How can I improve my residuals

tstensrud · April 11, 2013, 08:24

I'm very new to CFD and Star-CCM+ and I'm not sure how to improve my residuals. The image below shows my simulation after 2000 iterations.

I would very much like to improve on my energy and continuity. What causes the residuals to barely drop below 0.01?

I have tried to lower the under-relaxation factors and create lower base size on my mesh, but no help so far.

KyleA&M · April 11, 2013, 12:19

The convergence criteria differs between simulations, although .01 is rather large. I would run reports to check mass flow at inlet and outlet and compare, also do you have reversed flow at the outlet? Additionally maybe your physics model or volume meshing model isn't optimal for the simulations that you're doing.

tstensrud · April 11, 2013, 12:30

There are no massflow inlet or outlets. The velocities are purely because of natural convection. I'm using a polyhedral meshing model with a 2-layer prism layer.

KyleA&M · April 11, 2013, 14:10

Your meshing model should be fine. Maybe check if the flow is turbulent or laminar. If you run a turbulent model as a laminar model then it won't converge; I'm not sure if laminar flow under turbulent model would converge or not although I feel like it would.

KyleA&M · April 11, 2013, 14:50

Also if its turbulent try running implicit unsteady instead of steady.

cwl · April 29, 2013, 08:39

Describe the mesh, please. From what i see - i would suppose this as a result of coarse mesh.

tstensrud · April 29, 2013, 09:55

It worked out fine when I changed from turbulent to laminar model, so everything is all right now. Thanks.

siara817 · May 8, 2013, 09:20

Hi
No one can say that the results of the simulation are wrong because of high residuals. As Kylea&M told, you need to check mass conservation or energy conservation.
If you are simulating air movements in the room caused by a heat source, the pattern is substantially turbulent and simulating by laminar give wrong results.

rwryne · May 8, 2013, 11:03

With natural convection problems, I believe it is best to switch to a bouyancy driven "two-layer type" when using the k-epsilon turb. model.

This is under Continua->Physics->Models->Realizable K-Epsilon Two Layer

disregard if you are using k-omega.

April 11, 2013, 08:24	How can I improve my residuals	#1
tstensrud New Member Torbjørn Stensrud Join Date: Mar 2013 Posts: 4 Rep Power: 2	I'm very new to CFD and Star-CCM+ and I'm not sure how to improve my residuals. The image below shows my simulation after 2000 iterations. I would very much like to improve on my energy and continuity. What causes the residuals to barely drop below 0.01? I have tried to lower the under-relaxation factors and create lower base size on my mesh, but no help so far.

May 8, 2013, 09:20		#8
siara817 Senior Member siamak rahimi ardkapan Join Date: Jul 2010 Location: Copenhagen, Denmark Posts: 167 Rep Power: 5	Hi No one can say that the results of the simulation are wrong because of high residuals. As Kylea&M told, you need to check mass conservation or energy conservation. If you are simulating air movements in the room caused by a heat source, the pattern is substantially turbulent and simulating by laminar give wrong results. __________________ Good luck Siamak

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April 11, 2013, 12:19		#2
KyleA&M New Member Kyle McVay Join Date: Apr 2013 Posts: 4 Rep Power: 2	The convergence criteria differs between simulations, although .01 is rather large. I would run reports to check mass flow at inlet and outlet and compare, also do you have reversed flow at the outlet? Additionally maybe your physics model or volume meshing model isn't optimal for the simulations that you're doing.

April 11, 2013, 12:30		#3
tstensrud New Member Torbjørn Stensrud Join Date: Mar 2013 Posts: 4 Rep Power: 2	There are no massflow inlet or outlets. The velocities are purely because of natural convection. I'm using a polyhedral meshing model with a 2-layer prism layer.

April 11, 2013, 14:10		#4
KyleA&M New Member Kyle McVay Join Date: Apr 2013 Posts: 4 Rep Power: 2	Your meshing model should be fine. Maybe check if the flow is turbulent or laminar. If you run a turbulent model as a laminar model then it won't converge; I'm not sure if laminar flow under turbulent model would converge or not although I feel like it would.

April 11, 2013, 14:50		#5
KyleA&M New Member Kyle McVay Join Date: Apr 2013 Posts: 4 Rep Power: 2	Also if its turbulent try running implicit unsteady instead of steady.

April 29, 2013, 08:39		#6
cwl New Member Chaotic Water Join Date: Jul 2012 Posts: 4 Rep Power: 2	Describe the mesh, please. From what i see - i would suppose this as a result of coarse mesh.

April 29, 2013, 09:55		#7
tstensrud New Member Torbjørn Stensrud Join Date: Mar 2013 Posts: 4 Rep Power: 2	It worked out fine when I changed from turbulent to laminar model, so everything is all right now. Thanks.

May 8, 2013, 11:03		#9
rwryne Super Moderator Ryne Whitehill Join Date: Aug 2009 Posts: 297 Rep Power: 7	With natural convection problems, I believe it is best to switch to a bouyancy driven "two-layer type" when using the k-epsilon turb. model. This is under Continua->Physics->Models->Realizable K-Epsilon Two Layer disregard if you are using k-omega.