finite elements and twophase flow ?
I am trying to simulate a  what I thought  rather simple testproblem. Consider a closed container, the lower half filled with water, the upper half with air. Gravity is acting downward. No walls are moving, so you would expect this configuration to be steady. Any simulation should give you zero velocities, nearly constant pressure in air and (with depth) increasing pressure in water.
Using a standard finiteelement Galerkin method (Taylor Hood triangle) in combination with the VOFapproach (linear approximation of density and viscosity, both fluids simulated), I found this problem can *not* be handled. During the first timesteps the velocities remain zero and the pressure solution is exact. Later the surface becomes unsteady and with increasing velocities the simulation "explodes". Simulating the same problem without gravity results in no problems. The code without VOF has been validated in lots of problems including gravityrelated ones. Computing other problems with the VOFapproach without gravity gives again no problems. The VOFfunction is approximated by a linear function within the triangle. Summing up I wonder whether there is some bug in my code or if this is some other (deeper) problem related to the linear VOFapproximation in combination with finite elements. Does anyone have similar problems  or (even better) could hint at some inconsistency with the above approach ? Any ideas are welcome ... Thanks, Chris 
Re: finite elements and twophase flow ?
I am no expert in VOF, but it sounded like that when you turn on the gravity, the water is on the top and the air is on the bottom. Change the direction of your gravitational field to see whether your bugs are still there.

Re: finite elements and twophase flow ?
Thanks for responding John. This was of course one of the first things I tested when I first saw the problem. Unfortunately the whateveritisproblem lies somewhat deeper.

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