Bubble deformability is crucial for strong drag reduction in turbulent Taylor-Couette flow

Bubbly Taylor-Couette flow in the turbulent regime is studied both globally and locally at Reynolds numbers of $5.1 \times 10^5 - 2.0 \times 10^6$ for pure inner cylinder rotation. We measure the drag reduction (DR) based on the global torque for global gas volume fractions ($\alpha_{global}$) up to 4$\%$, and observe a moderate DR for Re $= 5.1 \times 10^5$, and a strong DR for Re $= 1.0 \times 10^6$ and $2.0 \times 10^6$. Remarkably, more than 40$\%$ of DR is achieved for $\alpha_{global} = 4\%$ at Re $= 2.0 \times 10^6$. We investigate the statistics of the liquid flow velocity, and directly measure the local bubble concentration and Weber number for two Reynolds numbers in different drag reduction regimes, i.e. Re $ = 1.0 \times 10^6$ (strong DR) and $5.1 \times 10^5$ (moderate DR). By combining global and local measurements we reveal that bubble deformability is crucial for strong drag reduction in bubbly turbulent Taylor-Couette flow.