Split end-cap operation at Re = 3 × 10⁵. The dotted line is the ideal Couette profile. The dashed lines are the solid-body speeds of the end rings. Based on the cylinder speeds, the flow should be stable by the Rayleigh criterion. Our beta measurement indicates that the bulk flow is in a turbulent state. Only Rayleigh-unstable flows with the outer cylinder at rest were observed to have greater levels of transport.