An invoiced, irrigational flow field of free vortex motion has a circulation $\Omega$ . The tangential velocity at any point in the flow field is given by $\Omega /r$ where r, is the radial distance from the centre. At the centre, there is a mathematical singularity which can be physically substituted by a forced vortex. At the interface of the free and forced vortex motion (r=rc), the angular velocity $\omega$ is given by
