how to understand the pressure equation in potentialFoam
 

Foamers:

I have two questions about the pressure equation in potentialFoam:
1) how does the pressure equation derived?
and how to understand fvm::laplacian(rUAf,p) == fvc::div(phi) ?
2)what does the line of "phi-=pEqn.flux()" mean ?

thanks!

jianye