# Gravitational Acceleration Problem???

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 March 30, 2012, 10:58 Gravitational Acceleration Problem??? #1 New Member   Mehmet Ak Join Date: Jun 2011 Location: Turkey Posts: 6 Rep Power: 8 I just trying to model radiation in a box with one wall is hot and the opposite of that wall is cold. I wanna compare the results by using different radiation models (Rosseland , P-1 , DTRM ,S2S ). I also want to set the gravitational acceleration as 9.81 m/s2 but there ıs a note ın the tutorıal that ı m tryıng to learn .. That note says that '' The gravitational acceleration is adjusted to yield the correct dimensionless quantities for Prandtl , Rayleigh , and Planck numbers. '' And ın that tutorial ıt ıs been set to 6.94e-5 m/s2 and ıt made my mınd confused. whıch one do ı have to use ? 9.81 or 6.94e-5 ??

March 31, 2012, 16:18
#2
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Lucky Tran
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Quote:
 Originally Posted by fakerangel I just trying to model radiation in a box with one wall is hot and the opposite of that wall is cold. I wanna compare the results by using different radiation models (Rosseland , P-1 , DTRM ,S2S ). I also want to set the gravitational acceleration as 9.81 m/s2 but there ıs a note ın the tutorıal that ı m tryıng to learn .. That note says that '' The gravitational acceleration is adjusted to yield the correct dimensionless quantities for Prandtl , Rayleigh , and Planck numbers. '' And ın that tutorial ıt ıs been set to 6.94e-5 m/s2 and ıt made my mınd confused. whıch one do ı have to use ? 9.81 or 6.94e-5 ??
Use whatever gravity you want! Whatever is most meaningful. It is up to you! What are you testing? What areyour dimensionless numbers? And what parameters do you need to test it? Ask yourself that and then use the parameters that you need!

In their case, they changed gravity to match the correct dimensionless numbers. You can change many other parameters to get the same dimensionless numbers.

Dimensionless groupings are a basic consequence of the buckingham pi theorem, which should've been taught in an introductory fluid mechanics course. i.e. how to do scaled model testing.

 April 2, 2012, 06:07 #3 New Member   Mehmet Ak Join Date: Jun 2011 Location: Turkey Posts: 6 Rep Power: 8 Thanks for ur reply and sorry bec I dıdnt see ıt untıl now. I am working on Radiation Models on Fluent.. I want to compare radiation models by using a room.. I actually dont know whıch dimensionless numbers I have to use.. after I decided which one of them I will use , how wıll I change the gravity? And I wanna see the differences between laminar and turbulent flows at my project. But I don't know how to make the flow in the room laminar or turbulent.. which dimensıonless number I have to change? In my work , there is a room (2.78 m x 2.78 m x 2.30 m ) and this room is heating from the wall.. I m using 3D mesh and made it on Gambit. There is a hot wall and a cold wall and I want to see the temperature pattern , velocity vectors and contours of stream function as a result of my work.. Gosh , I have so many qouestions hope u can answer... Thanks for ur answers already.

April 2, 2012, 08:10
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Lucky Tran
Join Date: Apr 2011
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Quote:
 Originally Posted by fakerangel Thanks for ur reply and sorry bec I dıdnt see ıt untıl now. I am working on Radiation Models on Fluent.. I want to compare radiation models by using a room.. I actually dont know whıch dimensionless numbers I have to use.. after I decided which one of them I will use , how wıll I change the gravity? And I wanna see the differences between laminar and turbulent flows at my project. But I don't know how to make the flow in the room laminar or turbulent.. which dimensıonless number I have to change? In my work , there is a room (2.78 m x 2.78 m x 2.30 m ) and this room is heating from the wall.. I m using 3D mesh and made it on Gambit. There is a hot wall and a cold wall and I want to see the temperature pattern , velocity vectors and contours of stream function as a result of my work.. Gosh , I have so many qouestions hope u can answer... Thanks for ur answers already.
Sorry, I have a firm believer of the pre-analysis philosophy. You should research and find all those dimensionless numbers yourself and familiarized yourself with your own topic. Otherwise, why are you doing 3D CFD? The answers to all your questions will also help you understand the limits of what you can do.

You cannot mesh up a geometry without knowing these dimensionless numbers beforehand, since they determine what the flow looks like and will determine whether or not your mesh is "good enough".

Also, the dimensionless number that tells whether the flow is laminar or turbulent is well-known and can easily be found through any search engine. Do your homework! We are not here to do your homework for you, you must do [most] of it.

I will give you some hints and abbreviations:
Gr, Ra, Pr
are the two dimensionless numbers you need to lookup. You should also look up,

Re, Nu, Pr for forced convection flows and note their equivalence

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