  The phenomenon of drag reduction induced by injection of bubbles into a
turbulent carrier fluid has been known for a long time; the governing control
parameters and underlying physics is however not well understood. In this
paper, we use three dimensional numerical simulations to uncover the effect of
deformability of bubbles injected in a turbulent Taylor-Couette flow on the
overall drag experienced by the system. We consider two different Reynolds
numbers 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 which
is varied in the range $We=0.01 - 2.0$. Our numerical simulations show that
increasing the deformability of bubbles i.e., $We$ leads to an increase in drag
reduction. We look at the different physical effects contributing to drag
reduction and analyse their individual contributions with increasing bubble
deformability. Profiles of local angular velocity flux show that in the
presence of bubbles, turbulence is enhanced near the inner cylinder while
attenuated in the bulk and near the outer cylinder. We connect the increase in
drag reduction to the decrease in dissipation in the wake of highly deformed
bubbles near the inner cylinder.
