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Meaning of Negative Residuals

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Old   April 8, 2021, 00:37
Default Meaning of Negative Residuals
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Kira
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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.
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Old   April 8, 2021, 10:58
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" 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)
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Old   April 8, 2021, 11:21
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Quote:
Originally Posted by arjun View Post
" 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?
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Old   April 8, 2021, 11:48
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I ask because I am trying to figure out why my velocity profile is wrong.

Last edited by aero_head; April 8, 2021 at 14:25.
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Old   April 8, 2021, 13:10
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Residuals must be evaluated in some norm, the sign is not relevant to ensure the convergence.
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Old   April 8, 2021, 13:48
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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.
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