Ultimate Turbulent Taylor-Couette Flow

The flow structure of strongly turbulent Taylor-Couette flow with Reynolds numbers up to Rei=2×106 of the inner cylinder is experimentally examined with high-speed particle image velocimetry (PIV). The wind Reynolds numbers Rew of the turbulent Taylor-vortex flow is found to scale as Rew∝Ta1/2, exactly as predicted by Grossmann and Lohse [ Phys. Fluids 23 045108 (2011)] for the ultimate turbulence regime, in which the boundary layers are turbulent. The dimensionless angular velocity flux has an effective scaling of Nuω∝Ta0.38, also in correspondence with turbulence in the ultimate regime. The scaling of Nuω is confirmed by local angular velocity flux measurements extracted from high-speed PIV measurements: though the flux shows huge fluctuations, its spatial and temporal average nicely agrees with the result from the global torque measurements