Velocity imbalance above and below the null point. Panel a: slice of the pressure through y = 0 overlaid with fluid velocity, where the longest arrows correspond to a fluid velocity of approximately 0.1. Panel b: |u_x(x)| − |u_x(−x)|, the difference in u_x between the left and right sides of the plane x = 0. This gives a measure of the asymmetry in the velocity around the null point.