[pmo]

I'm trying to model an oscillating flow (due to wave action) over a subsea structure at the seabed. The aim is to calculate the resulting hydrodynamic in-line/horizontal force and the vertical/lift force on the structure. The total forces will be depending on both fluid acceleration (inertia force) and velcoity. I'm running transient simulations where the horizontal velocity is set up to oscillate sinusoidally with time (oscillating period equal to wave period). The problem is single phase, and buoyancy is not included as the hydrostatic pressure can be assumed to be the same both over- and under the structure.

I've tried different boundary condition, and the one that seems to work best is opening with prescribed velocity (sine function) in both ends. My problem is that the amplitude of the oscillating pressure (and the hence the lift force) varies almost linearly with the length of the domain. Is this realistic? In that case; how should I set the boundary conditions to simulate my problem in a realistic way? I have attached a plot showing the pressure distribution at a time instant around the structure (simplified 2D-case). As can be seen in this plot, there's a pressure gradient in the flow direction, but the pressure is always constant (0 Pa) in one end.

Have anybody dealt with similar problems? Any advice would be appreciated.