We address a problem of potential motion of ideal incompressible fluid with a free surface and infinite depth in two dimensional geometry. We admit a presence of gravity forces and surface tension. A time-dependent conformal mapping z(w, t) of the lower complex half-plane of the variable w into the area filled with fluid is performed with the real line of w mapped into the free fluid's surface. We study the dynamics of singularities of both z(w, t) and the complex fluid potential Π(w, t) in the upper complex half-plane of w. We show the existence of solutions with an arbitrary finite number N of complex poles in zw(w, t) and Πw(w, t) which are the derivatives of z(w, t) and Π(w, t) over w. We stress that these solutions are not purely rational because they generally have branch points at other positions of the upper complex half-plane. The orders of poles can be arbitrary for zero surface tension while all orders are even for nonzero surface tension. We find that the residues of zw(w, t) at these N points are new, previously unknown constants of motion, see also Ref. V. E. Zakharov and A. I. Dyachenko, arXiv:1206.2046 (2012) for the preliminary results. All these constants of motion commute with each other in the sense of underlying Hamiltonian dynamics. In absence of both gravity and surface tension, the residues of Πw(w, t) are also the constants of motion while nonzero gravity g ensures a trivial linear dependence of these residues on time. A Laurent series expansion of both zw(w, t) and Πw(w, t) at each poles position reveals an existence of additional integrals of motion for poles of the second order. If all poles are simple then the number of independent real integrals of motion is 4N for zero gravity and 4N − 1 for nonzero gravity. For the second order poles we found 6N motion integral for zero gravity and 6N − 1 for nonzero gravity. We suggest that the existence of these nontrivial constants of motion provides an argument in support of the conjecture of complete integrability of free surface hydrodynamics in deep water. Analytical results are solidly supported by high precision numerics.
|Original language||English (US)|
|State||Published - Sep 25 2018|
- Conformal map
- Constants of motion
- Fluid dynamics
- Water waves
ASJC Scopus subject areas