Effects of wind characteristics on natural ventilation of buildings

Prirodna ventilacija je proces dobave svježeg zraka u zatvoreni prostor i uklanjanje ustajalog zraka iz prostorije bez rada mehaničkog uređaja. Za izmjenu zraka koriste se prirodne sile gibanja zraka i uzgona. U vrijeme sve veće potrošnje električne energije i zagađenja okoliša, ovaj ekološki prihvatljiv način izmjene zraka je vrlo poželjan. Za iskorištavanje punog potencijala prirodne izmjene zraka i konstrukciju uređaja koji će je pospješiti, važno je poznavati karakteristike parametara koji utječu na efikasnost ventilacije. S ciljem detaljnijeg analiziranja parametara koji utječu na prirodnu ventilaciju zgrada su provedeni modelski eksperimenti u zračnom tunelu za generiranje modela atmosferskog graničnog sloja na Institutu za mehaniku fluida i prijenos topline Tehničkog sveučilišta u Grazu, Austrija. Cilj istraživanja bio je odrediti utjecaj karakteristika vjetra na intenzitet prirodne ventilacije samostalne zgrade, kao i one smještene u urbanom okruženju, i to za atmosferski granični sloj modeliran za strujanje nad ruralnim i prigradskim terenom. Karakteristike strujanja zraka su određene na temelju mjerenja brzine sustavom užarene žice, a za izmjenu zraka u modelu zgrade je korišten sustav praćenja koncentracije plina. Ispitivanje je provedeno za jednostranu i poprečnu ventilaciju modela zgrade, pri čemu se na modelu zgrade nalazio jedan odnosno dva prozora, mijenjajući brzinu i kut nastrujavanja zraka i udaljenost među modelima zgrada. Za oba je modela atmosferskog graničnog sloja izmjena zraka veća prilikom poprečnog nastrujavanja zraka i raste s porastom brzine. Također, izmjena zraka je prilikom poprečnog nastrujavanja intenzivnija ako je model zgrade samostalan, tj. ako nije dio urbanog naselja.Natural ventilation is a process of supplying the fresh air in the indoor space and removing the saturated air from the indoor space without using a mechanical device. The driving forces for this air exchange are the buoyancy and the flow momentum. At a time of increasing electrical consumption and environmental pollution, it is important to promote this environmentally friendly technology. To exploit its full potential, it is necessary to properly understand the relationship between parameters that affect its efficiency. Therefore, small-scale model experiments were carried out in a boundary layer wind tunnel at the Institute of Fluid Mechanics and Heat Transfer at the Graz University of Technology, Austria. It was attempted to determine the influence of inflow characteristics on the characteristics of a natural ventilation of a stand-alone cubic building as well as of a building situated in an urban environment. The atmospheric boundary layer was successfully modeled to simulate flow over rural and suburban terrain types. A hot-wire anemometry system was used to measure turbulent flow characteristics, while tracer gas system was employed to determine air exchange rates. Experiments were conducted for single-side and cross-ventilation arrangements of the studied building model. The analyzed parameters include the flow velocity and its incidence angle as well as a distance between the buildings. The experimental results indicate that the natural ventilation has similar patterns for both studied atmospheric boundary layer models, whereas the rural-type atmospheric boundary layer model is more sensitive to the changes of input parameters. Air change rate is larger for larger wind velocities, as well as for cross-ventilated buildings compared to the single-side ventilation arrangement