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Static Pressure Inlet  Static Pressure Outlet 

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August 26, 2016, 18:25 
Static Pressure Inlet  Static Pressure Outlet

#1 
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Hello,
My FDM code simulates Backward Facing Step flow when I use conventional BCs such as defining velocity profile at inlet and fully developed condition at outlet. I have validated the results and it seems functioning correctly. However, when I want to impose a pressurepressure condition (both static), it only converges when a zerogradient condition is selected for inlet. When I try to use mass balance to update inlet normal velocity at each iteration, the solution eventually blows up. I am integrating outflow and use the mean velocity as a uniform inlet velocity. Also I am interested to know if there is any difference to use momentum balance and mass balance to update inlet normal velocity. Has anyone faced this problem before? I appreciate your help 

August 26, 2016, 22:22 

#2 
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duri
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Solution will not converge when Neumann boundary condition is used for a variable at all boundaries. Static pressurestatic pressure boundary condition has no meaning, one of the boundary condition should contain velocity. When four equations are solved four variables should be specified as Dirichlet boundary condition.


August 27, 2016, 03:58 

#3  
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Filippo Maria Denaro
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Well, you should provide more details... 1) are you using Neumann at walls and Dirichlet at inflow and outflow? what about the pressure difference ? 2) are you using staggered or colocated grid? 3) how do you discretize the pressure equation? 

August 29, 2016, 23:40 

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Thanks Duri and Filippo for your comments. I am using a collocated explicit MacCormack (predictorcorrector) scheme for incompressible flow with pseudocompressibility coupling for pressure. The BCs that converge for pressurepressure are listed below:
@inlet: p=p_in , du/dx=0 , dv/dx=0 @outlet: p=p_out , du/dx=0, dv/dx=0 @walls: dp/dy=0 (or the extended version) , u=0, v=0 The BCs that won't converge: @inlet: p=p_in , u=U_in , v=0 @outlet: p=p_out , du/dx=0, dv/dx=0 @walls: dp/dy=0 (or the extended version) , u=0, v=0 I find "U_in" from mass conservation at the end of each iteration and update it. I have used pressurepressure condition in CFX without zerogradient condition so many times, so there should be a way for convergence. 

August 30, 2016, 02:49 

#5 
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Filippo Maria Denaro
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The error is that you can not prescribe both pressure and velocity as Dirichlet bc


August 31, 2016, 19:33 

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Thanks Filippo. I did not know about this. Could you kindly introduce me a book or paper that discusses what kind of Dirichlet and Neumann conditions go well together?


August 31, 2016, 22:59 

#7  
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duri
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September 1, 2016, 03:35 

#8  
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Filippo Maria Denaro
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Quote:
https://www.researchgate.net/publica..._viscous_flows 

September 2, 2016, 02:29 

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September 2, 2016, 05:19 

#10 
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Filippo Maria Denaro
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compressible or incompressible???


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