Dependence of outdoor thermal comfort on street design in hot and dry climate
The present work addresses the contribution of street design toward the development of a comfortable microclimate at street level for pedestrians. The work is design-oriented and seeks to provide a quantitative knowledge readily interpretable from the perspective of urban designers. Street geometries are investigated, including various aspect ratios, i.e. height-to-width ratio H/W, solar orientations and a number of design details. First, symmetrical urban canyons with H/W equal to 0.5, 1, 2 and 4 and for different solar orientations (i.e. E-W, N-S, NE-SW and NW-SE) are studied. Secondly, asymmetrical profiles with different openness to the sky are investigated together with the role of ar-chitectural details such as galleries, horizontal overhangs on façades and rows of trees, considered as possible ways to improve the outdoor thermal comfort further in the summertime. Moreover, the analysis focuses on the local differences in the thermal sen-sation across the street, i.e. street centre vs. street sides, which influence the frequenta-tion of the street. A special emphasis is placed on a human bio-meteorological assess-ment of these microclimates by using the thermal index PET, Physiologically Equiva-lent Temperature. The investigation is carried out by using the three-dimensional numerical model ENVI-met 3.0, which simulates the microclimatic changes within urban environments in a high spatial and temporal resolution. Model calculations are run for typical summer conditions in Ghardaia, Algeria (32.40° N, 3.80° E), a subtropical region characterized by a hot and dry climate. Additionally, short-term field measurements are carried out in Freiburg, Germany, and in Ghardaia (Beni-Isguen), Algeria, during the summer 2003. In the former site, the microclimate changes due to geometry and the effects of the street irradiation patterns on the heat gained by a human body are dealt with in detail. In the latter site, a quantitative evaluation of the thermal effectiveness of existing architectures in a hot-dry climate is the focus. The simulations show that the thermal comfort is difficult to reach passively in such an extreme climate but improvements are possible by means of appropriate geometrical forms. All investigated urban describers are found to influence the final thermal sensa-tion. Contrasting patterns in the comfort situation are found between shallow and deep urban streets as well as between the various orientations studied. Wide streets (H/W  0.5) are highly uncomfortable for both orientations. Yet, N-S ori-entation shows some advantage over E-W orientation, and this benefit increases as the aspect ratio increases. Explicitly, this is expressed by a shorter period of heat stress and lower PET maxima. Moreover, heat stress can effectively be mitigated if galleries, trees or textured façades are appropriately combined with the aspect ratio and solar orientation. A comparison of all case studies reveals that the duration, the period of day of extreme heat stress, as well as the spatial distribution of PET across the canyon depend strongly on aspect ratio and on street orientation. This is crucial since this will directly influence the design choices in relation to street usage, e.g. streets exclusively planned for pedes-trian use or including motor traffic, and also the time of frequentation of urban spaces. The simulations as well as the on-site measurements also confirmed the dominant role of the radiation fluxes expressed by the mean radiant temperature Tmrt for summer con-ditions. The human body absorbs energy from the irradiated surrounding surfaces and from a direct exposure of his body. This fact points out the necessity of shading as a main strategy for keeping the street area in comfort range. Air temperature and wind speed are secondary factors with respect to comfort as these vary less with urban ge-ometry changes in comparison to Tmrt. The issue of solar access indoors has been briefly discussed as an additional criterion in designing the street by including winter needs and draw attention on the double role of the street, i.e. as interface of urban and architectural scales. Design recommendations are also outlined for designing a comfortable urban street. Methodologically, ENVI-met revealed to be a good tool for the prognosis of the urban microclimate changes within urban areas, and also in the assessment of outdoor comfort through a satisfactory estimation of the mean radiant temperature. A number of eventual refinements of the model are mentioned to improve its accuracy. The work also highlights the necessity of more on-site measurements and more subjec-tive votes of people for validating the simulations results and in order to strengthen a practice-oriented knowledge about comfort in urban areas.
Straßenschlucht, human-biometeorologie, PET, Himmelrichtung, Freiräume, urban canyon, aspect ratio, street orientation, ENVI-met, gallery, overhangs, urban vegetation
Networked Digital Library of Theses and Dissertations