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_______pressure----please help

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Old   July 18, 2006, 04:12
Default _______pressure----please help
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I want to keep inlet pressure fixed and outlet pressure equal to atmosphereic pressure. I use inlet pressure and inlet mass flow rate at outlet pressure. I hooked an udf so that I can get atmospheric pressure by adjusting mass flow rate. But when I use mass flow inlet at outlet then face pessure F_P(f,t) and cell pressure C_P(c0,t0) adjacent to face at outlet is equal.Which is not physical. can any body tell what is the problem. All pressures are total pressures. I can use inlet and outlet pressure but in that case solution diverges so want to control the solution by adjusting massflow rate. This is why I am using mass flow inlet at outlet. Thanks in advance. mech
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Old   July 18, 2006, 08:58
Default Re: _______pressure----please help
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I don't think that what you try to do is right. Keep your first inputs with inlet and pressure outlets. Define correctly the operating pressure (define->operating conditions). Be aware that all your inputs in Fluent about the pressure is a gauge pressure and not the total pressure (total pressure=gauge_pressure+operating pressure). Furthermore you must NOT include the hydrostatic pressure in the gauge pressure because Fluent includes it automatically in the body force if it's an incompressible flow (have a look at Fluent manual about pressure outlets and inlets.....)

If the solution still diverges, you should review your scaled residuals, your mesh, discretization schemes.

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Old   July 18, 2006, 21:12
Default Re: _______pressure----please help
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Hi Fabrice Thank you for your good advice. I am simulating 2D micro parallel channel flow. It is laminar but compressible flow. I am using Nitrogen gas. In my case the channel width is 1.2 micron. when I use 12 mesh in normal direction to the flow it is converging. But when I use 23 mesh then it is diverging.This is my problem. So I wanted to fix inlet pressure and to initialize the solution from outlet with outlet pressure and mass flow rate so that I can fix outlet pressure by adjusting mass flow rate so that I can control the divergency. In my case gause pressure=total pressure because I am using operating pressure 0. To do the above I need staggered grid. I do not know how to use staggered grid in fluent. If you have any idea please suggest me. Mech
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