Fig. 1. Diagram of a vortex ring in cross section with the X' and Y' axes indicated, velocity components and velocity profiles. (A) Cross section of vortex ring left behind by the frog foot just disappearing on the left. Red, clockwise vorticity; blue, anti-clockwise vorticity; Y'-axis is normal to the vortex ring; X'-axis is perpendicular to the normal axis, parallel to the ring plane. (B) Predicted v velocity profile along the Y'-axis. (C) Predicted u velocity profile through vortex core along the Y'-axis. (D) Predicted v velocity profile along the X'-axis through both vortex cores.