Spontaneous Swirling
 

I have been writing my own CFD code, and I've come across some problems. I create a simple rectangle and just set the velocity to zero on each side, and then I applied gravity in the -y direction. The pressure gradient I find is reasonable, but I get a weird vortex pattern inside. If you have any idea about what I should be looking at, that would be extremely helpful.

Here's an link to the picture:
http://thebluekoala.com/images/swirling.png

By the way, I'm using the equations for Stokes' flow (which is the really weird part!!). I've been writing my own code for fun, so I'm not too worried about finding more accurate and faster solvers, which I know are out there.

UPDATE:::
I found out that if I change my initial guess before using biCGSTAB, the swirling pattern changes. It's all very strange!!

UPDATE::::
It's fixed!! I had a silly problem with some elemental matrices. I switched up the order of the block matrix structure. Hard to find!!