Regions of flow within cylinder gap MRI operation, in this case the outer ring is rotating slower than the bulk flow and the inner ring corotates with the bulk flow at r = 120 mm. The lower half of the apparatus is shown. A) The Taylor-Proudman theorem holds in the bulk flow, which closely approximates the ideal Couette profile. B) Fluctuations generated by the vertical boundaries extend approximately 40 mm into the flow. The sign of radial circulation within the “boundary zones” are indicated by the horizontal arrows. At r = 120 mm, the boundary corotates with the bulk flow. The radial velocity of the Ekman circulation changes sign at this radius as the ring is removing (adding) angular momentum to the bulk at smaller (larger) radii. The Ekman circulations penetrate into the bulk flow, see Fig. 8. Near the ring gap a vertical scan of v_θ fluctuations indicates that the discontinuity in velocity produced at the ring gap does not propagate in to the bulk flow. C) Centrifugally stable boundary which transitions from the bulk flow to the outer cylinder. (This region C may be centrifugally unstable when the cylinder speeds exceeds the Rayleigh criterion as in the flow CUS 1 listed in Table 2. This leads to a lower q than 2 in the region A), and may explain the small measured Reynolds stress there as shown in Fig. 10). D) Ekman layer detaches from walls with higher fluctuations visible in the 5 mm radial scan, see Fig. 7. The maximum fluctuation amplitude due to the detachment is approximately 37% and occurs at r ≈ 105 mm (Schartman 2008). E) Measurement of q indicates a centrifugally-unstable region exists between the inner cylinder and r₁ ≈ 90 mm. F) The unstable flow relaminarizes by r = 100 mm.