CHARACTERISTICS OF TURBULENCE MODELS
k-ε model: Velocity scale: k1/2, Length scale: k3/2/ε, Eddy viscosity: ρ Cμ k2/ε
Advantages:
Good results for many industrial applications dealing with wall-bounded flows with low pressure gradients.
Stable and numerically robust.
Computationally less expensive, no sensitivity of free-steam conditions.
Realizable version of k-ε model is good for complex flows with large strain rates, recirculation, rotation, separation, strong pressure gradients.
Disadvantages:
Limited ability to predict secondary flow characteristics such separation and reattachment (such as flow over aerofoil with non-zero angle of attack, cross flow over a cylinder).
Not accurate in flows with high streamline curvature and sudden changes in the mean strain rate (such as back-facing step), strong swirling flow (such as cyclones, hydrocyclones and stirred tanks).
Like any eddy-viscosity model, fails to predict the cases where turbulence transport or non-equilibrium effects are important in flows. It is based on isotropic turbulence.
Spalart-Allmaras model
Key characteristics:
In standard form, it is a Low-Re number model and hence no wall function is used. That further imposes a restrictions to have mesh fine enough so that y+ is < 1.0 everywhere.
Since this is a Low-Re number model, it can be used with "Automatic Wall Treatment" or "All y+ treatment" methods of turbulence modeling.
This turbulence model had especially been developed for aerodynamic flow simulation for aerospace industry.
This models is a good choice for applications in which the boundary layers are largely attached and separation is not present or mild separation is expected. Typical examples would be flow over a wing, ship-hulls, missiles, fuselage or other aerospace external-flow applications.
The Spalart-Allmaras model for RANS equations is not recommended for flows dominated by free-shear layers (such as jets), flows where complex recirculation occurs (especially with heat transfer) and natural convection.
The Spalart-Allmaras models with DES can be used for flows dominated by free-shear layers (such as jets), flows where complex recirculation occurs (especially with heat transfer) and natural convection.
Reynolds Stress Model (RSM) or Second Moment Closure Methods
Key characteristics:
Reynolds stresses are not modeled as Boussinesq Hypothesis. However, modeling is still required for many terms in the transport equations.
Recommended for complex 3-D turbulent flows with large streamline curvature and swirl, but the model is computationally intensive, difficult to converge than eddy-viscosity models such as k-ε or Spalart-Allmaras models.
Anisotropy of turbulence is accounted for, quadratic pressure-strain option improves performance for many basic shear flows.
Most suitable for curved ducts say U or S-bends, rotating flow passages, combustors with large inlet swirl and cyclone separators.
Standard k-ω Model (SKO): Velocity scale: k1/2, Length scale: k1/2/Cμω, Eddy viscosity: ρ k2/ω
Key characteristics:
Specific dissipation rate ω = k/ε solved instead of ε
Demonstrates better performance for wall bounded and low-Re flows and potential to handle transitional flows (though tend to predict the transition early).
Suitable for complex boundary layer flows with adverse pressure gradient and separation (external aerodynamics and turbomachinery).
Separation is typically predicted to be higher and earlier than experimentally observed values.
Shear Stress Transport (SST) k-ω Model
Key characteristics:
Specific dissipation rate ω = k/ε solved in inner layer (log-layer and viscous sub-layer) and transitions to a k-ε model away from the wall (but not same as standard k-ε equations).
The boundary conditions for SST model are the same as the k-ω model and is relatively less sensitive to the free stream value of ω.
Suitable for complex boundary layer flows with adverse pressure gradient and separation (external aerodynamics and turbomachinery).
Large Eddy Simulation (LES):
Key characteristics:
This model resolves all eddies with scales larger than grid scale and hence recommended for wide-band aeroacoustic noise predictions.
Time step size is governed by the time scale of the smallest resolved eddies which requires the local Courant-Friedrichs-Lewy (CFL) number to order of 1.
In ANSYS FLUENT, inlet perturbations at velocity inlets can be imposed while using LES turbulence model.
FLUENT also recommends "Bounded Central Differencing" for momentum in case of LES on unstructured mesh.
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