Drag Reduction in Bubbly Taylor-Couette Turbulence

In Taylor-Couette flow the total energy dissipation rate and therefore the drag can be determined by measuring the torque on the system. We do so for Reynolds numbers between Re=7×10⁴ and Re=10⁶ after having injected (i) small bubbles (R=1  mm) up to a volume concentration of α=5% and (ii) buoyant particles (ρ_p/ρ_l=0.14) of comparable volume concentration. In case (i) we observe a crossover from little drag reduction at smaller Re to strong drag reduction up to 20% at Re=10⁶. In case (ii) we observe at most little drag reduction throughout. Several theoretical models for bubbly drag reduction are discussed in view of our findings.