Theory of the almost-highest wave. Part 2. Matching and analytic extension

Most methods of calculating steep gravity waves (of less than the maximum height) encounter difficulties when the radius of curvature R at the crest becomes small compared with the wavelength L , or some other typical length scale. This paper describes a new method of calculation valid when R/L is small. For deep-water waves, a parameter e is defined as equal to q /2 ½ c 0 , where q is the particle speed at the wave crest, in a frame of reference moving with the phase speed c . Hence e is of order ( R/L ) ½ . Three zones are distinguished: (1) an inner zone of linear dimensions e 2 L near the crest, where the flow is described by the inner solution found previously by Longuet-Higgins & Fox (1977); (2) an outer zone of dimensions O(L) where the flow is given by a perturbed form of Michell's solution for the highest wave; and (3) a matching zone of width O(L) . The matching procedure involves complex powers of e. The resulting expression for the square of the phase velocity is found to be \[ c^2 = (g/k)\{1.1931-1.18\epsilon^3\cos(2.143\ln \epsilon + 2.22)\} \] (see figures 5 a, b ), which is in remarkable agreement with independent calculations based on high-order series. In particular, the existence of turning-points in the phase velocity as a function of wave height is confirmed. Similar expressions, valid to order e 3 , are found for the wave height, the potential and kinetic energies and the momentum flux or impulse of the wave. The velocity field is extended analytically across the free surface, revealing the existence of branch-points of order ½, as predicted by Grant (1973).