How the trace term of SGS stress tensor implemented in Openfoam
 

How does the trace term of SGS stress tensor, 2/3kI, implement in OpenFoam. I can't find the code in solvers, such as pisoFoam. And in the Forum, I remenber that someone said OF add the term of 2/3kI in the pressure gradient. But where can I find it in the code?