The computation of turbulent natural convection flows - Turbulence ...

421
results of the 3D numerical simulation were mostly two-dimensional. The 3D
simulation using the k-ε-AWF produced similar but weaker movement near
the top of the cavity, compared with the experimental data, but it was not
strong enough to penetrate all the way down to the bottom of the cavity, as
was seen in the experimental data. The numerical simulation also produces
similar circulations at the bottom with the opposite direction. Comparisons
of normal stresses showed that while near the end walls the normal stresses
were severely under-estimated, over the rest of the cavity thek-ε-AWF returns
the same fluctuation levels as those found in the measurements. The inconsis-
tency is due to the existence of 3-dimensional flow conditions reported in the
experimental study of the 15 ◦ stable test case.
The version of the k-ε-AWF which included a limiter for the turbulent vis-
cosity based on a realisability criteria was then employed to simulate the flow.
The calculation without the turbulent viscosity limit produced steady-state re-
sults even in a time dependent simulation. The calculation with the turbulent
viscosity limit, on the other hand, resulted in a time-dependent solution. The
reason or this behaviour is that in the original model the level of turbulent
viscosity was sufficient to damp out any instabilities which might have led to
large-scale unsteady structure. The limiter became active in certain regions
of the flow, acting to reduce the turbulent viscosity, and hence allowing such
structure to develop. The turbulent viscosity limit is based on a physical con-
straint (Section 3.3.2). In general the flow pattern was similar to the results of
the k-ε-AWF without the limiter. These comparisons revealed that the inclu-
sion of the turbulent viscosity limit does not make significant changes to the
temperature profiles. The comparisons showed that introduction of the viscos-
ity limiter, in addition to enabling the k-ε-AWF to return unstable flow condi-
tions, led to some improvements in the predicted time-averaged velocity field,
but there were large deviations between the predicted and measured flow
fields. Improvements in the predicted rms velocities were evident, most no-
tably near the end walls, where the predicted levels were significantly higher
than those of k-ε-AWF predictions without the viscosity limiter, and close to
the measured values. These predictive improvements are most likely to re-
sult from the fact that the predicted rms velocities included both the modelled