# the different types of boundary conditions in CFD

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 March 22, 2015, 04:49 the different types of boundary conditions in CFD #1 New Member   dengli Join Date: Mar 2015 Posts: 10 Rep Power: 2 I'm confused by the different boundaries in CFD,such as pressure inlet ,mass flow inlet and so on.Can somebody give me some brief description about this?or can send me some information about this.(books or papers).My email is dengli_njust@126.com Thank you.

 March 24, 2015, 11:36 #2 Senior Member   Matt Join Date: Aug 2014 Posts: 426 Rep Power: 6 This is a gross-oversimplification but... think of the equation a*x=12. Without specifying a, you can never solve for x, right? Boundary conditions are known values that you apply at the boundaries of your model to allow you to solve for x, basically. The different types of boundary conditions are there to allow you to model different physical scenarios. A mass flow inlet might be used for setting flow rate through a system of pipes. A pressure outlet might be specified at the end of the pipe to define downstream conditions (such as amibent). Velocity inlet... welll you get the idea. I do a lot of external aerodynamics so I might specify a velocity inlet (with a specific temperature, pressure and direction) for my freestream boundaries. Downstream I would put a pressure outlet so that the flow has somewhere to go. Then I might also specify a mass flow rate to set flow thorugh an engine. Does that answer your question? Also, I am not sure what your level of experience is but if BC's are confusing you its a good bet its not much. I would work some tutorials and things to get your bearings before jumping in with both feet.

March 24, 2015, 20:38
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dengli
Join Date: Mar 2015
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 Originally Posted by MBdonCFD This is a gross-oversimplification but... think of the equation a*x=12. Without specifying a, you can never solve for x, right? Boundary conditions are known values that you apply at the boundaries of your model to allow you to solve for x, basically. The different types of boundary conditions are there to allow you to model different physical scenarios. A mass flow inlet might be used for setting flow rate through a system of pipes. A pressure outlet might be specified at the end of the pipe to define downstream conditions (such as amibent). Velocity inlet... welll you get the idea. I do a lot of external aerodynamics so I might specify a velocity inlet (with a specific temperature, pressure and direction) for my freestream boundaries. Downstream I would put a pressure outlet so that the flow has somewhere to go. Then I might also specify a mass flow rate to set flow thorugh an engine. Does that answer your question? Also, I am not sure what your level of experience is but if BC's are confusing you its a good bet its not much. I would work some tutorials and things to get your bearings before jumping in with both feet.
I knew that there are general three types of equations need specify the different BCs,but we restrict to the hyperbolic equations. I am focusing on the compressible pipe flow .I specify the pressure inlet ,but I don't know if there are back-flows（because there are shocks or detonations propagating around the pipe). My question is if there are back-flows at the inlet ,the pressure-inlet BC can still work?or how can i implement this type of BC into my code？

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