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Boundary conditions on inlets have big effect

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I have a simple 2D model of a reservoir with a membrane connected to an inlet and outlet.
It is filled with water.
I want to simulate the flow throught the inlets as function of membrane movemement. For that I use an FSI model.

I try to simulate the transient behaviour when a smoothed force step function is applied to the membrane from outside.

I can simulate this reasonably well if I choose the boundary conditions for the fluid inlets as: open-boundary, no viscous stress.

However, when I want to simulate more complicated transient behaviour, this boundary condition results in a non-stable oscilation that over time will crash the simulation. It works for a simple step function, but for periodic behaviour the unstable oscillation will get larger and larger.

When I set the boundary conditions of the open-boundarys to 'normal stress' f0 = 0 N/m^2, then the simulation will work initially, but over time the time steps will get to small and the result will not converge.

By plotting during the simulation I can see that the new boundary conditions make more sense than the old, but I don't understand why it runs into a wall.

Could somebody help with this?

I've included the model using the old boundary conditions, so you can run a test (5 minutes on Core i3). But I would like to change the boundary conditions of the open boundary 1 to 'zero normal stress'.


1 Reply Last Post Apr 13, 2012, 9:24 a.m. EDT

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Posted: 1 decade ago Apr 13, 2012, 9:24 a.m. EDT
The problem seems to go away when I reduce the applied force.
When the force is reduced by a factor of 2, the simulation finishes correctly and gives results as expected.
The problem seems to go away when I reduce the applied force. When the force is reduced by a factor of 2, the simulation finishes correctly and gives results as expected.

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