How the Vortex Motion of Gravity Waves on the Surface of Water is Formed

Abstract

The formation of vortex motion by nonlinear gravity waves on the surface of water is studied experimentally in a bath with dimensions of 70 × 70 cm. Gravity waves are excited by two plungers installed perpendicularly to each other at a distance of 1 cm from the bath walls. The pumping frequency is 4 Hz, and the excitation wavelength is 9.6 cm. The liquid flow is visualized by polyamide decorating particles. After pumping is switched on, the traveling waves propagate over the surface and, at first, form a system of bores, which then transforms into a vortex lattice when a standing wave is formed on the surface. The ideal vortex lattice is broken down by intense vortex interaction with time. The energy distribution over the wave vector can be described by a power function with the variable subscript n, E ~ k^-n, 1.5 < n < 3. The scale of the vortex with the maximum size is close to the size of the bath. It is assumed that a forward energy cascade is formed in the system of vortices; however, the nonlinear interaction of vortices is weak. After the pumping of waves is switched off and the waves are damped on the surface, vortex motion in a viscous sublayer produced by nonlinear waves remains. The smaller scale vortices damp more rapidly with time, and one or two large vortices remain on the surface and are dominant.