Free Vortices

Singularities that are free to move in the flow do not behave in response to F = ma (what is m?). Rather they move with the local flow velocity. Thus, vortices and sources are convected downstream with the flow. And interacting singularities can produce complex motions due to their mutual induced velocities.

A pair of counter-rotating vortices moves downward because of their mutual induced velocities.

Co-rotating vortices orbit each other under the influence of their mutual induced velocities.