Selection of Gas-Based Drag Reduction Technology

Drag reduction in water has been a goal of numerous efforts based on different technologies. The gas-based drag reduction technologies include supercavitation, partial cavitation and microbubble ejection. The objective of this study is to clarify the vehicle speed and size ranges for the technology effective application. The provided analysis is based on both our results and publications by other authors. Cavitation can reduce drag because a surface under the cavity is practically free of friction. Usually, there is also a significant drag penalty to create a cavity. For supercavitation, this drag becomes smaller than friction reduction only for extremely low cavitation numbers (less than 0.06). Ventilation would be necessary to maintain such cavitation numbers at speed around 100 knots, but there is no confident basis for any forecast: Model test data on air supply to supercavities are affected by flow blockage and a reasonable scaling law for air flux was not found yet. For partial cavitation, there are the possibilities to have no drag penalty and to achieve a total drag reduction in moderate ranges of cavitation number. The penalty-free partially cavitating flow can exist with suppression of cavity pulsation by a pressure gradient downstream of cavity tail. A 25%–30% drag reduction by partial cavitation was measured in model tests with our specially designed hydrofoil (in a water tunnel at the University of Minnesota) and in sea with a 100-ton boat. The friction reduction is proportional to areas covered by cavities. The attainable cavity lengths and covered areas depend on the vehicle speed. Requesting moderate air supply rates at design conditions, partial cavitation looks as the most promising and widely applicable drag reduction technology. Oppositely to supercavitation, microbubble drag reduction decreases with flow speed (the best results were obtained at 4.7m/s) and its effect critically depends on the surface orientation. An opportunity to apply this technology may exist for slow flat-bottom ships.