Ground effects can be modelled with the Lifting Line Free Vortex Wake method by mirroring all vortex elements, bound and free, at the ground plane (Leishman1). A mirror image (see Fig. 12) of all bound and free vortices is created at every time step using the ground as a symmetry plane. Such a treatment doubles the number of times that the Biot-Savart equation is calculated and thereby doubles the computational time needed for the evaluation of the convection step.
J. G. Leishman. Principles of Helicopter Aerodynamics. Cambridge University Press, 2000.