The relative importance of vegetation terms was analysed for flow and dispersion in an urban street canyon with avenue-trees. To this end, simulations with three k-e turbulence models and different approaches to model vegetation were performed. The different approaches resulted in rather slight differences in mean flow velocities, turbulence kinetic energies and dissipation rates, but in more pronounced differences in pollutant concentrations. Key words: Pollutant Dispersion, Street Canyon, Avenue-Trees, Computational Fluid Dynamics, k-e Turbulence Models, Vegetation Terms
Large modern sports stadia are often multifunctional buildings that are not only used for sports purposes but also for other events such as concerts, conferences and festivities. Some of the stadia that have been built in recent years in north-western Europe are equipped with a semi-transparent roof that can be opened and closed, depending on the weather conditions and on the type of event. Whereas the roof is often open for sports events, it is often closed for concerts, conferences and festivities. This allows sheltering the indoor stadium environment from wind, rain and snow. A matter of concern related to such facilities is the natural ventilation, since HVAC systems are often not incorporated. This paper presents a numerical (CFD) and an experimental analysis of natural ventilation in a large semi-indoor multifunctional stadium in the Netherlands. CFD validation is performed based on full-scale wind speed measurements. Different alternative ventilation configurations are studied, including widening the existing openings and adding new openings at a few positions. It is shown that adding small openings near roof-height can increase the natural ventilation rate by up to 43%. A particular feature of this study is the coupled simulation of the wind flow in the urban environment around the stadium and the air flow inside the stadium on a high-resolution grid.
Pollutant dispersion is of great relevance for people living in urban areas. High levels of pollutant can usually result from the combination of poor natural ventilation and high-traffic volumes of vehicles. Idealized point and line sources are commonly used to reproduce traffic emissions in simplified portions of urban areas, as street canyons. However, a limited number of studies focuses on the usage of realistic sources, as real car geometries which can influence the flow characteristics and the pollutant distribution inside the canyon. This is also the goal of the present paper for which Computational Fluid Dynamics (CFD) simulations were performed by means of scale-adaptive simulation (SAS) on a street canyon to investigate the impact of idealized and realistic sources. In stage 1 , SAS simulations were performed with idealized line sources by reproducing reduced-scale wind-tunnel (WT) experiments. In stage 2 , SAS simulations were carried out on a street canyon using idealized line sources and realistic sources with different levels of simplification. The results showed that the use of realistic sources can result in an increased concentration of 1.03 - 6.76 (at z = 0.33 m above the ground), with respect to the use of idealized line sources. Overall, at the lower level of the street canyon (e.g. z < 1.5 m), the concentration can be strongly affected by the presence of the car bodies. The results of the present study are expected to help urban planners as well as governmental institutions to reduce pollutant concentrations in the street canyon.
Air curtains can be applied to aerodynamically separate two environments. Air curtains are plane impinging jets at high-Reynolds numbers, preventing the transfer of heat and mass from one environment to another. The performance of an air curtain is called the separation efficiency, which depends on a wide range of jet and environmental parameters, such as jet velocity and turbulence intensity, jet thickness, air temperature differences and pressure differences over the air curtain. This study presents the first results of ongoing research on the optimization of air curtain performance. The first results consist of reduced-scale experiments in a water channel using Particle Image Velocimetry (PIV), and of steady Reynolds-averaged Navier-Stokes Computational Fluid Dynamics (CFD) simulations. The PIV measurements are used to validate the CFD model. Comparison of the experimental results with the results obtained with steady RANS CFD simulations in combination with three different turbulence models showed a fairly accurate agreement.
The accuracy and reliability of 3D steady RANS CFD simulations of wind flow in urban environments can be affected by numerical settings including the turbulence model and the imposed roughness heights. In that regard, various k-ε and k-ω turbulence models and roughness height (ks) values are commonly used when predicting wind flow in urban environments. However, it is insufficiently known to which extent the CFD results may be influenced by these settings when simulating wind flows in complex urban environments with large changes in surface roughness. This is the scope of the present paper, for which wind-tunnel (WT) measurements and CFD simulations were performed on a reduced-scale model (1:300) of a district of Livorno (Italy). Mean wind speed (U), turbulent kinetic energy (k) and turbulence dissipation rate (ε) profiles from WT measurements and CFD simulations were compared at 25 positions and deviations between experimental and numerical results were quantified by three metrics: fractional bias, correlation coefficient and fraction of data within a factor of 1.3. The turbulence model selection had a larger impact compared to the surface roughness selection on U, k and ε values. The best and worst performing turbulence models (e.g. for α = 240° at 0.02 m above the bottom) showed a deviation in terms of correlation (0.89 and 0.61, respectively) of about 0.28. Conversely, the best and worst performing roughness set, (e.g. for α = 240° at 0.02 m above the bottom), showed a deviation in terms of correlation (0.77 and 0.78, respectively) of only 0.01.
