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November 24, 2012, 16:54 |
residual values >> scale
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#1 |
Senior Member
mohamed khedr emam
Join Date: Oct 2011
Location: Egypt
Posts: 121
Rep Power: 14 |
what the difference in conversion when i check (or uncheck) scale box?
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November 25, 2012, 18:52 |
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#2 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
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Quote:
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 00:22. |
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November 26, 2012, 14:20 |
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#4 |
Senior Member
Emre G
Join Date: May 2011
Location: Turkey
Posts: 126
Rep Power: 14 |
scale is not too much important..
dont bother this. |
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November 27, 2012, 02:46 |
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#5 |
Senior Member
Rick
Join Date: Oct 2010
Posts: 1,016
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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". |
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November 27, 2012, 03:18 |
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#6 | |
Senior Member
Rick
Join Date: Oct 2010
Posts: 1,016
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Quote:
Daniele |
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November 27, 2012, 18:55 |
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#7 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
Rep Power: 66 |
Quote:
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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August 17, 2019, 07:23 |
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#8 |
New Member
Join Date: May 2019
Posts: 18
Rep Power: 6 |
Hello,
i have a general question about the calculation of the scaled residual. In Fluent, there is the following defintion of it: Please see attached picture. Maybe someone could explain it to me becasue the documentation of Fluent doens't help me. What is calculated exactly? Is it the difference of a governing variable from the actual iteration minus the last iteration in relation to the last iteration?? Thanks for your help, bumper |
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August 18, 2019, 22:33 |
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#9 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
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Quote:
On the top and bottom is a summation over all cells p. Each summand is the difference difference in fluxes of the current cell and neighboring cells + sources (b). The summand in the denominator is the flux in each cell. Hence, it is the sum of all imbalances divided by the sum of all fluxes. And since the number of cells in numerator and denominator are the same, it is the average/mean imbalance divided by the avg/mean flux. |
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August 19, 2019, 03:50 |
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#10 |
New Member
Join Date: May 2019
Posts: 18
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Okay, so to sum it up in my words:
In general, the scaled residuals compares the differences to neighboring cells in relation to all in one iteration step, right ? And with that, you can rate the convergence behaviour? Isn't it necessary to consider the behaviour betweeen iteration steps? |
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August 19, 2019, 13:31 |
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#11 | |
Senior Member
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,674
Rep Power: 66 |
Quote:
Yes it is only for 1 iteration. Residuals measure imbalance, they don't tell you anything about convergence except that a converged solution should have low residuals. It still is necessary to compare something between iterations. What and how that is done, is up to the user to be creative. |
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August 25, 2019, 02:49 |
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#12 | |
New Member
Join Date: May 2019
Posts: 18
Rep Power: 6 |
Quote:
And can you tell me what is represented by the coefficient a in the equation of residuals? Ansys documentation says that is is a center coefficient or an influence coefficient. But what does it mean exactly? Last edited by bumper; August 26, 2019 at 07:28. |
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