2.1.2. Hodograph transform
To fix the conceptual framework, let us assume throughout this paper that the wave crest is located at in the moving frame. The condition (11) of irrotational flow allows us to define the velocity potential by
Equation (21)
Setting
we ensure that on the crest line, that is, for . Note that is odd and periodic with period L in the x-variable. In particular, for every integer j. Moreover, due to (19).
The stream function ψ, defined in section 2.1.1, and the velocity potential phgr are harmonic conjugate functions, with the complex mapping analytic throughout the fluid domain. It is convenient to perform the orientation-preserving conformal hodograph change of variables
Equation (22)
that transforms the free-boundary problem (17) into a nonlinear boundary-value problem for the harmonic function
Equation (23)