
[Sponsors] 
May 22, 2010, 06:17 
Understanding fvm::Sp()

#1 
New Member
Join Date: May 2010
Posts: 2
Rep Power: 0 
I was reading the code for the LaunderSharmaKE model (incompressible), then I ran into this:
Code:
tmp<fvScalarMatrix> epsEqn ( fvm::ddt(epsilonTilda_) + fvm::div(phi_, epsilonTilda_)  fvm::laplacian(DepsilonEff(), epsilonTilda_) == C1_*G*epsilonTilda_/k_  fvm::Sp(C2_*f2()*epsilonTilda_/k_, epsilonTilda_) + E ); 

May 22, 2010, 06:43 

#2 
Member
Simon Lapointe
Join Date: May 2009
Location: Québec, Qc, Canada
Posts: 33
Rep Power: 9 
fvm::Sp makes the source term implicit so it contributes to the diagonal. This can help convergence when the source term is negative on the rhs (sink term). This is the case in the equation you've shown.
The two other source terms are positive on the rhs and therefore left explicit. Making them implicit would reduce the diagonal dominance of the matrix and probably cause problems to the solver. Hope this helps 

May 22, 2010, 08:38 

#3 
New Member
Join Date: May 2010
Posts: 2
Rep Power: 0 
Thank you for your fast and complete answer. That is wat I wanted to know.


May 26, 2010, 09:21 

#4 
Member

On the basis of the above explanation, is it advised to have the pressure on the right hand side of the equation to solve, in an expression that is treated implicitly, or the opposite? For instance the hEqn.H of buoyantSimpleFoam, has pressure under fvc:
Code:
fvm::div(phi, h)  fvm::Sp(fvc::div(phi), h)  fvm::laplacian(turbulence>alphaEff(), h) == fvc::div(phi/fvc::interpolate(rho)*fvc::interpolate(p))  p*fvc::div(phi/fvc::interpolate(rho)) 

February 9, 2016, 13:44 
Question about the source term

#5 
Member
Saurabh Tandon
Join Date: Nov 2015
Location: Austin
Posts: 41
Rep Power: 2 
Hi
I have another question about the fvm::Sp() term in the equation. If you this source term in any equation: fvm::Sp(C2, u), where u is velocity, can C2 be a tensor or a tensor field? If not, can anyone suggest how to code anisotropic, hetorgeneous term in the equation. Thank you. 

July 25, 2016, 23:11 
I am having the same problem

#6 
New Member
Mateus Palharini Schwalbert
Join Date: May 2011
Posts: 18
Rep Power: 7 
Hello,
I have the same doubt as STutexas: how to code an anisotropic, heterogeneous term in the source term in the momentum equation. I see the fvm::Sp(tensor,vector) does not work. I took a look at the porousSimpleFoam code, which does this in adding the porous resistance term. But porousSimpleFoam code deals with this in a different manner, through adding an addResistance function, which I don't quite understand (I'm new here). Any ideas on how to code an anisotropic, heterogeneous term in the source term in the momentum equation? Did you solve this problem, STutexas? 

July 26, 2016, 01:09 

#7 
Member
Saurabh Tandon
Join Date: Nov 2015
Location: Austin
Posts: 41
Rep Power: 2 
Hi Mateus
It seems that fvm::Sp() can only take in vectors and not tensors. Defining a tensor does not work . I tried the following approach: Use fvc::Sp() and define your term as constant source term in the RHS of the vector equation. Then iterate over the equation several times until the values converge. I hope it helps. Let me know if you come up with another method to solve this. Thank you. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Problem understanding rhoPisoFoam  MatP  OpenFOAM Running, Solving & CFD  1  May 15, 2010 13:21 
Help using an understanding y+  siw  CFX  4  October 4, 2009 10:23 
Understanding of channelOodles and oodles solver  fs82  OpenFOAM  1  September 23, 2009 10:36 
understanding dualtime stepping  srinath  Main CFD Forum  1  August 11, 2008 11:05 
Help understanding Memory Allocation Factor  Todd  CFX  1  March 6, 2008 14:27 