far-field boundary condition for full potential fl
 

Hi friends I am working in potential flow around airfoil. I use finite difference method to solution the laplace equation of full potential flow with C-type mesh. The boundary condition on the body is DPhi/Dn=0. what is the boundary condition at far-field. Notes the circulation on the body has arelation to this boundary as I think to avoid vortex singularity at trailling and wake.

thanks to any notes about above laith