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April 30, 2020, 08:36 
DDPM with RSM convergence problem

#1 
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mohamad ali mirzaei
Join Date: Feb 2017
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I am trying to simulate a cyclone using DDPM and RSM with structured hexahedral mesh. although the mesh quality seems to be fine (Minimum Orthogonal Quality = 0.53), the solution diverges immediately after the start with the following warning:
turbulent viscosity limited to viscosity ratio of 1.000000e+05 in many cells and then: reverse flow at the outlet the turbulent boundary condition is assigned at inlet using intensity and hydraulic diameter. is there any solution method that might help? at the moment I am using phase coupled SIMPLE, greengauss node based, presto!, secondorder upwind for momentum, quick for volume fraction and firstorder upwind for the rest. I ran the same simulation using sstcc and it was fine. I tired converged solution of sstcc as initial for RSM simulation bit it did not help. can you help find the problem? 

April 30, 2020, 09:13 
Rsm

#2 
Senior Member

is not a good starting point for RSM; is. But even with to begin with, you should be able to converge. Firstly, when the model is changed, URFs should be very very low, of the order of 0.01, so that the matrix could be developed. Use only firstorder numerics and disable volume fraction for a few iterations. If the mesh is hex and structured, node based gradient does not add anything, so, prefer using LSQ.
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April 30, 2020, 10:19 

#3 
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mohamad ali mirzaei
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Quote:


April 30, 2020, 10:23 
URFs

#4 
Senior Member

You should increase the URFs otherwise there could be conservation issues. You can increase in two to three steps, say, every 10 timesteps.
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April 30, 2020, 10:26 

#5 
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mohamad ali mirzaei
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April 30, 2020, 10:32 
URFs

#6 
Senior Member

Yes, usually default values are good. However, if convergence is good within each timestep for given values of URF and URFs are not very small, say below 0.1, then its alright. With very small URFs, the changes in the coefficient matrix are so small and numerical solver considers everything converged. What happens in reality is that high frequency errors are reduced but low frequency errors remain. So, using high URFs are required but 0.5 and 0.9 won't make a difference as long as solution is well converged.
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May 1, 2020, 05:33 

#7  
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mohamad ali mirzaei
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Quote:
in general is it ok to continue the simulation with 0.2 for URFs? how can we ensure that the final converged result is not affected by this value? 

May 1, 2020, 15:02 
URFs

#8 
Senior Member

If it is diverging with 0.3, then you can continue with 0.2 or 0.25. Once converged, which will take longer due to small URF, solution will be more or less independent of the URF used. Most importantly, you should set up some monitors. If those monitors become constant, then you can consider it converged.
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convergence, ddpm, rsm 
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