Drag reduction by microbubbles - Current research status

The experimental data concerning the effect of microbubbles on skin friction in turbulent boundary layers are reviewed and compared on a common basis. All available data indicate microbubbles reduce skin friction. Maximum reductions are typically 80 percent although the required airflow rates differ by an order of magnitude. Appropriate airflow normalizations are discussed, but the recommended nondimensionalization does not bring all the data into agreement. Bubble sizes appear to be controlled by the shear layer characteristics, and to be qualitatively the same in all experiments. The persistence of the Cf reduction downstream of the injection section increases with freestream velocity, but is also affected by buoyancy. Improved understanding of bubble trajectories, the persistence of the Cf reduction and the effects of freestream velocity are necessary to improve understanding and to scale up these laboratory results to possible applications. In addition, information on the effects of pressure gradient and multiple injection locations is needed.