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. These experiments and the data collected are
described in SAFL Report 492.
In the first experiment we used several layers of chicken wire with a total height of 5cm,
a porosity of 98% 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 setup
was considered to be a crude representation of a canopy of trees 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 with a smooth surface
which could be a simplified representation of a high bank or buildings on the upwind side
of a 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 roughness of the
two canopies was determined to be 1.3 cm and 0.5 cm, respectively, and the
displacement height was determined to be 2.3cm and
6.68cm. The roughness of the wind tunnel floor downstream from the canopy was
determined to be 0.00003m =0.03mm. 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 well-known logarithmic velocity profile should
form. The shear stress on the surface downwind from the canopy was unaffected by
wind sheltering after x/h=100, and the effect was less than 10% after x/h=60. A
separated flow region formed downstream of each of the three canopies. The distance to
reattachment was about 8 times the displacement height in the canopy. After a distance
x/h=25 an internal boundary layer could be identified. It was characterized by rising
shear stresses with distance from the canopy. Between x/d=8 and x/h=25 a turbulent
shear layer touches down on the surface. Shear stresses in this range are highly
variable, depending on canopy roughness and porosity. The velocities profiles downwind
from the canopy are shaped by two attributes: the canopy roughness (z0) and the
canopy porosity: the 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. Wind velocity profiles took a much longer
distance than x/h=100 to overcome the canopy effect. This leads to the conclusion that
surface shear stresses make a much faster transition than velocity profiles downstream
from the canopy. In other words, momentum transfer is faster than mass transfer
downstream from the canopy.
Jaster, Dane A.; Perez, Angel L. S.; Porte-Agel, Fernando; Stefan, Heinz G..
Wind velocity profiles and shear stresses on a lake downwind from a canopy: Interpretation of three experiments in a wind tunnel.
St. Anthony Falls Laboratory.
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