MIL-HDBK-1011/2
APPENDIX A (continued)
penetrate. Figure A-4a compares wind velocity profiles in a ponderosa pine
stand to those in the open; Figure A-4b shows the influence of foliage from
seasonal wind measurements in a deciduous oak-beech forest.
The extent of the sheltered area produced by a windbreak varies
with the physical dimensions and porosity of the barrier. Porous barriers
cause less turbulence and can create a greater area of total shelter than
solid barriers. The more solid the barrier, the shorter the distance to the
point of minimum wind velocity and the greater the reduction in velocity at
that point. The velocity, however, increases more rapidly downwind of the
minimum point than behind a more porous barrier. Figure 10 in Section 4 of
this handbook shows a cross section of the airflow near a screen of 50 percent
porosity. Figure 10 also shows the effect of varying porosity in shelter at
ground level downwind.
A porosity of 40 to 50 percent has been found to provide maximum
extent of sheltered area. This reduction in leeward velocity occurs without
appreciable disturbance of the airflow. Windbreaks with higher porosities
(greater than 50 percent) do not form a turbulent wake and the airflow pattern
is dependent on the velocity of the flow. These windbreaks provide more
protection from 5H to 20H with velocities reduced to 30 percent of the free
stream velocity, but less protection up to 5H. Windbreaks with lower
porosities (less than 35 percent) exhibit a turbulent wake that provides more
protection up to 5H with velocities reduced to 10 percent of the free stream,
but provides less protection from 5H to 20H with velocities up to 60 percent
of the free stream. The large-scale eddies within the wake are sensed as
gusts and may be disruptive to outdoor uses in the wake area.
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