convergence, mesh quality, and dispersion

Ed Schaub

I didn't get a response on the fluent forum, so I thought I would try here. Here is the original message

...Okay folks, I'm fixing to get in an argument with colleaque. Anyway, I am trying to achieve similar results on a slightly different geometry. We have a molten glass(highly viscous, temperature dependent viscosity and density), with multiple outlets. The glass surface is conditioned by a 'radiation temperature profile' boundary on the glass surface. These profiles come from other models. I run a steady-state solution and it converges smoothly to the fluent default residuals. I then indicate a transient solution with the time step he specifies. At each step, it converge within 2 iterations. It can't be transient! I say to myself. I then look at the transient solution my colleague obtained. His mesh quality is quite a bit poorer. For equi-angle skew, his model has 0.94 while mine is 0.86, and for equi-volume skew, he has 0.99 while I have 0.93. Is the problem with his mesh, or should I be looking for something else? Your feedback is appreciated. Ed ...

Anyway, clarifying further, his solution is getting vortex rolls through the distributor or manifold. Due to a poor mesh and 1st order implicit time method, I believe he is getting dispersion error. My flow is smooth as silk, and converges nicely. Feed back would be appreciated before I go and stick my foot in my mouth. Ed