Physical Modeling of Ground Effects on Vortex Wakes

Towing-tank experiments were conducted to investigate ground effects on vortex wakes. Two methods were used to generate the vortex wakes: 1) a towed NACA 0012 wing and 2) a slotted-jet vortex generator. Trajectories derived from trailing vortex wakes, tagged with a fluorescent dye and released near the simulated ground surface, have confirmed the inadequacy of the two-dimensional inviscid solution. The generation of a relatively weak secondary vortex causes the primary vortex to rebound from the surface as a result of unsteady separation. There is no significant difference between the trajectories, whether the ground is simulated by a rigid surface or a free surface. The introduction of a rigid surface close to the exit of the slotted-jet vortex generator results in the formation of secondary vortices at the ground outboard of the primary vortices. The secondary vortices generated by the slotted-jet vortex pair are more coherent and persistent than those generated by the trailing vortex pair. A simple modification of the two-dimensional inviscid theory—namely, adding a secondary vortex to the system—recreates the rebound phenomenon, which agrees qualitatively with the trend seen in the experiments.