
[Sponsors] 
May 15, 2014, 10:27 
Body force (gravity term)

#1 
New Member
iggy iggy
Join Date: May 2014
Posts: 2
Rep Power: 0 
Hello, I am working on a structured inhouse code on natural convection flow, and I have a doubt regarding the body force (gravity term).
This term is generally encountered as rho*g with rho density and g gravity acceleration, however most of the time I see it implemented as (rhorho_ref)*g with rho_ref is a reference density. Can anyone tell me why should rho_ref be necessary? Thanks in advance. 

June 10, 2014, 04:04 

#2  
Senior Member
Rami BenZvi
Join Date: Mar 2009
Posts: 154
Rep Power: 14 
Quote:
Hi Iggy, The body force is indeed rho*g. Sometimes it is reasonable to assume the changes of the density due to temperature small (the Boussinesq approximation), so that the reference density is used as an approximation to the density for all terms except the body force that becomes rho_ref*g + rho_ref*beta*(TT_ref)*g , where beta is the expansion coefficient and T_ref is the reference temperature for which the density is rho_ref. 

August 13, 2014, 18:08 

#3  
New Member
iggy iggy
Join Date: May 2014
Posts: 2
Rep Power: 0 
Hi Rami,
Thank you very much for your answer. In case the Boussinesq approximation is not used, which means that in all the other terms the corresponding rho (function of temperature) is used, should the body force term stay as just rho(T)*g? I think I'm missing one detail regarding the buoyancy effects. If we imagine a stationary warmer fluid region which is in the middle of colder fluid region, we expect the warmer region to rise (if density reduces with temperature increase). So, what is the term which makes the warmer fluid rise? Both the warm and cold regions experience a rho*g term downwards, although the magnitude of this term is somewhat less for the warm fluid. Is it this mere difference which makes the warmer fluid rise? Quote:


Tags 
boussinesq approximation, density, gravity 
Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
coupling vof with single phase flow and gravity term  alame005  Main CFD Forum  0  August 2, 2013 14:37 
Gravity source term in PisoFoam  DanM  OpenFOAM Running, Solving & CFD  4  May 24, 2013 11:30 
Convergence Problem with body force driven flow  Jinfeng  FLUENT  1  December 9, 2009 03:54 
DPM Body Force  Sandilya Garimella  FLUENT  1  April 8, 2008 03:30 
Body force  Jacob  FLUENT  0  August 4, 2003 16:38 