
In terms of computation, the SpalartAllmaras model is the least expensive
turbulence model of the options provided in
FLUENT, since only one turbulence
transport equation is solved.
The standard

model clearly requires more computational effort than the
SpalartAllmaras model since an additional transport equation is solved. The
realizable

model requires only slightly more computational effort than the
standard

model. However, due to the extra terms and functions in the
governing equations and a greater degree of nonlinearity, computations with the
RNG

model tend to take 1015% more CPU time than with the standard

model. Like the

models, the

models are also twoequation
models, and thus require about the same computational effort.
Compared with the

and

models, the RSM requires additional
memory and CPU time due to the increased number of the transport equations for
Reynolds stresses. However, efficient programming in
FLUENT has reduced the
CPU time per iteration significantly. On average, the RSM in
FLUENT requires 5060% more CPU time per iteration compared to the

and

models. Furthermore, 1520% more memory is needed.
Aside from the time per iteration, the choice of turbulence model can affect the
ability of
FLUENT to obtain a converged solution. For example, the standard

model is known to be slightly overdiffusive in certain situations, while
the RNG

model is designed such that the turbulent viscosity is reduced in
response to high rates of strain. Since diffusion has a stabilizing effect on
the numerics, the RNG model is more likely to be susceptible to instability in
steadystate solutions. However, this should not necessarily be seen as a
disadvantage of the RNG model, since these characteristics make it more
responsive to important physical instabilities such as timedependent turbulent
vortex shedding.
Similarly, the RSM may take more iterations to converge than the

and

models due to the strong coupling between the Reynolds stresses and
the mean flow.