Numerical simulations were conducted to investigate the influence of nonlinear vertical shear on the trajectories of a trailing vortex pair. Descending vortices which approach a shear layer begin to tilt and diverge. It is shown that the vortex whose vorticity is of opposite sign to the shear-layer vorticity can stall or even rebound, whereas the other vortex penetrates through the shear layer. The behaviour of the vortices is highly sensitive to the vorticity distribution within the shear layer and cannot be described by circulation or maximum vorticity relations.
When the vorticity distribution of the shear layer varies along the axis of the vortices, the interaction between shear and vortex is also modified in axial direction and presumably triggers a quickly growing instability of the vortex pair.