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Old   June 17, 2005, 15:19
Default Pressure Boundary Problems
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I am using a pressure boundary at the inlet set to a prescribed pressure with the Environment option set to on, and all the other options set to default.

The problem is that the inlet pressure appears to be held by the outlet flowrate, not the pressure boundary itself. If I set the outlet flow to 0.1 kg/s the pressure boundary at the inlet is at a higher pressure than it's supposed to be. If I set the outlet flowrate to 0.38 kg/s, the inlet pressure looks OK.

The problem is that my outlet flowrate should only be 0.055 kg/s which causes the pressure at the inlet to be significantly higher than where I set the pressure boundary.

Can someone help me with this and help explain why the pressure boundary isn't holding the pressure I set it to??

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Old   June 21, 2005, 06:03
Default Re: Pressure Boundary Problems
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I guess you want to predict the right pressure drop (you know for example from experiments) at a given mass flow rate. As a rule: You can either impose mass flow (but in this case the pressure drop is a solution of the simulation) or the pressure drop (but in this case the mass flow is a result of the simulation). Since the results are very dependent on mesh size, turbulence model used (if any) and discretization scheme, you will need some time to find out the right combination of the cited things above (hence fine enough mesh, good wall resolution adapted to the right choice of the turbulence model and discretization scheme with as much as possible second -order contribution) to give you the results approaching at best the experimental data. As a rule of thumb: Make a good mesh, set y+ values to be remain between 30 and 60 when using a High-Re turbulence model, do not use UD as a differencing scheme for momentum, converge the results (0.001 may not be sufficient, especially if the number of unknowns is large).

Hope this helps...
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Old   June 21, 2005, 08:20
Default Re: Pressure Boundary Problems
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Thanks for the info 4xF! I'll think about this and talk with my professor about it too.
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