Water exchange between adjacent vortices under an additional oscillatory flow

It is shown that the coupling effect of the steady vortices and the Eulerian oscillatory flow yields the 8-shaped Lagrangean motion through which adjacent vortices intercommunicates, inducing water exchange between them. The water exchange coefficient is fairly large. This coupling effect is considered to play an important role in the water exchange across a narrow strait which is accompanied with a strong tidal current and a pair of tidal residual circulations.