Forme urbaine et environnement thermoaéraulique en climat chaud et sec

Abstract

Our research tends to explore the impact of a complex topography and irregular compact urban structure on the airflow mechanisms at street level, examine the effect of these phenomena on outdoor thermal comfort during the daily cycle, on both summer and winter periods, and produce recommendations for urban planners and architects. This case study is based on the old city of Ghardaïa, in the northern part of the Algerian Sahara, under hot and dry climate. Built on a rocky hill, the traditional establishment is characterised by a strong compact urban structure. This original morphology is particularly interesting for studying the parameters influencing urban airflow patterns. Five extensive measurement campaigns of the major climatic parameters that affect thermal comfort have been performed between June 2003 and August 2006. The main results highlighted significant air movements even in very deep street canyons under certain conditions. Data analysis showed that the air movements within the streets were closely related to the upwind conditions above the roofs which are dependent on the slope exposure to the wind. Furthermore, two fundamentally different in-flow patterns were identified depending on the street length and the street asymmetry: a significant channeling phenomenon within the long street canyon and hypothetical tri-dimensional in-flows in both short and asymmetric streets. Results also showed the effect of curved streets and covered walkways on air movements. Finally, conclusions of this research confirm previous studies on the effect of urban morphology (street geometry, orientation and horizontal protection) in reducing summer heat stress in the desert cities. They also show the importance of wind action on comfort conditions during both summer and winter seasons.