Variable Composition Mixture

Hamilton · January 10, 2014, 01:06

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?

Regards

ghorrocks · January 12, 2014, 06:58

Can you post an image of what you are seeing and your CCL?

Hamilton · January 15, 2014, 05:22

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.

Regards

ghorrocks · January 15, 2014, 06:48

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.

Hamilton · January 15, 2014, 09:20

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?

Regards

ghorrocks · January 15, 2014, 17:36

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.

Hamilton · January 16, 2014, 06:06

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.

Regards

ghorrocks · January 16, 2014, 08:18

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.

safikhani_hamed · January 16, 2014, 09:16

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?

Regards

ghorrocks · January 16, 2014, 18:03

This is discussed in the documentation on obtaining convergence, and this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

Hamilton · January 17, 2014, 00:27

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

January 10, 2014, 01:06	Variable Composition Mixture	#1
Hamilton Member Alireza Safikhani Join Date: Dec 2010 Posts: 36 Rep Power: 16	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? Regards

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January 12, 2014, 06:58		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	Can you post an image of what you are seeing and your CCL?

January 15, 2014, 06:48		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

January 15, 2014, 09:20		#5
Hamilton Member Alireza Safikhani Join Date: Dec 2010 Posts: 36 Rep Power: 16	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? Regards

January 15, 2014, 17:36		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

January 16, 2014, 06:06		#7
Hamilton Member Alireza Safikhani Join Date: Dec 2010 Posts: 36 Rep Power: 16	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. Regards

January 16, 2014, 08:18		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	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.

January 16, 2014, 09:16		#9
safikhani_hamed Member Hamed Join Date: Oct 2009 Posts: 64 Rep Power: 17	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? Regards

January 16, 2014, 18:03		#10
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,870 Rep Power: 144	This is discussed in the documentation on obtaining convergence, and this FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

January 17, 2014, 00:27		#11
Hamilton Member Alireza Safikhani Join Date: Dec 2010 Posts: 36 Rep Power: 16	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