
[Sponsors] 
March 3, 2013, 01:25 
Implementing an Equation

#1 
New Member
Join Date: Apr 2012
Posts: 21
Rep Power: 5 
Hi FOAMERS
Do you know the simplest way to implement equation "dC/dt + grad(CL).V = 0", where C and CL are scalar and V is the velocity vector? 

March 3, 2013, 17:02 

#2 
Senior Member
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 546
Rep Power: 18 
This could be done as
Code:
fvScalarMatrix CEqn ( fvm::ddt(C) + U & fvc::grad(CL) ); CEqn.solve(); Code:
fvScalarMatrix CEqn ( fvm::ddt(C) + fvm::div(phi,C) + fvm::SuSp(fvc::div(phi),C) + U & fvc::grad(C0) ); CEqn.solve(); 

March 5, 2013, 13:44 

#3 
New Member
Join Date: Apr 2012
Posts: 21
Rep Power: 5 
Daniel
Thanks for your answer. In my case C = (gL + kp*(1gL))CL, where gL is a volScalaerField and kp is constant. You have any ideas of the best way to implement this? By the way, can you explain why adding term "fvm::SuSp(fvc::div(phi),C)" makes it more conservative? Last edited by Yahoo; March 5, 2013 at 14:01. 

March 5, 2013, 14:33 

#4  
Senior Member
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 546
Rep Power: 18 
Quote:
the Code:
fvm::SuSp(fvc::div(phi),C) If there is incomplete convergence in your momentum portion of your solver, this will account for that fact. For a completely converged velocity field, this term will approach zero for an incompressible fluid (i.e. by the nature of continuity). Take a look at the thread origin of fvm::Sp(fvc::div(phi_), epsilon_) in kepsilon Eqn? and the post ScalarTransportFoam for RTD calculations. This is also implemented in the kepsilon turbulence models if you would like to look at them. Last edited by chegdan; March 5, 2013 at 15:13. 

March 6, 2013, 02:35 

#5 
Senior Member
Mahdi Hosseinali
Join Date: Apr 2009
Posts: 124
Rep Power: 8 
Dear chegdan,
I believe the left side of your equation is conservative form, the right side is called primitive form. The reason for the name comes from gas dynamics and passing through the shocks. When you use the left hand side (consider C as as rho) passing through the shock your momentum will remain constant, however on your right hand side every term experiences a severe gradient which makes numerical instabilities in presence of shock. PS: I'm speaking in the context of gas dynamics, please let me know if I'm wrong. 

March 12, 2013, 13:08 

#6  
Senior Member
Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
Posts: 546
Rep Power: 18 
Quote:


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Calculation of the Governing Equations  Mihail  CFX  7  September 7, 2014 06:27 
error message  cuteapathy  CFX  14  March 20, 2012 07:45 
Constant velocity of the material  Sas  CFX  15  July 13, 2010 08:56 
mass flow in is not equal to mass flow out  saii  CFX  2  September 18, 2009 08:07 
Implementing additional equation?  Pratap  FLUENT  1  December 4, 2004 21:32 