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Wind-induced single-sided natural ventilation in buildings was widely investigated based on isolated buildings. However, owing to the presence of surrounding buildings, the wind flow pattern around a building in an urban area becomes very different from that around an isolated building. Considering an urban context, this study investigates the wind-induced single-sided natural ventilation in buildings near a long street canyon under a perpendicular wind direction using CFD method. Four aspect ratios (AR) of the street canyon, from 1.0, 2.0, 4.0 to 6.0, are investigated to examine the influence of street configuration, while eight envelope features are compared to explore the possibility of envelope design in improving natural ventilation performance of urban buildings. Ventilation rate of rooms in buildings is particularly analyzed. AR influences ventilation rate and its distribution among rooms along height of buildings. The percentage decrease of ventilation rate of buildings reaches 67% when AR of a street canyon is increased from 1.0 to 6.0. Envelope design provides a possibility to enhance the adaptability of buildings to dense urban environments. A good envelope design, such as a horizontal feature at the middle of an opening, can break effectively the along-facade flow and thus create a large pressure difference to drive ventilation. The findings of this study are intended to increase the understanding of natural ventilation performance in urban buildings and thus provide information for urban planning and building design. [ABSTRACT FROM AUTHOR]

A street canyon is an important platform for the understanding of local atmospheric flow and other related processes in the built environment. Many previous studies focused on long street canyons under a perpendicular wind direction, as they represent the worst street canyon microclimate, such as stagnation of wind and accumulation of pollutants. While CFD simulations were widely applied to investigate atmospheric processes in street canyons, appropriate computational settings are important factors influencing the predictive reliability. A non-exhaustive literature review of CFD studies on atmospheric processes in long street canyons indicates an arbitrary selection of three important computational settings, namely computational domain configuration, domain dimensions and inflow boundary conditions. Based on previous water tunnel experimental data for street canyons with aspect ratio equal to 0.5, 1.0 and 2.0, this study evaluates the influence of the three computational settings on CFD prediction of isothermal flow field inside the street canyons. Flow field inside an urban street canyon cannot be reasonably predicted using an isolated street canyon included in a conventional computational domain, which, however, can be well predicted using a T-shape computational domain where a street canyon is connected to a free flow layer above the canyon. A T-shape domain with the upstream length, downstream length and height above a street canyon all equal to the height of the street canyon is appropriate when considering both computational cost and predictive accuracy. It is reasonable to use uniform inflow boundary conditions to represent the free layer above street canyons. [ABSTRACT FROM AUTHOR]

This study hypothesizes that reduced-scale models for computational fluid dynamics (CFD) simulations of wind flow and related processes can save numerical resources. To test this hypothesis, both theoretical analysis and a numerical case study are conducted. The CFD simulations are validated against a set of wind tunnel experimental data. Both theoretical and numerical results support the hypothesis. It is found that a reduced-scale model requires fewer cells than a full-scale model to achieve a target near-wall z + value and prediction accuracy and therefore has the potential to save numerical resources. Quantitative analysis shows that this potential is very large, depending on many factors, such as the scaling factor, the target z + value, and the flow problem. The findings of this study should be useful for CFD simulations of wind flow and related processes, particularly over large areas. However, special attention should be paid to the disadvantages of using reduced-scale models, such as the difficulty to fulfill the similarity requirements. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the interunit dispersion characteristics in and around multistory buildings under wind-induced single-sided ventilation conditions using computational fluid dynamics (CFD) method, under the hypothesis that infectious respiratory aerosols exhausted from a unit can reenter into another unit in a same building through opened windows. The effect of balconies on the interunit dispersion pattern is considered. The RNG k − ε model and the two-layer near-wall model are employed to establish the coupled indoor and outdoor airflow field, and the tracer gas technique is adopted to simulate pollutant dispersion. Reentry ratios from each unit to other units under prevailing wind directions are quantified and the possible interunit dispersion routes are then revealed. It is found that many reentry ratios appear to reach around 10.0%, suggesting that the interunit dispersion is an important pollutant transmission route. The interunit dispersion pattern is highly dependent on the incident wind direction and the fact whether the building has protrusive envelope features. On average, the strongest dispersion occurs on the windward wall of the buildings under oblique wind direction, owing to high ACH (air change per hour) values and unidirectional spread routes. Except under a normal incident wind, the presence of balconies intensifies the interunit dispersion by forming dispersion channels to increase the reentry ratios. [Copyright &y& Elsevier]

Highlights: [•] On-site measurement is conducted to verify the existing empirical models. [•] Previous empirical models are not applicable to multistory buildings. [•] CFD method is suitable for predicting ventilation rate in multistory buildings. [•] Ventilation characteristics are different between rooms in a multistory building. [•] A larger incident k profile leads to a decrease of ventilation rate to leeward rooms. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a study of the effect of the inhomogeneous atmospheric boundary layer (ABL) and near-wall treatment on a computational fluid dynamics (CFD) simulation of the flow and dispersion around an isolated building. Based on a revised k−ɛ model (MMK), the effect of the inhomogeneous ABL is studied by comparing several sets of inhomogeneous inlet conditions with a homogeneous one, while the roughness-adapted standard wall functions and two-layer model are compared to examine the effect of the near-wall treatment of rough ground. Wind tunnel experimental data from previous investigations are used to validate the numerical simulations. The inhomogeneous ABL has a significant effect on the prediction of the flow and dispersion fields, depending on the percentage deviation of the incident from the inlet conditions. A decreased incident k profile results in a longer reattachment length on the roof and a larger recirculation region in the wake, which in turn produce decreased and increased nondimensional velocity and concentration fields, respectively. An increased incident U profile enlarges the reattachment lengths and raises the k field, which has a similar effect on the nondimensional velocity and concentration fields as decreased k. It is observed that the use of the two-layer model to solve the near-wall viscous sublayer clearly improves the prediction of flow and dispersion, depending on the role of the low-Reynolds-number effect of the near-wall region on the entire wall-bounded turbulent flow. [Copyright &y& Elsevier]

The purpose of this study was to investigate the effect of a new building which can impose on its immediate ambient environment, on air movement and thermal comfort of people using the area. The effect of eight prevailing wind directions around the building was simulated using computational fluid dynamics (CFD). The CFD simulation was validated by two wind tunnel experiments. The CFD results were used to conduct an air ventilation assessment using velocity ratio (VR) indicator and to assess thermal comfort by an extended predicted mean vote (PMV) evaluation. An analysis of the relationship of VR and PMV with the to-be-built building (the front area index λf and plan area density λp) was also conducted. The result indicates that the PMV trend was generally opposite to the VR trend. The λp would increase due to the new building and the λf would increase in all wind directions except East-South-East and South-East. There could be a significant change in thermal comfort when the λf became smaller than 12% and a significant change in ventilation efficiency if the λf was smaller or larger than 12%. It means that λf can be used as an urban planning parameter for the thermal comfort study in the natural ventilated urban environment. [ABSTRACT FROM AUTHOR]

Previous studies on inter-unit dispersion are limited to isolated buildings. The influence of an upstream interfering building may significantly modify the indoor airflow characteristics of the wind-induced natural ventilated downstream interfered building. Motivated by the findings in previous studies, namely that infectious respiratory aerosols exhausted from a unit can re-enter into another unit in the same building through building envelope openings, this study investigates the inter-unit pollutant dispersion around a multi-story building in two wind directions by employing the computational fluid dynamics (CFD) method. The CFD model employed in this study has been validated against previous experimental data. The results show that the presence of an upstream building greatly changes the path lines around the downstream target building and the pollutant transportation routes around it. The presence of a low upstream building also greatly increases the average air exchange rate (ACH) values and the pollutant re-entry ratios ( R k ) below the source unit on the windward side of the downstream target building for normal wind incidence. However, the presence of a high upstream building greatly increases the average ACH values on the windward side and increases the R k on the leeward side of the downstream building for oblique wind incidence. [ABSTRACT FROM AUTHOR]