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Old   January 10, 2014, 01:06
Default Variable Composition Mixture
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Alireza Safikhani
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Dear friends,
I am simulating the smoke in air using Variable Composition Mixture. My problem has one inlet and one outlet. Unfortunately after converging the problem when I see the mass fraction of smoke in post process, the smoke in inlet is suddenly eliminated, please help me what is the fault?

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Old   January 12, 2014, 06:58
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Glenn Horrocks
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Can you post an image of what you are seeing and your CCL?
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Old   January 15, 2014, 05:22
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Alireza Safikhani
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Dear Glenn
Thanks for the consideration, I have uploaded a contour of mass fraction, as it is obvious the mass fraction at the inlet suddenly is vanished. Please help me in fact this problem is very important for me.

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File Type: jpg Mass Fraction.jpg (53.5 KB, 11 views)
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Old   January 15, 2014, 06:48
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What does the flow vectors look like? Is the simulation converged? Has it run for enough pseudo time? If your time is too short that might be as far as the inlet fluid has got in the time you have run.
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Old   January 15, 2014, 09:20
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Alireza Safikhani
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Dear Glenn
Yes the problem is converged and the velocity vectors are good. In fact I have tried both "Additional Variables" and "Variable Composition Mixture" methods and this fault is occurred in both methods. What do you mean by this sentence "If your time is too short that might be as far as the inlet fluid has got in the time you have run" would you please define more?

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Old   January 15, 2014, 17:36
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CFX uses a psuedo transient approach for steady state simulations. That means that the fluid is convects from the inlet to the outlet in a fashion similar to a true transient flow. So if you start with initial condition AV=0 everywhere and AV=1 at the inlet, it will take some time for the AV to convect through the domain and reach the outlet, and a bit more time to establish equilibrium (ie achieve steady state convergence).

This convection process is often not well captured by the residuals. The best way to check that you have converged in this case is to add the imbalances as a convergence criteria. This is because the imbalances will pick this up as an imbalance in the AV.

So you say the problem is converged - have you checked the imbalances? I suggest you have not converged yet and the imbalance for the AV equation will be poor.
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Old   January 16, 2014, 06:06
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Alireza Safikhani
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Dear Glenn
Yes in fact the imbalance is poor, what should be done to have a good imbalance?
Does AV is the abbreviation of Additional Variables or not?
In fact I have simulated with initial value of mass fraction other than zero (for example 0.04) but this fault was still observed unfortunately.

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Old   January 16, 2014, 08:18
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Poor imbalances means you have not converged yet. You need to run the simulation longer.

By AV I mean additional variable.

If your simulation is not converged (and poor imbalances means it is not converged) then do not worry about what it is giving for results. Until it is converged the results are rubbish.
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Old   January 16, 2014, 09:16
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Thank you very much glenn for very good guidance and sorry for consuming your time let me ask you the final question. In new simulations do you believe in that larger pseudo time should be used? How can I reach the pseudo time value and does the pseudo time the same as time scale ? now the time scale is set on " auto time scale" should I set the time scale on "Physical time scale" or "Local" time scale? How can I calculate the appropriate value of time scale?

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Old   January 16, 2014, 18:03
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This is discussed in the documentation on obtaining convergence, and this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
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Old   January 17, 2014, 00:27
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Alireza Safikhani
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Thank you very much glenn for very good guidance, in fact I changed the time scale and the mass fraction shows very good distribution.

Sincerely
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