I'm basically a novice with FLUENT and am having some problems getting the solution for a 3D model to converge. The geometry is pretty simple, basically just flow through a rectangular test section, over a step. The air velocities are pretty high (around 140 m/s). I've gone through all of the FLUENT tutorial sections for compressible flow, but nothing seems to be working. For some reason my energy equation and/or continuity equation solutions are always divergent. Is there a common cause and solution to this?

Common cause is the mesh. Look at the diverged solution to find where the model is falling apart and look at mesh size in that area. Are the cells small enough in that area to approximate the curvature?

Other possible causes are solver settings, solution process (i.e. what settings you change and when), boundary conditions, or model setup. Post more about your setup (solver, models, BCs, etc...) and people will be able to comment on that.

Good luck, Jason

agreed with Jason. which model you are using? Solver? convergence criterion settings, etc.....