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6.1.1 Choosing a Turbulence Model

It is an unfortunate fact that no single turbulence model is universally accepted as be-ing superior for
all classes of problems. The choice of turbulence model will depend on considerations such as the
physics encompassed in the flow, the established practice for a specific class of problem, the level of
accuracy required, the available computational resources, and the amount of time available for the
simulation. To make the most ap-propriate choice of model for your application, you need to
understand the capabilities and limitations of the various options.

The purpose of this section is to give an overview of issues related to the turbulence models provided
in ANSYS FLUENT. The computational effort and cost in terms of CPU time and memory of the
individual models is discussed. While it is impossible to state categorically which model is best for a
specific application, general guidelines are presented to help you choose the appropriate turbulence
model for

6.1.2 Mesh Considerations for Turbulent Flow Simulations

Successful computations of turbulent flows require some consideration during the mesh generation.
Since turbulence (through the spatially-varying effective viscosity) plays a dominant role in the
transport of mean momentum and other parameters, you must as-certain that the turbulence
quantities in complex turbulent flows are properly resolved if high accuracy is required. Due to the
strong interaction of the mean flow and turbu-lence, the numerical results for the turbulent flows tend
to be more susceptible to mesh dependency than those for laminar flows.
It is therefore recommended that you resolve, with sufficiently fine meshes, the regions where the
mean flow undergoes rapid changes and the shear layers with large strain rates.

You can check the near-wall mesh by displaying or plotting the values of y+, y∗, and Rey, which are all
available in the postprocessing dialog boxes. It should be remembered that y+, y∗, and Rey are not
fixed, geometrical quantities. They are all solution-dependent. For example, when you double the
mesh (thereby halving the wall distance), the new y+ does not necessarily become half of the y+ for the
original mesh.

6.1.3 Near-Wall Mesh Guidelines

Wall Functions

The log-law, which is valid for equilibrium boundary layers and fully developed flows, provides upper
and lower limits on the acceptable distance between the near-wall cell centroid and the wall. The
distance is usually measured in the dimensionless wall units, y+ (≡ ρuτ y/µ), or y∗. Note that y+ and y∗
have comparable values when the first cell is placed in the log-layer but are different
by Cµ1/4 i.e. ≈ 0.5.

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