The effects of microbubbles on turbulent boundary layer skin friction

The effects of microbubbles on the skin friction of a zero pressure gradient turbulent boundary layer generated on the test section wall of a water tunnel are investigated. Microbubbles are created by injecting air through a porous plate immediately upstream of a floating element force balance. Several surface hot film probes mounted on the force balance plate are also used to provide another independent measure of the skin friction. At the downstream edge of the force balance the length Reynolds number is as high as 10 million. Integrated skin friction reductions of as much as 80% are observed. The ratio of the air to water volumetric flow rate is shown to be a relevant parameter in the integrated skin friction reduction. Tests show that skin friction reductions can be realized with a variety of porous materials, with the size of the pores having only a secondary effect. The effects of buoyancy on the skin friction reduction are also investigated. Measurements with the surface hot film probes confirm the force balance results.