City, wind, energy: limits for applying natural ventilation for thermal comfort in its relation to urban densification in the hot-humid tropical climate

The object of this research is the changes in natural ventilation conditions and sunlight caused by the urban environment. The aim is to prove the hypothesis that higher urban density levels thru vertical buildings is compatible with thermal comfort in naturally ventilated residential buildings during 80% of total year´s hours. Surrounding obstacles modify wind patterns, reducing or increasing the pressure field over façades. On the other hand, tall buildings located nearby might reduce significant solar radiation portions over vertical planes. This study focus the hot-humid climate of Fortaleza, Ceará, Brazil (3° S) and analyses four different representative urban forms within this city. The investigation uses computational simulations in different stages and approaching scales in a sequential and complementary methodology in which each phase supplies the necessary data to the next level. Starting point was to evaluate the urban ventilation using a CFD tool according to two dominant wind directions, which provided Cp data over buildings openings. Then, internal flows were simulated in order to determine air changes rates and velocity field in the apartment. Annual thermodynamic performance in threelong permanence rooms was calculated and the results were evaluated using ASHRAE (2004) adaptive model. Cooling effect due to air movement was considered to extend the limits of thermal comfort zone. The hypothesis proved to be valid since more vertical urban forms such as scenarios 2 and 3 obstructed significant solar radiation portions over lower floors compared to urban settlements with lower buildings. Even with airflow rates about 40% lower in some cases with southeast wind, scenario 2 had more hours within the comfort zone. The same occurred with scenario 3, in which airflow rates for east wind were slightly higher than that obtained in scenarios 1 and 4, but achieving greater comfort periods in lower floors, reinforcing the combined effect of obstructing solar radiation and maintaining the conditions to naturally ventilate building´s façades. Considering the cooling effect due to air movement made possible to achieve thermal comfort situations in 85% of the cases. Yet, lower correlations between airflow rates and land use in each scenario or average height of buildings reinforce the premise that wind variability and the dynamism of the urban form prevent the immediate determination of which spatial configuration may reduce or enhance natural ventilation conditions, also indicating that high-density urban levels are compatible with satisfactory environmental conditions within buildings. (AU)