# residual values >> scale

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November 24, 2012, 17:54
residual values >> scale
#1
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mohamed khedr emam
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what the difference in conversion when i check (or uncheck) scale box?
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November 25, 2012, 19:52
#2
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Lucky Tran
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Quote:
 Originally Posted by m5edr what the difference in conversion when i check (or uncheck) scale box?
The "scale" option enables/disables the scaling of the residuals that are reported. If not on, the residuals are not scaled. If enabled, then the residuals are scaled.

The unscaled residuals are the sum of the imbalance (mass, momentum, energy, etc.) over all cells in the domain. The scaled residual, is the unscaled residual divided by the appropriate scaling factor.

For most variables (all but continuity), the unscaled residual (sum of imbalance) is divided by the sum of the variable. Sum of momentum imbalance divided by sum of momentum. The sum of the quantity over all cells is the scaling factor.

Continuity is treated differently. For continuity, the unscaled residual is divided by the worst residual encountered during the first five iterations.

It is explained in detail in the Fluent manual 29.15.1.1

Last edited by LuckyTran; November 26, 2012 at 01:22.

 November 26, 2012, 01:15 #3 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,352 Blog Entries: 6 Rep Power: 45 How you look at the unscaled residuals? What should be the criteria to judge the unscaled residuals?

 November 26, 2012, 15:20 #4 Senior Member   Emre G Join Date: May 2011 Location: Turkey Posts: 123 Rep Power: 8 scale is not too much important.. dont bother this.

November 27, 2012, 03:46
#5
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Daniele
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Quote:
 Originally Posted by emreg scale is not too much important.. dont bother this.
Directly from Fluent's guide:

"In general, it is difficult to judge convergence by examining the residuals defined by Equation 25.18-3 since no scaling is employed. This is especially true in enclosed flows such as natural convection in a room where there is no inlet flow rate of with which to compare the residual."

"
The scaled residual is a more appropriate indicator of convergence for most problems".

November 27, 2012, 04:18
#6
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Daniele
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Quote:
 Originally Posted by Far How you look at the unscaled residuals? What should be the criteria to judge the unscaled residuals?
Fluent's guide says that unscaled residuals should decrease by 3 orders of magnitude, but in my opinion it is not enough to look at the residuals to judge convergence; absolute values of residuals, behaviour and monitoring of key parameters, I think these are the factors we have to look at.

Daniele

November 27, 2012, 19:55
#7
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Lucky Tran
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Quote:
 Originally Posted by ghost82 Fluent's guide says that unscaled residuals should decrease by 3 orders of magnitude, but in my opinion it is not enough to look at the residuals to judge convergence; absolute values of residuals, behaviour and monitoring of key parameters, I think these are the factors we have to look at. Daniele
Just to further the points made:

If the solver is initialized with the exact solution to the problem. The unscaled residuals will never decrease. The scaled residuals likewise will also not decrease. Hence searching for a reduction in three orders of magnitude of the residuals is not a very good criteria to judge convergence/accuracy.

The residuals do tell the amount of imbalance in the governing equations being solved. However, the residuals do not tell whether the solution has converged. Monitoring actual solution values is definitely needed.

The scaled residuals are suitable in my opinion for considering the amount of imbalance since the scaling factor is the sum of the variables. The scaled residuals are therefore like % imbalances and can be compared directly for different problems. The unscaled residuals can be the same for two different problems and have different meanings depending on magnitude of the solution values. Consider the laboratory flow around a cylinder versus interstellar flow around a galaxy, the difference in length and velocity scales are tremendous!

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