## New VOF multiphaseStabilizedTurbulence fvOption

### Summary

Applies corrections to turbulence kinetic energy equation and turbulence viscosity field for incompressible multiphase flow cases.

Turbulence kinetic energy is over-predicted in incompressible VOF solvers at the phase interface and throughout the water column in nearly-potential flow regions beneath surface waves.

This fvOption applies corrections based on the references:

```
Buoyancy source term in turbulence kinetic energy equation:
Devolder, B., Rauwoens, P., and Troch, P. (2017).
Application of a buoyancy-modified k-w SST turbulence model to
simulate wave run-up around a monopile subjected to regular waves
using OpenFOAM.
Coastal Engineering, 125, 81-94.
Correction to turbulence viscosity:
Larsen, B.E. and Fuhrman, D.R. (2018).
On the over-production of turbulence beneath surface waves in
Reynolds-averaged Navier-Stokes models
J. Fluid Mech, 853, 419-460
```

### Resolved bugs (If applicable)

See #1433 (closed)

### Details of new models (If applicable)

The implementation is based on the form for the k-epsilon turbulence model.

Example usage:

```
multiphaseStabilizedTurbulence1
{
type multiphaseStabilizedTurbulence;
active yes;
multiphaseStabilizedTurbulenceCoeffs
{
// Optional coefficients
lambda2 0.1; // A value of 0 sets the nut correction to 0
Cmu 0.09; // from k-epsilon model
C 1.51; // model coefficient from k-omega model
alpha 1.36; // 1/Prt
}
}
```

The model `C`

coefficient for the k-epsilon model equates to C2/C1 = 1.33; the (default) value of 1.51 comes from the k-omega model and is more conservative.

### Risks

Modular - low risk

### Credits

Thanks go to the Turbulence Technical Committee, and the useful discussions with and code testing by Bjarke Eltard-Larsen and David Fuhrman (Technical University of Denmark).