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Static Pressure Inlet - Static Pressure Outlet |
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#1 |
New Member
Join Date: Feb 2016
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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 pressure-pressure condition (both static), it only converges when a zero-gradient 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 ![]() |
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#2 |
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duri
Join Date: May 2010
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Solution will not converge when Neumann boundary condition is used for a variable at all boundaries. Static pressure-static 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.
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#3 | |
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Filippo Maria Denaro
Join Date: Jul 2010
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Quote:
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? |
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#4 |
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Thanks Duri and Filippo for your comments. I am using a collocated explicit MacCormack (predictor-corrector) scheme for incompressible flow with pseudo-compressibility coupling for pressure. The BCs that converge for pressure-pressure 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 pressure-pressure condition in CFX without zero-gradient condition so many times, so there should be a way for convergence. |
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#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
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#6 |
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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?
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#7 | |
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duri
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#8 | |
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Filippo Maria Denaro
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Quote:
https://www.researchgate.net/publica..._viscous_flows |
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#9 | |
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#10 |
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Filippo Maria Denaro
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compressible or incompressible???
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pressure-pressure bc |
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