
Overview
The simplest "complete models'' of turbulence are twoequation models in which
the solution of two separate transport equations allows the turbulent velocity
and length scales to be independently determined. The standard

model in
FLUENT falls within this class of turbulence model and has become the
workhorse of practical engineering flow calculations in the time since it was
proposed by Launder and Spalding [
196]. Robustness, economy, and
reasonable accuracy for a wide range of turbulent flows explain its popularity
in industrial flow and heat transfer simulations. It is a semiempirical model,
and the derivation of the model equations relies on phenomenological
considerations and empiricism.
As the strengths and weaknesses of the standard

model have become known,
improvements have been made to the model to improve its performance. Two of
these variants are available in
FLUENT: the RNG

model [
408] and
the realizable

model [
330].
The standard

model [
196] is a semiempirical model based on
model transport equations for the turbulence kinetic energy (
) and its
dissipation rate (
). The model transport equation for
is derived
from the exact equation, while the model transport equation for
was
obtained using physical reasoning and bears little resemblance to its
mathematically exact counterpart.
In the derivation of the

model, the assumption is that the flow is fully
turbulent, and the effects of molecular viscosity are negligible. The standard

model is therefore valid only for fully turbulent flows.
Transport Equations for the Standard

Model
The turbulence kinetic energy,
, and its rate of dissipation,
, are
obtained from the following transport equations:
Modeling the Turbulent Viscosity
The turbulent (or eddy) viscosity
,
, is computed by combining
and
as follows:
Model Constants
The model constants
and
have the following default values [
196]: