The Effect of Localized Air Emission on the Drag of a Slender Surface Craft
1991
Coughran, Mark T. | Coder, David W.
In an attempt to reduce skin friction drag, air bubbles were emitted from a porous girthwise section near midships on a 28 ft-(8.5 m-) long rowing shell at speeds up to 20 ft/s (6.1 m/s). Air flow rates were scaled up from previous studies which yielded large skin friction reductions on small, submerged geometries. Increases in total drag occurred in the present experiment at most conditions. The smallest percentage drag increases occurred at the highest towing speed. Several possible reasons for the difference from the previous work are discussed. Drag reduction using localized air emission from a long, slender hull does not appear to be possible at low absolute speeds, due to the influence of buoyancy. The apparent requirement of high speed leads to difficulties in model-scale testing for surface ships. The governing parameter for microbubble drag reduction in most of the ARL/PSU configurations is the ration of injected air flow to the water flow rate in the boundary layer. This parameter is applicable for air microbubbles confined to the boundary layer either by buoyancy (body geometry) or high speeds. It is not clear that the same parameter will correctly scale up air flow rate requirements for higher Reynolds numbers on arbitrary geometries.
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