Urban ventilation and the compact Mediterranean city numerical analysis of the dynamic relationships between density, morphology, and wind flow

Combining urbanization and climate change provides new and complex challenges to our cities since they represent a vulnerable element at the center of human life. Cities have been regarded as the emblem of scientific and technological progress, health, and wealth for centuries by humankind. They have gradually attracted the majority of the world population and economic activities. In this process, cities increased in number, size, and density, leading to gradual and inexorable environmental and social degradation. Consequently, the challenge towards more sustainable and resilient built environments has become urgent, especially in existing cities. In this framework, investigations of wind flow in the urban environment are of primary importance. They involve various topics and associated processes, such as human health and comfort, energy consumption and production, durability and robustness of materials, buildings, and infrastructures. Weak wind conditions, often experienced in compact urban areas, can be responsible for low air quality, hence higher potentially hazardous air pollution levels, and higher temperatures, thus increased amplification of heatwaves and human thermal stress. These phenomena cause augmented morbidity, mortality, and energy demand for air cooling and purification. The detrimental effects of air pollution and heat stress on human health and comfort may be mitigated by exploiting urban ventilation. Urban ventilation represents the capacity of a built area for introducing fresh air within its tissue and diluting pollutants and heat within its canyons. The phenomenon is closely related to urban morphology, i.e., the form and the structure of the urban area, the physical characteristics of the buildings, and their mutual arrangement. Conversely, urban morphology can be an effective tool in renewal plans for improving urban ventilation in urban areas. For this purpose, detailed investigations of the impact of actual urban morphologies on urban ventilation are fundamental and represent an essential step towards more sustainable and resilient cities, especially in the Mediterranean area, where a continuous increase in the mean air temperature can already be observed. However, given the complexity of the phenomena in play, the majority of these studies used simplified models of actual urban areas, i.e., arrangements of generic building geometries forming idealized urban structures. Albeit useful, these simplified models may not reproduce the complexity of the built environment. In this regard, the main objective of the present research is to identify and validate a methodology for analyzing the effects of urban density and morphology on urban ventilation performance in actual urban areas. The research is structured in three main phases. The first phase involves describing and quantifying the physical structure of urban areas. The second phase covers the investigation of wind flows at the relevant observation scale for the selected case study. The final phase regards the correlation of the results of the analyses performed during the previous two stages. In the first stage, through an extensive literature review, indices used by practitioners and city planners are selected to describe and quantify the densities and morphologies of different urban tissues. The district scale is used as the operational scale to perform morphological analyses because it is suitable to describe the morphological characteristics of urban areas. The Tuscolano-Don Bosco district, an area in the Southeast of Rome, is selected as the case study. The district presents morpho-typological features typical of compact cities in the Mediterranean basin. Morphological analyses are conducted for the selected area using GIS software, and several scripts and algorithms have been used to combine the raw data to calculate the morphological parameters. In the second stage, appropriate tools, techniques, and indicators for investigating wind flows and assessing urban ventilation performance at the district scale in actual urban areas are selected. Then, 3D steady-state RANS simulations are performed for 12 different wind directions to investigate the impact of different urban morphologies on wind flows and urban ventilation in the selected area. The validation of the computational setting is performed using experimental data from wind-tunnel measurements found in literature and conducted for an idealized case study with physical features similar to the actual case study, applying the so-called sub-configuration approach. The results are presented in terms of non-dimensional mean wind velocity and local age of air, i.e., the time a particle of external fresh air spends replacing a particle of the pollutant in a specific location within the area of interest. The abovementioned parameters and their derivatives are employed in the research as key performance indicators for urban ventilation. In the third stage, the wind environment in the entire case study is described and analyzed. The non-dimensional mean wind velocity calculated at two different heights is correlated to the selected morphological parameters to provide linear, easy-to-use models for highlighting areas potentially vulnerable to poor air conditions without running computationally expensive simulations. Finally, the key performance indicators are used to assess the effectiveness of the different morphologies within the case study in enhancing urban ventilation. The results show that, in a compact urban area, a drastic reduction in the mean wind velocity, up to 60%, can be experienced at the pedestrian level with a consequent worsening of thermal comfort conditions and air quality. The mean wind velocity reduction is dependent on the urban density and increases monotonically with increases in the morphological parameters. Moreover, the linear models between urban ventilation indicators and morphological parameters show remarkable correlations: coefficients of determination up to 89%. Furthermore, results demonstrate that specific morphologies depending on the wind direction can determine locally pronounced increases in the mean wind velocity, up to 135%, and enhance urban ventilation. The research results confirm the potential of urban morphology in enhancing urban ventilation and the need for approaching regeneration plans according to a climate-sensitive/climate-aware way since sustainability and resilience are the ultimate goals. This research contributes to establishing a knowledge base of the wind environment in compact cities and developing guidelines for prioritizing regeneration plans in existing urban areas. This work represents a further step to integrate different disciplines to ease the management of the urban environment complexity. The research outcomes are of interest to stakeholders, practitioners, policymakers, and researchers.