
[Sponsors] 
March 30, 2012, 10:58 
Gravitational Acceleration Problem???

#1 
New Member

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 , P1 , 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 6.94e5 m/s2 and ıt made my mınd confused. whıch one do ı have to use ? 9.81 or 6.94e5 ?? 

March 31, 2012, 16:18 

#2  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,420
Rep Power: 22 
Quote:
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

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 

#4  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,420
Rep Power: 22 
Quote:
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 wellknown 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 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem with interFoam; Wave/wiggle alpha1 behavior  JonW  OpenFOAM  3  February 23, 2013 21:41 
UDF compiling problem  Wouter  Fluent UDF and Scheme Programming  6  June 6, 2012 04:43 
problem of kappat in buoyantBoussinesqSimpleFoam  jignesh_thaker2007  OpenFOAM  0  October 2, 2011 05:45 
Gambit  meshing over airfoil wrapping (?) problem  JFDC  FLUENT  1  July 11, 2011 05:59 
Adiabatic and Rotating wall (Convection problem)  ParodDav  CFX  5  April 29, 2007 19:13 