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Boundary Conditions : Total Pressure or Velocity 

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February 12, 2010, 16:36 
Boundary Conditions : Total Pressure or Velocity

#1 
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Hello,
I have to simulate a curved pipe. Here are the caracteristics of the flow : Incompressible Re = 40 000 Hydraulic diameter = 0.08m So we found U characteristic = 15.7m/s k = 0.3676 epsilon = 0.3328 (with a turbulent intensity of 6, don't remember the characteristical length used to calculate this) Since I begin in CFD, I've asked help to a professor wich told me that I should impose these BC : Outlet = Ambient pressure Inlet = Total pressure To calculate the total pressure, we used empirical relation for pressure loss and we find pstatic by adding the pressure loss to the outlet pressure and we just have to add rho u² to have total pressure (u characteristic is used for u) But in all the examples I found over the web, openfoam, ... BC used are always : Inlet : velocity Outlet : static pressure I don't understand why I have to fix different BC for my case. My professor told us that this was more physical and it probably will cause less numerical problems but he is used to work in compressible flows and said he was not familiar with incompressible solver. Also in openfoam as well as in Fluent, the BC with the total pressure seem to cause problem (convergence in openfoam) When I fix the BC with the velocity @15.7m/s at the inlet, my total pressure is quite different from the one we calculate with empirical formulas and I don't understand why (We found 102 Pa total pressure and the numerical simulation with velocity inlet give us 500 Pa in fluent) Can someone help me to solve my problem with the boundary conditions with the total pressure? Our professor told us it could be the mesh ... Thanks for the help!! 

February 28, 2011, 16:37 

#2 
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Mark Beal
Join Date: Feb 2011
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Did you find the solution to this?
Mark 

February 28, 2011, 21:18 

#3  
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Santiago Marquez Damian
Join Date: Aug 2009
Location: Santa Fe, Santa Fe, Argentina
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The differences are probably due meshing issues, you need to satisfy y+ parameters in near wall regions in order to use kepsilon model appropriately (check these Guidelines). Once you have a good mesh then results for pressurepressure and velocitypressure should be slightly different for Re outside of laminar region (region of validity of kepsilon model and short development zone) Regards.
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Santiago MÁRQUEZ DAMIÁN, Ph.D. Research Scientist Research Center for Computational Methods (CIMEC)  CONICET/UNL Tel: 543424511594 Int. 7032 Colectora Ruta Nac. 168 / Paraje El Pozo (3000) Santa Fe  Argentina. http://www.cimec.org.ar 

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