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March 18, 2019, 13:05 
pressure inlet/outlet BC

#1 
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ali
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Hi all,
Is it ok if I set pressure at both inlet and outlet boundaries, and do NOT specify the mass flow rate? A CFD expert told me once that is not acceptable because the solution would not be unique, while I confirmed with ANSYS rep and they said it is ok and there is nothing wrong with that. The solution would be unique either way. Any guidance is much appreciated. 

March 18, 2019, 13:16 

#2  
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Filippo Maria Denaro
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Quote:
For incompressible flow you can set the pressure difference between inlet and outlet even if prescribing an absolute value for the pressure is not relevant for the solution. You would get the same solution with different values provided that the difference is the same. 

March 18, 2019, 13:20 

#3 
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ali
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Thanks Filippo,
So it is not like that multiple solutions may exist for the same pressure difference? right? 

March 18, 2019, 13:24 

#4 
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Filippo Maria Denaro
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March 18, 2019, 13:32 

#5 
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ali
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Assume the problem is steady. And dp is fixed. would there exist more than 1 solution to the velocity field inside the domain?


March 18, 2019, 13:43 

#6  
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Filippo Maria Denaro
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Quote:
Not for the velocity but for the pressure this issue exists indipendently from the steady/unsteady case. A specific flow problem where multiple solutions in the velocity can appear is the case of one inlet and two outlets 

March 18, 2019, 13:46 

#7 
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ali
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can you explain why?
I am interested to learn more. 

March 18, 2019, 13:52 

#8 
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Filippo Maria Denaro
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why about what, the pressure? It is very simple, in the incompressible flow model the pressure acts only in the momentum equation by means of the gradient. There is no pressure law that defines one operating value. Therefore any function of time added to the pressure field produces the same velocity field. 

March 18, 2019, 14:16 

#9 
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ali
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So it is better to say that we are allowed to have pressure inlet and pressure outlet in a domain with one inlet and outlet. But we are not allowed to have velocity inlet and velocity outlet combination for boundaries of the same domain, since many pressure solutions might exist.
Am I correct? 

March 18, 2019, 14:29 

#10 
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Filippo Maria Denaro
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We must ensure that the problem is mathematically well posed, prescribing on a boundary either the normal component of the velocity or a condition for the pressure (Dirichlet or Neumann).
The pressure field is however always determined up to a time function. In case of multiple outlets you must prescribe the mass flow rate to determine a single solution for the velocity. 

March 18, 2019, 14:43 

#11 
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ali
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Maybe you did not understand my question fully.
Is it mathematically well posed if we have velocity at the inlet and outlet(incase of one inlet and one outlet). Or is it illposed? Having velocity at both BCs is Dirichlet type BC. If it is wellposed, then would there be more than one solution to the pressure field.(Assuming the flow is steady state) 

March 18, 2019, 14:47 

#12  
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Filippo Maria Denaro
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Quote:
As I wrote above, it is correct to prescribe the velocity BC instead of the pressure. No matter about that, the pressure is determined up to a constant and the velocity has a single solution. See https://books.google.it/books?id=RvB...%20CFD&f=false 

March 18, 2019, 16:48 

#13 
Senior Member

Point is that Fluent doesn't actually fix the static pressure at inlet, but the total one, which includes the dynamic pressure. Try running the same case with two pressure outlet (both at inlet and outlet) and see what happens.


March 19, 2019, 08:40 

#14  
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Lucky
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There's nothing wrong with using a pressure inlet and pressure outlet if those are your boundary conditions.
There's also nothing wrong with a velocity inlet and velocity outlet mathematically. I question how a velocity field gets imposed at an outlet physically but mathematically this is allowed. Quote:
For incompressible flows only the pressure gradient matters. There are always infinitely many pressure fields (+/ a constant or +/ a function of time) that will satisfy a given velocity field. But that's not a problem and nobody cares. It just means any pressure gauge will work. The pressure field is unique up to a constant (up to a gauge). In Fluent this translates to you can use any value in the operating pressure and get the same flow and pressure field (because Fluent solves for the gauge pressure field and not the absolute Field). 

April 9, 2019, 03:14 

#15 
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Steve
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If you make your B.C for pressure inlet and outlet (static)
That problem will be indeterminate. You should give more B.C for example reference pressure at the point. (You can get many solutions, if you change I.C in your problem) 

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