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# Which Turbulence Model Should You Use For Your CFD Analysis?

If you’re new to CFD, you might be wondering why there are so many turbulence models.

What CFD software does? It can solve the Navier-Stokes equation and the energy and mass conservation equations. When Prandtl solved these equations, he discovered there was no direct mathematical solution for turbulent flow. Because the equations for turbulent flow cannot be closed, this is known as the turbulence closure problem.

As is customary, the engineer’s solution to unsolvable equations entails simplification methods and observations. As is always the case, these methods vary depending on the problem and what is deemed essential or conservative.

So, how do you decide which model to buy?  CAE analysis is also used in engineering analysis tasks like these.

Choosing the correct model is critical and can significantly impact the results. In this post, we’ll look at some of the most popular models and their advantages and disadvantages.

## RANS – Reynolds Averaged Navier Stokes Models

RANS-based models are closed using viscosity. The Navier-Stokes equations are closed using thickness in these models. RANS models assume that the turbulence is isotropic and is not stretched by proximity to a wall or swirling or shear flow. RANS models come in different flavours.

### ●     RANS Single Equation Model: Spalart-Allmaras (SA)

The Spalart-Allmaras models are primarily used in aerospace and turbomachinery. It is effective for supersonic and transonic flows over airfoils, boundary layer flows, etc.

### ●     RANS Two Equation Model: k-ε

This is one of the most widely used turbulence models. They’re adequate for a wide range of applications.

### ●     RANS Two Equation Model: Realizable k-ε

This is the turbulence model’s workhorse. It is the most tested, well-documented, and widely available turbulence model. This model is the default in many commercial software packages.

### ●     RANS Two Equation Model: RNG k-ε

The RNG k- model improves on the standard k- a model for shear, swirling, and separation. It also works well for high strain rate flows.

### ●     RANS Two Equation Model: Standard k-ω

This model includes the rate of kinetic energy dissipation. The goal here is to better model near-wall interactions than the k- models.

### ●     RANS Two Equation Model: Shear Stress Transport k-ω

The SST k- model crosses k- and k-. This model employs the k- model near walls and switches to the k- model in the open flow field. It is widely used in aerospace and turbomachinery applications.

### ●     Large-Eddy Simulation (LES)

LES models are significantly more complex than RANS models and are better suited for high-end applications (combustion, mixing, external aerodynamics like flow around bluff bodies).

Large-eddy simulation (LES) models resolve the largest turbulence scales and model the rest with sub-grid turbulence models or blend with a RANS model.

### ●     Reynolds Stress Model

The Reynolds Stress Model is the complete turbulence m representing the turbulent flow model. This is typically reserved for the most complex flows in which the CFD analyst has no idea what to expect (unstudied flows) or for highly complex flows.

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