Template:Equations for fixed bodies in the time domain

Equations for fixed bodies in the time domain

We consider a two-dimensional fluid domain of constant depth, which contains a finite number of fixed bodies of arbitrary geometry. We denote the fluid domain by [math]\displaystyle{ \Omega }[/math], the boundary of the fluid domain which touches the fixed bodies by [math]\displaystyle{ \partial\Omega }[/math], and the free surface by [math]\displaystyle{ F. }[/math] The [math]\displaystyle{ x }[/math] and [math]\displaystyle{ z }[/math] coordinates are such that [math]\displaystyle{ x }[/math] is pointing in the horizontal direction and [math]\displaystyle{ z }[/math] is pointing in the vertical upwards direction (we denote [math]\displaystyle{ \mathbf{x}=\left( x,z\right) ). }[/math] The free surface is at [math]\displaystyle{ z=0 }[/math] and the sea floor is at [math]\displaystyle{ z=-h }[/math] (the equations would be almost identical if the sea floor depth varied). The equations of motion in the time domain are

[math]\displaystyle{ \Delta\Phi\left( \mathbf{x,}t\right) =0,\ \ \mathbf{x}\in\Omega, }[/math] [math]\displaystyle{ \partial_{n}\Phi=0,\ \ \mathbf{x}\in\partial\Omega, }[/math] [math]\displaystyle{ \partial_{n}\Phi=0,\ \ z=-h, }[/math]

where [math]\displaystyle{ \Phi }[/math] is the velocity potential for the fluid. At the free surface we have the kinematic condition

[math]\displaystyle{ \partial_{t}\zeta=\partial_{n}\Phi,\ \ z=0,\ x\in F, }[/math]

and the dynamic condition (the linearized Bernoulli equation)

[math]\displaystyle{ \zeta = -\partial_{t}\Phi,\ \ z=0,\ x\in F, }[/math]

where [math]\displaystyle{ \zeta }[/math] is the free-surface elevation. These equations are in non-dimensional form (so that the fluid density and gravity are both unity). They are also subject to initial conditions

[math]\displaystyle{ \left.\zeta\right|_{t=0} = \zeta_0(x)\,\,\,\mathrm{and}\,\,\, \left.\partial_t\zeta\right|_{t=0} = \partial_t\zeta_0(x). }[/math]