We are interested in the shear stresses exerted by wind on a lake surface,
especially if a lake has a small surface area. We have therefore begun to study
the development of the atmospheric boundary layer over a small lake surrounded
by a vegetation canopy of trees or cattails. Wind tunnel experiments have been
performed to simulate the transition from a canopy to a flat solid surface. In the
first experiment we used several layers of chicken wire with a total height of 5cm,
a porosity of 98.8% and a length of 2.4m (8 ft) in flow direction to represent the
vegetation canopy, and the floor of the wind tunnel consisting of plywood was
used to represent the lake. The chicken wire represents a porous step that ends
at x=0. This experimental set-up was considered to be a crude representation of
a canopy of tress or other vegetation that ends at the shore of a lake. In a second
experiment we used an array of pipe cleaners inserted in a styrofoam board to
represent the canopy. The porosity of that canopy was 78%. In a third
experiment we used a solid step which could be a simplified representation of a
high bank or buildings on the upwind side of the lake.
Wind velocity profiles were measured downstream from the end of the canopy or
step at distances up to x=7m. Using the velocity profile at x=0, the absolute
roughnesses of the two canopies were determined to be 1.3 cm and 0.5 cm,
respectively, and the displacement heights were determined to be 2.3 cm and
6.6 cm. The roughness of the wind tunnel floor downstream from the canopy
was determined to be 0.00001m =0.01mm.
Three distinct layers were identified in the measured velocity profiles downstream
from the canopy: the surface layer in response to the shear on the wind tunnel
floor, an outer layer far above the canopy, and a mixing/blending layer in
between. With sufficient distance downwind from the canopy the mixing layer
should disappear, and the remaining two layers should form the well-known
logarithmic velocity profile. In other words the memory of the canopy should
become erased from the velocity profile with sufficient distance downstream. The
shear stress on the wind tunnel floor was found to approximately double from x=0
to about x/h=100. Changes of mass fluxes, momentum fluxes and energy fluxes
integrated with height above the wind tunnel floor were related to the distance
from the edge of the canopy (x=0). The aerodynamically rough and porous
canopy made the velocity profiles and the associated fluxes substantially
different from those downstream from a solid step. One significant difference was
the absence of a separated flow region downstream from the highly porous
canopy. Instead, the velocity profile coming out of the rougher and more
permeable canopy was linear with distance above the wind tunnel floor.
Essentially the velocities profiles differ because of two attributes: the canopy
roughness (z0) and the canopy porosity. We believe that the (initial) shape of the
each velocity profile at the end of the canopy is given by the canopy roughness,
while the velocity profiles downwind from the canopy are shaped by both
roughness and the porosity of the canopy.
Perez, Angel L. S.; Jaster, Dane A.; Thill, James; Porte-Agel, Fernando; Stefan, Heinz G..
Wind velocity profiles and shear stresses downwind from a canopy: Experiments in a wind tunnel.
St. Anthony Falls Laboratory.
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