fv::OUForce: New random Ornstein-Uhlenbeck) process force fvModel

Description
    Calculates and applies the random OU (Ornstein-Uhlenbeck) process force to
    the momentum equation for direct numerical simulation of boxes of isotropic
    turbulence.

    The energy spectrum is calculated and written at write-times which is
    particularly useful to test and compare LES SGS models.

Note
    This random OU process force uses a FFT to generate the force field which
    is not currently parallelised.  Also the mesh the FFT is applied to must
    be isotropic and have a power of 2 cells in each direction.

Usage
    Example usage:
    \verbatim
    OUForce
    {
        type    OUForce;

        libs    ("librandomProcesses.so");

        sigma   0.090295;
        alpha   0.81532;
        kUpper  10;
        kLower  7;
    }
    \endverbatim

The tutorials/incompressibleFluid/boxTurb16 tutorial case is an updated version
of the original tutorials/legacy/incompressible/dnsFoam/boxTurb16 case,
demonstrating the use of the OUForce fvModel with the incompressibleFluid solver
module to replicate the behaviour of the legacy dnsFoam solver application.

tutorials/legacy/incompressible/dnsFoam/boxTurb16/constant/momentumTransport

@@ -14,13 +14,9 @@ FoamFile
 }
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
-UOsigma         0.090295;
-
-UOalpha         0.81532;
-
-UOKupper        10;
-
-UOKlower        7;
-
+sigma   0.090295;
+alpha   0.81532;
+kUpper  10;
+kLower  7;
 
 // ************************************************************************* //