Accurate CFD simulation of coupled outdoor wind flow and indoor air flow is essential for the design and evaluation of natural cross-ventilation strategies for buildings. It is widely recognized that ...CFD simulations can be very sensitive to the large number of computational parameters that have to be set by the user. Therefore, detailed and generic sensitivity analyses of the impact of these parameters on the simulation results are important to provide guidance for the execution and evaluation of future CFD studies. A detailed review of the literature indicates that there is a lack of extensive generic sensitivity studies for CFD simulation of natural cross-ventilation. In order to provide such a study, this paper presents a series of coupled 3D steady RANS simulations for a generic isolated building. The CFD simulations are validated based on detailed wind tunnel experiments with Particle Image Velocimetry. The impact of a wide range of computational parameters is investigated, including the size of the computational domain, the resolution of the computational grid, the inlet turbulent kinetic energy profile of the atmospheric boundary layer, the turbulence model, the order of the discretization schemes and the iterative convergence criteria. Specific attention is given to the problem of oscillatory convergence that was observed during some of these coupled CFD simulations. Based on this analysis, the paper identifies the most important parameters. The intention is to contribute to improved accuracy, reliability and evaluation of coupled CFD simulations for cross-ventilation assessment.
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► Extensive literature review on numerical cross-ventilation studies in overview table. ► Detailed sensitivity analysis for CFD simulations of a cross-ventilated building model. ► The SST k–ω turbulence model shows the best agreement with PIV measurements. ► The turbulent kinetic energy strongly influences the convergence and the results. ► The prediction of the outdoor standing vortex largely affects the indoor air flow.
Urban microclimate studies are gaining popularity due to rapid urbanization. Many studies documented that urban microclimate can affect building energy performance, human morbidity and mortality and ...thermal comfort. Historically, urban microclimate studies were conducted with observational methods such as field measurements. In the last decades, with the advances in computational resources, numerical simulation approaches have become increasingly popular. Nowadays, especially simulations with Computational Fluid Dynamics (CFD) is frequently used to assess urban microclimate. CFD can resolve the transfer of heat and mass and their interaction with individual obstacles such as buildings. Considering the rapid increase in CFD studies of urban microclimate, this paper provides a review of research reported in journal publications on this topic till the end of 2015. The studies are categorized based on the following characteristics: morphology of the urban area (generic versus real) and methodology (with or without validation study). In addition, the studies are categorized by specifying the considered urban settings/locations, simulation equations and models, target parameters and keywords. This review documents the increasing popularity of the research area over the years. Based on the data obtained concerning the urban location, target parameters and keywords, the historical development of the studies is discussed and future perspectives are provided. According to the results, early CFD microclimate studies were conducted for model development and later studies considered CFD approach as a predictive methodology. Later, with the established simulation setups, research efforts shifted to case studies. Recently, an increasing amount of studies focus on urban scale adaptation measures. The review hints a possible change in this trend as the results from CFD simulations can be linked up with different aspects (e.g. economy) and with different scales (e.g. buildings), and thus, CFD can play an important role in transferring urban climate knowledge into engineering and design practice.
•A total of 183 CFD studies on urban microclimate are investigated.•Yearly number of studies on the topic increased rapidly from 1998 till 2015.•Investigated cities, target parameters and keywords are summarized for every study.•Past achievements on the field are documented and future perspectives are provided.•The keywords used in the reviewed studies are evaluated based on their popularity.
Accurate and reliable computational fluid dynamics (CFD) simulations are essential for the assessment of cross-ventilation of buildings. To determine which CFD models are most suitable, validation ...studies are required. A detailed review of the literature indicates that most CFD validation studies only employed the 3D steady Reynolds-averaged Navier-Stokes (RANS) approach and/or focused on a limited set of flow parameters. Therefore, the objective of this paper is the validation of both 3D steady RANS simulations and large eddy simulation (LES) of cross-ventilation in a generic isolated enclosure with wind-tunnel measurements. The evaluation is based on five parameters: mean velocity, turbulent kinetic energy, ventilation flow rate, incoming jet angle and incoming jet spreading width. The RANS simulations are conducted with the standard k-ε (SKE), RNG k-ε, realizable k-ε (RLZ), SST k-ω and RSM turbulence models, whereas the LES is performed with the dynamic Smagorinsky subgrid-scale model. SST/RNG/RSM reproduce the experimentally observed direction of the incoming jet, but all RANS models fail in reproducing the turbulent kinetic energy, which is too low especially above and below the jet, because steady RANS does not capture the vertical flapping of the jet. This transient feature is reproduced by LES, resulting in a better reproduction of all three measured parameters (velocity, turbulent kinetic energy, volume flow rate). It is concluded that choice of the model (RANS vs. LES) actually depends on which parameter is the target parameter, noting that the use of LES entails an increase in computational demand with a factor of ≈80–100.
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•Assessment of five steady RANS turbulence models and LES for cross-ventilation flow.•SST/RNG/RSM reproduce the experimentally observed direction of the incoming jet.•All RANS models underpredict turbulent kinetic energy inside the enclosure.•LES reproduces the mean velocity and turbulent kinetic energy with higher accuracy.•Better performance of LES due to the reproduction of transient flow features.
Previous research indicated that the surface-averaged forced convective heat transfer coefficient (CHTC) at a windward building facade can vary substantially as a function of building width and ...height. However, existing CHTC expressions generally do not consider the building dimensions as parameters and are therefore strictly only applicable for the building geometry for which they were derived. Most CHTC expressions also categorize facades only as either windward or leeward. This indicates the need for new and more generally applicable CHTC expressions. This paper presents new generalized expressions for surface-averaged forced CHTC at building facades and roofs that contain the reference wind speed, the width and the height of the windward building facade as parameters. These expressions are derived from CFD simulations of wind flow and forced convective heat transfer for 81 different isolated buildings. The 3D Reynolds-averaged Navier-Stokes equations are solved with a combination of the high-Re number realizable k-ε model and the low-Re number Wolfshtein model. First, a validation study is performed with wind-tunnel measurements of surface temperature for a reduced-scale cubic model. Next, the actual simulations are performed on a high-resolution grid with a minimum near-wall cell size of 400 μm to resolve the entire boundary layer, including the viscous sublayer and the buffer layer, which dominate the convective surface resistance. The new CHTC expressions are analytical formulae (trivariate polynomials) that can easily be implemented in Building Energy Simulation (BES) and Building Envelope Heat-Air-Moisture (BE-HAM) transfer programs. The accuracy of the expressions is confirmed by in-sample and out-of-sample evaluations.
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•New CHTC expressions in analytical form (trivariate polynomials).•Taking into account wind speed, building width, building height.•Taking into account differences between windward, leeward, side facade, roof.•Accuracy evaluation based on in-sample and out-of-sample data.•Easy to implement in Building Energy Simulation and HAM programs.
Knowledge of the pressure distribution on building walls is important for the evaluation of wind loads and natural ventilation. Wind-induced pressure distributions are influenced by a wide range of ...factors including approach-flow conditions, urban surroundings and building geometry. Computational Fluid Dynamics (CFD) can be a valuable tool for determining mean wind pressure coefficients on building facades. However, while many CFD studies of mean wind pressure on buildings have been performed in the past, the vast majority of these studies focused on simple building geometries without facade details such as balconies. These details however can drastically influence the flow pattern and the overall pressure distribution on the facade. This paper presents a systematic evaluation of 3D steady Reynolds-Averaged Navier–Stokes (RANS) CFD for predicting mean wind pressure distributions on windward and leeward surfaces of a medium-rise building with and without balconies. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements. It is shown that building balconies can lead to very strong changes in wind pressure distribution, because they introduce multiple areas of flow separation and recirculation across the facade. The results show that steady RANS, in spite of its limitations, can accurately reproduce the mean wind pressure distribution across the windward facade of the building. The average deviations from the wind-tunnel measurements are 12% and 10% for the building with and without balconies, respectively. In addition, also the important impact of the reference static pressure and the turbulence model are demonstrated.
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► Steady 3D RANS CFD simulations for wind pressures on buildings with balconies. ► Detailed grid-sensitivity analysis and validation with wind tunnel measurements. ► Building balconies strongly influence mean wind pressures at building facades. ► Local differences in mean wind pressure coefficients: increase of 0.6, decrease of 0.7. ► 3D steady RANS gives very accurate results: 10–15% accuracy at windward facade.
Façade geometrical details can substantially influence the near-façade airflow patterns and pressures. This is especially the case for building balconies as their presence can lead to multiple ...separation and recirculation areas near the façades and hence large changes in surface pressure distribution. Computational fluid dynamics (CFD) has been widely used to investigate the impact of building balconies, mainly based on the steady Reynolds-averaged Navier-Stokes (RANS) approach. The objective of the present study is to evaluate the performance of steady RANS and large-eddy simulations (LES) in predicting the near-façade airflow patterns and mean surface pressure coefficients (Cp) for a building with balconies for three wind directions θ = 0°, 90°, 180°, where 0° is perpendicular to the façade under study. The evaluation is based on validation with wind-tunnel measurements of Cp. The results show that both RANS and LES can accurately predict Cp on the windward façade for θ = 0° with average absolute deviations of 0.113 and 0.091 from the measured data, respectively. For the other two wind directions, LES is clearly superior. For θ = 90°, the average absolute deviations for RANS and LES are 0.302 and 0.096, while these are 0.161 and 0.038 for θ = 180°. Large differences are found in the computed flow fields on the balcony spaces. Because RANS systematically underestimates the absolute values of both Cp and mean wind speed on the balconies, it is suggested that building design based on RANS might result in excessive ventilation and in too high wind nuisance level.
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•Performance of RANS and LES is evaluated for a high-rise building with balconies.•RANS and LES accurately predict mean static pressure for wind direction 0°.•LES performs better than RANS for wind directions 90° and 180°.•Prediction of wind speed ratios on balcony spaces is systematically investigated.•Design based on RANS will result in too high wind nuisance level.
Computational Fluid Dynamics (CFD) is increasingly used to study a wide variety of complex Environmental Fluid Mechanics (EFM) processes, such as water flow and turbulent mixing of contaminants in ...rivers and estuaries and wind flow and air pollution dispersion in urban areas. However, the accuracy and reliability of CFD modeling and the correct use of CFD results can easily be compromised. In 2006, Jakeman et al. set out ten iterative steps of good disciplined model practice to develop purposeful, credible models from data and a priori knowledge, in consort with end-users, with every stage open to critical review and revision (Jakeman et al., 2006). This paper discusses the application of the ten-steps approach to CFD for EFM in three parts. In the first part, the existing best practice guidelines for CFD applications in this area are reviewed and positioned in the ten-steps framework. The second and third part present a retrospective analysis of two case studies in the light of the ten-steps approach: (1) contaminant dispersion due to transverse turbulent mixing in a shallow water flow and (2) coupled urban wind flow and indoor natural ventilation of the Amsterdam ArenA football stadium. It is shown that the existing best practice guidelines for CFD mainly focus on the last steps in the ten-steps framework. The reasons for this focus are outlined and the value of the additional – preceding – steps is discussed. The retrospective analysis of the case studies indicates that the ten-steps approach is very well applicable to CFD for EFM and that it provides a comprehensive framework that encompasses and extends the existing best practice guidelines.
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► Application of the ten-steps approach to CFD in Environmental Fluid Mechanics (EFM). ► Existing best practice guidelines (BPG) in CFD mainly focus on only last three steps. ► Reason: specific character and disadvantages of CFD: need for verification and validation. ► More attention should be given to the other steps in CFD studies of EFM. ► Two case studies show that ten-steps approach is very well applicable to CFD in EFM and that it extends existing BPG.
High-rise buildings can significantly increase the wind speed at pedestrian level, and knowledge of building aerodynamics and pedestrian-level wind (PLW) conditions is therefore imperative in their ...design. This study aims at evaluating different building geometry modifications to reduce PLW speed around an isolated high-rise building. Numerical simulations with computational fluid dynamics (CFD) are performed to evaluate the effect of canopies, podiums and permeable floors. To the best knowledge of the authors, a systematic study on the impact of these modifications on PLW conditions using validated CFD simulations has not been reported before. Grid-sensitivity analyses are performed and sub-configuration validation is applied using wind-tunnel measurements from the literature. It is shown that a canopy or a podium can significantly reduce the area-averaged PLW speed (up to 29%) and maximum PLW speed (up to 36%) around the high-rise building. In general, the PLW speeds decrease with increasing canopy or podium size. The introduction of a permeable floor to the building can reduce the maximum and area-averaged mean wind speed. However, when low-floor building layers are removed, adverse effects are noted, i.e. the average PLW speed increases (up to 21%) and the lower-speed wake region behind the building is reduced in size.
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•A systematic parametric CFD study on reducing pedestrian-level wind speeds.•Canopies, podiums and permeable floors in various configurations are evaluated.•Mean wind speeds decrease with increasing canopy or podium size.•Permeable floors at mid-lower heights can decrease the maximum mean wind speeds.•Smaller podia and permeable floors at low heights can increase mean wind speeds.
Wind comfort and wind safety for pedestrians are important requirements in urban areas. Many city authorities request studies of pedestrian wind comfort and wind safety for new buildings and new ...urban areas. These studies involve combining statistical meteorological data, aerodynamic information and criteria for wind comfort and wind safety. Detailed aerodynamic information can be obtained using Computational Fluid Dynamics (CFD), which offers considerable advantages compared to wind tunnel testing. However, the accuracy and reliability of CFD simulations can easily be compromised. For this reason, several sets of best practice guidelines have been developed in the past decades. Based on these guidelines, this paper presents a general simulation and decision framework for the evaluation of pedestrian wind comfort and wind safety in urban areas with CFD. As a case study, pedestrian wind comfort and safety at the campus of Eindhoven University of Technology are analysed. The turbulent wind flow pattern over the campus terrain is obtained by solving the 3D steady Reynolds-averaged Navier–Stokes equations with the realisable
k–
ε model on an extensive high-resolution grid based on grid-convergence analysis. The simulation results are compared with long-term and short-term on-site wind speed measurements. Wind comfort and wind safety are assessed and potential design improvements are evaluated. The framework and the case study are intended to support and guide future studies of wind comfort and wind safety with CFD and, this way, to contribute to improved wind environmental quality in urban areas.
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► CFD is increasingly used for assessment of wind comfort and safety in urban areas. ► Framework with integration of CFD best practice guidelines in iterative procedure. ► Graphical representation of the framework by flow-chart including three cases. ► Application for complex case study with grid-convergence analysis and validation. ► Support and guidance for future studies of wind comfort and wind safety with CFD.
Wind flow in urban environments is an important factor governing the dispersion of heat and pollutants from streets, squares and buildings. This paper presents a coupled CFD modelling approach for ...urban wind flow and indoor natural ventilation. A specific procedure is used to efficiently and simultaneously generate the geometry and the high-resolution body-fitted grid for both the outdoor and indoor environment. This procedure allows modelling complex geometries with full control over grid quality and grid resolution, contrary to standard semi-automatic unstructured grid generation procedures. It also provides a way to easily implement various changes in the model geometry and grid for parametric studies. As a case study, a parametric analysis of natural ventilation is performed for the geometrically complex Amsterdam ArenA stadium in the Netherlands. The turbulent wind flow and temperature distribution around and inside the stadium are solved with the 3D steady Reynolds-averaged Navier–Stokes equations. Special attention is given to CFD solution verification and validation. It is shown that small geometrical modifications can increase the ventilation rate by up to 43%. The coupled modelling approach and grid generation procedure presented in this paper can be used similarly for future studies of wind flow and related processes in complex urban environments.
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