Experimental Investigation on Shear-Stress Partitioning for Flexible Plants with Approximately Zero Basal-to-Frontal Area Ratio in a Wind Tunnel

Abstract

Shear-stress partitioning is investigated for one type of flexible plant for very small values of the basal-to-frontal area ratio σ (0.001–0.007). The plant model is made of plastic with irregular structures, which are different from previously investigated rigid regular or flexible roughness elements with larger σ values. The distribution of the surface shear stress and the total shear stress at four plant densities with five plant heights are measured in a wind tunnel using Irwin-type sensors and a load cell, respectively. The wind-tunnel experiments prove that, for these flexible plants, the plant height and lateral cover usually decrease with increasing friction velocity, especially for taller plants, while the plant coverage generally increases. However, these characteristics may be inconsistent with flexible roughness elements with very large σ values (and usually very low aspect ratios) because these elements are less flexible. The present flexible plants generally result in lower shear-stress ratios compared with other roughness elements, which is also proven by the higher values of β (the ratio of the drag coefficient of an isolated roughness element to that of the bare surface) and a constant m (accounting for the difference between the average and peak surface shear stresses) from the present experiments (β = 184–210 and m = 0.68–0.79). The peak mean stress ratio of the present flexible plants is not a constant (1.07–1.54) because it is affected by the lateral cover, which is different from previous studies that consider the ratio to be constant without regard for the lateral cover.