Radial profiles of v_θ at several axial locations for an optimized flow (QK(q = 1.9)) at Re ≈ 3 × 10⁵. To facilitate viewing the profiles have been offset. The increased fluctuation levels in the 5 mm scan near the inner cylinder are due to the disruption of the boundary layer circulation which occurs for differential rotation of the inner cylinder with respect to the inner ring. For the innermost radial point at 72 mm, the measurement volume of the diagnostic overlaps the inner cylinder and is therefore a measure of the surface speed of the cylinder rather than a flow velocity.