The importance of bubble deformability for strong drag reduction in bubbly turbulent Taylor-Couette flow

Drag reduction (DR) in two-phase turbulent Taylor-Couette (TC) flow is studied for Reynolds number up to Re = 2$\times 10^6$ for pure inner cylinder (IC) rotation, thus extending the previously explored range. DR based on the global torque as a function of the global gas volume fraction ($\alpha$) over the range $0\%$ up to $4\%$ is obtained. We observe two DR regimes: moderate DR up to $7\%$ for Re $= 5.1 \times 10^5$ and stronger DR for Re $= 1.0 \times 10^6$ and $2.0 \times 10^6$, remarkably finding more than $40\%$ of DR for $\alpha = 4\%$ at Re $= 2.0 \times 10^6$. Furthermore, TC flow is locally studied in each regime (Re $= 5.1 \times 10^5$ and $1.0 \times 10^6$) at a fixed $\alpha = 3\%$: statistics of the local liquid flow azimuthal velocity and the local gas concentration are obtained. The local bubble Weber number ($We$) is computed close to the IC showing that the crossover from the moderate to the strong DR regime occurs roughly at the crossover of $We \sim 1$. We find that a larger local gas volume fraction close to the inner wall has a positive effect on the azimuthal velocity decrease, which is responsible for the observed DR. However for strong DR what is more important for the $\alpha$ values explored here is bubble deformability close to the boundary layer.