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ABSTRACT

Wind tunnel tests were carried out to investigate the change in the flow regime in relation to cylindrical element concentration. Wood dowels of a uniform diameter, 2.3 cm, and of one of two heights, 2.3 and 4.6 cm, were placed along the length of the wind tunnel working section (250 cm) in a diagonal array and in rows normal to the flow using two different spacings, 3.45 and 9.2 cm. The array was then exposed to three preselected shear velocities (28.8, 32.9 and 45.1 cm/s).

The velocity close to the top surface of the dowels was greatly reduced by their presence. The height (z) of the zone in which winds were slightly modified can be found, at least when z above the plane of dowels approximately equals 0.16D + H where H is the height of the obstacle and D is the spacing between obstacles. Due to the formation of a wake zone and vortices to the rear of each cylindrical element, the element lee was characterized by a strong local mean velocity deficit below the element height. The results of the study showed that an increase in the concentration of the elements caused a significant flow deceleration along and above the top surface of the cylinders. An outcome of the study is a simple approximate estimate of the value of the complex modification of the roughness height (Z_o) due to the presence of an array of cylinders; that is, Z_o = Hë, where ë is the element concentration.