
[Sponsors] 
September 13, 2012, 21:42 
Simple question about icoFoam

#1 
New Member
Yuri Almeida
Join Date: Jan 2012
Location: Rio de Janeiro, Brazil
Posts: 21
Rep Power: 7 
Dear all,
The momentum equation in icoFoam.C (OF21) is the following: fvVectorMatrix UEqn ( fvm::ddt(U) + fvm::div(phi, U)  fvm::laplacian(nu, U) ); solve(UEqn == fvc::grad(p)); If I understood right, the equation would be: d(U)/dt + div(rho*U*U)  laplacian((mu/rho)*U) = grad(p) (1) But the equation found in books is: d(rho*U)/dt + div(rho*U*U)  laplacian(mu*U) = grad(p) (2) or, dividing by rho: d(U)/dt + div(U*U)  laplacian((mu/rho)*U) = (1/rho)*grad(p) (3) My question is: Why are equation 2 and 3 different from 1? Probably they are not, but, where is "rho" in fvc::grad(p) and why is it present in div(phi, U)?? Thx in advance! 

September 14, 2012, 03:51 

#2  
Senior Member
Gerhard Holzinger
Join Date: Feb 2012
Location: Austria
Posts: 200
Rep Power: 19 
Hello,
in (1) you have rho in the convective term Quote:
d(U)/dt + div(U*U)  laplacian((mu/rho)*U) = grad(p) (1.1) Quote:
In an icoFoam case, the dimension of pressure is dimensions [0 2 2 0 0 0 0]; So, although in eq (1.1) and eq (3) the symbol p is used, the meaning of p is not the same in this equations. In icoFoam p is a pressure that's been divided by rho. 

September 14, 2012, 08:34 

#3 
New Member
Yuri Almeida
Join Date: Jan 2012
Location: Rio de Janeiro, Brazil
Posts: 21
Rep Power: 7 
Hi Gerhard,
Now I got it, I knew that the equation was right, but I didn't understand how. So, phi is flux, this makes sense, however, I saw that phi = rho*U from here: http://www.openfoam.org/docs/user/fvSchemes.php "The divSchemes subdictionary contains divergence terms. Let us discuss the syntax of the entry in reference to a typical convection term found in fluid dynamics div(rhoUU), which in OpenFOAM applications is commonly given the identifier div(phi,U), where phi refers to the flux phi = rhoU." Thanks for the attention, Gerhard. 

September 14, 2012, 09:04 

#4 
Senior Member
Gerhard Holzinger
Join Date: Feb 2012
Location: Austria
Posts: 200
Rep Power: 19 
I see, maybe that is the case for compressible solvers. In an incomressible case rho is eliminated.


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Simple Beginner User Question on 'geometry'  scottimus  FLUENT  5  March 27, 2012 13:17 
Simple Question  or not?  Florian2  Main CFD Forum  10  November 17, 2011 17:48 
Question about PISO in IcoFoam  titio  OpenFOAM Running, Solving & CFD  5  September 17, 2010 13:31 
Simple Question Regarding Symmetry Planes  Atella  Main CFD Forum  0  April 9, 2010 10:58 
Simple icoFoam simulation gone wrong  rieuk  OpenFOAM Running, Solving & CFD  3  March 5, 2010 03:24 