# Defining Boundary Conditions in GAMBIT

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 April 11, 2008, 12:51 Defining Boundary Conditions in GAMBIT #1 Tim R Guest   Posts: n/a Hello, I am doing a study on a free jet exiting a 2D orifice into an open space. The pipe length, diameter are all defined and have associated boundary conditions. The open space is the same. I created an outlet pipe and want to create a suction force on the end, to essentially pull the jet. I was wondering if I could define the outlet as an exhaust fan to create the suction force, or if I just need to create a pressure gradient between the inlet and outlet pipes. Any help would be greatly appreciated, and I can give further details if needed. Thanks. -Tim

 April 12, 2008, 20:58 Re: Defining Boundary Conditions in GAMBIT #2 AAA Guest   Posts: n/a Hi If you know the mass flow rate at the outlet (usually equal to that at the inlet), specify the outlet BC as a "mass flow inlet" with an opposite sign. Choose "Direction vector" as the "direction specification method" and the proper direction in the "X-, Y- Component of flow direction" to have the flow exit from the outlet. Regards AAA

 April 13, 2008, 17:33 Re: Defining Boundary Conditions in GAMBIT #3 Tim R Guest   Posts: n/a I don't see where to specify the "Mass Flow Inlet" with an opposite sign. Do you mean have a negative mass flow rate? The flow rate has to be a positive value in fluent, otherwise errors occur. Also, should I set the upstream inlet as a mass flow inlet as well, or as a velocity inlet? How important is the "Supersonic/Initial gauge pressure" when setting the mass flow inlet for the downstream BC's? Also, I don't think the mass flow rate in equals the mass flow rate out, because there is a large open space for the jet to spread, followed by the outlet tube, where the suction occurs. Thanks for the help, Tim

 April 14, 2008, 04:30 Re: Defining Boundary Conditions in GAMBIT #4 AAA Guest   Posts: n/a Trust me in this Tim, it works! I use it all the time and it is "computationally" acceptable too. I presume you don't know the velocity or the pressure at the outlet. So specifying the mass flow rate is your best bet. Specify the inlet as a velocity inlet (if you know the velocity there) or as a mass flow inlet (if you know the mass flow rate). As for the outlet, 1) choose Mass flow inlet as the BC, 2) specify the mass flow rate (of course with a positive sign), 3) specify the x- and y- components to give you a vector pointing out of the domain at the boundary (i.e., if the boundary is vertical, and the flow is hitting it from the left, set the x- component to 1 and the y- component to 0). If you are not dealing with a supersonic flow, leave the "Supersonic/Initial gauge pressure" setting as it is. If the mass flow "in" is higher than the mass flow "out", the pressure in the domain will be high, and vise versa. Please get back to me if this gives you and problem! AAA

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