Civil Engineering Reference
In-Depth Information
Figure 3.13
Mean velocity profile and r.m.s.
longitudinal turbulence velocity near the crest
of a steep escarpment (
H
=480m, upwind
slope=0.48).
Thunderstorm downdrafts also have 'boundary layers' with peaks in the velocity
profiles at 50-100m. They also do not have fully developed boundary-layer velocity
profiles. There have been some basic studies using wind-tunnel jets impinging on a flat
board (Letchford and Illidge, 1999; Wood
et al.,
1999) to indicate considerably lower
topographic multipliers compared with developed thick boundary-layer flows. However,
the effect of forward motion of the storm is uncertain.
3.5
Change of terrain
When strong winds in a fully developed boundary layer encounter a change of surface
roughness, e.g. winds from open country flowing over the suburbs of a town or city, a
process of adjustment in the turbulent boundary-layer flow properties develops. The
adjustment starts at the ground level and gradually moves upwards. The result is the
development of an internal boundary layer over the new terrain as shown in Figure 3.14.
Deaves (1981), from numerical studies, developed the following relationships for the
horizontal position of the inner boundary layer as a function of its height,
z:
For flow from smooth terrain (roughness length
z
01
) to rougher terrain (
z
02
) with
z
01
>z
02
:
(3.36)
