Abstract. We determine the phase portrait of a Hamiltonian system of equations describing the motion of the particles in linear deep-water waves. The particles experience in each period a forward drift which decreases with greater depth.
IntroductionThe motion of water particles under the waves which advance across the water is a classical problem in this field. Watching the sea it is oft possible to trace a wave as it propagates on the water's surface, but what one observes traveling across the sea is not the water but a wave pattern. Although the wave travels from one place to another, the substance through which it travels moves very little. As the wave advances across the water and can be followed for a long way, a typical water particle moves slightly up and down, forward and backward as the wave passes it. If an object hovered in the water, like a water particle, its motion would be synchronized with that of a floating object lying on the water's surface, with its orbit diminishing with the distance from the surface. As waves generated by wind in an area move towards a region where the wind has ceased, we observe swell-long crested two-dimensional waves approaching a smooth sinusoidal shape and moving over long distances. Deep-water waves are modelled mathematically as periodic two-dimensional waves in water of infinite depth. The motion of the water particle in the fluid below swell is of great interest. The classical description of these particle paths is obtained within the framework of linear water wave theory [1,15,16,23,24,25,29,30]: all water particles trace a circular orbit, the diameter of which decreases with depth so that the orbital motion practically ceases at depth equal to one-half the wavelength. These features have important practical consequences. For example, a submarine at a depth below half a wavelength would hardly notice the motion of the surface wave, for this reason submarines dive during storms in the open sea.The only known explicit solution with a non-flat free surface of the governing equations for gravity water waves is Gerstner's wave [17]: a deep-water wave solution for which all particle paths are circles of diameters decreasing with the distance from the free surface (see the discussion in [2,3]). Due to the mathematical intractability of the governing equations for water waves, for irrotational water waves (Gerstner's wave has a peculiar nonvanishing vorticity) the classical approach [15,20,23,24,28,29,30] relies on analyzing the particle motion after linearization of the governing equations. However, even within the linear water wave theory, the ordinary differential equations system describing the motion of the particles is nevertheless nonlinear and explicit solutions of this system are not available, but qualitative features of the underlying flow 2000 Mathematics Subject Classification. 76B15, 34C25, 35Q35.