[MultiPhy]

I'm trying to simulate a rigid object moving in a rigid fluid-filled channel in COMSOL. The inlet provides a velocity for the non-compressible fluid, which pushes the object. No problem so far, this works fine for time-dependent studies in 2D.

The issue is that I want to move to 3D, and I think that will become very computationally expensive. It was suggested that I use snapshots of static studies (it's Stokes flow, so supposedly that's ok -- regardless, that's a different issue), rather than doing time-dependent studies. But, a static study won't converge (which makes sense because there is no steady state given the continuous movement of the object down the channel).

I was told the solution to this was to use a moving wall. I have spent hours trying to figure out how to do this, and learned nearly nothing. The 1000+ page COMSOL reference guide has one paragraph on moving walls, and essentially says "If you use a moving wall, u = u sub w." Not very informative.

I would greatly appreciate it if someone could explain the theory of a moving wall in general (why and when is it used), and how it applies to this problem specifically (where does the wall go, and what are the parameters).

Thank you!

[saeedsh]

the moving wall is used whenever you want to model a wall with speed. for example for a ball with speed of 2 m/s in air you want to evaluate drag force; one way is to set wall velocity equal to 2 and start your simulation. right?