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

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moves down from the top of the cold wall with high velocity and decelerates
as it approaches the bottom of the cavity. All of the near-wall treatments pro-
duce results which are broadly in agreement with the experimental data. One
inconsistency exists at the top of the cold wall where the turbulence models
predict higher velocities compared with the experiment. Both numerical and
experimental data show higher velocity at the bottom of the hot wall. The hy-
drodynamic flow pattern at the bottom of the hot wall is similar to that at the
top of the cold wall. Therefore some portion of this inconsistency might be
from the fact that at those regions the flow involves impingement and change
of direction, the flow is more complicated at the two ends than at the other
regions. The models employed therefore, possibly due to the use of the ef-
fective viscosity approximation, are unable to fully reproduce the measured
mean flow development at the two ends. Moreover, AWF predicts higher ve-
locity compared with the other models. As it shown, at the bottom of hot wall,
AWF predicted higher velocity which continued to the top of the hot wall. This
may be caused due to higher turbulence resulted fromk-ε-AWF adjacent to the
walls (Figure 6.32).
In Figure 6.32, the rms velocity fluctuations resulting from the k-ε using
LRN, AWF and SWF are compared for the vertical and inclined cavities. Mea-
surements for the inclined case are also included. The experimental measure-
ments reveal that the turbulent stresses are higher at the positions near the
mid-height of the tall cavity compared to those near to the top and bottom of
the cavity. Turbulence is generated due to the mean velocity gradients (mean
flow shear) and buoyancy (mean temperature gradients). As can be seen in
the mean velocity profiles of Figure 6.31, the mean flow shear is stronger at
the middle of the cavity. These comparisons show that all turbulence models
significantly under-predict the near-wall turbulence levels by as much as 20
percent in the core region. On a more positive note, all predictions reproduce
the severe reduction in turbulence levels near the end walls.
Generally the predictions from the LRN, AWF and SWF approaches are
quite similar. This is because of the use of the eddy-viscosity approximation in
all three sets of computations, which is a rather simple relation and is not able