Shielding Forces On Structural Arrays In Turbulent Boundary Layers

The characteristics of wind and wind pressures in arrays of structural obstructions is important in the design of these structures. As the wind impacts a structural array, the loading patterns are altered. This thesis investigates the interference effects between structures of varying geometries and aspect ratios. A series of wind tunnel tests were carried out to measure the drag forces experienced by structures in the wakes of other obstructions. The approach flow was always configured to reflect an open country terrain. Additionally, a sensitive force balance, with a small threshold value, was designed and used to measure the forces acting on the structures.;The study examined shielding effects produced in the wakes of buildings having circular, triangular, and rectangular planar cross sections. Each arrangement consisted of two inline structures. Some configurations examined identical structures, while others studied combinations of different geometries. The drag force was then measured on the downstream structure. Universal equations emerged, describing the shielding, as a function of separation. These equations were solely dependent on the planar geometry of the upstream structure. The form of these equations is given by: {dollar}\gamma=1-\rm ae\sp{lcub}-b\Delta{rcub}.{dollar} {dollar}\gamma{dollar} being the shielding factor, {dollar}\Delta{dollar} the non-dimensional separation, and a and b are empirically found parameters that depend on the planar geometry of the upstream structure.;Second, the influence of two cylinders, separated by a distance S{dollar}\rm \sb{lcub}y{rcub},{dollar} on an identical structure in their wake was investigated. Analytical functions were found to describe the shielding. At small separations, the flow between the upstream structures behaved like a jet. At large separations, the downstream body did not distinguish between the separate wakes of the elements, and regarded them as one body.;Third, drag development over inline and staggered arrays was investigated. The structures considered were cylindrical, rectangular and combinations of the two geometries. The spacing between consecutive rows also was varied. The results obtained from the homogeneous arrays were then used to develop an analytical method to predict the drag attenuation over arrays. The numerical method was validated by considering eight irregular arrays. Experimental measurements and predictions agreed well. Comparisons with experimental results reported by Marshall (1971), and Hussain & Lee (1980) were also well.;Finally, velocity profiles were measured upstream, inside and downstream of cylindrical arrays. When plotted as a function of fetch, the friction velocity, displacement height, and roughness length increased until they reached equilibrium, inferring stability of the drag force on the constituent elements. Equilibrium values of Z{dollar}\rm \sb{lcub}o{rcub},{dollar} d, and U{dollar}\*{dollar} compared very well with theoretical predictions, developed by Raupach (1992), Lettau (1969), and McNamara (1976)