Wind velocity profiles and shear stresses downwind from a canopy: Experiments in a wind tunnel

Abstract

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.