Rate of strain modification due to microbubbles injection in a channel flow

Injection of microbubbles within the turbulent boundary layer in a channel flow to achieve drag reduction is investigated. Experimental data for a fully developed channel flow, obtained using Particle Image Velocimetry (PIV) technique for single phase and two-phase flow are compared to elucidate the effects of microbubbles presence within the boundary layer, specifically the modification on the strain rate, which is directly related to the energy dissipation in turbulent flows. A notorious decrease in the strain rate is observed in two-phase flow. There seem to be a direct relationship between the void fraction and the decrease in the strain rate for turbulent channel flows. These results are significant and will help to unfold the physical mechanism of drag reduction by injection of microbubbles. (author)