Extended Abstract (336K)2.5
Patterns of mean wind and turbulence inside a square porous windbreak
John D. Wilson, Univ. of Alberta, Edmonton, AB, Canada; and T. K. Flesch
Mean windspeed and turbulence were sampled in a square plot that was sheltered by a porous plastic fence. The side length of the plot was D=20m, while the height and the resistance coefficient of the fence were h=1.25m, kr =2.4. The experiments took place in a large, uniform field of low, thin stubble (roughness length z0 order 1 – 2 cm). Measurements, mostly with an array of 7 cup anemometers placed at z/h=0.5, cover all incident mean wind directions (b ) and a wide range in atmospheric stability, as determined from vertical profiles of the mean wind S0(z) and temperature T0(z) on a tower standing outside the shelter.
When the mean wind blew at nearly normal incidence (b=0) across a nearby fence to any particular anemometer in the shelter, a strong fractional wind reduction
FWR={ S0(z) – S(x,y,z) } / S0(z)
was recorded, its numerical value being (roughly) that expected in the near lee of a long straight fence. FWR was not very sensitive to stability, but very sensitive to mean wind direction: indeed when the wind blew over any corner of the plot in order to reach the anemometer (b=45o), there was no reduction in mean speed!
This talk will also cover turbulence measurements from a sonic anemometer within the shelter, and speculate on the aerodynamics causing the dependency of FWR on wind direction. A companion poster presentation will discuss tracer gas trials from an area source on ground inside this plot.
Session 2, turbulence and dispersion in canopies Part 1
Monday, 20 May 2002, 1:30 PM-3:30 PM
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