Fractional Fourier approximations for potential gravity waves on deep water

Abstract

In the framework of the canonical model of hydrodynamics, where fluid is assumed to be ideal and incompressible, waves are potential, two-dimensional, and symmetric, the authors have recently reported the existence of a new type of gravity waves on deep water besides well studied Stokes waves (Phys. Rev. Lett., 2002, v. 89, 164502). The distinctive feature of these waves is that horizontal water velocities in the wave crests exceed the speed of the crests themselves. Such waves were found to describe irregular flows with stagnation point inside the flow domain and discontinuous streamlines near the wave crests. Irregular flows produce a simple model for describing the initial stage of the formation of spilling breakers when a localized jet is formed at the crest following by generating whitecaps. In the present work, a new highly efficient method for computing steady potential gravity waves on deep water is proposed to examine the above results in more detail. The method is based on the truncated fractional approximations for the velocity potential in terms of the basis functions $1/\bigr(1-\exp(y_0-y-ix)\bigl)^n$, $y_0$ being a free parameter. The non-linear transformation of the horizontal scale $x = \chi - \gamma \sin\chi, 0<\gamma<1,$ is additionally applied to concentrate a numerical emphasis on the crest region of a wave for accelerating the convergence of the series. Fractional approximations were employed for calculating both steep Stokes waves and irregular flows. For lesser computational time, the advantage in accuracy over ordinary Fourier expansions in terms the basis functions $\exp\bigl(n (y+ix)\bigr)$ was found to be from one to ten decimal orders depending on the wave steepness and flow parameters.