[photon137]

Natural convection in a system heated from below - try and detect when the heat transfer switches from conduction to convection - maybe compare Nusselt numbers computed versus what's predicted by Navier-Stokes (no inter molecular interactions) ?

Edit: Another one - model the human circulatory system - trying to find optimal postures for maximum heart efficiency. Also simulate blood-flow keeping track of anti-coagulant factor concentration to detect high-risk areas for blood-clot formation.