# Follow the flowing of a gas

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 February 1, 2012, 08:17 Follow the flowing of a gas #1 New Member   jean-françois jerier Join Date: Dec 2011 Posts: 13 Rep Power: 6 Hi all, In my simulation, a gas (G1) is injected in a domain via a pump and it is purged via a vacuum pump (steady state phase). Then, I inject during a short period an other gas (G2) by a transient phase. Please, can you tell me how can I follow the flowing of the second gas (G2) in the CFD-post, so as to know if G2 is well evacuated in a fixed time. Thank you in advance for your help

 February 1, 2012, 15:17 #2 New Member   Dustin Join Date: Jan 2012 Posts: 18 Rep Power: 6 syens, This sounds like a straight-forward transient analysis (maybe I'm missing something). You should calculate roughly the time it would take for one particle of Gas 2 to enter and exit the pipe. That will govern your timestep. Your initial conditions can be set up so that it starts with 100% Gas 1 and 0% Gas 2 (Mass Fraction 1 and 0), then giving Gas 1 and Gas 2 an initial condition (velocity maybe). You will also want to stop injecting Gas 2 after a preset time (someone else will have to tell you how to do this). In CFX-Post, you could then animate the transient, or look at each time step. I skipped over some things, since this stuff is in the tutorials, but correct me if this is more complicated than I thought.

 February 2, 2012, 03:28 #3 New Member   jean-françois jerier Join Date: Dec 2011 Posts: 13 Rep Power: 6 Hi Trues, Thank you for your reply, I think that everything in the CFX-Pre is defined : _ The steady state phase (P1) to install the gas G1 _ The transient phases to inject G2 (P2) and evacuate (P3) the gases. During the phase P3, the injection of gas G2 is stopped and G1 is injected. In the domain, I have solid parts and I want to follow the flowing of G2 through the domain. Do you know how can I visualize with the streamline or vector the flowing of G2 during P2 and P3 ? Short version: Is it possible in the CFD-Post to visualize by streamlines or vectors the flowing of defined material for a transient phase ?

 February 2, 2012, 05:41 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,715 Rep Power: 99 Yes, you can define streamlines to be calculated from any vector variable field. But note streamlines are a bit weird in transient flows. You might want to use massless particles or additional variables (passive scalars) if you want to track where stuff gets to. But if you already have a multiphase or multicomponent mixture simulation you can simply do isosurfaces of the phases/components.

 February 2, 2012, 12:04 #5 New Member   jean-françois jerier Join Date: Dec 2011 Posts: 13 Rep Power: 6 Hi Thank you for your reply !!! I read this previous discussion (Massless particle tracking in CFX-5.7.1 ), but in the CFX-Pre, into the output section for transient phase, the option: the monitor particles section is full. I cannot choose the density of the particles !!! In CFX-Post, I can visualise the flowing with the symbol BALL in the streamline section. Is this type of visualisation, you mentionned in your post ? Regards JF

 February 2, 2012, 12:49 #6 New Member   Dustin Join Date: Jan 2012 Posts: 18 Rep Power: 6 You could visualize isosurfaces of a specified gas 2 concentration (if you want a 3D picture), as ghorrocks suggested. If you want a 2D visualization of the flow, why don't you just look at gas 2 mass fraction? you can step through time to watch gas 2 enter and exit

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