## Turbulence modeling

It
is crucial to obtain low numerical and convergence errors in turbulent
flow simulations. This is because the relevance of turbulence modelling
only becomes significant in CFD simulations when other sources of error,
especially the numerical and convergence errors, have been eliminated
or properly controlled.

It
is clear that no proper evaluation of the merits of different turbulence
models can be made unless the discretisation error of the numerical algorithm
is known, and grid sensitivity studies become crucial for all turbulent
flow computations.

Unfortunately,
there is no universally valid general model of turbulence that is accurate
for all classes of flows. Validation and calibration of the turbulence
model with test data are necessary for all applications*.

(*
Calibration in this sentence refers to testing the ability of a CFD code
to predict global quantities of interest for specific geometries of engineering
design interest (see definition in the section about errors and uncertainties)
and not to tuning the coefficients of a turbulence model, which cannot
be recommended for a novice user).

If
possible, to select the right turbulence model, examine the effect and
sensitivity of results to the turbulence model by changing the turbulence
model being used.

When
using a particular turbulence model, check the published literature to
find out if there are any known weaknesses of the model concerned.

The
weaknesses of the standard k- model (Launder and
Spalding [1974]), which is the most commonly used model in industrial
applications, are listed below together with some indications of possible
palliative actions which might be fruitfully considered.

Choose
a suitable near-wall model. Decide whether to use a wall function method,
in which the near-wall region is bridged with wall functions, or a low
Reynolds number model, in which the flow structure in the viscous sub-layer
is resolved.

What
model to choose depends on the available resources and the requirements
for resolution of the boundary layer.

The
validity of the wall function approach or the use of a low Reynolds number
model should be examined for the flow configuration under study.

Wall
function methods are not valid in the presence of separated
regions and/or strong three-dimensional flows.