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

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The reverse feature is observed at the top end, where the core fluid is at a
temperature closer to that of the hot side. Figure 7.73 shows large differences
between the numerical and experimental data. The numerical simulation is
mostly similar to the two-dimensional simulation in which within the central
longitudinal plane there is a circulation cell with fluid moving upwards along
the hot, upper side and then returning downwards along the cold, lower side.
In contrast, the experimental data show that within this plane the predomi-
nant motion is in the downward direction, which suggests that within other
longitudinal planes, closer to the side walls, there must be a compensating
downward motion and thus strongly three-dimensional flow conditions. The
experimental data suggests that probably because at this angle of inclination
the buoyancy force is weak, even with a spanwise aspect ratio as high as 6.8,
the presence of the end walls causes the development of a three-dimensional
flow, with downward flow in the middle and upward flow along the sides.
This is a feature that the k-ε-AWF model, is unable to reproduce. The rea-
son may be that EVM tends to produce too high values for turbulent viscosity
which prevent the flow breaks down to smaller cells. Corresponding compar-
isons for the velocity fluctuations in the longitudinal and normal directions
are presented in Figures 7.75 and 7.76. Comparisons show that while near the
end walls the velocity fluctuations are severely under-estimated, over the rest
of the cavity the k-ε-AWF returns the same fluctuation levels as those found
in the measurements, though the distribution is different. Nevertheless, the
large deviation between the predicted and measured flow fields, makes these
comparisons less meaningful.
Figures 7.77-7.80 present comparisons between measured and predicted
temperature profiles in the off-centre longitudinal planes, Z=0.25, 0.655, 0.75
and 0.875. The comparisons show that, as is also the case for the central longi-
tudinal plane, Z=0.5, the core temperature is close to that of the cold side near
the bottom end wall and close to that of the hot side near the top end wall and
also that the predictions are in reasonable agreement with the measurements.
The fact that measured temperature field, in agreement with the predicted one
is mainly two-dimensional, suggests that the three-dimensional flow features
exert a weak influence on the temperature field. Figures 7.81 and 7.82 present