###### Abstract  
Different urban block morphologies can greatly influence the air quality inside the buildings of the block. The model presented in this paper determines the correlation between block morphology and air quality, and outputs the indoor air quality via computational fluid dynamics (CFD) simulations. In this study, stagnant air was assumed to have a velocity lower than 0.15 m/s and considered to be low-quality air in the context of human health. The geometry of the urban blocks was simplified based on real-life buildings. Doors and windows were not 3D-modeled, and all the vertical surfaces of the buildings were considered as potential locations for them. Eight of the highest-frequency wind directions out of sixteen main directions per block were used. Wind directions and velocities were determined based on the weather data for one location chosen for testing. The simulation used the Reynolds-averaged Navier-Stokes (RANS) equations with the k-\(\epsilon\) turbulence model. The results were then interpreted through the specific algorithm using 3D graphic software. The surface of the building envelope was divided into smaller meshes. For each mesh, the average velocity was calculated and meshes were marked for values below the stagnant air threshold. The eight results, one from each wind direction, were synthesized into one final result. The model was tested on eight different urban block morphologies based on real-life blocks, i.e., blocks in Novi Sad, Serbia. The pressure on the building surfaces determined via CFD analyses is presented alongside results from the method described in this paper. The results show that urban block morphologies with clustered buildings inside the urban block, which are typical for the most newly built structures in Novi Sad, have areas on the facades where windows cannot provide elemental natural ventilation throughout the year. To interpret the results obtained in this research, graphs and 3D color-coding models were used. The best results show a 1-tower urban block morphology with only 0.7% of all vertical faces of the model registering a wind velocity lower than the set minimum. The worst results were measured for a traditional urban block typical in old city centers. A total of 54.5% of all the vertical surfaces show no problems with air stagnation in close proximity to them. 3D model visualization; air quality; computational fluid dynamics (CFD); natural ventilation; urban microclimate; urban morphology; urban wind flow +
Footnote †: journal: Medical  
mechanical ventilation. Stagnant outdoor airflow near windows can influence the natural ventilation capacity inside the building and increase the density of air pollutants. The airflow that is observed as stagnant does not have to have a velocity value of 0 m/s. Very slow velocities can have similar effects, and those are common in outdoor urban environments. An urban city block's density and its morphological characteristics are crucial for the local ventilation performance and pollutant dispersion [4]. Airflow simulations in an urban environment are usually limited to representing only how wind intensity and direction influence the outdoor space of the buildings. More research needs to be done in order to highlight the problematic areas in terms of air quality around individual houses or apartments. The geometrical characteristics of the block can change, but most parts are immutable. It is possible to change the type of ventilation in critical areas from natural to mechanical, even in cases where there is no possibility of removing structures inside the block. Developing a method for low air-quality area analysis is very important. Methods used for the detection of critical stagnant air areas have complexity regarding wind velocities and directions. The issue cannot be solved by simulation of one wind-tunnel scenario.