LES — Large Eddy Simulation

Instead of trying to resolve the entire energy spectrum of turbulence, LES model defines a grid that can capture the largest turbulent scales, the eddies, in the flow down to a spatial scale Δ, called the cutoff length scale.

Large eddies (1) are dependent on the nature of the flow and its boundary conditions. They represent the left-hand side of the turbulent energy spectrum.

The smallest eddies (2) are smaller than the cutoff length scale. LES models these small eddies by taking advantage of the fact that they are homogenous and universal in any turbulent flows. A physical or empirical model called a subgrid scale model accounts for their effect in the flow.

The resulting velocity field is a filtered velocity field where the small, subgrid, scales are approximated using one of the following models.

Smagorinsky-Lilly model

Assumes that the energy production and dissipation of small scales are in equilibrium. This is the oldest and most used LES subgrid scale model.

WALE model

Takes into account the dissipative effect of the turbulent structures with a high rate of deformation, a high rate of rotation, or both, and reproduces proper asymptotic variation of turbulent viscosity close to the wall. WALE stands for Wall Adapting Local Eddy-viscosity.

Vreman model

Captures the transition of flows from a laminar to a turbulent regime.

When you use the LES turbulence method, it is recommended that you:

• Select one of the high-order advection schemes on the 3D Flow Solver page in the Solver Parameters dialog box.

• Use the Convective Outflow type of the Flow Boundary Condition simulation object to specify the outlet boundary conditions on your fluid domain.

• Select Fractional Step from the Parallel Flow Solver Scheme list.

• Use a very small timestep to resolve the turbulence.

• When you specify turbulent inlet conditions, consider the flow geometry to compute a proper length scale. This insures that pressure instability is not created due to poor length scale computation during the run. You also need to specify turbulent intensities in the range of 1 to 10% for internal flows.

How do I

Define Advanced Parameters

Define Generic Entities

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Solution options

Understanding thermostat options for steady state analyses

Thermal initial conditions

3D flow initial conditions

Understanding ambient conditions

Joining fluid meshes

Understanding the turbulence models

Understanding out-of-bound options for time-dependent tables

Setting turbulence scale values

Turbulence Characteristics

Including additional input files for thermal analysis

Restarting a solution

Understanding time-varying time step

Target Temperature and Target Temperature Change

Solving the model

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Adaptive time stepping method

Parallel flow solver schemes

Computed quantities for multiple convective BCs

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LES — Large Eddy Simulation, Simcenter 3D 2021.1 Series

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