# inletOutlet and outletInlet boundary condition

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 April 4, 2018, 11:26 inletOutlet and outletInlet boundary condition #1 Member   Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 9 Dear Foamers I am trying to get your attention to this age-old problem of inletOutlet boundary conditions. I hope many of you have successfully used this boundary condition. Now PLEASE help me. I have a backflow situation at the outlet where the backflow velocity is unknown. Now, for the inlet velocity and pressure I am using- inlet_velocity { type fixedValue;; value uniform (0.13 0 0); } inlet_pressure { type zeroGradient; } Now, in order to take care of the backflow at the outlet, I am using- outlet_velocity { type inletOutlet; inletValue uniform (0 0 0); value uniform (0 0 0); } outlet_pressure { type outletInlet; outletValue 101325; value 101325; } What I understood is that, during back flow, the backflow velocity will be calculated based on the pressure difference. I used the backflow pressure as atmospheric. Also, I am using the inletOutlet for the outlet temperature as well where the backflow temperature is 300K. PLEASE let me know if this is correct or not. If not, please correct me. Thanks in advance.

 April 4, 2018, 22:57 #2 Senior Member   Taher Chegini Join Date: Nov 2014 Location: Houston, Texas Posts: 125 Rep Power: 11 Regarding the inlet, If you have gravity and surface tension in your problem you should use fixedFluxPressure for pressure instead of zeroGradient. Regarding the outlet, usually pressure is specified explicitly through a boundary condition such as totalPressure and for velocity pressureInletOutletVelocity is imposed.

April 4, 2018, 23:19
#3
Member

Sheikh Ahmed
Join Date: Dec 2015
Location: South Carolina, USA
Posts: 88
Rep Power: 9
Quote:
 Originally Posted by Taataa Regarding the inlet, If you have gravity and surface tension in your problem you should use fixedFluxPressure for pressure instead of zeroGradient. Regarding the outlet, usually pressure is specified explicitly through a boundary condition such as totalPressure and for velocity pressureInletOutletVelocity is imposed.
Thank you very much Taher for your reply. I am not considering gravity and surface tension in the inlet, that means zeroGradient pressure is ok right?
For the outlet pressure, I could understand the pressureInletOutletVelocity boundary for the outlet velocity, but for pressure, I saw the totalPressure bc mostly for inlets. That is why I was confused.
Could you PLEASE tell me what should be the field entries (rho, p0, value) in this case? I am expecting backflow air coming inside the domain from atmospheric condition. So should I use the following for subsonic compressible condition-
{
type totalPressure;
p0 uniform 101325;
value uniform 101325;
}

I WOULD HIGHLY APPRECIATE YOUR SUGGESTION.
Thanks

 April 4, 2018, 23:39 #4 Senior Member   Taher Chegini Join Date: Nov 2014 Location: Houston, Texas Posts: 125 Rep Power: 11 You can find good and reliable explanation here. EDIT (Sections 5.2.3.1 and 5.2.3.2) Yes, only p0 is mandatory. So it should work well. I meant gravity and surface tension in your whole domain not just the boundary. If gravity and/or surface tension play a role in your simulation you should use that BC.

April 5, 2018, 00:55
#5
Member

Sheikh Ahmed
Join Date: Dec 2015
Location: South Carolina, USA
Posts: 88
Rep Power: 9
Quote:
 Originally Posted by Taataa You can find good and reliable explanation here. EDIT (Sections 5.2.3.1 and 5.2.3.2) Yes, only p0 is mandatory. So it should work well. I meant gravity and surface tension in your whole domain not just the boundary. If gravity and/or surface tension play a role in your simulation you should use that BC.
Thanks once again for the link. it clearly explains what I need.
Just one quick question- in the damBreak tutorial of the link where totalPressure boundary is used for the outlet pressure, the P0 value is specified as 0 which I didnt understand. If the backflow is coming from atmosphere, shouldnt it be 101325 instead of 0?

April 5, 2018, 11:44
#6
Senior Member

Taher Chegini
Join Date: Nov 2014
Location: Houston, Texas
Posts: 125
Rep Power: 11
Quote:
 Originally Posted by sahmed Thanks once again for the link. it clearly explains what I need. Just one quick question- in the damBreak tutorial of the link where totalPressure boundary is used for the outlet pressure, the P0 value is specified as 0 which I didnt understand. If the backflow is coming from atmosphere, shouldnt it be 101325 instead of 0? Thanks in advance.
If the solver is incompressible gauge pressure matters not absolute pressure so that's why atmospheric pressure is set to zero whereas in compressible solvers absolute pressure should be used hence the value of 101325 for atmospheric pressure.

 April 5, 2018, 11:54 #7 Member   Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 9 Got it. Thanks a lot Taher.

 April 5, 2018, 11:54 #8 Senior Member   Taher Chegini Join Date: Nov 2014 Location: Houston, Texas Posts: 125 Rep Power: 11 You're welcome.

 April 13, 2018, 10:18 #9 Member   Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 9 Hi Taher Can I have your email address so that I can share something related to my simulation with you with some attachment? Thanks in advance. ~SFA

 Tags backflow, inletoutlet, outletinlet