The Different Turbulence Models
The eight RANS turbulence models differ in how they model flow near walls, the number of additional variables solved for, and the meaning of these variables. These models add turbulence eddy viscosity terms to the Navier-Stokes equations, but they do so differently. A mechanical design engineer is a good career option you may want to check out.
- yPlus and L-VEL
The eddy viscosity is computed by the L-VEL and algebraic yPlus turbulence models using algebraic expressions based solely on the local fluid velocity and distance to the nearest wall. They don’t solve any other transport equations. These models, which are the most robust and least computationally intensive of the eight turbulence models, solve for flow everywhere.
The Spalart-Allmaras model includes a single extra variable for undamped kinematic eddy viscosity. It is a low Reynolds number model capable of resolving the entire flow field down to the solid wall. The model was initially designed for aerodynamics applications and was relatively robust with low-resolution requirements.
The k- model solves for two variables: k, the turbulence kinetic energy, and (epsilon), the turbulence kinetic energy dissipation rate. Because this model employs wall functions, the flow in the buffer region is not simulated. Because of its high convergence rate and low memory requirements, the k- the model has historically been prevalent for industrial applications.
The k-model is similar to the k-model, but it solves for (omega) — the specific kinetic energy dissipation rate. It is a low Reynolds number model that can also be used with wall functions. It is more nonlinear and thus more challenging to converge than the k-model, and it is susceptible to the initial guess of the solution.
- k- Low Reynolds Number
The low Reynolds number k- model is similar to the k- model in that it doesn’t require wall functions to solve for flow everywhere. It’s a logical extension of the k-model and shares many of its benefits, but it requires a denser mesh at walls and about. It can be helpful first to compute a good initial condition for solving the standard Reynolds number k- model using the k- model.
The SST model is a hybrid of the k-model in free flow and the k-model near the walls. It is a low Reynolds number model commonly used in industrial applications. It has the exact resolution requirements as the k- model and the standard Reynolds number k- model, but its formulation eliminates some of the shortcomings of the pure k- and k- models. IN A TUTORIAL MODEL EXAMPLE, the SST model solves for flow over a NACA 0012 airfoil. The results are shown to be consistent with the experimental data.
Close to wall boundaries, velocity fluctuations are typically more significant in parallel directions to the wall than in perpendicular directions. The changes in velocity are said to be anisotropic. The fluctuations are the same magnitude in all orders as you move away from the wall. The fluctuations in speed become isotropic.
Knowledge of turbulence modelling will help you choose which Turbulence Model you should select for your CFD Application.