Turbulence is the three-dimensional unsteady random motion observed in fluids at moderate to high Reynolds numbers. As technical flows are typically based on fluids of low viscosity, almost all technical flows are turbulent. Many quantities of technical interest depend on turbulence, including:
Mixing of momentum, energy and species
Heat transfer
Pressure losses and efficiency
Forces on aerodynamic bodies
While turbulence is, in principle, described by the Navier-Stokes equations, it is not feasible in most situations to resolve the wide range of scales in time and space by Direct Numerical Simulation (DNS) as the CPU requirements would by far exceed the available computing power for any foreseeable future. For this reason, averaging procedures have to be applied to the Navier-Stokes equations to filter out all, or at least, parts of the turbulent spectrum. The most widely applied averaging procedure is Reynolds-averaging (which, for all practical purposes is time-averaging) of the equations, resulting in the Reynolds-Averaged Navier-Stokes (RANS) equations. By this process, all turbulent structures are eliminated from the flow and a smooth variation of the averaged velocity and pressure fields can be obtained. However, the averaging process introduces additional unknown terms into the transport equations (Reynolds Stresses and Fluxes) that need to be provided by suitable turbulence models (turbulence closures). The quality of the simulation can depend crucially on the selected turbulence model and it is important to make the proper model choice as well as to provide a suitable numerical grid for the selected model. An alternative to RANS are Scale-Resolving Simulation (SRS) models. With SRS methods, at least a portion of the turbulent spectrum is resolved in at least a part of the flow domain. The most well-known such method is Large Eddy Simulation (LES), but many new hybrids (models between RANS and LES) are appearing. As all SRS methods require time-resolved simulations where the time step size is relatively small, it is important to understand that these methods are substantially more computationally expensive than RANS simulations.
Ansys Fluent provides the following choices of turbulence models:
Spalart-Allmaras model
- models
Standard - model
Renormalization-group (RNG) - model
Realizable - model
- models
Standard - model
Baseline (BSL) - model
Shear-stress transport (SST) - model (enabled by default)
Generalized - (GEKO) model
model (add-on)
Transition -- model
Transition SST model
Reynolds stress models (RSM)
Linear pressure-strain RSM
Quadratic pressure-strain RSM
Stress-Omega RSM
Stress-BSL RSM
Scale-Adaptive Simulation (SAS) model, which can be used in combination with one of the following -based URANS models:
SST - model
Standard - model
BSL - model
Transition SST model
-based Reynolds stress models (RSM)
Detached eddy simulation (DES) model, which includes one of the following RANS models.
Spalart-Allmaras RANS model
Realizable - RANS model
SST - RANS model
BSL - RANS model
Transition SST model
Shielded Detached Eddy Simulation (SDES) model, which includes one of the following RANS models.
SST - RANS model
BSL - RANS model
Transition SST model
Stress-Blended Eddy Simulation (SBES) model, which includes one of the following RANS models.
SST - RANS model
BSL - RANS model
Transition SST model
Large eddy simulation (LES) model, which includes one of the following subgrid-scale models.
Smagorinsky-Lilly subgrid-scale model (with or without dynamic stress enabled)
WALE subgrid-scale model
Dynamic kinetic energy subgrid-scale model
Wall-Modeled LES (WMLES)
Wall-Modeled LES - (WMLES -)