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March 18, 2013, 05:53 
Opening Boundary Condition

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Xingyuan Chen
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I want to simulate a flow domain having such a boundary, where the pressure value is known, and the fluid can flow into or out of the domain at this boundary. It means this boundary can be an inlet or outlet boundary conditions with fixed pressure value. I am wondering how to treat the pressure and velocity on this boundary, since the flow direction can be both in and out. Is it correct that I treat the boundary as normal inlet pressure static BC when the fluid flows in and as normal outlet pressure static BC when the fluid flows out? I have found such a boundary condition in the software CFX, which is called Opening Boundary Condition. I am wondering the principle and mathematical background of this boundary condition. However, I could not find any details in the CFX manual. Could anyone tell me some papers or books about this kind of boundary condition? Thanks. 

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March 18, 2013, 09:00 

#2 
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Filippo Maria Denaro
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I don't understand your flow problem...what about your computational domain and the forces driving the main flow?


March 18, 2013, 09:30 

#3 
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Xingyuan Chen
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There is a movable part in the domain, which makes the volume of the whole domain changeable. When the volume decreases, the fluid flows out through this 'opening boundary'. When the volume increases, the fluid flows in through this 'opening boundary'.


March 18, 2013, 11:00 

#4 
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Filippo Maria Denaro
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if the flow is modelled as incompressible, you can set also some bc. on the velocity instead of pressure, for example setting a vanishing second derivative. If the flow is fully compressible, you need some more complex nonreflecting bc.s.


March 18, 2013, 13:54 

#5  
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Xingyuan Chen
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Quote:
I am simulating an incompressible fluid. But I wondering why we can set a vanishing second derivative of the velocity as a boundary condition. What is the physical meaning of it? 

March 18, 2013, 14:01 

#6  
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Filippo Maria Denaro
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Quote:
For incompressible flows you can prescribe either velocity or pressure to have the problem well posed. What the physics says you about the inlet/outle? I don't think you know the velocity profile or the pressure gradient in time ... therefore some "free condition" on the velocity is better to let the flow to go in and out. You cna also prescribe some fixed pressure value if you have two large tank at the end... What is really required is that you enforce correctly the divergencefree constraint 

March 19, 2013, 00:05 
Open boundary condition

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Jonas T. Holdeman, Jr.
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Quote:


March 19, 2013, 07:11 

#8  
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andy
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Quote:
I am not familiar with the current set of CFX boundary conditions and so can offer little help here. 

March 19, 2013, 07:14 

#9  
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Xingyuan Chen
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Quote:
Do you also know the mathematical description of such boundary condition? 

March 19, 2013, 07:20 

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Xingyuan Chen
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March 19, 2013, 11:08 
open boundary condition

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Jonas T. Holdeman, Jr.
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I thought about this a little more and did a Google search on open boundary conditions. Most of the hits seemed to involve atmospheric or oceanic flows and radiation, and the methods seem mostly heuristic. My examples involve a specific method, but perhaps there are general considerations.
Quote:
In my case, the equations for the nodes on the exit face are only partially assembled because here are no elements outside. This may be called a "do nothing" boundary condition. I am not sure why it works, but it does. Any mathematicians out there? This open condition has further utility. Consider flow over a cylinder. One usually uses a mesh wide enough that the effects of truncation of the space might be neglected. Usually one has to compromise between computational work and accuracy. But suppose you leave the top and bottom of the mesh open as well. Then the fluid could expand outside the mesh near the cylinder, and then reentering downstream, reducing the influence of truncation without additional work. This is shown for Re=50 in the figure, where the top, bottom and right end are open, but net flow through the top and bottom constrained to be zero. 

March 19, 2013, 17:46 

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Yes zero gradient is an option, from a physical point of view it indicates that the velocity is the same on each side of the boundary. A better one is, as Filippo stated it, second derivative of the velocity =0. In this case you specify that you want to keep the same gradient of the velocity on each side of the boundary. This BC is less constraining for the solution.


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