We consider a viscous fluid of finite depth below the air, occupying a
three-dimensional domain bounded below by a fixed solid boundary and
above by a free moving boundary. The fluid dynamics are governed by the
gravity-driven incompressible Navier–Stokes equations, and the effect of surface
tension is neglected on the free surface. The long-time behavior of solutions
near equilibrium has been an intriguing question since the work of Beale
(1981).
This is the second in a series of three papers by the authors that answers the
question. Here we consider the case in which the free interface is horizontally infinite;
we prove that the problem is globally well-posed and that solutions decay to
equilibrium at an algebraic rate. In particular, the free interface decays to a flat
surface.
Our framework utilizes several techniques, which include
a priori estimates that utilize a “geometric” reformulation of the equations;
a two-tier energy method that couples the boundedness of high-order
energy to the decay of low-order energy, the latter of which is necessary
to balance out the growth of the highest derivatives of the free interface;
control of both negative and positive Sobolev norms, which enhances
interpolation estimates and allows for the decay of infinite surface waves.
Our decay estimates lead to the construction of global-in-time solutions to the surface
wave problem.
Keywords
Navier–Stokes equations, free boundary problems, global
existence