![]() The rest of the RANS models that reproduced the turbulent kinetic energy or normal stresses upwind of the building relatively well showed significant underestimation of the turbulent kinetic energy or normal stresses above the center or downwind of the building. Most of the RANS models showed significant overestimation of the turbulent kinetic energy or normal stresses upwind of the building. and Mohamed and Wood compared 11 and 6 RANS models, respectively, against the measurements of the flow around the building, which Kono and Kogaki also used to validate their LES approach. With respect to the CFD turbulence models for this kind of analyses, it is preferable to use large-eddy simulation (LES) type turbulence models, which have higher accuracy for the behavior of the separated flows around buildings than the Reynolds-averaged Navier-Stokes (RANS) turbulence models. To formulate these standards and guidelines, it is important to understand the effects of various factors on the wind conditions above the roof of a high-rise building. Therefore, in addition to guidelines for the micro-siting of roof-mounted SWTs, design standards are needed for roof-mounted SWTs that can be set up near the roof surface. Furthermore, in dense urban areas, it is common for the turbulence intensity at the heights of high-rise buildings to exceed the characteristic value of the NTM of IEC 61400-2. On the other hand, due to the high turbulence intensity near the roof surface, high-rise buildings can require a higher tower and stronger support structure which can be very expensive and can rule out most installations of SWTs. This is because the speed of the approaching wind generally increases with height, and there is less shielding and turbulence from the surrounding buildings. Wind conditions above the roof of a high-rise building can be more favorable for installing SWTs than those of mid- to low-rise buildings. However, there is limited information regarding wind conditions just above building roofs because most of the studies have focused on the wind load on the buildings, the natural ventilation of the buildings, or on the wind environment near the ground surface. There are many published studies on the behavior of wind near buildings (e.g., ). In addition, in order to avoid fatigue failures, wind turbines should ideally be installed in regions where the turbulence intensity is low. Thus, in order to achieve the ideal capacity factors, wind turbines need to be installed in regions where the mean wind speed is sufficiently high. Here, the capacity factor of a wind turbine at a given site is defined as the ratio of the energy actually produced by the turbine to the energy that could have been produced if the machine ran at its rated power over a given time period. With the increase in the number of small wind turbines (SWTs) installed on the roofs of buildings, there have been reports of some capacity factors being very low. When there is no prevailing wind direction, the center of the roof is more favorable for installing SWTs than the corners or the edge midpoints of the roof. At leeward representative locations of the roof, the bottoms of the height range of favorable wind conditions are typically higher than those at the windward representative locations, but the favorable wind conditions are much better at the leeward representative locations. In contrast, at the midpoint of the roof's windward edge, wind conditions are generally not favorable at relatively low heights. Moreover, at windward corners of the roof, wind conditions are generally favorable at relatively low heights. This tendency is more prominent as the angle between the wind direction and the normal vector of the building’s leeward face with longer roof edge increases. ![]() ![]() The LES results confirmed that as HAR decreases (i.e., as the building width decreases), the variation in wind velocity over the roof tends to decrease. From the viewpoint of installing small wind turbines (SWTs) on rooftops, this study investigated the effects of wind direction and horizontal aspect ratio (HAR = width/length) of a high-rise cuboid building on wind conditions above the roof by conducting large eddy simulations (LESs). ![]()
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