mechanism of microbubble drag reduction. The results indicate that the diameters of microbubbles basically obey the normal distribution at various conditions, and the microbubble flow presents the uniform microbubble, roll-up and cavity. As the air injection rate increases, the drag reduction respectively shows microbubble drag reduction, mixture drag reduction and air-layer drag reduction. Besides, the drag reduction ratio correspondingly presents the increasing stage, rapidly increasing stage and stable stage. At the same void ratio, the smaller microbubbles show the higher efficiency in drag reduction. The microbubbles injected into the boundary layer can reduce the turbulence intensity and the frequency of the bursting events in the flow field, which results in the drag reduction.