Boundary Conditions

Gearb0x · February 6, 2010, 12:17

Hello,

I'm new to fluent and I'm working on a case with a "U" pipe. I want to predict the pressure drop for a certain mass flow. I have problems with openfoam so I try to run the case on fluent to see where I did wrong with openfoam.

So I'd like to set the static pressure at the inlet & outlet
(I know, fluent has BC to match a mass flow but I'd like to compare results with OpenFoam wich doesn't have this type of boundary conditions so I do it this way)

In openfoam, there is no droplist for the boundary conditions and you can do whatever you want, it won't tell you if you do wrong things.
Here in fluent, I see that I can't set "static pressure" at the inlet but only total pressure and initial gauge pressure.
Why? Is it an error to only give static pressure @inlet & oulet? You absolutly have to set total pressure? Because in openfoam I set static pressure at the inlet & outlet and maybe my mistake is here so I'd like to know ...

I don't know if my question is very clear but thanks for your help!

jack1980 · February 7, 2010, 03:50

the initial gauge pressure is used for supersonic flows only

Gearb0x · February 7, 2010, 07:30

Ok thanks

but why isn't there an option "inlet static pressure" for boundary condition? Why am I forced to set "inlet total pressure" ?

jack1980 · February 7, 2010, 08:28

i think it has to do with the fysical aspect of your boundary condition. i can imagine a boundary at a fixed total pressure (like the atmospheric pressure or something). but i can't imagine a fysical boundary that can discriminate static pressure from dynamic pressure. a fysical boundary will just 'see' the total pressure?

anyway i'm not sure, just a gues ...

Gearb0x · February 7, 2010, 08:33

Thanks for the hint.

I see things like this : Imposing a static pressure difference between inlet/outlet and you will have a flow ... This is why I don't understand why you abslutly have to set total pressure... There must be a reason but I don't understand ...

In fact I try to compare fluent with openfoam because in openfoam you can set whatever you want as boundary conditions even things wich are completely wrong so I try to "copy/paste" fluent's BC to openfoam to be sure that my case is OK at boundary condition level.

joe_star · February 7, 2010, 19:37

Hmm....perhaps some manual calculations to get a rough idea of the pressure drops involved would be helpful. But yeah, thats how FLUENT works

February 6, 2010, 12:17	Boundary Conditions	#1
Gearb0x Senior Member Join Date: Nov 2009 Posts: 111 Rep Power: 16	Hello, I'm new to fluent and I'm working on a case with a "U" pipe. I want to predict the pressure drop for a certain mass flow. I have problems with openfoam so I try to run the case on fluent to see where I did wrong with openfoam. So I'd like to set the static pressure at the inlet & outlet (I know, fluent has BC to match a mass flow but I'd like to compare results with OpenFoam wich doesn't have this type of boundary conditions so I do it this way) In openfoam, there is no droplist for the boundary conditions and you can do whatever you want, it won't tell you if you do wrong things. Here in fluent, I see that I can't set "static pressure" at the inlet but only total pressure and initial gauge pressure. Why? Is it an error to only give static pressure @inlet & oulet? You absolutly have to set total pressure? Because in openfoam I set static pressure at the inlet & outlet and maybe my mistake is here so I'd like to know ... I don't know if my question is very clear but thanks for your help!

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February 7, 2010, 03:50		#2
jack1980 Senior Member Jouke de Baar Join Date: Oct 2009 Posts: 127 Rep Power: 16	the initial gauge pressure is used for supersonic flows only

February 7, 2010, 07:30		#3
Gearb0x Senior Member Join Date: Nov 2009 Posts: 111 Rep Power: 16	Ok thanks but why isn't there an option "inlet static pressure" for boundary condition? Why am I forced to set "inlet total pressure" ?

February 7, 2010, 08:28		#4
jack1980 Senior Member Jouke de Baar Join Date: Oct 2009 Posts: 127 Rep Power: 16	i think it has to do with the fysical aspect of your boundary condition. i can imagine a boundary at a fixed total pressure (like the atmospheric pressure or something). but i can't imagine a fysical boundary that can discriminate static pressure from dynamic pressure. a fysical boundary will just 'see' the total pressure? anyway i'm not sure, just a gues ...

February 7, 2010, 08:33		#5
Gearb0x Senior Member Join Date: Nov 2009 Posts: 111 Rep Power: 16	Thanks for the hint. I see things like this : Imposing a static pressure difference between inlet/outlet and you will have a flow ... This is why I don't understand why you abslutly have to set total pressure... There must be a reason but I don't understand ... In fact I try to compare fluent with openfoam because in openfoam you can set whatever you want as boundary conditions even things wich are completely wrong so I try to "copy/paste" fluent's BC to openfoam to be sure that my case is OK at boundary condition level.

February 7, 2010, 19:37		#6
joe_star New Member joe star Join Date: Oct 2009 Posts: 9 Rep Power: 16	Hmm....perhaps some manual calculations to get a rough idea of the pressure drops involved would be helpful. But yeah, thats how FLUENT works