Airflow patterns within a complex urban topography under hot and dry climate in the Algerian Sahara

Abstract

This research aims to explore the impact of a complex topography and irregular compact urban forms on wind environment and airflow mechanisms at street level and examine the effect of these phenomena on outdoor thermal environment during the daily cycle, on both summer and winter periods. This case study is based on the old city of Ghardaïa, in the northern part of the Algerian Sahara, under hot and dry climate. Three winter and summer measurement campaigns were carried out between February 2005 and August 2006. Extensive on-site measurements of air temperature, horizontal wind speed and direction were collected simultaneously throughout the Ksar, within the streets and above the roofs. 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 dependant 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, the thermal environment was found strongly influenced by airflow patterns during both summer and winter seasons.