[immortality]

in header file of totalPressure in 2.2.0 version its explained briefly about categories of totalPressure bc.
Why for subsonic compressible flow we should use p0=p+.5*rho*U^2 and not the isentropic equation that includes mach in it like in gas dynamics texts?

[immortality]

whats the reason of choosing different formula in subsonic and supersonic for compressible flow?

[immortality]

it is what has been written in totalPressureFvPatchField.H

The modes of operation are set via the combination of \c phi, \c rho, and
    \c psi entries:
    \table
        Mode                    | phi   | rho   | psi
        incompressible subsonic | phi   | none  | none
        compressible subsonic   | phi   | rho   | none
        compressible transonic  | phi   | none  | psi
        compressible supersonic | phi   | none  | psi
    \endtable

and:

compressible subsonic:
        \f[
            p_T = p_0 + 0.5 \rho |U|^2
        \f]
        where
        \vartable
            p_T     | total pressure [Pa]
            p_0     | reference pressure [Pa]
            \rho    | density [kg/m3]
            U       | velocity
        \endvartable

while in all educational texts I have seen that this relation is for incompressible flow and for compressible flow we should use isentropic relations.
could anyone help me with this doubt on the concept?