Minimum Allowable Wall Distance

rks171 · January 21, 2012, 12:07

I setup a model in starccm+. I get a warning "wall distance limited in n cells". I ran the simulation anyway and my solution seems to converge nicely. The residuals all decrease. I read in the manual that if you get that warning about wall distance, you should remedy it. So I tried reducing the minimum allowable wall distance so that less cells are limited. Now my turbulent residuals and energy residuals seem to be oscillating, though the other physical parameters that I'm monitoring in my solution still seem to converge correctly. So does it matter if a small number of my cells have their wall distance limited (~1,000 in a 6.5 million cell mesh)?

cfdnewbie · January 21, 2012, 12:11

are you doing RANS?

rks171 · January 21, 2012, 12:16

yes, it is RANS

cfdnewbie · January 21, 2012, 12:21

what kind of model are you using then?

rks171 · January 21, 2012, 12:26

It is a steady simulation using segregated flow and segregated energy equations. My turbulence model is realizable k-epsilon two layer with all y+ wall treatment.

rks171 · January 27, 2012, 13:58

Okay, so after some time, I found out another difference between these two times I ran this case. The first time, my mesh had 2 prism layers adjacent the walls in the model. The second time, I somehow (accidently) used a different outdated mesh that did not have prism layers included. The absence of the prism layers seems to cause the funny behavior in the residuals. But despite the residual behavior, the actual physical solution parameters (flow velocity and temperature) does not seem to change between using a prism layer and not using a prism layer (for my modeling scenario, anyway).

fisicas1 · June 16, 2012, 16:18

I have this warning " wall distance limited to 1e-6 in 678640 cells in region A" ... What can I do?

rks171 · June 16, 2012, 16:39

You can change the minimum allowable wall thickness (reduce it) and not as many cells should be limited. Does it cause you a problem or does your simulation still converge?

January 21, 2012, 12:07	Minimum Allowable Wall Distance	#1
rks171 Member Join Date: Dec 2011 Location: State College, PA Posts: 87 Rep Power: 14	I setup a model in starccm+. I get a warning "wall distance limited in n cells". I ran the simulation anyway and my solution seems to converge nicely. The residuals all decrease. I read in the manual that if you get that warning about wall distance, you should remedy it. So I tried reducing the minimum allowable wall distance so that less cells are limited. Now my turbulent residuals and energy residuals seem to be oscillating, though the other physical parameters that I'm monitoring in my solution still seem to converge correctly. So does it matter if a small number of my cells have their wall distance limited (~1,000 in a 6.5 million cell mesh)?

June 16, 2012, 16:18	warning	#7
fisicas1 Member David L Join Date: May 2012 Posts: 43 Rep Power: 14	I have this warning " wall distance limited to 1e-6 in 678640 cells in region A" ... What can I do?

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January 21, 2012, 12:11		#2
cfdnewbie Senior Member cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20	are you doing RANS?

January 21, 2012, 12:16		#3
rks171 Member Join Date: Dec 2011 Location: State College, PA Posts: 87 Rep Power: 14	yes, it is RANS

January 21, 2012, 12:21		#4
cfdnewbie Senior Member cfdnewbie Join Date: Mar 2010 Posts: 557 Rep Power: 20	what kind of model are you using then?

January 21, 2012, 12:26		#5
rks171 Member Join Date: Dec 2011 Location: State College, PA Posts: 87 Rep Power: 14	It is a steady simulation using segregated flow and segregated energy equations. My turbulence model is realizable k-epsilon two layer with all y+ wall treatment.

January 27, 2012, 13:58		#6
rks171 Member Join Date: Dec 2011 Location: State College, PA Posts: 87 Rep Power: 14	Okay, so after some time, I found out another difference between these two times I ran this case. The first time, my mesh had 2 prism layers adjacent the walls in the model. The second time, I somehow (accidently) used a different outdated mesh that did not have prism layers included. The absence of the prism layers seems to cause the funny behavior in the residuals. But despite the residual behavior, the actual physical solution parameters (flow velocity and temperature) does not seem to change between using a prism layer and not using a prism layer (for my modeling scenario, anyway).

June 16, 2012, 16:39		#8
rks171 Member Join Date: Dec 2011 Location: State College, PA Posts: 87 Rep Power: 14	You can change the minimum allowable wall thickness (reduce it) and not as many cells should be limited. Does it cause you a problem or does your simulation still converge?