Andrew Heather authored Dec 11, 2019 and Mark OLESEN committed Dec 11, 2019

Calculates the acoustic power due to the volume of isotropic turbulence
using Proudman's formula

The acoustic power \f$ P_A \f$ [W/m3] in terms of turbulence \f$ k \f$
and \f$ \epsilon \f$ is given as:

    \f[
        P_A = alpha_\epsilon \rho \epsilon M_t^5
    \f]

where \f$ alpha_\epsilon \f$ is a constant (0.1) and

    \f[
        M_t = \frac{\sqrt{2 k}}{a_0}
    \f]

with \f$ a_0 \f$ the speed of sound.  The acoustic power is also output in
dB using:

    \f[
        L_P = 10 \log \frac{P_A}{P_ref}
    \f]

where \f$ P_ref \f$ is a constant (1e-12 W/m3)

Usage
    Example of function object specification to calculate the Proudman acoustic
    power

    proudmanAcousticPower1
    {
        type        proudmanAcousticPower;
        libs        ("libfieldFunctionObjects.so");
        ...

        // Required additional entries for incompressible calculations
        rhoInf      1.225;
        aRef        340;
    }

    Where the entries comprise:
        Property     | Description                 | Required   | Default value
        type         | type name: proudmanAcousticPower         | yes        |
        rhoInf       | Freestream density for incompressible cases  | no |
        aRef         | Reference spped of sound for incompressible cases | no |
        alphaEps     | Model coefficient           | no         | 0.1

Note
- The freestream density and reference speed of sound are only necessary
  when a thermodynamics package is unavailable, typically for incompressible
  cases.