Turbulence Modelling

The FLuD group is actively developing turbulence models for both applied and fundamental problems. The development of accurate and efficient models is critical to improve simulation capability, and thus provide a greater insight into the physics of fluid flow in a wide range of applications

Reynolds Averaged Navier Stokes (RANS) models are required to run steady state turbulent flow computations. Within the group we utilise a range of RANS methods available in commercial algorithms, and within our very high order accurate algorithms. These are employed for rapid iteration of designs or initial scoping of a problem.A very high order accurate compressible method applied to RANS of a high lift configuration is shown here.
Large Eddy Simulation (LES) is a time accurate unsteady method of simulating high Reynolds number turbulence. The key modelling challenge is that the small scales of turbulence can no longer be resolved, hence they must be modelled. Within the group we are exploring modelling approaches for both reacting and non-reacting flows.

The image on the right shows the velocity field within a 1 billion point LES of homogeneous decaying turbulence.

Implicit LES

Implicit LES refers to a turbulent modelling strategy where the numerical dissipation of the numerical scheme provides the sub-grid model. To use this approach, a carefully designed numerical algorithm must be used along with an appropriate mesh to ensure that the large scales can evolve without influence of viscosity, thus in a physical manner. The USyd algorithm FLAMENCO utilises Implicit LES for the computation of free turbulence and turbulent mixing at very high Reynolds number. The implicit subgrid model is provided by the fifth order accurate scheme in space, coupled with a higher order accurate scheme in time. There is continuous research within the group into algorithms for Implicit LES.

The image shows an instantaneous snapshot of vorticity magnitude in an LES of the University of Sydney bluff body burner.

Detached Eddy Simulation (DES) is a combination of the RANS approach for wall bounded flows, and LES for detached shear layers. The aim is to provide a computationally efficient means of resolving the thin boundary layer with very sharp gradients, yet also capture unsteady shear layers and recirculation zones. These approaches are very promising for high Reynolds industrial problems and so improved modelling strategies are a focus of our research.The image shows the development of unsteady vortices over a generic road vehicle simulated using Hybrid RANS-ILES.
Reacting Flows
While the bulk of combustion capability lies within the Thermofluids Group, FluD group members have also undertaken unsteady turbulent combustion modelling utilising Flame Surface Density approaches and Conditional Moment Closure for premixed and non-premixed combustion. The USyd algorithms also have traditional Arhennius based sources implemented.An LES of a premixed air-methane flame using CMC model is shown.