how to add a damping in the momentum equation
 

Recently, I'm using the groovyBC to make wave with the interFoam solver,so it must build a field as the relaxationZone,and I choose to add a damping in the momentum equation,the special attention is that I use the RAS turbulence model，I have looked some papers about it,but all the papers are in Laminar flow model,so I must solve the question.the momentum equation with damping which compiled by myself as follow:
fvVectorMatrix UEqn
(
fvm::ddt(rho, U)
+ fvm::div(rhoPhi, U)
+ turbulence->divDevRhoReff(rho, U)
- fvm::laplacian(muf,U)
-(fvc::grad(U)&fvc::grad(muf))
+U*rho*thetaField
);
and the part
"- fvm::laplacian(muf,U)
-(fvc::grad(U)&fvc::grad(muf))
+U*rho*thetaField"
is added by me,Among them，the part
"+U*rho*thetaField"
is the damping，but I don't know the parts
"- fvm::laplacian(muf,U)
-(fvc::grad(U)&fvc::grad(muf))"
and
" + turbulence->divDevRhoReff(rho, U)"have question,because in the Laminar flow model paper,only have the part
"- fvm::laplacian(muf,U)
-(fvc::grad(U)&fvc::grad(muf))",so I want somebody give me a guide.