The velocity fields at the point of peak tangential momentum (): (a) ψA = 30°; (b) ψA = 15°. At each cap angle ψA, the radius ratio rA/rB = 2, 3, 4.5 from (i) to (iii), and the corresponding , 2.50, 2.48 in (a) and , 2.52, 2.56 in (b). The conditions are otherwise identical to those in figure 4, although the orientation is rotated 90°. The unit vectors (black arrows) represent uci. See also electronic supplementary material, movies S2–S7. (Online version in colour.)