Reduction of turbulent skin friction by microbubbles

Measurements of the effect of microbubbles on a zero pressure gradient turbulent boundary layer generated on the test section wall of a water tunnel are described. Microbubbles are created by injecting air through a 0.5 μm sintered stainless steel plate immediately upstream of a floating element drag balance. At the downstream edge of the balance the length Reynolds number is as high as ten million. Integrated skin friction reduction of greater than 80% is observed. The drag balance results are confirmed by measurements with a surface hot-film probe. For the case in which buoyancy tends to keep the bubbles in the boundary layer, the skin friction data are shown to collapse when plotted against the ratio of air to water volume flow rate. The effects of buoyancy on skin friction reduction are also documented.