A weakly-compressible two-dimensional axisymmetric isentropic vortex model is employed to stimulate interactions between single vortex and single optical wavefront. For near-field aero-optics at low beam energies geometrical optics is adopted. Optical-path-difference of propagated distorted wavefront is computed. It is confirmed that transform of beam wavefront is accelerated as beam passing through the vortex with low density. Single vortex defocuses the beam. Computation on COIL indicates that strength of vortex has strong effect on Strehl rate.