How to understand the new Source terms at LHS in Ueqn [for cfdemSolverPiso]

Hi Chris,

I notice that the UEqn.h has some changes in the latest version of cfdemSolverPiso:

fvVectorMatrix UEqn
(
fvm::ddt(voidfraction,U) + fvm::Sp(fvc::ddt(voidfraction),U)
+ fvm::div(phi,U) + fvm::Sp(fvc::div(phi),U)
// + turbulence->divDevReff(U)
+ particleCloud.divVoidfractionTau(U, voidfraction)
==
- fvm::Sp(Ksl/rho,U)
);

Thre are no source terms in the old version (2.5.x)

fvVectorMatrix UEqn
(
fvm::ddt(voidfraction,U)
+ fvm::div(phi,U)
// + turbulence->divDevReff(U)
+ particleCloud.divVoidfractionTau(U, voidfraction)
==
- fvm::Sp(Ksl/rho,U)
);

Literally fvm::Sp(a,b) returns a*b in the format of matrix. But compared to the orignal CFD-DEM equation in many journal papers, I feel a bit confused, so how to understand these terms and their roles?

best regards.

David