# Boundary conditions

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 March 4, 2006, 16:50 Boundary conditions #1 Ruchi Guest   Posts: n/a Is it possible to impose 2 boundary conditions on the same boundary? And impose no BC on an outlet? I mean, for example I have a geometry with an inlet and 2 outlets. I want to fix the velocity and the presure of my inlet, pressure of an outlet and let the software calculate the corresponding pressure at the second outlet. Can anyone please help me?

 March 5, 2006, 17:04 Re: Boundary conditions #2 Eike Guest   Posts: n/a You have to define at least one characteristic for every boundary, because the conditions on your second outlet will influence the flow as well.

 March 5, 2006, 22:46 Re: Boundary conditions #3 TB Guest   Posts: n/a If you specified inlet velocity & outlet static pressure, inlet static pressure will be part of the solution. No point to make redudant BC. Besides, I have doubt if you can leave one of your outlet boundary undefined anyway. If you aren't sure, just give it a go and see if the solver accept this condition.

 March 5, 2006, 23:07 Re: Boundary conditions #4 Ruchi Guest   Posts: n/a Yes, its right that I can't leave a boundary condition undefined. In real world I can measure the pressure at my outlet using a pressure gauge. All I want to do is to mimick the pressure gauge at my outlet to get the outlet pressure. Do you have any suggestion how a pressure gauge can be represeted in the software? I can't do real experiments at this stage because my real device requires microfabrication in clean room.

 March 5, 2006, 23:12 Re: Boundary conditions #5 TB Guest   Posts: n/a Remember you have to define the reference pressure? If you measure pressure with respect to atm, just define reference pressure as atm pressure. All pressure calculations will be based on this reference pressure.

 March 5, 2006, 23:26 Re: Boundary conditions #6 Ruchi Guest   Posts: n/a Let me re-phrase my question. May be I wasn't so clear. How do we measure pressure at a point in a pipe? Using a pressure gauge, right? What I meant is that my outlet is connected to a pressure gauge and I want to record what is the reading of my pressure gauge.

 March 6, 2006, 08:56 Re: Boundary conditions #7 siv Guest   Posts: n/a Hello Ruchi, If you want to measure any variable at some point create monitor points and solve. then you can even find the variation of your variable with time step.

 March 6, 2006, 09:11 Re: Boundary conditions #8 Ruchi Guest   Posts: n/a Hi, Thanks for the suggestion. But I am wondering if I have set my pressure at the outlet to atmospheric pressure, I should measure the pressure to be the same as what I have set. Thats the reason why I didn't want to set the outlet pressure and let the software calculate it for me. And because I have 2 outlets, I don't know how the mass flow will split in each of the outlets.

 March 6, 2006, 15:58 Re: Boundary conditions #9 Eike Guest   Posts: n/a I think your system is under-determind, if you define only the pressure of one outlet. How do you want to run your experiments without setting the pressure of both outlets?

 March 7, 2006, 02:14 Re: Boundary conditions #10 TB Guest   Posts: n/a Try look at user manual about "boundary condition" and think about how those NS equations are being solved. I remember there're suggestion about what type of inlet/outlet boundary condition you can use. For a simple flow, you need a driving force, say static pressure drop, for flow to occur. Unrealistic boundary condition gives unrealistic solution. You need to give solver some idea about the conditions at outlet to get a "unique" solution.

 March 8, 2006, 23:44 multiphase domain #11 Avillyd Guest   Posts: n/a HI.. I'm doing simulation in engine chamber now and i found some difficulties. I appreciate whoever can help me to solve the problem. Here is the problems i've faced. 1) How to define two material in a chamber. i not found the solution to define inlet air as air idea gas and inlet fuel as fuel. What i have is, Cfx define both condition is already in a chamber. What i need is they mixing after we define air anfd fuel are come in. Can anybody help me? i really need to solve this problem immediately.. tq

 March 14, 2006, 07:33 Re: Boundary conditions #12 M Guest   Posts: n/a Hi , I understand what you are trying to say, how about if you have your pipe in a box or enclosure. Instead of having an outlet have a hole so that flow will be coming into the inlet and then out of a hole into some sort of enclosure(box) maybe you could set a pressure condition on one of the far faces of the enclosure and this way you can determine the pressure at the hole. M

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