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-   -   what boundary condition should i set (http://www.cfd-online.com/Forums/fluent/114607-what-boundary-condition-should-i-set.html)

 saurabhjoshi2011 March 14, 2013 00:01

what boundary condition should i set

dear friends
my problem involves air flow at inlet with static pressure of 8 bar, total temperature of 572 k, velocity of 130 m/sec and constant density of 5.011kg/m3.
what should i set as Supersonic/Initial gauge pressure in the mass flow inlet BC.
as i dont know the outlet BC, and my aim is to calculate them(pressure,temperature,velocity,massflow rate) should i select OUTFLOW as the outlet BC.
what does exactly supersonic/initial gauge pressure means and why during hybrid initialization this pressure is not getting recognised by Fluent.
plz help
saurabh

I assume your problem is incompressible.
- So, supresonic initial guage pressure is not important. Forget it.
- For the outlet, It's better to use pressure outlet boundary condition with a pressure that is near the real outlet pressure.
- In incompressible flows, the only important thing is the pressure difference not the local exact pressure (except in reactions).

 saurabhjoshi2011 March 14, 2013 02:15

regards
If i do not specify the Supersonic/initial gauge pressure , then from where does the Fluent calculate the outlet pressure.
actually i can specify the outlet pressure as 101325 pascal, that is for the begining but when flow of 8 bar takes place from the inlet, the exit pressure must be higher than what i had set of101325 bar.
ok how do we calculate the pressure drop from the inlet to exit
actually it is a combustion chamber which we are modelling.
yes the flow is subsonic,however on running the iterations the convergence history of static pressure at inlet is showing some 5 bar and at exit around 1bar.
should i set outflow as the exit BC?
in case when the parameters are not known before hand setting up of outflow BC is recommended.
regards
Saurabh joshi

 saurabhjoshi2011 March 14, 2013 02:20

Dear sir
in your post you replied that
For the outlet, It's better to use pressure outlet boundary condition with a pressure that is near the real outlet pressure
does it means that i use a guessed pressure lets say around 8 bar
as such the combustion is supposed to be at constant pressure.
however, there will be pressure losses during the flow and the presssure will be less.
does using a pressure which is near the real outlet pressure(which i dont know beforehand) will not amount to forcing the conditions/forcing the solution.
regards
saurabh Joshi

In these problems, it is important to set overal pressure of the combustion chamber as operating condition. Then set outlet pressure as zero.

 saurabhjoshi2011 March 14, 2013 03:24

Sir
my problem has constant density
in that case the operating pressure is not used and i had set it to zero
i could set operating pressure to the average pressure as you suggested and set the outlet pressure to zero, actually it is the other way of doing the same thing.by this we are just specifying the pressure and the system is not calculating the pressure.
actually what we just know is the inlet pressure. the outlet pressure fluent has to tell us so that we can know what is the pressure drop which has occured due to the geometry of the combustion chamber, which will help us to redesign the chamber