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Wind tunnel experiments of spatial distribution of surface shear stress in flexible plant canopy

Liqiang Kang 1 ,2, Junjie Zhang1 ,3, Xueyong Zou1 ,2, Chunlai Zhang1 ,2, Hong Cheng1 ,2, Zhicheng Yang1 ,3
1State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing Normal University, Beijing 100875, China, 2MOE Engineering Research Center of Desertification and Blown-sand Control, Beijing Normal University, Beijing 100875, China, 3Academy of Disaster Reduction and Emergency Management, Ministry of Civil Affairs & Ministry of Education, Beijing 100875, China

Plant plays an important role in reducing the wind erosion which is a serious global environmental problem. Plant can absorb a part of the total shear stress of the wind by extracting the momentum from air flow, and thereby results in the lower surface shear stress acting on the bare ground between the plants. The spatial distribution of surface shear stress is a key factor to determine the sheltering effect of plant and degree of soil erosion. In the present paper, the spatial distribution of surface shear stress in artificial flexible plant canopy is measured in a wind tunnel with the 16 m long working section and the cross-sectional area of 1m1m (width height) at Beijing Normal University. In order to enable instantaneous measurement of the spatial distribution of surface shear stress, a custom measurement system of surface shear stress is designed and consists of 32 Irwin-type sensors, 32 differential pressure transducers and a data collection system. The artificial flexible plant is made of flexible plastic material and its shape is different from the previous plant model. The plants are arranged in staggered configuration on the wind tunnel floor with the covered region of 8m long and 1m width. Three plant heights are selected: 3cm, 5cm and 8cm, four plant number densities are studied: 18, 32, 50 and 98 plants per square meter for each plant height, and 9 different freestream wind velocities between 5 and 16.6m/s are used to systematically determine the distribution characteristics of surface shear stress beneath the flexible plant canopy. The results show that the spatial distribution of surface shear stress is very non-uniform. The minimum surface shear stress usually lies in front and back of the plant, while the maximum value is on the side of the plant. With the increase of friction wind velocity, the mean surface shear stress increases, while with the increase of plant number density and plant height, the mean surface shear stress decreases. The standard deviation of surface shear stress generally increases with friction wind velocity, while it decreases with plant number density except for the plant with 3cm height. The average surface shear stress ratio decreases with the increase of plant number density and plant height, but it is almost independent of friction wind velocity. Due to the flexibility of the plant, the frontal area of the flexible plant changes with the variation of friction wind velocity, and with the increase of plant height, the changed range of the frontal area of the flexible plant increases. Hence, the flexibility of the plant should be considered in the study of surface shear stress.