# Some Problems about the Boundary Conditions in OpenFoam

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April 16, 2017, 10:50
#41
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 Originally Posted by piu58 > total pressure = 6 Bar I think you mean the pressure difference between inlet and outlet. You may set this difference and calculate the velocity or set the velocity and calculate the pressure difference. If you have measured values and the results of your calculation differ, you have a good example to learn how OF needs to be set and which degree of accuracy can be reached with fluid simulations. Please keep in mind: When simulating incompressible, OF takes the kinematic pressure.
thank you so much. Please can you tell me why time step continuity error increases very rapidly. Because of that solver terminates as floating point error.

 April 16, 2017, 14:34 #42 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15 If you need more help we need your model (all files). __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 April 17, 2017, 12:47 #43 New Member   Bharadwaj Bhushan Join Date: Mar 2017 Posts: 19 Rep Power: 9 https://drive.google.com/drive/folde...00?usp=sharing https://drive.google.com/drive/folde...00?usp=sharing thank you for that Mr. Uwe Pilz Please take a look at the link of google drive where you can get the files I used. The parameters I know are inlet velocity - 1.46 m/s inlet static pressure - 6 Bar fluid - water and I used the dynamic viscosity of water at room temperature. According to the experiment the outlet velocity should be 20 m/s. I am solving for steady state using kOmega, but kEpsilon is also ok for me. with that settings I am getting 35 m/s and negative pressure. Don't know why. Please tell me what is the problem. Cheers Bharadwaj

 April 17, 2017, 13:44 #44 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15 As far as I can see it at a quick view your boundary conditions are contradicting: You give both pressures AND an inlet velocity. That cannot work. If you give both pressures, the simulation calculates the resulting velocity. If you give a velocity and a reference point for pressure the simulation calculates the pressure difference between inlet and outlet. You cannot have both. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

April 17, 2017, 13:51
#45
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 Originally Posted by piu58 As far as I can see it at a quick view your boundary conditions are contradicting: You give both pressures AND an inlet velocity. That cannot work. If you give both pressures, the simulation calculates the resulting velocity. If you give a velocity and a reference point for pressure the simulation calculates the pressure difference between inlet and outlet. You cannot have both.

Oh OK OK. thank you for the quick reply. But I tried by giving just velocity, still not getting the velocity of around 20 m/s. Can you set the BC's of all and run the file once, so that it will give correct results. Because I am stuck there.

 April 18, 2017, 01:04 #46 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15 If nobody else answers you have to wait for some days. I am jsut go for a short holiday. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

 April 22, 2017, 11:00 #47 Senior Member     Uwe Pilz Join Date: Feb 2017 Location: Leipzig, Germany Posts: 744 Rep Power: 15 I looked at your b.c. You used the combination of Code: `totalPressure` condition on pressure and Code: `pressureInletOutletVelocity` on velocity, which is thought for situations where pressure and velocity are unknown (mostly used for fluid / gaseous areas). If you use Code: `zeroGradient` for U at inlet and Code: `fixedValue` / some value but not zero for the pressure inlet your simulation runs. You get some value of U which corrsponds to the pressure difference. As I mentioned before you my set a value of U at the inlet instead and give a reference print for the pressure, you get the pressure difference which corrsponds to the velocity selected. __________________ Uwe Pilz -- Die der Hauptbewegung überlagerte Schwankungsbewegung ist in ihren Einzelheiten so hoffnungslos kompliziert, daß ihre theoretische Berechnung aussichtslos erscheint. (Hermann Schlichting, 1950)

October 10, 2017, 08:33
#48
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Quote:
 Originally Posted by piu58 As far as I can see it at a quick view your boundary conditions are contradicting: You give both pressures AND an inlet velocity. That cannot work. If you give both pressures, the simulation calculates the resulting velocity. If you give a velocity and a reference point for pressure the simulation calculates the pressure difference between inlet and outlet. You cannot have both.
Can I give both inlet pressure and inlet velocity? But no outlet conditions? DOF seems like it would be zero.