Poor Convergence for Transient Subsonic Diffuser sims with flow separation
I am modeling a diffusing air inlet that sees high subsonic velocities (0.3 - 0.9). The flow separates off the inlet walls under certain flow conditions that I need to model.
So far, none of my high resolution steady state sims has converged, regardless of how I refine or adjust the mesh. I think this is due to the inherently transient nature of the separated regions.
I have therefore switched to transient simulations. However, I am finding that I need to use a very small timestep size (1.0e-7 s) for the simulation to converge. Using a timestep size of 1.0e-7 the courant number ranges between 0.01 - 0.9. I am using the results from a converged Upwind scheme solution as my initial conditions.
As I increase the time step size, the solution does not converge, and at a certain point, the solver fails (usually preceeded by a huge max Mach number warning). The courant number increases greatly as I increase time step size and the solver fails at a time step size of about 1.0e-4.
I believe part of the reason is that the inlet geometry I am using has a relatively sharp lip. For the flow conditions I am modeling, the local velocity around the lip is very high relative to the freestream (but still subsonic). This high velocity combined with the small cell size around the lip contributes to a very high courant number.
I would like to use a larger time step since I don't have a long time to run these simulations. Using the current time step size it will take months to run a sim.
Does anyone have any suggestions that may allow for the use of a larger time step size?
Some details of my simulation are provided below
Using SST turbulence model, Ideal Gas, Total Energy model
- subsonic velocity inlet
- opening pressure outlet. Using the entrain setting since I expect there to be regions of separated flow crossing the outflow plane.
Time step size: 1.0e-7 s
Advection scheme: High Resolution
Transient Scheme: 2nd order backward Euler
Any suggestions would be greatly appreciated!
1) In a steady state run, use local timestep factor=5.0 or so until convergence looks good and finish off with some physical timescale iterations.
2) Extend the inlet and/or outlet lengths if there is recirculations through the inlet/outlet. This will make convergence much more difficult.
3) Have you tried these ideas: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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