Second Order Upwind: Residuals

zhaopeng · March 27, 2009, 09:58

This may seem a bit strange,i advise you slove the case you upload again.

enricokr · March 27, 2009, 11:03

Yes i'm using double precision...
Your results at the number of iterations you indicate aren't the same of mine... I don't know why!
Changing URF of pression from 0.3 to 0.1 i have the result i want, but is it a correct method?
And what means to decrease URF?
Excuse for my bad knowledge...

paka · March 27, 2009, 12:35

Maybe it is completely wrong approach, but what happens if you try to use transient solver?

enricokr · March 27, 2009, 13:29

Why i have to use a transient solver if the problem is steady?

paka · March 27, 2009, 14:49

I'm didn't say you have to, I said that maybe something unexpected happens if it breaks so early. I don't know

enricokr · March 29, 2009, 15:25

I repost the former question:
Changing URF of pression from 0.3 to 0.1 i have the result i want, but is it a correct method?
And what means to decrease URF?

Does anyone know it?

mr_fluent · March 29, 2009, 18:22

i was about to ask you to reduce mom urf little bit. Or reducing pressure urf would help but might not be the case always.

Quote:

Originally Posted by enricokr

I repost the former question:
Changing URF of pression from 0.3 to 0.1 i have the result i want, but is it a correct method?
And what means to decrease URF?

Does anyone know it?

urf of pressure results in slower update of pressure.
pressure_new = pressure_old + urf * change in pressure.

anyway the main reason of your problem is your grid. it is not refined enough to handle fast change that is coming in flow direction. If this situation happens the gradient of u,v,w varries very fast. And often breaking local maxima minima. When you use second order scheme there is a contribution of this term in momentum equation. If your gradients are false this term might be shooting up and down. By reducing pressure unerrelaxation the change due to pressure change is restricted (which is main change per iteration) and behaviour of second order contribution is controled better.

for steady state using low urf is no cheating. Its just a way to get converged solution.

Achilleas · January 2, 2018, 12:30

A few years later, which is the solution to this problem? I encounter the same difficulties and I did all the above things proposed..

DEd · January 2, 2018, 17:41

Achillieas, I don't think it is an unusual experience to get crappier convergence when switch to a less robust discretisation scheme but I don't think it is something special with a pipe expansion (as the testimonies above afirm) . Therefore I would advise you to do as in every case - describe as much as possible about your case, display the mesh and ask for help.

March 27, 2009, 09:58		#21
zhaopeng Member zhao peng Join Date: Mar 2009 Posts: 41 Rep Power: 17	This may seem a bit strange,i advise you slove the case you upload again. Last edited by zhaopeng; March 27, 2009 at 10:14.

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March 27, 2009, 11:03		#22
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	Yes i'm using double precision... Your results at the number of iterations you indicate aren't the same of mine... I don't know why! Changing URF of pression from 0.3 to 0.1 i have the result i want, but is it a correct method? And what means to decrease URF? Excuse for my bad knowledge...

March 27, 2009, 12:35		#23
paka Senior Member Join Date: Mar 2009 Posts: 225 Rep Power: 18	Maybe it is completely wrong approach, but what happens if you try to use transient solver?

March 27, 2009, 13:29		#24
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	Why i have to use a transient solver if the problem is steady?

March 27, 2009, 14:49		#25
paka Senior Member Join Date: Mar 2009 Posts: 225 Rep Power: 18	I'm didn't say you have to, I said that maybe something unexpected happens if it breaks so early. I don't know

March 29, 2009, 15:25		#26
enricokr New Member Enry Lorna Neil Join Date: Mar 2009 Posts: 11 Rep Power: 17	I repost the former question: Changing URF of pression from 0.3 to 0.1 i have the result i want, but is it a correct method? And what means to decrease URF? Does anyone know it?

January 2, 2018, 12:30		#28
Achilleas New Member Join Date: Sep 2015 Posts: 15 Rep Power: 10	A few years later, which is the solution to this problem? I encounter the same difficulties and I did all the above things proposed..

January 2, 2018, 17:41		#29
DEd Member Daniel Edebro Join Date: Feb 2016 Location: Gothenburg Posts: 41 Rep Power: 10	Achillieas, I don't think it is an unusual experience to get crappier convergence when switch to a less robust discretisation scheme but I don't think it is something special with a pipe expansion (as the testimonies above afirm) . Therefore I would advise you to do as in every case - describe as much as possible about your case, display the mesh and ask for help.