Experimental Investigation of the Aerodynamic Roughness Length for Flexible Plants

Abstract

The aerodynamic roughness length z₀ is modelled as a function of the zero-plane displacement d, plant drag coefficient C_dR, the ratio β of the drag coefficient for an isolated roughness element and the surface drag coefficient, and the lateral cover λ. The model is investigated using wind-tunnel experiments on a ground surface covered by artificial plastic flexible plants of very small basal-to-frontal area ratios (0.001–0.007). The plant drag coefficient at different plant densities is inferred from measurements of the total shear stress and the average surface shear stress for five plant heights at four plant densities. The aerodynamic roughness length and zero-plane displacement are estimated by logarithmic regression of the measured velocity profile with a predetermined friction velocity. With the increase of lateral cover λ, the plant drag coefficient can be considered as a constant for λ < 0.01, and decreasing for λ > 0.01. With an increase in the friction velocity, the roughness length z₀ generally decreases because of plant flexibility, while with increases in plant density and plant height, the value of z₀ increases because the surface becomes physically rougher. Our model for the roughness length z₀ is verified using experimental data for flow over flexible plants. Compared with rigid roughness elements, such as cylinders, cubes and blocks, the normalized roughness z₀/h (where h is the plant height) for the flexible plants is smaller because of the porosity and the larger value of the ratio d/h.