Convergence and Solver Selection

cfduser9 · January 9, 2012, 22:38

I have created a gas distributor with high back pressure and a choked inlet distributing into a vacuum (the operating pressure is not quite zero, but very close).

Using the pressure-based solver, I have attempted to solve. The model always diverges. No matter how much I play with the under-relaxation factors, it simply won't converge.

Should I be using the density solver over the pressure solver? I am aware of the relative merits of each, but don't know what to do for this situation. The flow is slow near the inlet, but definitely compressible by the exit. It seems to fall between the two solvers, but maybe one is better than the other?

I should mention that the model is very complicated, as the distributor has many exits. A simpler model does converge using the pressure solver, but the real one does not.

Thanks!

duri · January 10, 2012, 02:38

Operating pressure doesn't matter for pressure based solver, if it is zero it will take a negative value. When compressibility effects are large (discharging into vacuum) you need to move to density based solver.

LuckyTran · January 14, 2012, 00:34

The density based solver is preferred over the pressure based solver, especially for compressible flows. The density based solver is more complicated and costly compared to the pressure based solver but is much more physically accurate.

The pressure based solver can still be used but requires a lot more care. If you are using the pressure based solver, did you remember to account for the viscous heating effects caused by the shear?

cfduser9 · January 22, 2012, 13:21

What do you mean by this? I did turn on the energy model and did then chose low-pressure boundary slip because it is exiting into what is essentially a vacuum. Is this what you are referring to?

LuckyTran · January 22, 2012, 13:34

You should be using the density solver.

If you want to use the pressure based solver, in addition to simply 'turning on energy' you need to account for viscous heating effects.

January 9, 2012, 22:38	Convergence and Solver Selection	#1
cfduser9 New Member Join Date: Nov 2011 Posts: 5 Rep Power: 14	I have created a gas distributor with high back pressure and a choked inlet distributing into a vacuum (the operating pressure is not quite zero, but very close). Using the pressure-based solver, I have attempted to solve. The model always diverges. No matter how much I play with the under-relaxation factors, it simply won't converge. Should I be using the density solver over the pressure solver? I am aware of the relative merits of each, but don't know what to do for this situation. The flow is slow near the inlet, but definitely compressible by the exit. It seems to fall between the two solvers, but maybe one is better than the other? I should mention that the model is very complicated, as the distributor has many exits. A simpler model does converge using the pressure solver, but the real one does not. Thanks!

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January 10, 2012, 02:38		#2
duri Senior Member duri Join Date: May 2010 Posts: 245 Rep Power: 16	Operating pressure doesn't matter for pressure based solver, if it is zero it will take a negative value. When compressibility effects are large (discharging into vacuum) you need to move to density based solver.

January 14, 2012, 00:34		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	The density based solver is preferred over the pressure based solver, especially for compressible flows. The density based solver is more complicated and costly compared to the pressure based solver but is much more physically accurate. The pressure based solver can still be used but requires a lot more care. If you are using the pressure based solver, did you remember to account for the viscous heating effects caused by the shear?

January 22, 2012, 13:21		#4
cfduser9 New Member Join Date: Nov 2011 Posts: 5 Rep Power: 14	What do you mean by this? I did turn on the energy model and did then chose low-pressure boundary slip because it is exiting into what is essentially a vacuum. Is this what you are referring to?

January 22, 2012, 13:34		#5
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,674 Rep Power: 65	You should be using the density solver. If you want to use the pressure based solver, in addition to simply 'turning on energy' you need to account for viscous heating effects.