A comparison of shear stress fluctuation statistics between microbubble modified and polymer modified turbulent boundary layers

Shear stress fluctuation statistics, as measured by flush mounted hot film probes, are presented for both microbubble and polymer injection into the same zero pressure gradient turbulent boundary layer. At equivalent values of drag reduction, the moments of the probability distribution are remarkably similar. For drag reductions larger than about 40%, the deviation of the skewness and kurtosis values from their pure water values argue against simple scaling explanations of the drag reduction phenomenon.