GROUND AND CROSS-SHEAR EFFECTS ON WING-TIP VORTICES
Dr. (Pete) H.-T. Liu
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 . 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).
 Liu, H.-T. (1991) "Tow Tank Simulation
of Vortex Wake Dynamics," Proc. FAA
Inter. Sym. on Wake Vortices, Washington, D. C., October, 28-31.
 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.