Idealized pollutant sources are widely used by the scientific community to replicate traffic emissions in wind tunnel tests and CFD simulations. However, it is unclear to what extent such idealized sources can adequately reproduce the emission and dispersion by cars in idle (static) or moving situations in streets. This study investigates the impact of a static idealized point source (S-IPS) versus an idling or static (S-) and a moving or dynamic (D-) realistic car source (RCS) on the pollutant dispersion in a street canyon. First, 3D steady RANS and LES simulations are performed with a S-IPS and validated by means of wind-tunnel tests. Next, LES simulations are performed to analyze the impact of S-IPS versus S-RCS. Finally, three D-RCS with different car speeds are simulated and compared with S-RCS. The results show that using a S-RCS increases the plane-averaged concentration by 11%–140% at z/H = 0.03 and by 30%–50% at y/H = 0, with respect to S-IPS. The comparison of S-RCS and D-RCS shows that car movement can also have a large impact on pollutant dispersion along the canyon.
Wind-driven rain is one of the most important boundary conditions for numerical Heat-Air-Moisture (HAM) transfer models. Due to the complexity of WDR however, the current HAM models generally incorporate it in a very simplified way. Recent research has shown that CFD can provide accurate estimates of WDR on building facades. Therefore, in this paper, a combined CFD-HAM approach is presented. It is applied for a simplified wall model. It is shown that the accuracy of the HAM-simulation results is to a large extent determined by the time resolution of the meteorological input data and by the data-averaging technique used for these data. Some guidelines for accurate HAM analyses with WDR are provided.
Wind energy is a highly promising resource to approach a sustainable built environment. Vertical axis wind turbines (VAWT) offer the advantage of omni-directional operation over horizontal axis wind turbines (HAWT). This makes them ideal for utilization in urban environments which are characterized by frequently varying wind direction. However, a comparatively small amount of research on VAWTs has resulted in low power coefficients (CP) compared to HAWT. The pitch angle (β) is a parameter which is commonly used in HAWTs to enhance their performance and is a potential optimization parameter for VAWT as well. However, a recent study based on inviscid modeling states that it will not have any significant effect on CP . Therefore, in order to elucidate this claim using a viscous calculation, performance optimization of VAWTs by varying β is investigated in the current paper. CP, moment and thrust coefficient (Cm and CT) and angle of attack are obtained from a CFD simulation of a straight-bladed H-type VAWT using 2D Detached Eddy Simulations (DES). The turbine is operating at a tip speed ratio (TSR) of 4 and β-values of 0˚, +3˚, and -3˚ are investigated. The results show that unlike the inviscid results, increasing β from 0˚ to +3˚ would increase C P by 4% while decreasing it to -3˚ will result in a 16% reduction in CP. Furthermore, a change in β shifts the instantaneous loads between the upwind and downwind halves of the turbine. These results can be helpful for designing more optimized VAWTs.
The term “impinging jet” refers to a high-velocity fluid stream that is ejected from a nozzle, a narrow opening or an orifice, and which impinges on a surface. As applied to the built environment, impinging jets are used in air curtains to separate two environments subjected to different environmental conditions with the purpose of improving thermal comfort, air quality, energy efficiency and fire protection in buildings. The design and application of state-of-the-art air curtains requires detailed knowledge of the relationship between the separation efficiency of air curtains—their main performance criterion—and a wide range of jet and environmental parameters involving air curtain design. In order to address the current knowledge gaps in the field, this project encompasses an investigation into the impact of different jet and environmental parameters on the performance of air curtains while giving special attention to the study of innovative jet excitation techniques by means of optimizing the separation efficiency of air curtains. This project is being carried out in close collaboration with the air curtain manufacturer ‘Biddle B.V.’.
Information on pedestrian-level wind (PLW) speed for wind comfort assessment can be obtained with wind-tunnel measurements or Computational Fluid Dynamics (CFD). Wind-tunnel measurements for PLW are routinely performed with low-cost techniques such as hot-wire or hot-film anemometers, Irwin probes or sand erosion, while Laser-Doppler Anemometry (LDA) and Particle-Image Velocimetry (PIV) are less often used because they are more expensive. CFD simulations are routinely performed by the relatively low-cost steady Reynolds-Averaged Navier-Stokes (RANS) approach. Large-Eddy Simulation (LES) is less often used because of its larger complexity and cost. This paper first briefly addressed wind-tunnel and CFD techniques to determine PLW speed, and some comparative studies that systematically indicate that the low-cost wind-tunnel techniques and steady RANS simulations can provide accurate results (?10%) in high wind speed regions while their accuracy strongly deteriorates in low wind speed regions. Next, it is argued that this does not necessarily compromise the accuracy of PLW comfort assessment, because the higher wind speed regions provide the largest contribution to the discomfort exceedance probability in the comfort criterion. Although LDA, PIV and LES are inherently and potentially more accurate techniques, this paper supports the continued use of faster and cheaper techniques for PLW comfort assessment.
Accurate modeling of urban wind flow is important for the assessment of the natural ventilation of the outdoor environment and therefore for outdoor air quality. Different geometrical levels of analysis can be used, ranging from regular arrays of obstacles to real and complex case studies. In order to avoid the complexity of real case studies and to obtain more generic results, regular arrays of obstacles represent a suitable level to study the relation between urban morphology and outdoor ventilation. This paper presents the first results of a numerical study with Computational Fluid Dynamics (CFD) of isothermal wind flow in generic urban configurations. Nine urban configurations are defined with regular arrays of obstacles by increasing the plan area density (from 0.1 to 0.6) and the frontal area density (from 0.02 to 0.45). By this variation, the flow structure varies from isolated obstacle flow over wake interference flow to skimming flow. Among the nine configurations defined, four test cases are selected for a preliminary study. 3D steady Reynolds-Averaged Navier-Stokes (RANS) CFD simulations are performed for the four selected configurations and the simulations are compared with experimental wind-tunnel data. An overall fairly good agreement is found between the experimental and numerical results obtained with RANS simulations. The results allow establishing some relationships between morphological and fluid dynamics parameters of urban wind flow and outdoor ventilation. It is found that the upstream building density can cause a decrease of up to the 70% of the ventilation rate in the central street. Further work will focus on extending this parametric analysis and extrapolating it to real cities.
The development of advanced materials and technologies for application to new and existing structures, infrastructures, and equipment, aiming at improving their response to service and extreme loads, represents an emerging issue both from an academic and a professional viewpoint.Relevant performance assessment procedures, based on effective experimental verification methods [1] and refined numerical simulation models [2], are quickly evolving as well.New or improved building materials and innovative protection technologies offer updated solutions to the constantly increasing capacity requirements in structural, geotechnical, plant, and mechanical engineering design and rehabilitation.At the same time, they open novel and challenging research and application perspectives.The objective of this special issue was to provide readers with a representative outlook of the latest achievements in this field, including emerging aspects in modelling, testing, manufacturing, and practical implementation studies.The response of the scientific community was encouraging, with over 100 manuscripts submitted.The published papers offer a selected and articulated overview of the examined topics.
Outdoor ventilation is very important for a healthy and livable urban environment. It is strongly influenced by wind speed and direction, which in turn are affected by urban morphology. This paper first provides a detailed review of the literature for CFD studies of outdoor ventilation for generic urban configurations. The review indicates that there is a clear lack of studies for urban configurations where not all parallel streets have equal street widths. Next, the paper presents Computational Fluid Dynamics (CFD) simulations of outdoor ventilation for generic configurations with parallel streets of equal and unequal street widths. The 3D steady RANS equations with the standard k–ɛ model and the passive scalar transport equation are used to calculate the effective local mean age of air at pedestrian level as an indicator of pollutant removal efficiency. The study is based on grid-convergence analysis and on validation with previously published wind-tunnel measurements. The influence of a central and wider main street on the wind-velocity pattern and on the effective local mean age of air of the surrounding area is analyzed for different wind directions. For wind directions oblique or perpendicular to the main street, the presence of this main street generally improves the ventilation efficiency because the main street acts as a sink of clean air. However, this is generally not the case for the parallel wind direction, where the higher flow rate through the main street reduces the flow rates through the parallel narrower streets, negatively affecting their ventilation efficiency.
