Intermittent boundary layers and torque maxima in Taylor-Couette flow

Turbulent Taylor-Couette flow between counter-rotating cylinders develops intermittently fluctuating boundary layers for sufficient counter-rotation. We demonstrate the phenomenon in direct numerical simulations for radius ratios η=0.5 and 0.71 and propose a theoretical model for the critical value in the rotation ratio. Numerical results as well as experiments show that the onset of this intermittency coincides with the maximum in torque. The variations in torque correlate with the variations in mean Taylor vortex flow, which is first enhanced for weak counter-rotation and then is reduced as intermittency sets in. To support the model, we compare it to numerical results, to experiments at higher Reynolds numbers, and to Wendt's data.