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Derek Jing May 15, 2002 11:28

Help on prescribe pressure
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

bala May 15, 2002 11:58

Re: Help on prescribe pressure
The value in the initiation planel should be relative to operating pressure(gauge pressure).


cwflying May 16, 2002 00:57

Re: Help on prescribe pressure
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?

Jin-Wook LEE May 16, 2002 20:23

Re: Help on prescribe pressure
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

cwflying May 17, 2002 03:04

Re: Help on prescribe pressure
You mean I might set zero value to the "operating pressure"? or let the pressure at "pressure_outlet" panel as zero value? Thanks!

Peter May 18, 2002 15:23

Re: Help on prescribe pressure
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.


cwflying May 19, 2002 10:23

Re: Help on prescribe pressure
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!

Peter May 19, 2002 12:11

Re: Help on prescribe pressure
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.

Jin-Wook LEE May 19, 2002 21:44

Re: Help on prescribe pressure
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

Alammar May 21, 2002 14:07

Re: Help on prescribe pressure
Is this case steady and/or compressible?

lingo June 6, 2002 09:24

Re: Help on prescribe pressure
What's the meaning of "outlet is located far down stream"? How far

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