Civil Engineering Reference
In-Depth Information
Figure 4.14
Mean pressure coefficients on a
cube in turbulent boundary-layer flow (Baines,
1963).
The mean pressure distribution on a tall prism of square cross-section in a turbulent
boundary-layer flow is shown in Figure 4.15 (Baines, 1963). This is representative of the
pressure distribution on a tall building in the atmospheric boundary layer. The mean
pressure coefficients are again based on the dynamic pressure calculated from the mean
wind speed at the top of the prism. The effect of the vertical velocity profile on the
windward wall pressure is clearly seen. The maximum pressure occurs at about 85% of
the height. On the windward face of unshielded tall buildings, the strong pressure
gradient can cause a strong downwards flow, often causing high wind speeds which may
cause problems for pedestrians at ground level.
4.4.5
Jensen number
For bluff bodies such as buildings immersed in a turbulent boundary-layer flow, the ratio
of characteristic body dimension, usually the height, h, in the case of a building, to the
characteristic boundary-layer length, represented by the roughness length,
z
0
,
is known as
the Jensen number. In a classic series of experiments, Jensen (1958) established the need
for equality of
(h/z
0
)
in order for wind-tunnel mean pressure measurements on a model of
a small building to match those in full scale. The effect is greatest on the roof and side
walls, where the increased turbulence in the flow over the rougher ground surfaces
promotes shorter flow re-attachment lengths.
