interFoam problem: Movement in wrong direction
 

Hi everybody,

I have encountered a problem using interFoam. I am simulating the behaviour of a droplet of water in a small windtunnel. The alpha field is initialized so that the droplet forms a semi-circle at the bottom wall of the tunnel (see the attached picture). The simulation is 2D and the channel is 1mm high and 20mm long. Air enters from the left and exits on the right side.

My problem is that in many cases, the droplet starts moving to the left, instead of the right as I would expect.
For example, using a uniform mesh of 400x20 cells, for U=0.1 or U=2, the droplet moves to the right and eventually exits the channel, but for U=0.5 or U=1, it moves to the left and stops just before (i.e. to the right of) the inlet (see pictures alpha1 and U).

If i use a finer mesh of 800x40 cells, the results differ in that the droplet moves to the right for U=1, but to the left for U=0.1.
I have also run tests on a larger channel (1cm high and 20cm long) and the same phenomenon occurs. I also tried giving the droplet an initial velocity to the right in some cases, but it would slow down and change direction.

I'm using OpenFOAM version 2.1.1.

I have no idea what couses this counter-intuitive behaviour or if it is an error in interFoam. One thought that occured to me was that it might have something to do with the Laplace pressure, due to different contact angles because of deformations of the droplet, but since I'm using a fixedContactAngle I don't think this is the answer.

Any help would be greatly appreciated!
Thanks

Moritz