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 Harrison July 1, 2003 19:04

questions about the pathlines generated by Fluent

Dear Sir,

I am simulating the ventilation system in an animal house. I have two inlets (top ceiling inlet and floor inlet) and one outlet. I assume the flow is incompressible idea gas. No chemical reactions. I use Standard k-e model first and change to RNG model after it begin to converge. The model gives me good results including temperature gradient, ammonia gradient, and air velocity vectors profiles. However, when I use the pathline to show the airflow pattern in the house, I find that all pathlines entering the top ceiling inlet stay at the ceiling and end at the corner. They do not flow down and to the outlet. I check the total mass balance of all inlets and outlet. It is good.

So, I am wondering if this pathline pattern means something special. Or do you have the same situation as me? Does anybody know how fluent define the pathline? How does fluent calculate it? Based on our regular knowledge, the particles entering the inlet should leave through the outlet if there is no sink in the domain. Am I right?

Thank you very much! Harrison

 Alex Munoz July 1, 2003 21:58

re: questions about the pathlines generated by Fluent

Hi

I don't know too much about pathline, I have never used it. However, i sugguest that you use DPM model

See the tutorial for particle tracking basically, create the injections one for each inlet, then the material (your material could be the default antracite). finally, request particle tracking under under display>particle tracking.

Use all the default parameters of DPM except the maximum number of step, which are determined by the distance between the inlet and the outlet and the lenght step that you assing the default 0.01m if you select specify length scale.

For the injection properties just provide a velocity normal to the inlet surface equal to the average velocity of the flow at that surface and the particle diameter 1e-03 or 1e-04m.

Good luck

Alex Munoz

 xiang July 2, 2003 00:38

re: questions about the pathlines generated by Fluent

Dear Harrison,

Are you sure that your setting for the max. number of steps is sufficient so that the calculation of the path lines can be completed? If not, the pathline will terminate somewhere in your domain without reaching the outlet. In this case, the number of incomplete pathlines will be reported in the console window. Of course, since your solution was converged, the mass balance was kept for all your outlet and inlets.

Another way to check the pathline is that your enable "reverse" option in the pathline panel and set the outlet as release surface, then FLUENT will track back the pathline from the outlet to inlet. Hope I am giving you some help without wasting your time. regards, Xiang rongbiao

 Harrison July 3, 2003 00:31

re: questions about the pathlines generated by Fluent

Hi, Alex and Xiang,

Thank you so much.

I believe that I set to the max number of steps. I will try the DMP and reverse model. Thanks again!

Harrison

 kike September 1, 2011 11:05

I think that I have a similar situation to what Harrison had. I have a simulation with one inlet and one outlet and after convergence of all equations to 1e-6 the pathlines are incomplete. The path the particles take though seem to make good sense until they stop without escaping. Thus I think that the problem is merely that more time is needed to finish calculating the pathlines, I decided to continue to iterate disregarding the convergence criteria but it seems it will take a long time before all the pathlines are calculated. If there is a way to reduce the time step for the pathline calculation that would be great, is this only available in unsteady? my simulation is steady.

 stuart23 September 1, 2011 12:22

Kike,
You should not need to iterate the solver any more. Try inceasing the step size and number of steps options in the pathline dialogue. Your pathlines are probably just not traced far enough

Stu

 kike September 1, 2011 13:23

Stuart, thanks for your response. You are right, after a long time of iterations, the results do not seem to be different. I tried to increase the stepsize as you said (by deactivating the accuracy control) and although this brought more detail, the path lines seem to remain trapped within my geometry. When I Click reverse on the outflow some pathlines are completed and the particles escape, unfortunately not all the particles escape in this option either.

 kike September 1, 2011 14:14

As an update, I took the results into CFD-post and the streamlines are displayed through a more complete path, almost half or more go from inlet to outlet, however, there are some still that (although taking the path that seems to make sense) they end up suspended in between.. any suggestions?

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