Since people want to do something serious with 2d simulations I thought I would mention MicroCFD. Note it isn't exactly free.
MicroCFD - Computational Fluid Dynamics Software & Consulting
It does have a 3 day free trial however.
Note if you want to cheat this guy out of his money you can install it in a virtualbox image (It will run slower). Just installing it there won't let you cheat him and I'm not going into details on how to do it. It can be done though.