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Old   May 15, 2002, 11:28
Default Help on prescribe pressure
  #1
Derek Jing
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I know in prescribing pressure-outlet B.C., the inputed gauge pressure value should be relative to the pressure value defined in the operating condition panel.

However, when I prescribe the initial value for the whole flow field, the value inputed in the initiation panel should be absolute value or relative value?

Thank you very much.

Derek Jing
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Old   May 15, 2002, 11:58
Default Re: Help on prescribe pressure
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bala
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The value in the initiation planel should be relative to operating pressure(gauge pressure).

bala.
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Old   May 16, 2002, 00:57
Default Re: Help on prescribe pressure
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cwflying
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Then if the media density is constant (in this case, operating pressure is not used at all). How to prescribe the gauge pressure under pressure-outlet boundary conditions?
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Old   May 16, 2002, 20:23
Default Re: Help on prescribe pressure
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Jin-Wook LEE
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Then set the value as your arbitary value, e.g. zero. And consider it as reference value and the computed pressure field is relative value to that of your arbitary value.

Sincerely, Jinwook

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Old   May 17, 2002, 03:04
Default Re: Help on prescribe pressure
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cwflying
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You mean I might set zero value to the "operating pressure"? or let the pressure at "pressure_outlet" panel as zero value? Thanks!
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Old   May 18, 2002, 15:23
Default Re: Help on prescribe pressure
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Peter
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If you are considiring the flow as incompressible, what you should do is using outflow boundary condition instead pressure outlet boundary condition. If your model has several outlets, you have to prescribe the the percentage of the mass flow going to each outlet.

Cheers
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Old   May 19, 2002, 10:23
Default Re: Help on prescribe pressure
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cwflying
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but you can not describe the percentage before carrying out the simulation. In this forum,it is recommended to apply pressure-outlet conditions under this case. But the detail is not given(when in terms of the incompressible flow and gague pressure) Thanks anyway!
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Old   May 19, 2002, 12:11
Default Re: Help on prescribe pressure
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Peter
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Outlet pressure is important when you are running compressible flow calculations. In those cases you need to specify conditions for the pressure at inlet and outlet of your model. What you could do is run your model with zero gauge pressure at the outlet and then run it again with a different valu of gaug pressure. Then you can see how sensible you model is with regards to the gauge pressure specified at the outlet.
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Old   May 19, 2002, 21:44
Default Re: Help on prescribe pressure
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Jin-Wook LEE
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If you have only one outlet, you can use both of 'PRESSURE OUTLET' and 'OUTFLOW'. Both results give nearly same if your outlet is located far down stream.

If you have multiple outlets, you should be careful of selecting the boundary type. But first of all, you should know one of 'pressure' or 'the ratio of the mass flow rate' at each outlet. For the first case(pressure is known for each outlet), then you use 'pressure outlet'. For the other case, you use 'outflow.

If you do not know both(pressure or mass flow ratio), then you do not have enough boundary condition. So you have only two choices. Assume the value(pressure or mass flow ratio), or give up the simulation. Recall that CFD is based on the BVP(Boundary Value Problem).

For incompressible flow with constant density, when you are using pressure-outlet, you can set any arbitary value for both of operating pressure and pressure-outlet gauge pressure.

Sincerely, Jinwook

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Old   May 21, 2002, 14:07
Default Re: Help on prescribe pressure
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Alammar
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Is this case steady and/or compressible?
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Old   June 6, 2002, 09:24
Default Re: Help on prescribe pressure
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lingo
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What's the meaning of "outlet is located far down stream"? How far
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