# Problem with BC + ReactingFOAM + gravity

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 November 27, 2012, 06:29 Problem with BC + ReactingFOAM + gravity #1 New Member   Guillaume Join Date: Nov 2012 Posts: 10 Rep Power: 6 Hi guys, I searched for a problem looking like mine on the forum, but I could not find such a topic, so here is a new one ! My computation domain is a channel, in which air is flowing, while an other gas is injected in the middle. To adress this problem, I use reactingFOAM. For my first runs, I forgot to set the gravity vector to -9.81 u_z. I obtained physical pressure and velocity fields. But when I realized my mistake, I 'activated' gravity. Doing so, I got weird velocity and pressure fields for the corresponding runs. Especially close to outlet BC where a recirculation zone appeared. Big differences between theoretically very close runs. (If useful : outlet BC are zeroGradient for U and fixedValue (1 atm.) for p) How do you explain the differences ? Which solution could you advise to me ? I was considering the option to try to create a simplified version of reactingFOAM, as my case is non-reactive, but I'm not sure that this work would help me ... Thanks in advance for your help ... Gloq

 November 27, 2012, 06:48 #2 Member   Robert Join Date: Aug 2012 Location: Berlin Posts: 74 Rep Power: 6 Could you please post your files from the 0-directory and a screenshot of the strange flow behaviour you're talking about? I'm dealing with a similar problem and perhaps I can help you. But I need a little more detailed information.

 November 27, 2012, 08:49 #3 New Member   Guillaume Join Date: Nov 2012 Posts: 10 Rep Power: 6 I'll try to upload more precise elements. For the fields, I have roughly : - a recirculation zone at the outlet - a depression at the outlet. In the channel, pressure is not homogeneous. I mean, pressure is slightly higher in the lower part than in the upper part of the channel. For the case ran w/o gravity, pressure is homogeneous.

 November 27, 2012, 09:08 #4 Member   Robert Join Date: Aug 2012 Location: Berlin Posts: 74 Rep Power: 6 The pressure gradient is typical of gravity-influenced cases. It should be something like dp/dy = g*rho [Pa/m]. (Please correct me, if I'm misled.) You will find this pressure gradient in our atmosphere, too. Concerning the recirculation zone I can't say anything until I've seen your BC files and a screenshot would really help my imagination, too.

 May 9, 2013, 21:02 #5 New Member   Join Date: Feb 2013 Posts: 17 Rep Power: 5 Hello. I am very interested in the solution to this matter. I am also doing a case on air flow and gas injection, using reactingFoam. Like Gloq I obtain this circulation zone at the outlet when gravity is set. Did you discover a solution to the matter? Could it be possible to use f.ex. fireFoam, or rewrite solver to solve for p_rgh? Lasse

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