Physical modeling of ground and cross-shear effects on wing-tip vortices was conducted in a towing tank to demonstrate the vortex rebound phenomenon [1, 2]. A slotted-jet vortex generator was used to form a 2-dimensional vortex pair.

A rigid plate was placed below the exit slot to simulate the ground. As the vortex approaches the ground, the vortex separation increases (left pictures). A counter rotating secondary vortex begins to form beneath and outboard of the right primary vortex around t = 12 s. Its initial formation is not detectable, however, because there is no dye there to trace it. The presence of the secondary vortex becomes evident at t = 18 s as it grows in strength and size and entrains dye from the primary vortex. The perturbations of the primary vortex in the inviscid region of the boundary layer create an adverse wall pressure gradient slightly ahead of the primary vortex core, which retards the flow in the viscous local shear layer at the viscous-inviscid interface. The vorticity in the shear layer combines to form the secondary vortex and protrudes into the inviscid region, leading to local flow separation. As a result, the primary vortex begins to rebound at t > 12 s under the influence of the secondary vortex.

Cross shear was generated by recirculating a weakly stratified fluid in the tank [1]. The stratification also helps suppress the ambient turbulence. The cross shear causes the vortex pair to rotate and changes its course of movement. In the presence of a strong shear, the vortex pair rotates nearly 180 while moving sideway (right pictures). The cross shear strengthens one of the vortices while annihilating the other. As a result, the vortex pair remains close to the ground and poses a potential hazard to aircraft operating along a parallel runway (an upside down model).

[1] Liu, H.-T. (1991) "Tow Tank Simulation of Vortex Wake Dynamics," Proc. FAA Inter. Sym. on Wake Vortices, Washington, D. C., October, 28-31.
[2] Liu, H.-T., Hwang, P. A., and Srnsky, R. A. (1992) "Physical Modeling of Ground Effects on Vortex Wakes," AIAA J. of Aircraft 29, 1027-34.