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-   -   why totalPressure formulas are different? (http://www.cfd-online.com/Forums/openfoam-solving/116003-why-totalpressure-formulas-different.html)

immortality April 10, 2013 19:48

why totalPressure formulas are different?
 
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 April 11, 2013 04:41

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

immortality April 11, 2013 08:37

it is what has been written in totalPressureFvPatchField.H
Code:

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:
Code:

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?


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