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The phenomenon of drag reduction induced by injection of bubbles into aturbulent carrier fluid has been known for a long time; the governing controlparameters and underlying physics is however not well understood. In thispaper, we use three dimensional numerical simulations to uncover the effect ofdeformability of bubbles injected in a turbulent Taylor-Couette flow on theoverall drag experienced by the system. We consider two different Reynoldsnumbers for the carrier flow, i.e. $Re_i=5\times 10^3$ and $Re_i=2\times 10^4$;the deformability of the bubbles is controlled through the Weber number whichis varied in the range $We=0.01 - 2.0$. Our numerical simulations show thatincreasing the deformability of bubbles i.e., $We$ leads to an increase in dragreduction. We look at the different physical effects contributing to dragreduction and analyse their individual contributions with increasing bubbledeformability. Profiles of local angular velocity flux show that in thepresence of bubbles, turbulence is enhanced near the inner cylinder whileattenuated in the bulk and near the outer cylinder. We connect the increase indrag reduction to the decrease in dissipation in the wake of highly deformedbubbles near the inner cylinder.

Physical mechanisms governing drag reduction in turbulent Taylor-Couette flow with finite-size deformable bubbles