Mahalingam, R., Funk, R.B., Komerath, N.M., "Flow Visualization of Low-Speed Perpendicular Vortex-Airfoil Interaction". AIAA Paper 96-2387, 14th Applied Aerodynamics Conference, New Orleans, LA June 1996. 

The interaction of a wing tip vortex with a downstream airfoil is visualized.  The tip vortex from a straight wing was moved vertically in a wind tunnel. Laser sheet videography was used to document the quasi-steady interaction of the vortex  with full-span airfoils in two test cases: a 21% thick symmetric airfoil,  and a thin flat plate of 2.5% thickness, with rounded leading edge and sharp trailing edge. The strength of the vortex was computed from the measured velocity profile upstream.  Vortex trajectories above and below both airfoils were measured. The vortex filament drifts spanwise above and below the airfoil as expected.  During the downward traverse, the vortex filament stays above the airfoil long after the generating wing has gone below the airfoil and then flips across the airfoil surface in a very short time. The vortex switching from the top to the bottom surface is initiated by the splitting of the primary vortex into two.  During the upward traverse,  the trajectory is exactly duplicated: there is no delay in switching from below to above, but there is a delay in vortex appearance after switching. Pressure gradient effects are observed in the thick airfoil interaction and near the leading and trailing edges for the flat plate.  The  trajectories continue to diverge behind the trailing edge. In both experiments, the remaining vortex filaments downstream of the airfoil are too weak and far apart to recombine.