The influence of the type of gas on the reduction of skin friction drag by microbubble injection

The influence of the type of gas on the performance of microbubble skin friction reduction was investigated on an axisymmetric body. Gases were selected which covered a wide range of densities and solubilities. Integrated skin friction measurements, which span a range of velocities (U _∞) from roughly 10 to 20 m/s and tunnel pressures from 1 to 2.6 atm, are presented as a function of gas flow rate. All gases show qualitatively similar behavior. The gas volume flowrate, referenced to injector ambient conditions (tunnel temperature and pressure), is shown to correlate the drag reducing behavior of all the gases at one velocity, independent of pressure. A normalization based on the volume flowrate through the turbulent boundary layer is shown to nearly collapse all the results independent of velocity or pressure. The results indicate that high ambient pressures may degrade the drag reducing capabilities of highly soluble gases.