Problem with BC + ReactingFOAM + gravity

Gloq · November 27, 2012, 05:29

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

vainilreb · November 27, 2012, 05:48

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.

Gloq · November 27, 2012, 07:49

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.

vainilreb · November 27, 2012, 08:08

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.

redhoax · May 9, 2013, 21:02

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

November 27, 2012, 05:29	Problem with BC + ReactingFOAM + gravity	#1
Gloq New Member Guillaume Join Date: Nov 2012 Posts: 10 Rep Power: 13	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

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November 27, 2012, 05:48		#2
vainilreb Member Robert Join Date: Aug 2012 Location: Berlin Posts: 74 Rep Power: 13	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, 07:49		#3
Gloq New Member Guillaume Join Date: Nov 2012 Posts: 10 Rep Power: 13	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, 08:08		#4
vainilreb Member Robert Join Date: Aug 2012 Location: Berlin Posts: 74 Rep Power: 13	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
redhoax New Member Join Date: Feb 2013 Posts: 17 Rep Power: 13	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