# The effects of BCs and the initial conditions on the final results

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 August 27, 2013, 22:31 The effects of BCs and the initial conditions on the final results #1 New Member   mj Join Date: Apr 2011 Posts: 24 Rep Power: 7 Dear Friends Does anyone know any paper which investigated the boundary conditions or initial condition effects on final results of two phase flow simulation? I have simulated ship motion and its wave propagation by Fluent software with different BCs (velocity inlet or pressure inlet) and initial conditions (zero velocity or equal inlet velocity). The results of the simulations are completely different. In addition I did simulate by MRF approach and stationary, again the results are different. Thanks for your help in advance

 August 28, 2013, 00:46 #2 Member   Jingchang.Shi Join Date: Aug 2012 Location: Xi'an, China Posts: 46 Rep Power: 6 based on theoretical knowledge of your focused problem, you can select suitable BC. I never saw any papers on this kind of research. So, maybe to take time to study theoretical knowledge is what you should do. My advice.

 August 28, 2013, 03:53 #3 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 2,503 Rep Power: 31 ...but theoretically I see correct that different BC and initial conditions drive to different solutions

 August 28, 2013, 08:37 #4 Member   Join Date: Jul 2013 Posts: 49 Rep Power: 5 Assuming the uniqueness of the steady-state solution for 3d N-S (which there is no proof for that fact to be true...) the initial condition should not have an influence on the steady-solution. That being said, the transient domain will obviously be altered by a different initial condition. In my opinion, there is no reason to think different boundary conditions will lead to the same solution, unless you specify an equivalent boundary. As exemple : after running a velocity inlet solution, you could export the pressure field on the inlet, and specify a pressure inlet with that pressure field, which should therefore lead to the same solution.

August 28, 2013, 10:39
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duri
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Quote:
 Originally Posted by mrji8011 I have simulated ship motion and its wave propagation by Fluent software with different BCs (velocity inlet or pressure inlet) and initial conditions (zero velocity or equal inlet velocity).
Velocity inlet BC and pressure inlet BC are different in fluent, it preserves velocity and total pressure respectively. So difference in results are expected.
Initial condition will not lead to different solution. But the path of convergence will get affected. Probably one of the conditions is not properly converged. (For convergence, don't just go by residual, some times residual settle faster than the flow quantities)

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