# what boundary condition should i set

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 March 14, 2013, 00:01 what boundary condition should i set #1 New Member   saurabh joshi Join Date: Feb 2013 Posts: 12 Rep Power: 6 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

 March 14, 2013, 01:35 #2 Senior Member   SSL Join Date: Oct 2012 Posts: 226 Rep Power: 8 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).

 March 14, 2013, 02:15 #3 New Member   saurabh joshi Join Date: Feb 2013 Posts: 12 Rep Power: 6 Dear Mr Saeed Sadeghi 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. plz advise regards Saurabh joshi

 March 14, 2013, 02:20 #4 New Member   saurabh joshi Join Date: Feb 2013 Posts: 12 Rep Power: 6 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. plz advise regards saurabh Joshi

 March 14, 2013, 02:21 #5 Senior Member   SSL Join Date: Oct 2012 Posts: 226 Rep Power: 8 In these problems, it is important to set overal pressure of the combustion chamber as operating condition. Then set outlet pressure as zero.

 March 14, 2013, 03:24 #6 New Member   saurabh joshi Join Date: Feb 2013 Posts: 12 Rep Power: 6 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 plz advise regards saurabh joshi

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