Please help: Question regarding inletOutlet/outletInlet boundary conditions

sahmed · February 10, 2017, 16:54

Dear Foamers
I have a very basic question about inletOutlet and outletInlet boundary conditions. I have to solve a reactive flow where there is possibility of potential backflow. I am sure that for backflow case, I have to use inletOutlet boundary condition. I have to solve pressure, Temperature, velocity and species concentration.
My first question is- I have to specify inletValue and Value with this BC, where inletValue is the value of the backflow. Then what should I give for 'Value'? Can I just skip it, I found that the simulation runs even without this 'Value'.
My second question is- since I am specifying inletOutlet boundary for velocity, I am using the opposite outletInlet for pressure, shouldnt that be the typical case backflow simulation?
My third question is- for velocity inletOutlet boundary condition, the backflow value (specified as 'Value') should be uniform (0 0 0) right?
My last question or rather confusion is that, I am specifying a Dirichlet boundary at the inlet and at the same time specifying a fixed value for backflow (inletValue) condition. How does two Dirichlet condition work at the same time at opposite directions?

sahmed · February 11, 2017, 13:45

Anyone PLEASE help me with any info. This will help the community as well.

jherb · February 27, 2017, 13:08

Value is just used as initial condition for the first iteration of your simulation.
For pressure you can set fixedValue at your outlet.
If you want to block backflow, then yes set (0 0 0) for velocity.
Numerically most stable is to set some fixedValue condition at one boundary and something like zeroGradient (or inletOutlet) for the opposite boundary.

sahmed · February 27, 2017, 13:53

Thank you very much Herb for your reply. I just have a quick question regarding your last answer (where I still have confusion). Yes numerically the most stable BCs are fixedValue and zeroGradient/inletOutlet. But for inletOutlet (or outletInlet), I have to specify an inletValue/outValue which is again a fixedValue boundary. Then how do two Dirichlet conditions at two opposite directions can work?
Please share your thoughts.

~SFA

jherb · March 2, 2017, 17:27

I am not sure, what you want to do. If you set inletOutlet at the side, you expect the flow to leave the domain, then the inletValue is used only for those faces, where there is (local) inward flow. I am not really sure, what is the purpose of outletInlet. Can't you use fixedValue for your inlet?

February 10, 2017, 16:54	Please help: Question regarding inletOutlet/outletInlet boundary conditions	#1
sahmed Member Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 10	Dear Foamers I have a very basic question about inletOutlet and outletInlet boundary conditions. I have to solve a reactive flow where there is possibility of potential backflow. I am sure that for backflow case, I have to use inletOutlet boundary condition. I have to solve pressure, Temperature, velocity and species concentration. My first question is- I have to specify inletValue and Value with this BC, where inletValue is the value of the backflow. Then what should I give for 'Value'? Can I just skip it, I found that the simulation runs even without this 'Value'. My second question is- since I am specifying inletOutlet boundary for velocity, I am using the opposite outletInlet for pressure, shouldnt that be the typical case backflow simulation? My third question is- for velocity inletOutlet boundary condition, the backflow value (specified as 'Value') should be uniform (0 0 0) right? My last question or rather confusion is that, I am specifying a Dirichlet boundary at the inlet and at the same time specifying a fixed value for backflow (inletValue) condition. How does two Dirichlet condition work at the same time at opposite directions?

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February 11, 2017, 13:45		#2
sahmed Member Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 10	Anyone PLEASE help me with any info. This will help the community as well.

February 27, 2017, 13:08		#3
jherb Senior Member Joachim Herb Join Date: Sep 2010 Posts: 650 Rep Power: 21	Value is just used as initial condition for the first iteration of your simulation. For pressure you can set fixedValue at your outlet. If you want to block backflow, then yes set (0 0 0) for velocity. Numerically most stable is to set some fixedValue condition at one boundary and something like zeroGradient (or inletOutlet) for the opposite boundary.

February 27, 2017, 13:53		#4
sahmed Member Sheikh Ahmed Join Date: Dec 2015 Location: South Carolina, USA Posts: 88 Rep Power: 10	Thank you very much Herb for your reply. I just have a quick question regarding your last answer (where I still have confusion). Yes numerically the most stable BCs are fixedValue and zeroGradient/inletOutlet. But for inletOutlet (or outletInlet), I have to specify an inletValue/outValue which is again a fixedValue boundary. Then how do two Dirichlet conditions at two opposite directions can work? Please share your thoughts. ~SFA

March 2, 2017, 17:27		#5
jherb Senior Member Joachim Herb Join Date: Sep 2010 Posts: 650 Rep Power: 21	I am not sure, what you want to do. If you set inletOutlet at the side, you expect the flow to leave the domain, then the inletValue is used only for those faces, where there is (local) inward flow. I am not really sure, what is the purpose of outletInlet. Can't you use fixedValue for your inlet?