Modifying default k-epsilon equation
 

Hi,
I use Openfoam Version 1.5.
I was trying to edit the k-epsilon equation for buoyancy case.
Basically I have to add a term on the k equation:
mueff/sigma*g/rho*di(rho)/di(xj).
I got mueff/sigma*rho part, but having some trouble with g and di(rho)/di(xj).
I think I have to add a nonscalar variable g in the kEpsilon.C file, but need some help. Can somebody give me some resources to make this work?
Thank you in advance.

Kevin