There are many methods for simulating turbulent flow. One of the most complex and advanced methods is Large Eddy Simulation (LES). In this article, after introducing LES, this method is compared with RANS methods and it is stated when it is better to use each method. In addition, ANSYS Fluent’s features and capabilities in simulating turbulent flows have been investigated.

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## What is Large Eddy Simulation?

LES is a method for simulating turbulent flows in CFD simulations. Reynolds-Averaged Navier-Stokes (RANS) methods have the lowest accuracy and the lowest computational cost. The Direct Numerical Simulation (DNS) method is the most accurate method, but it has a heavy computational cost. LES method is placed between RANS and DNS in terms of accuracy and computing power. In this method, large eddies are simulated directly, each eddy needs at least 4 cells of the mesh. To consider the effect of eddies, special equations are used in each cell. The name of the LES method is derived from these features.

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## Large Eddy Simulation Vs Rans

RANS and LES methods are used to simulate turbulent flows. The RANS method was invented by Osborne Reynolds. He concluded from experimental experiments that in turbulent flows, and found out that the average of different parameters is constant. Based on this, he made changes in the Navier-Stokes equation and invented the RANS equation, which is the cornerstone of all RANS simulation models such as** k-omega **and **k-epsilon**. Although RANS methods have complex equations at first glance, they are the simplest methods and have the lowest computational cost. From an industrial and academic point of view, RANS methods are used more than other methods and are suitable methods for many engineering applications.

The LES method was first presented by Joseph Smagorinsky in 1963 and has been greatly developed since then. In this method, large eddies are directly simulated and the effect of small eddies is considered by equations inside each cell. This method provides **more accurate details of the turbulent flow** than RANS** at the cost of higher computational cost.** LES is suitable for time dependent problems, while RANS is more suitable for stable problems and problems where average parameters are considered. Choosing the right model depends on the characteristics of the problem and computing power.

LES calculates well the time-dependent fluctuations of turbulent flow, which is a suitable feature for simulating complex flows. RANS models usually do not have the ability to simulate these fluctuations. As you can see in the next two figures, the LES simulation results are much more similar to reality than the RANS results.

To choose between LES and RANS, it is recommended to first refer to previous works in your desired field and see what model they have used that has been validated with experimental results. If you do not have access to previous simulation works, it is recommended to use RANS methods first, and if their results do not agree with experimental results, then simulate with LES.

Streamlines of a backward-facing step CFD simulation, using the k-omega standard model to simulate turbulent flow. Note that k-omega is a RANS model.

Streamlines of a backward-facing step CFD simulation, using the LES model to simulate turbulent flow.

## Challenges of Using LES

The **computational cost** of LES is very high, typically requiring high-performance computing resources rather than home computers. One reason for this high cost is that LES simulations must be performed in **three dimensions**. LES is highly sensitive to **mesh resolution**; structured meshes are often recommended, with cell sizes determined relative to the entire domain and target area, usually based on experimental or theoretical guidelines.

Due to LES’s sensitivity to initial conditions, it is common to initialize an LES simulation using results from a converged RANS simulation. This approach provides more realistic initial conditions.

LES uses **Subgrid-Scale (SGS)** to apply the effects of small eddies. It is difficult to develop and validate these models, especially since their effects are applied in small dimensions. The efficiency of LES is strongly influenced by the SGS that uses it.

All CFD simulation methods require **validation** **with experimental results**. In the case of LES and turbulence models, it is very difficult to conduct experimental tests and record results. If after the LES simulation the result does not agree with the experimental results, it is necessary to change the LES or mesh settings and perform the simulation again. These simulations, which are done again and again, are very time-consuming and require a lot of computing power.

LES simulation is performed only in the 3D domain, which, along with the details of the simulation, makes the generated data and results very voluminous. The transfer of this data and its post-processing requires very high computing power. The 3D nature, high production data volume, accuracy in meshing, small time steps, and complex equations all make the simulation long and use a lot of computing power.

## Applications of Large Eddy Simulations

Large Eddy Simulations (LES) have a wide range of applications across various fields due to their ability to accurately capture the dynamics of turbulent flows. Some key applications include:

### Aerospace Engineering

Turbulent flow simulations based on LES are widely used in aerospace engineering applications. For instance, to visualize the combustion process inside a jet engine, to know the noise factor in an airplane or its engine, and to check the drag and lift forces applied to the structure of the airplane. These cases are suitable for simulation with LES because they require time dependent simulation with turbulent and complex flow regime where details are important.

The movement of an airplane creates vortices in the air. Studying these vortices helps in better understanding drag and lift forces, including their separation phenomena. In the image above, the eddies have altered the shape of the clouds. Adapted from “A Gallery of Fluid Motion” by M. Samimy et al.

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### Automotive Engineering

LES is used to simulate combustion that occurs in internal combustion engines. It is also possible to simulate air and fuel mixing with LES. Some automobile companies use the LES method to check the air flow around the car and reduce the drag force, which greatly affects the fuel consumption. LES is used a lot in academic research in the field of automobiles, these simulations require much more powerful computers than home computers, so the simulations are done by big companies or universities.

### Energy Sector

Investigating fluid flow around turbines is one of the most popular and well-known applications of LES simulations. This is particularly true for scenarios involving several wind turbines arranged in a row, where the effect of each turbine on the wind flow significantly impacts the performance of the downstream turbines that are in the same wind path.

### Industrial** Processes**

Turbulent flow plays a critical role in numerous industrial processes involving material mixing. Additionally, in HVAC systems, which are widely used in industrial environments, turbulent airflow significantly influences the path and distribution of air within environments. Engineers utilize CFD simulations based on the LES method to design, assess, and optimize these scenarios.

## Large Eddy Simulation Equations

In LES, each field variable (ϕ), which is typically velocity or pressure, consists of two components: the Filtered Component (ϕ ̅) and the Subgrid Scale Component (ϕ ̀). So for each field variable such as velocity (u) and pressure (p):

The Filtered Component in LES represents the resolved, large-scale motions of the flow. It captures the dynamics of larger eddies and structures that are directly computed in the simulation, providing a detailed representation of the dominant, energy-containing turbulent features. The filtering operation in LES is typically expressed in the following integral form:

Where ϕ ̅ is the filtered variable, which usually is the velocity of pressure, ϕ is the original variable, x and r denote spatial coordinates and G is the filter kernel, which defines the characteristics of the filter, such as a Gaussian, box, or top-hat filter.

The **Subgrid Scale (SGS)** equations include the effect of that part of the turbulent flow that is smaller than the mesh cells into the simulation. There are various SGS models such as the Smagorinsky model, the Algebraic Dynamic model, Dynamic Global-Coefficient model, Localized Dynamic model, Wall-Adapting Local Eddy-viscosity (WALE) model, RNG-LES model, and various structural modeling approaches. Choosing the right option is done based on the user experience of the type of problem and the help of the previous simulation results. It should be noted that the use of SGS reduces the computational cost and the difference between LES and DNS is more in the use of SGS.

Finally, by selecting the desired SGS model and finding ϕ ̀, ϕ is calculated, and the desired field component variable is obtained and placed in different equations, such as continuity and Navier-Stokes.

## Choice of the Subgrid-Scale (SGS) Modelling

To choose the right SGS model, different parameters should be considered. **WALE** or the **Dynamic Smagorinsky** model are suitable for starting simulation in many applications. The **Dynamic** **TKE** model is suitable for very non-equilibrium flows and reacting flows. In simulation, in many cases the fluid inlet boundary conditions are far from the experimental reality, in the **Dynamic** **Smagorinsky** and **Dynamic** **TKE** models, those simplified boundary conditions are less effective and the flow quickly approaches the real conditions. **DDES** or similar **hybrid** methods are recommended for streams with very high Re number. WMLES, a 0-equation algebraic Detached Eddy Simulation model, is suitable for scenarios with high near-wall cell aspect ratios. Each model has its advantages and disadvantages, and its choice depends on the type of problem and computational ability.

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## Large Eddy Simulation for Incompressible Flows

Whether fluid and flow are compressible or not, LES can be used to simulate them. The simulation of compressible flows is more complicated and has many difficulties in terms of convergence. It also requires a lot of computing power. Simulating such flows requires expertise and a lot of experience. It is very difficult to create and find experimental results to compare with simulation results in these cases.

## Detached Eddy Simulation

Detached Eddy Simulation (DES) is a **hybrid method that combines RANS and LES**. Depending on the design and programming choices, RANS and LES methods, which have better performance, are used in different areas of the fluid flow. For example, in the area near the wall and the boundary layer, RANS is usually employed because of its ability to simulate small vortices. In the separation region, LES is used due to its capability to simulate the details of the turbulent flow.

## Large Eddy Simulation by ANSYS Fluent

ANSYS Fluent is a CFD simulation software based on finite volume that has been used for several decades in various engineering fields and has shown brilliant performance. As can be seen in the figure below, Fleunt offers many methods for simulating **turbulent flow**, and it is possible to change the details and adjust each method. Rest assured that their algorithms and equations are the most advanced in this field. Whether you choose RANS or LES models, this software performs the simulation well.

In ANSYS Fluent, the viscous model setting window for LES includes extensive and detailed options for configuring the subgrid-scale (SGS) model, model constants, and user-defined constants. This setup is crucial for accurately capturing turbulent flows at varying scales.

Our experts at CFDLAND have done many simulations in the field of turbulent flows with RANS and LES and have a lot of experience. You can order your simulation CFD projects in the ORDER CFD PROJECT section with confidence in the quality of our work. Maybe one of them will help you.

## Conclusion

In conclusion, LES is a powerful method for turbulent flow simulation, especially suitable for examining flow details. Compared to RANS, it simulates many more characteristics of the flow, but it has a much higher computational cost. For LES simulations, it is recommended to use ANSYS Fluent, this software has proven its capabilities in this field so far.