Generation of a vortex flow by waves on the surface of a liquid

The formation of a vortex flow in a vessel with a liquid undergoing harmonic oscillations in a vertical direction has been studied experimentally. It has been found that the vortex flow does not occur in a cylindrical vessel until the amplitude of the oscillations exceeds a threshold value, at which the Faraday parametric instability develops and azimuthal modes emerge on the surface. The vortices appear in a square vessel and in a cylindrical vessel with broken symmetry at amplitudes below the parametric instability threshold. The formation of a vortex flow is presumably caused by the interaction of surface waves propagating at an angle with respect to each other.