Implementation of Higher-Moment Stratified RANS Turbulence Models in OpenFOAM

Simulation of engineering problems in an active ocean environment requires accounting for numerous environmental effects. Among these are the various effects of buoyancy due to temperature and salinity stratification. Additionally, it is important to acknowledge that the environment is not quiescent; turbulence occurs naturally due to a variety of environmental sources including breaking waves, wind shear, and overturning. In modeling stratified turbulent flows using a RANS approach, it is necessary to adopt models which account for the anisotropic effect of buoyant forcing. A second-moment closure is typically necessary, with the associated computational cost. The existence of background turbulence must also be accommodated. Source terms have been introduced to unstratified turbulence models which maintain the freestream turbulence to specified values; these source terms can be adapted to more extensive stratified turbulence models. The requirements for a Reynolds-averaged turbulence model which could be applied to problems in an active ocean can be determined with these considerations in mind. A pair of stress-transport models are implemented and applied to a variety of representative model problems using OpenFOAM. The models differ chiefly in the pressure-strain terms in the stress transport equation; a linear model and a more complex cubic model built to maintain realizable turbulence values. A set of model problems are then examined: a shear-driven mixed layer, an unstable temperature stratification, and a wake under the effects of stratification. Aspects of the implementation in OpenFOAM will be outlined, including discussion of freestream turbulence sources intended to allow the introduction of background environmental turbulence. The difficult task of verification and validation for such models will also be briefly discussed.