[kjetil]

Hello everybody,
I am trying to simulate stall in a diffuser (low liquid flow). Currently both steady state and transient simulations overpredicts by far - stall occurring at over twice the flow rate than what is found in experimental data. Quite a few parameteres have been checked, including mesh quality (y+), timestep and numerical schemes.

I am now experimenting a bit with the turbulence models, as I fear that the SST model might be causing parts of this. I read that SST might not work well in curved geometries, or with secondary flows, and here I have both. I have therefore tried to turn to RSM models, with the obvious difficulties to get this to cooperate at all.

Observing numerous "FINMES" errors, still with even lower timesteps, I conducted a planned near crash, and was able to store the .res file. Highest residuals are then found close to the interface. Mesh is overall good, though not quite uniform across the interface, and aspect ratios a bit on the high side in this area. Could this be causing the errors?

Though, looking at the partially converged RSM simulation, the flow pattern is much more what is expected, that is - without the blockage/stall. I was also trying to find some other differences between these two simulations, and found that the eddy viscosity plots to differ quite a bit. They are attached here. Turbulence numerics are first order, but advection "High resolution". Switching RSM simulation to "High resolution" turbulence cause only eddy dissipation to converge differently, and whole simulation to crash earlier than with first order.

So my questions are:
- should I try to improve the mesh near the interface?
- should I coarsen the mesh?
- other experiences with turbulence models for initial run, before running RSM? My current runs here are initialized from a fairly stable run on higher flow, then flow is gradually reduced in steps by an expression. Though do not converge, possibly due to the mesh problems mentioned. I did not have any good experience with initializing with SST runs on same flow.