# How to? Extra term in k-e equation. Implicit-Explicit

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#1
Senior Member

M. Montero
Join Date: Mar 2009
Posts: 140
Rep Power: 16 Hi all,

I am trying to implement a canopy model into OF.
With respect to the momentum equation, I am seeing several post where the code was similar to the following one:
Quote:
 // Solve the Momentum equation UEqn() == -fvc::grad(p)-fvm::Sp(mag(fvc::Sp(Cd*canopydensity*canopyleaf,U) ),U)
The point is when I need to implement the extra terms in k and epsilon equation.
I have introduced the following ones:
Quote:
 // Turbulent kinetic energy equation volScalarField Fk=0.8*canopydensity_*canopyleaf_*mag(U_)*mag(U_)* mag(U_); tmp kEqn ( fvm::ddt(k_) + fvm::div(phi_, k_) + fvm::SuSp(-fvc::div(phi_), k_) - fvm::laplacian(DkEff(), k_) == G - fvm::Sp(epsilon_/k_, k_) + Fk // Dissipation equation volScalarField Fe=(epsilon_/k_)*Cpepsilon1_*Fk; tmp epsEqn ( fvm::ddt(epsilon_) + fvm::div(phi_, epsilon_) + fvm::SuSp(-fvc::div(phi_), epsilon_) - fvm::laplacian(DepsilonEff(), epsilon_) == C1_*G*epsilon_/k_ - fvm::Sp(C2_*epsilon_/k_, epsilon_) + Fe
It compiles correctly and I obtain some physical results, but I have just found a master thesis where:

Quote:
 // Dissipation equation tmp epsEqn ( fvm::ddt(epsilon_) + fvm::div(phi_, epsilon_) - fvm::Sp(fvc::div(phi_), epsilon_) - fvm::laplacian(DepsilonEff(), epsilon_) == C1_*G*epsilon_/k_ - fvm::Sp(C2_*epsilon_/k_, epsilon_) + fvm::Sp(Cd_*alphaL_/k_*(CEps4_*betap_*pow(mag(U_),3)-CEps5_*betad_*k_*mag(U_)), epsilon_); //Source Term) // Turbulent kinetic energy equation tmp kEqn ( fvm::ddt(k_) + fvm::div(phi_, k_) - fvm::Sp(fvc::div(phi_), k_) - fvm::laplacian(DkEff(), k_) == G - fvm::Sp(epsilon_/k_, k_) + fvm::Sp(Cd_*alphaL_/k_*(betap_*pow(mag(U_),3)-betad_*k_*mag(U_)), k_); //Source term)
The question are:

Is correct my implementation?

what is the advantages to take the extra terms as implicit in the k and epsilon equation with respect to take them as explicit terms? Quick Convergence? Stability?

Thank you very much for your time.   March 19, 2013, 10:50 #2 Senior Member   M. Montero Join Date: Mar 2009 Location: Madrid Posts: 140 Rep Power: 16 Ok, It seems according to the other topic that it is related to the stability of the equations...  Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded Mode 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 OffTrackbacks are Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post Don456 Main CFD Forum 1 January 19, 2012 15:00 DaIN Main CFD Forum 0 October 6, 2011 15:11 MACFD FLUENT 4 January 4, 2011 14:16 Valdemir G. Ferreira Main CFD Forum 0 April 30, 2000 08:57 Michael Main CFD Forum 1 June 25, 1999 10:20

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