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

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reduces towards the sides and also with distance from the exit of the penetra-
tion. Comparison with the corresponding contour plots at the lower Rayleigh
number of Figure 8.11, shows that the main difference is that in the higher
Ra case, the downward motion diminishes more rapidly with distance from
the penetration exit. Contour comparisons for the normalised spanwise ve-
locity component, U, Figures 8.37 and 8.11, show that the change in Rayleigh
number has little effect on the weak spanwise velocity field outside the pene-
tration. The most detectable difference being the higher levels near the pene-
tration exit, at the higher Ra case. A similar conclusion can be reached about
the Rayleigh number effects on the axial velocity outside the penetration, from
Figures 8.38 and 8.12. One difference is that the axial velocity near the open
end of the penetration increases more substantially with Rayleigh number. As
already seen at the lower Ra case, conditions outside the penetration are prac-
tically isothermal, Figure 8.39, which is why there is very little deviation from
the hydrostatic conditions, Figure 8.38. The corresponding plots ofk contours,
Figure 8.39 , show that in common with the lower Ra case, Figure 8.7, the gen-
eration of turbulence occurs mainly within the boundary layer that develops
along the vertical wall, but in contrast to the lower Ra case, it is confined to a
narrow central region below the penetration exit.
Figure 8.40 shows the contour plots of V, T and P ” within off-centre longi-
tudinal planes, y-z, at locations X of 0.25 and 0.5. Comparison with the cor-
responding plots of Figure 8.14, for the lower Ra case, show that the thermal
and pressure fields remain isothermal and hydrostatic respectively, but while
at the lower Rayleigh number the vertical velocity is higher near the symmetry
plane and drops along the vertical wall, at the higher Ra the vertical velocity
is uniform within both longitudinal planes. At both Ra values the downward
flow becomes weaker with distance from the centre.
The plots of Figure 8.41, of the vertical velocity andk contours within hori-
zontal planes outside the penetration and comparisons with the corresponding
low Ra plots of Figure 8.16, confirm the earlier observations on the downward
motion, while for the turbulent kinetic energy, they indicate that this is mainly
generated at the wall region below the open end of the penetration.