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-   -   operation pressure and pressure outlet BC (http://www.cfd-online.com/Forums/fluent/96799-operation-pressure-pressure-outlet-bc.html)

Lilly February 1, 2012 08:34

operation pressure and pressure outlet BC
 
Dear all,

I am a bit confused concerning the operation pressure and the pressure as outlet boundary condition.
I want to simulate the flow of an incompressible blood-contrast agent mixture in a vessel geometry with several outlets. Is it better to leave the operation pressure at the default value of 101325 Pa or to set it to 0Pa in this case (I found both suggestions during my search)? And what is the concrete meaning of the operation pressure?
My plan was to do a simulation without contrast agent (just blood) at the beginning and set the outflow boundary condition which I estimated by the diameter of my vessels. Afterwards I wanted to check the pressure at the outlets and set these pressures as boundary condition for the main simulation then (since I have a mixture model, I shouldn’t use the outflow bc). But I am not sure about which kind of pressure…Is it the absolute pressure (If I set p(operation)=0 Pa it must be p(absolute)=p(gauge), I guess)? Or is it the static pressure? Or is the static pressure equivalent to the absolute pressure for p(operation)=0 Pa?
I read several threads and in the fluent manual, but got more and more confused.
It would be nice, if somebody could help me!
Thank you,
Lilly

ANTONY A R V February 1, 2012 09:37

Absolute pressure (above atmosphere)= Atmospheric pressure + Gauge pressure
Absolute pressure (below atmospheric) = Atmospheric pressure - vacuum pressure
The operating pressure will be atmospheric pressure if the flow process takes place in the standard atmospheric condition. If the flow process takes place anywhere other than this condition, we have to change the operating pressure.
Total pressure = Static pressure + velocity pressure
The total pressure and static pressures are relative to the operating pressure value given in the operating condition panel.
If the operating condition are already mentioned we can give the value of atmospheric pressure as 0 if the condition is atmospheric and any other value can be given relative to atmospheric pressure.

trong-khoa February 1, 2012 10:03

Hi Lilly,
I think Antony explained very clearly about the pressure in Fluent. I just add an information important cited in User Guide: "All pressures that you specify and all pressures computed or reported by FLUENT are gauge pressures."

For example, we use often "pressure outlet=0", it means the pressure at outlet is equal the "pressure operation" or "the pressure where the simulation take place"(usually atmospheric pressure"
Good luck for you, Lilly

sumeet kotak May 5, 2013 07:16

Hello friends

As discussed in earlier discussion I little bit clear with the idea of operating condition

But my question is suppose I have cylindrical chamber and want vacuum inside the chamber so I have to set operating pressure as 0.005 pa. it means my whole computational domain set to 0.005Pa which is vacuum and any species transport or reaction taking place at this pressure only..

Am I right?

Replay please it can help me to solve my problem....

Thanks in advance

ANTONY A R V May 8, 2013 09:57

pressure detail
 
The case you have discussed will be for a single species and single phase system

In species transport and reactions, each species will have a mass/mole fraction and corresponding partial pressures if it is gaseous. The resultant pressures will be total pressure of the system.

regards

Antony


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