Meaning of Negative Residuals

aero_head · April 8, 2021, 01:37

Hello Everyone,

I ran a simulation and then looked at the velocity profile in CFX-Post and it is not what I expected. I then looked at residual plots in all three planes and I noticed that some residuals were negative (i.e. -5E-10).

What do negative values for the residuals imply? Is this an issue? I have searched the forum/online but cannot find anything.

arjun · April 8, 2021, 11:58

" I then looked at residual plots in all three planes and I noticed that some residuals were negative (i.e. -5E-10)"

If you are plotting residuals cell by cell basis then they could be negative because

res = src - (A . solution)

aero_head · April 8, 2021, 12:21

Quote:

Originally Posted by arjun

" I then looked at residual plots in all three planes and I noticed that some residuals were negative (i.e. -5E-10)"

If you are plotting residuals cell by cell basis then they could be negative because

res = src - (A . solution)

Thanks Arjun,

So this is not an issue and would not affect my results, right?

aero_head · April 8, 2021, 12:48

I ask because I am trying to figure out why my velocity profile is wrong.

FMDenaro · April 8, 2021, 14:10

Residuals must be evaluated in some norm, the sign is not relevant to ensure the convergence.

arjun · April 8, 2021, 14:48

It will not affect your solution. Most modern CFD solvers now solve the equation in delta form that is first calculate the residual and then calculate the correction corresponding to it.

CFX is just showing you those residuals , same is what starccm does too.

April 8, 2021, 01:37	Meaning of Negative Residuals	#1
aero_head Senior Member Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 9	Hello Everyone, I ran a simulation and then looked at the velocity profile in CFX-Post and it is not what I expected. I then looked at residual plots in all three planes and I noticed that some residuals were negative (i.e. -5E-10). What do negative values for the residuals imply? Is this an issue? I have searched the forum/online but cannot find anything.

April 8, 2021, 11:58		#2
arjun Senior Member Arjun Join Date: Mar 2009 Location: Nurenberg, Germany Posts: 1,290 Rep Power: 34	" I then looked at residual plots in all three planes and I noticed that some residuals were negative (i.e. -5E-10)" If you are plotting residuals cell by cell basis then they could be negative because res = src - (A . solution) aero_head likes this.

April 8, 2021, 12:48		#4
aero_head Senior Member Kira Join Date: Nov 2020 Location: Canada Posts: 435 Rep Power: 9	I ask because I am trying to figure out why my velocity profile is wrong. Last edited by aero_head; April 8, 2021 at 15:25.

April 8, 2021, 14:10		#5
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,896 Rep Power: 73	Residuals must be evaluated in some norm, the sign is not relevant to ensure the convergence. aero_head likes this.

April 8, 2021, 14:48		#6
arjun Senior Member Arjun Join Date: Mar 2009 Location: Nurenberg, Germany Posts: 1,290 Rep Power: 34	It will not affect your solution. Most modern CFD solvers now solve the equation in delta form that is first calculate the residual and then calculate the correction corresponding to it. CFX is just showing you those residuals , same is what starccm does too. aero_head likes this.

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