I have read the dissertation "
 

I have read the dissertation "Error Analysis and Estimation for the Finite Volume Method with Application to Fluid Flows" from Dr. Jasak, where the PISO loop is described for the incompressible flow. But for the compressible turbulence flow in sonicTurbFoam, what is the mathematical expression for pressure equation?

From reading the code I cannot understand the pEqn:
fvScalarMatrix pEqn
(
fvm::ddt(psi, p)
+ fvm::div(phid, p, "div(phid,p)")
- fvm::laplacian(rho*rUA, p)
);

pEqn.solve();
What is rUA here, which is used to correct the velocity after pressure is updated and what is the expression for phid? From which book I can find these equations? Thank you very much!