CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM

RAS Turbulence Models - Convective Term

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 28, 2010, 12:18
Default RAS Turbulence Models - Convective Term
  #1
New Member
 
John Weaver
Join Date: Mar 2009
Posts: 6
Rep Power: 17
john_w is on a distinguished road
I was recently looking at the code for the RAS turbulence models within OF in order to relate the coding to the mathematical algorithms. As expected, with the standardized syntax within OF, this is not too hard to do. However, there is one expression within the kinetic energy and dissipation equations that is used in some cases in describing the left hand side (i.e. unsteady and convective terms), but not in all cases. For example if we look at the compressible kEpsilon.C kinetic energy equation, we find:

// Turbulent kinetic energy equation

tmp<fvScalarMatrix> kEqn
(
fvm::ddt(rho_, k_)
+ fvm::div(phi_, k_)
- fvm::laplacian(DkEff(), k_)
==
G
- fvm::SuSp((2.0/3.0)*rho_*divU, k_)
- fvm::Sp(rho_*epsilon_/k_, k_)
);

However, in the incompressible case, an additional term (in red) is also found:

// Turbulent kinetic energy equation
tmp<fvScalarMatrix> kEqn
(
fvm::ddt(k_)
+ fvm::div(phi_, k_)
- fvm::Sp(fvc::div(phi_), k_)
- fvm::laplacian(DkEff(), k_)
==
G
- fvm::Sp(epsilon_/k_, k_)
);

It appears to be part of the code which describes the convective term, but would appreciate more insight if anyone can help.

Thanks, John
john_w is offline   Reply With Quote

Old   April 29, 2010, 08:42
Default
  #2
Senior Member
 
Claus Meister
Join Date: Aug 2009
Location: Wiesbaden, Germany
Posts: 241
Rep Power: 18
idrama is on a distinguished road
Have a look into the ProgrammersGuide P-37. I consider it as a simple multiplication. However, it has something to do with the way how it is discretize.

Cheers
idrama is offline   Reply With Quote

Old   April 29, 2010, 08:55
Default
  #3
ngj
Senior Member
 
Niels Gjoel Jacobsen
Join Date: Mar 2009
Location: Copenhagen, Denmark
Posts: 1,902
Rep Power: 37
ngj will become famous soon enoughngj will become famous soon enough
Hi John

With respect to the incompressible formulation, what the red part states is that you include convection of k which is proportional to the continuity error (fvc::div(phi)). This term is also present in the differential form as

k \nabla \cdot U

however due to the continuity equation for incompressible flow this is identical equal to zero. The discrete formulation as you mention account for the error in the continuity equation, thus with small enough tolerances on your solvers, this term should also go to zero.

I have no knowledge of compressible flows, hence cannot be of any help in that regard.

Best regards,

Niels
ngj is offline   Reply With Quote

Old   May 29, 2010, 22:31
Default Source term. Implicit.
  #4
Senior Member
 
sahm's Avatar
 
Seyyed Ali H.M.
Join Date: Nov 2009
Location: Utah
Posts: 107
Rep Power: 16
sahm is on a distinguished road
As far as I have understood it is a source term calculated by simply multiplying the terms. Since It is fvm::Sp(a,b) it calculated the source term with Implicit method.
sahm is offline   Reply With Quote

Reply

Tags
convective term, ras, turbulence model


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Turbulence models become invalid at Re=2000~5000? liweif FLUENT 9 August 27, 2007 12:55
turbulence models? haider FLUENT 0 March 8, 2006 00:58
Turbulence Models and external flow. Alan FLUENT 3 November 22, 2005 05:46
How to deal with the nonlinear convective term? D.Yang Main CFD Forum 2 January 16, 2002 17:24


All times are GMT -4. The time now is 04:35.