Long Nonlinear Surface Waves in the Presence of a Variable Current

Abstract

We investigate the eigenvalue problem governing the propagation of long nonlinear surface waves when there is a currentū(y) beneath the surface, y being the vertical coordinate. The amplitude of such waves evolves according to the KdV equation and it was proved by Burns [1] that their speed of propagation c is such that there is no critical layer (i.e., c lies outside the range ofū(y)). If, however, the critical layer is nonlinear, the result of Burns does not necessarily apply because the phase change of linear theory then vanishes.In this paper, we consider specific velocity profiles and determine c as a function of Froude number for modes with nonlinear critical layers. Such modes do not always exist, the case of the asymptotic suction profileū = 1 − e −y being a notable example. We find, however, that singular modes can be obtained for boundary layer profiles of the Falkner-Skan similarity type, including the Blasius case. These and other examples are treated and we examine singular solutions of the Rayleigh equation to gain insight about the long wave limit of such solutions.