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A Cylindrical Meshing Methodology for Annual Urban Computational Fluid Dynamics Simulations
Abstract
For urban CFD simulations, it is considered a best practice to use a box-shaped simulation domain. Box-shaped domains, however, show drawbacks for airflow from several wind directions as remeshing and additional preprocessing steps become necessary. We introduce a routine to create a cylindrical mesh that expedites the simulation of arbitrary wind directions using OpenFOAM. Results computed with the cylindrical domain are validated against wind tunnel data. We report that the cylindrical method yields comparable results in terms of accuracy and convergence behaviour. Further, run time comparisons in a real-world scenario are conducted to discuss its advantages and limitations. Based on the findings, we recommend using the cylindrical approach if at least eight wind directions are analyzed for which we report 18% run time savings. The cylindrical domain along with automated best practice boundary conditions has been implemented in Eddy3D – a plugin for Rhinoceros.
... Partly for the same reason, most of these studies were conducted separately from annual or monthly energy evaluations, despite the insightful tradeoffs between them which have rarely been explored to date. Lately, Eddy3D -a new Grasshopper interface for the validated OpenFOAM Computational Fluid Dynamics (CFD) simulation engine has introduced new possibilities in this respect [37] -it allows for annual thermal comfort analyses by calculating the annual hourly wind speed for a given spatial context. This is done by using an annual interpolation method based on wind factors calculated for multiple wind directions (8,16, or more) conducted cylindrically around the geometry. ...
... Eddy3D uses OpenFOAM's blockMesh utility for the background mesh and snappyHexMesh to snap the background mesh to the building geometry. For the background mesh, a cylindrical simulation domain approach was used which allows reusing the same computational mesh for every wind direction, thus reducing the computation time and storage space [37]. Within the cylindrical mesh, the mesh was further refined with the help of three levels of refinement within a refinement box that surrounds the buildings of interest (Fig. 4). ...
... To justify the cylindrical simulation domain for such analyses, it has been validated and showed promising results when compared to wind tunnel data for a simple cross ventilation geometry, see Fig. 5 [37]. Additionally, we compared the high-rise and the courtyard geometry in FAR 4 for the three predominant wind directions found for our contexts (0°, 270°, and 315°) for both cylindrical and box meshing approaches (see Appendix B. Fig. 12), to confirm that the cylindrical meshing approach yields equally accurate results. ...
With the rise of awareness of health and well-being in cities, urban environmental analysis should expand from energy performance to new environmental quality-based considerations. The limited potential to annually evaluate outdoor thermal comfort, predominant among these considerations, has restricted the exploration of the interrelations between urban morphology and annual energy performance. This study aims to bridge this gap by capitalizing on the new capabilities of Eddy3D – a Grasshopper plugin which enables effective calculations of hourly microclimatic wind factors via OpenFOAM which in turn are used to generate annual outdoor thermal comfort plots. Using this method, a parametric study was conducted for different typology and density scenarios in three different hot climatic contexts in Israel. The automated analytical workflow evaluated a total of 60 design iterations for their energy balance, outdoor thermal comfort autonomy (OTCA) and self-shading levels using the shade index. The high correlation found here between the annual shade index and the OTCA, across all climatic contexts, shows the potential of the shade index to serve as an effective indicator, in these contexts, for comparative or optimization outdoor comfort studies. Further results are both the superiority of the courtyard typology in both energy and outdoor comfort studies, and the contrasting impact of higher density on the annual energy balance (lower performance) and outdoor thermal comfort (higher performance) in hot climates. The annual plots of both the energy balance and OTCA reveal various seasonal and monthly trends in the three different climatic zones which can lead to localized and seasonal urban design strategies.
... Subsequent variants incrementally consider the following elements: 1) Uniform scaling of wind speeds for urban wind shading, 2) shading from buildings, 3) shading from trees, 4) spatially resolved wind conditions due to building geometry, and 5) surface temperatures of the environment along the route. Tools used for the UTCI calculations include DIVAfor-Rhino for solar radiation simulations [17], PANDO for modeling tree seasonality and leaf temperatures [18], Eddy3D for Grasshopper for incorporating Computational Fluid Dynamics (CFD) wind results [19], and Surfer for simulating building and ground surface temperatures [20]. ...
... Air temperature, relative humidity and MRT inputs are identical to Variant 4, while wind speeds vary along the route based on computational fluid dynamics (CFD) simulations. Eddy3D, a Grasshopper plugin for airflow and microclimate simulations [19], is used to set-up and run CFD simulations in BlueCFD/OpenFOAM, a widely-used open source tool for CFD simulations [31]. The extent of the geometric models used in the CFD simulations are shown in Fig. 5. Table 3 summarizes the assumptions of the CFD simulations. ...
... With the dMRT that accounts for short-wave radiation, and the various surface temperatures and view factors that account for longwave radiation, the total MRT is calculated using Equation (19). ...
The Universal Thermal Climate Index (UTCI) has been linked to outdoor activity patterns and used to evaluate the effectiveness of urban interventions to improve thermal comfort. This study investigates how simulating the urban environment at increasing levels of physical accuracy impacts UTCI values along three cycling routes in Cambridge, Massachusetts. Baseline UTCI values are estimated using a local weather file, and the following increments in physical accuracy are considered: wind-scaling, shading from buildings, shading and cooling from trees, computational fluid dynamics simulations for wind speeds, and simulated surface temperatures. With bike ridership data from Bluebikes, Boston's bike-sharing program, the relationship between bike ridership patterns and UTCI values along each route is studied. Supervised machine learning models are applied to predict bike ridership based on UTCI and other predictors.
UTCI simulation results show that incorporating the various increments of accuracy influences hourly UTCI values at urban areas and exposed areas differently. Incorporating local wind speeds is especially impactful for urban areas. The statistical models trained to predict hourly bike trip counts based on UTCI and other demand and weather predictors achieved a root-mean-squared error of 1.06 trips. 47% of predictions were correct, and an additional 42% of predictions were off by 1 trip.
This study demonstrates the importance of spatial refinement in simulating UTCI, and motivates future research into efficient simulation methods or rules-of-thumb for deriving spatial-temporal UTCI values. Future work into building a robust predictive model would motivate the design of thermally comfortable environments for human-powered transportation in cities.
... Partly for the same reason, most of these studies were conducted separately from annual or monthly energy evaluations, despite the insightful tradeoffs between them which have rarely been explored to date. Lately, Eddy3D -a new Grasshopper interface for the validated OpenFOAM Computational Fluid Dynamics (CFD) simulation engine has introduced new possibilities in this respect (Kastner & Dogan, 2020) -it allows for annual thermal comfort analyses by calculating the annual hourly wind speed for a given spatial context. This is done by using an annual interpolation method based on wind factors calculated for multiple wind directions (8, 16, or more) conducted cylindrically around the geometry. ...
... Eddy3D uses OpenFOAM's blockMesh utility for the background mesh and snappyHexMesh to snap the background mesh to the building geometry. For the background mesh, a cylindrical simulation domain approach was used which allows reusing the same computational mesh for every wind direction, thus reducing the computation time and storage space (Kastner & Dogan, 2020). Within the cylindrical mesh, the mesh was further refined with the help of three levels of refinement within a refinement box that surrounds the buildings of interest (Fig. 5.4). ...
... To justify the cylindrical simulation domain for such analyses, it has been validated and showed promising results when compared to wind tunnel data for a simple cross ventilation geometry, see Fig. 5.5 (Kastner & Dogan, 2020). Additionally, we compared the high-rise and the courtyard geometry in FAR 4 for the three predominant wind directions found for our contexts (0°, 270°, and 315°) for both cylindrical and box meshing approaches (see Appendix C. Fig. 5. 12), to confirm that the cylindrical meshing approach yields equally accurate results. ...
Focusing on the urban block scale in hot climates, this dissertation offers new insights into the nexus between urban form and environmental performance. It introduces and explores a new set of harmonized workflows which by capitalizing on the benefits of a parametric environment open new possibilities in the pursuit of a sustainable urban form - going beyond energy considerations towards environmental quality and urban livability. Beyond the ability to reproduce these workflows for environmentally driven urban design in practice, this dissertation highlights current limitations and future outlooks which pave the way for further exploration.
... The authors implemented the proposed workflow as part of Eddy3D (Dogan and Kastner 2019), an easy-to-use tool that leverages OpenFOAM (Weller et al. 1998) to conduct isothermal exterior airflow simulations. The framework is fully automated, applies best practice guidelines (Tominaga et al. 2008a) to set up the simulation domain dimension as well as mesh resolution and then generates c p arrays for arbitrary building shapes and contextual situations using a novel cylindrical domain approach (Kastner and Dogan 2020). This approach avoids expensive re-meshing of the domain for alternating wind directions and can accelerate the simulation process by up to 18%. ...
... To circumvent the re-meshing of the simulation domain for every wind direction and to reduce manual and computational overhead, a cylindrical mesh is used to facilitate the simulation of arbitrary wind directions. Kastner and Dogan (2020) introduced a cylindrical meshing methodology that automatically adheres to the best practice dimensions proposed by Tominaga (2008b) while setting appropriate boundary conditions. This results in a mesh with a cell number of 28 × 10 6 . ...
Building energy modeling software generally comes with capable airflow network solvers for natural ventilation evaluation in multi-zone building energy models. These approaches rely on arrays of pressure coefficients representing different wind directions derived from simple box-shaped buildings without contextual obstructions. For urban or obstructed sites, or more complex building shapes, however, further evaluation is needed to avoid geometric oversimplification. In this study, we present an automated and easy-to-use simulation workflow for OpenFOAM-based exterior airflow simulations to generate arrays of pressure coefficients for arbitrary building shapes and contextual situations. The workflow is compared to other methods commonly used to obtain pressure coefficients for natural ventilation analysis. Finally, we assess for which climate zones and building types modelers should rely on more accurate CFD-based pressure coefficients and where it may be justifiable to rely on easier and readily available analytical approaches to determine pressure coefficients. Results suggest that existing workflows lead to significant error in predicted comfort hours for climates in the Global South and modelers should consider CFD-based facade pressure coeficients.
... The study used Eddy3d for the calculation which is a plugin for Grasshopper. It is a CFDbased UTCI calculation, which calculates the wind factors and MRT (Mean Radiant Temperature) [81]. Wind factor calculated based on the CFD simulation result of velocity and direction. ...
... To calculate the MRT, Eddy3d uses a method that assumes the building surface temperature equals the ambient temperature. Eddy3D runs a sky view factor analysis by Radiance to account for direct solar gain [81]. ...
Nowadays building performance optimization is extended to urban planning Multi-Objective Optimization (MOO). Most research focuses on the optimization of energy use and daylight performance of building design. Buildings optimized for performance metrics rarely consider different performances together. Without integrating different building performance areas, the solution found from optimization will not be a balanced or trade-off one. This paper proposes a method to extend the use of optimization to cover multi-discipline areas that optimize visual comfort and outdoor thermal performances on the layout of high-rise residential buildings.
Daylight, sunlight hours, the sky view, and outdoor thermal comfort were the performance objectives. A parametric building model was built to control the buildings’ layout and simulation tools were used to find the performance of objectives. To accelerate the simulation process, an Artificial Neural Network (ANN) was applied to the building simulation models to calculate the performance results rapidly.
ANN model had an average accuracy of 89.9% across all outcomes. The MOO method was conducted to find integrated solutions to the building layouts on site. By ranking the optimized solutions based on five combined performance targets, the top 10 out of 150 building layout options were identified, indicating an almost 21% better performance than the baseline case.
Moreover, the top 30 out of 150 optimum cases performed better than the baseline. The study demonstrates that the proposed MOO method that combines visual comfort and outdoor thermal measurements can improve and contribute to a sustainable building layout design.
... The urban context, the campus buildings, the trees and the ground were modeled in Rhinoceros [23] and instantiated into the parametric model realized in Grasshopper [24]. The weather file climatic data and the microclimatic data generated through Urban Weather Generator (UWG) [25] were used for the wind speed and the surface temperature simulations performed through the grasshopper plug-ins Eddy [26] and Honeybee of Ladybug Tools [27], respectively. The first used the Computational Fluid Dynamics (CFD) software OpenFOAM [28] and the latter used the energy simulation software EnergyPlus [29] to finally assess outdoor comfort using the Universal Thermal Climate Index (UTCI) [30] and the Outdoor Thermal Comfort Autonomy (OTCA) [31] metrics. ...
... Such an amount of wind directions was necessary to obtain accurate simulations of hourly wind velocities. However, the cylindrical domain allowed for the saving of computational time because only one domain mesh was generated, a process that requires considerable time [26]. Due to the uniformity of the areas surrounding the TalTech campus the terrain surface roughness parameter used for all the simulation directions was Z 0 = 1, which corresponds to an environment characterized by buildings with similar heights and green areas with trees. ...
Cities are one of the major contributors of climate change. The built environment urgently needs to significantly reduce its impact on resource depletion and its CO2 emissions. At the same time, urban environments must adapt to guarantee livability and safety in increasingly frequent severe conditions. To aid this process, assessment methods and indexes have been developed to help designers and researchers investigate optimal solutions for outdoor thermal comfort. Temperature increase during summer is a growing concern also in northern European cities such as Tallinn, Estonia. This paper presents a study on the comfort conditions of the outdoor areas of the TalTech campus in Tallinn during summer and investigates the cooling potential of vegetated surfaces and trees in the local microclimate. A parametric design workflow was developed that integrates building and climate modeling, environmental and building simulations and outdoor comfort assessment through the metrics of Universal Thermal Climate Index and Outdoor Thermal Comfort Autonomy. The results show that heat stress can be experienced on the outdoor areas of the campus. The quantity and the optimal location of vegetated surfaces and trees to provide comfort were determined through the developed algorithm. The methods and the generated vegetation patterns are presented and discussed.
... To quantify the solar exposure potential in urban areas, the seasonal radiation, sunlight hours maps are generated separately for summer and winter using the 3D models of the area as input for the Radiance simulation engine. The wind field is also simulated for the same area using OpenFOAM engine and Eddy3D tools (Kastner and Dogan 2020). The potential correlations between the raster results of the Space Syntax and microclimate modelling were analysed in QGIS program to decode, quantify and cluster the invisible qualities of outdoor environments driven by urban morphology. ...
... The tool utilizes Open-FOAM's ℎ utility for the background mesh and ℎ to subsequently snap the background mesh to the building geometry. For the background mesh, we used a cylindrical simulation domain approach which allows reusing the same computational mesh for every wind direction, thus reducing the computation time and storage space [11]. Within the cylindrical mesh, we further refined the mesh within a refinement box that surrounds the buildings of interest. ...
With the impending issues regarding global warming, urban design is considered a key driver to improve the microclimate in cities. For public spaces, studies suggest that outdoor thermal comfort may be seen as a proxy for space usage, and in turn, its attractiveness to people. Although the topic has gained interest in recent years, the discussion so far has focused on computing the metrics rather than deriving interventions from them. Here, we use the tool Eddy3D to model and analyze the outdoor thermal comfort of a designated area around a university campus. Further, we demonstrate how to estimate space usage from those results. Finally, we conduct a spatial sensitivity analysis of the underlying results as a step towards decision aiding. Our work demonstrates how decision-makers may derive areas where interventions will likely have the largest impact on outdoor thermal comfort performance.
... The cylindrical domain was used to automate wind simulations from multiple directions. It saves computational time because only one domain mesh is generated for all the simulations [64]. The wind velocities for all the hours of the analysis period were calculated via wind factors. ...
Considering climate change, controlling outdoor microclimates is an increasingly pressing concern. Microclimates have a significant effect on both outdoor and indoor comfort, and on the energy efficiency of buildings. This concern is particularly important as current climate conditions reveal that warmer summers are threatening the comfort of pedestrians and causing overheating in office environments, which is consequently increasing cooling energy consumption. A further concern is that this trend now extends to Nordic latitudes.
Existing literature demonstrates how a local microclimate depends on many factors such as urban density, shape and orientation of buildings, the types of materials present, the number of green areas and anthropogenic activities. However, there is little research focusing on how reciprocal distances among tall buildings, and their relative position, affect outdoor and indoor comfort, and the associated energy consumption of buildings. This paper presents a unique and comprehensive insight into the interconnected nature of indoor and outdoor comfort via coupled simulations.
It presents a study of clusters of tall commercial buildings located in the Nordic climate of Tallinn (Estonia) with different microclimates, and shows that the differences are due to variable shadowing and reflections and different wind patterns. The results, which focus on summer conditions, show that small variations of cluster layout strongly affect the local indoor and outdoor comfort, thus highlighting the need to conduct both studies simultaneously in research aiming to increase pedestrian and indoor comfort and resource efficiency.
... Both Rhinoceros and Grasshopper have their own C# API's (Rhinocommon (RC) and GrasshopperSDK (GS)), Where GS depends on RC, and allow custom components and parameters development for many purposes. Many CAE extensions were developed as Karamba3D [8], Kiwi3D [9] and Beaver [19] for structural analysis, Ladybug [20] for Thermar/Solar Analysis, and Butterfly [20] and Eddy3d [21] for CFD analysis. ...
The new paradigms of parametric modelling have been proving promising on the advance of systems for analysis and design of taut (or tensile) structures. With this premise, the presented work consist on the development with a form-finding tool for Computer Aided Design(CAE) and Computer Aided Engineering (CAE) integration using VPL (Visual Programming Language), in the context of parametric modelling. The methods used in the implementation are the Force Density Method (FDM) and the Natural Force Density Method (NFDM), taking advantage of the linear solution approach provided, suitable for fast form-finding computational procedures.
The program is implemented as a Grasshopper plug-in and it is named BATS (Basic Analysis of Taut Structures), which enables parametric definition of boundary conditions for the form-finding. The program structure and benchmarks with other available Grasshopper plug-ins for taut structures form-finding are presented, showing considerably superior performance using BATS.
... One of the most widely used tools is ENVImet, based on CFD simulation [32]. Other outdoor modeling software alternatives are Urban Weather Generator (UWG), based on energy conservation principles [33]; SOLWEIG, which can simulate spatial variations of 3D radiation fluxes and mean radiant temperatures [34]; Open FOAM, which has been used in previous research to simulate urban wind flows [35]; FreeFem++, employed to perform courtyard microclimate modelling [17,36]; and ANSYS Fluent, which has been applied for the simulation of wind flows in outdoor spaces [17,36]. Most of these tools present adequate accuracy for predicting urban outdoor microclimates, but they tend to show a larger error range when they are used to model the microclimate of smaller-scale spaces, such as courtyards, with greater dependence on the built environment [12,36]. ...
Currently, there is a lack of accurate simulation tools for the thermal performance modeling of courtyards due to their intricate thermodynamics. Machine Learning (ML) models have previously been used to predict and evaluate the structural performance of buildings as a means of solving complex mathematical problems. Nevertheless, the microclimatic conditions of the building surroundings have not been as thoroughly addressed by these methodologies. To this end, in this paper, the adaptation of ML techniques as a more comprehensive methodology to fill this research gap, covering not only the prediction of the courtyard microclimate but also the interpretation of experimental data and pattern recognition, is proposed. Accordingly, based on the climate zoning and aspect ratios of 32 monitored case studies located in the South of Spain, the Support Vector Regression (SVR) method was applied to predict the measured temperature inside the courtyard. The results provided by this strategy showed good accuracy when compared to monitored data. In particular, for two representative case studies, if the daytime slot with the highest urban overheating is considered, the relative error is almost below 0.05%. Additionally, values for statistical parameters are in good agreement with other studies in the literature, which use more computationally expensive CFD models and show more accuracy than existing commercial tools.
... While researchers have successfully integrated CFD in design workflows, creating tools such as RhinoCFD plugin for Rhino (CHAM | RhinoCFD, 2020), Butterfly plugin for Rhino (Ladybug Tools | Butterfly, 2020), Fast Fluid Dynamics (FFD) solver validated for limited design problems (Zuo and Chen, 2007) and Envi-Met ('ENVI-met Software Elements -Wind and Sun', 2021) limitations such as cost, fluid dynamics and programming knowledge remain a barrier to a broader implementation of CFD in design workflows. Eddy3D (Kastner and Dogan, 2020) is an example of an accessible and easy-to-use CFD design tool in Rhino/Grasshopper but currently only supports outdoor wind analyses. The goal of our research is to create an equally easy-to-use and accessible tool for indoor simulations. ...
As work environments struggle to reopen during the current COVID-19 pandemic, it is crucial to establish practical decision-aiding tools. While a strong emphasis has been placed on determining generic guidelines to reduce the risk of airborne viral spread, there is a lack of free and easy-to-use simulation workflows to quantify indoor air quality and the risk of airborne pathogens indoors at a spatial resolution that can take into account floor-plan layouts, furniture, and ventilation inlet-outlet positions. This paper describes the development of a new, free, early design tool that allows designers and other stakeholders to simulate and compare viral airborne concentration under different indoor conditions. The tool leverages OpenFOAM-based Computational Fluid Dynamics (CFD) and a passive scalar simulation approach to allow architects and interior designers to quantify airborne pathogens' exposure. The tool is integrated into the popular Rhino3d & Grasshopper CAD environment to facilitate its application in fast-paced design processes. We demonstrate good agreement compared to a CFD benchmark test. Further, we validate newly developed COVID-19 capabilities by comparing our results to an existing restaurant case study that included tracer gas measurements and validation using Fluent (Ansys). We demonstrate applications of the tool in a comparative study of a restaurant that investigates how plan and furniture layout interventions, ventilation strategies can impact the movement of airborne pathogens in indoor environments.
... the simultaneous action of multiple wind directions in airflow simulations (Kastner and Dogan, 2020a). It is worth mentioning that Butterfly, Swift, Eddy3D, ixCube CFD (ixCube, 2018), and ProceduralCS (Procedural, 2021) on the Grasshopper platform are all plug-ins that use the CFD solver OpenFOAM to perform CFD simulations. ...
The transformation of urban and building design into green development is conducive to alleviating resource and environmental problems. Building design largely determines pollutant emissions and energy consumption throughout the building life cycle. Full consideration of the impact of urban geometries on the microclimate will help construct livable and healthy cities. Computational fluid dynamics (CFD) simulations significantly improve the efficiency of assessing the microclimate and the performance of design schemes. The integration of CFD into design platforms by plug-ins marks a landmark development for the interaction of computer-aided design (CAD) and CFD, allowing architects to perform CFD simulations in their familiar design environments. This review provides a systematic overview of the classification and comprehensive comparison of CFD plug-ins in Autodesk Revit, Rhinoceros/Grasshopper, and SketchUp. The applications of CFD plug-ins in urban and building design are reviewed according to three types: single-objective, multi-objective, and coupling simulations. Two primary roles of CFD plug-ins integrated into the design process, including providing various micro-scale numerical simulations and optimizing the original design via feedback results, are analyzed. The issues of mesh generation, boundary conditions, turbulence models, and simulation accuracy during CFD plug-in applications are discussed. Finally, the limitations and future possibilities of CFD plug-ins are proposed.
... This feature gives the possibility of setting inflow or outflow boundary conditions for any wind incidence direction while reduces the total size of the computational domain. This kind of wind tunnel shape has several advantages, which are even more noticeable in problems where at least eight wind directions have to be studied, as was recently shown by Kastner and Dogan (2020). The platform simulates and predicts C p data for at least 12 wind directions, exploiting enough such advantages. ...
Natural ventilation (NV) is a key passive strategy to design energy-efficient buildings and improve indoor air quality. Therefore, accurate modeling of the NV effects is a basic requirement to include this technique during the building design process. However, there is an important lack of wind pressure coefficients (C p ) data, essential input parameters for NV models. Besides this, there are no simple but still reliable tools to predict C p data on buildings with arbitrary shapes and surrounding conditions, which means a significant limitation to NV modeling in real applications. For this reason, the present contribution proposes a novel cloud-based platform to predict wind pressure coefficients on buildings. The platform comprises a set of tools for performing fully unattended computational fluid dynamics (CFD) simulations of the atmospheric boundary layer and getting reliable C p data for actual scenarios. CFD-expert decisions throughout the entire workflow are implemented to automatize the generation of the computational domain, the meshing procedure, the solution stage, and the post-processing of the results. To evaluate the performance of the platform, an exhaustive validation against wind tunnel experimental data is carried out for a wide range of case studies. These include buildings with openings, balconies, irregular floor-plans, and surrounding urban environments. The C p results are in close agreement with experimental data, reducing 60%-77% the prediction error on the openings regarding the EnergyPlus software. The platform introduced shows being a reliable and practical C p data source for NV modeling in real building design scenarios.
Electronic supplementary material esm:
The appendix is available in the online version of this article at 10.1007/s12273-021-0881-9.
This study investigated outdoor thermal comfort within 5 × 5 idealized building arrays on five consecutive days by conducting unsteady computational fluid dynamic (CFD) simulations. The dynamic interacting effects of building height topology, building distance and building layout on airflow patterns, spatial distributions and spatially averaged outdoor thermal environment were evaluated by UTCI (Universal Thermal Comfort Index). CitySim Pro was used to simulate the transient solar-induced temperatures on the walls of buildings and grounds inside building arrays, which were then set as thermal boundary conditions in CFD simulations. The results showed that the influence of transient wind conditions on airflow patterns and urban thermal comfort was significant so steady simulations or unsteady simulations of one typical day might not provide a complete overview of the wind-thermal environment. The results also showed that presenting the results by average UTCI or spatial distribution could lead to different conclusions on the impacts of urban geometry on urban thermal comfort, especially the impact of building distance. Increasing the building height always provided positive effects on urban thermal comfort. The spatial distributions of UTCI in three-dimensional building arrays with equal aspect ratio diverse significantly, which was not observed in the pattern of average UTCI.
In this paper, the fundamentals of a 3D nested construction method for 3D-printing stackable tower-like structures are explained, taking into consideration the transportation, storage, assembly, and even disassembly of building components. The proposed method is called “PRINT in PRINT.” This paper also documents the authors’ experience of and findings from designing and printing a column erected out of a series of 3D printed components in a short stack. Employing the design principles of 3D printing in a nested fashion, the authors showcase the main parameters involved in dividing the column’s global geometry into stackable components. By converting formal, technical, and material restrictions of a robotic-assisted 3D printing process into geometric constraints, the paper describes how the column components are divided, namely that one component shapes the adjacent one.
In the physics-based simulation of urban geometries, the outdoor environment was usually simulated separately from buildings – until recently, when the holistic assessment of the urban environment began to attract more attention. Although analyzing design alternatives with multiple objectives is still a challenge, computational tools enable generating thousands of scenarios to rapidly assess performance corresponding to a specific goal. In this study, we developed a multi-phase optimization framework for conceptual urban design. We tested this framework for urban typologies in Syracuse. The energy performance of each alternative was compared with a baseline. The alternatives that generate wasteful energy performance were filtered out first, then remaining scenarios that performed better than the baseline were analyzed using outdoor thermal comfort autonomy (OTCA). Mid-rise multifamily buildings showed the best performance (55.8% energy improvement compared to the baseline). Although hot week outdoor comfort satisfaction among selected mid-rise typologies was high (92.9–98.5%), the satisfaction in cold week was very low (between 8.4–11.6%) among them. This framework contributes to identifying an acceptable range of design solutions by broadening the perspective of the field toward using a more customized optimization framework in early design that will further guarantee the requirements of energy efficient and sustainable cities.
Courtyards are a passive strategy to improve the energy performance of buildings. However, the accurate simulation of courtyards’ thermodynamic performance in the early design stage is still challenging, even though there has been an emergence of new methods to assess outdoor simulation. This paper tests a novel workflow using the Ladybug Tools that uses CFD for outdoor temperature and comfort simulation of courtyards and is suitable for the early stage of building design, comparing the results with monitored data and simulated data from ENVI-met. Results show high accuracy in the prediction of temperature from Ladybug Tools with error ranges from 3.8–7.5%, which is lower than with ENVI-met. In terms of comfort, the simulated Universal Thermal Climate Index values differ by up to 10°C between ENVI-met and the Ladybug Tools. The results show a significant improvement towards the design of the courtyard in the search for net-zero energy buildings.
The article is devoted to the modeling of wind flows when placing new construction objects in the existing building. The CFD technologies are used (computational fluid dynamics) to assess the influence of wind, to determine the direction of airflow velocity vectors for a given location and geometric characteristics of buildings, the earth's surface topography. When solving the problems of modeling wind flows in a residential area, a review of tools for architectural and climatic analysis is carried out. In addition, the possibilities of calculating each of them are described, and advantages and disadvantages are highlighted. It is concluded that most of the tools remain proprietary and not integrated into the main programs for architectural and urban design. The main stages of modeling and obtaining initial data for CFD analysis are described: digital models of the terrain and buildings obtained on the basis of a geoinformation model of the city, annual data on the actual direction and speed of the wind in the study area. The values of speeds, wind for the residential group under consideration, the trajectories of its movement in the building, projected onto vertical and horizontal planes are obtained. The simulation results allow to consider measures for planning and beautification of urban space and reducing the negative impact of wind on the built-up area
The construction of a building inevitably changes the microclimate in its vicinity. Many city authorities request comprehensive wind studies before granting a building permit, which can be obtained by Computational Fluid Dynamics (CFD) simulations. When performing wind simulations, the quality of the geometry model is essential. Still, no available studies examine different geometry inputs' impact on the wind flow through an urban environment. This study investigates the influence of the building geometry acquisition method on the simulated wind field in an urban area, focusing on the application of pedestrian wind comfort. A suburban area in the west coast of Norway was chosen as a case study. Four building model types were produced and used in the simulations for comparison. The simulations using a building model produced from data stored in the national general feature catalog (FKB) in Norway showed minor differences to the simulations using more detailed and accurate models based on remote sensing measurements. Prominent variations were seen using a model based on the extrusion of the building footprint. A greater understanding of the geometry acquisition method's influence may enable more efficient pedestrian wind comfort studies that recognize the uncertainty of different geometric model use in urban wind simulations.
The construction of a building inevitably changes the microclimate in its vicinity. Many city authorities request comprehensive wind studies before granting a building permit, which can be obtained by Computational Fluid Dynamics (CFD) simulations. When performing wind simulations, the quality of the geometry model is essential. Still, no available studies examine different geometry inputs' impact on the wind flow through an urban environment. This study investigates the influence of the building geometry acquisition method on the simulated wind field in an urban area, focusing on the application of pedestrian wind comfort. A suburban area in the west coast of Norway was chosen as a case study. Four building model types were produced and used in the simulations for comparison. The simulations using a building model produced from data stored in the national general feature catalog (FKB) in Norway showed minor differences to the simulations using more detailed and accurate models based on remote sensing measurements. Prominent variations were seen using a model based on the extrusion of the building footprint. A greater understanding of the geometry acquisition method's influence may enable more efficient pedestrian wind comfort studies that recognize the uncertainty of different geometric model use in urban wind simulations.
For environmental CFD simulations, it is considered best practice to use a box-shaped wind tunnel as simulation domain. A box-shaped wind tunnel, however, shows drawbacks when it comes to simulating air flow from several wind directions-remeshing and additional preprocessing steps may be necessary and can be considerable time constraints. We utilize a routine implemented in Grasshopper to create a cylindrical computational mesh that allows for the simulation of arbitrary wind directions in a streamlined manner with the open source software OpenFOAM. We estimate the time savings that are possible along with specific mesh properties to take advantage of the proposed method. For validation purposes, commonly used wind tunnel data are presented. A proof of concept tool is implemented in the Rhinoceros CAD modeling environment and will be released publicly.
Wind pressure coefficients (Cp) are influenced by a wide range of parameters, including building geometry, facade detailing, position on the facade, the degree of exposure/sheltering, wind speed and wind direction. As it is practically impossible to take into account the full complexity of pressure coefficient variation, building energy simulation (BES) and Airflow network (AFN) programs generally incorporate it in a simplified way. This paper provides an overview of pressure coefficient data and the extent to which they are currently implemented in BES–AFN programs. A distinction is made between primary sources of Cp data, such as full-scale measurements, reduced-scale measurements in wind tunnels and computational fluid dynamics (CFD) simulations, and secondary sources, such as databases and analytical models. The comparison between data from secondary sources implemented in BES–AFN programs shows that the Cp values are quite different depending on the source adopted. The two influencing parameters for which these differences are most pronounced are the position on the facade and the degree of exposure/sheltering. The comparison of Cp data from different sources for sheltered buildings shows the largest differences, and data from different sources even present different trends. The paper concludes that quantification of the uncertainty related to such data sources is required to guide future improvements in Cp implementation in BES–AFN programs.
With increased awareness of sustainability, natural ventilation has become a strategy to reduce energy consumption in the built environment while providing comfortable indoor air quality.
The main aim of this thesis was to apply the CFD software OpenFOAM, to investigate the wind-induced natural ventilation potential of classrooms. Relevant ventilation metrics such as air change rates, age of air, andCO2 concentrations were implemented.
External and internal wind flow was simulated in one domain to assess the natural ventilation potential via RANS turbulence modeling. A case setup for a simple cross-ventilation geometry was validated against wind tunnel measurements in accordance with CFD guidelines for the built environment. Based on the validation, a case study was conducted for BRAC University—located in subtropical Asia. The validation study revealed that OpenFOAM was able to accurately predict the flow field and flow rates for the cross-ventilation geometry. Further, OpenFOAM’s passive scalar transport method is able to predict CO2 concentrations with a reasonable error range, while the workflow can be highly automated. Applying these results to the case study, we suggest a number of geometry modifications to optimize the natural ventilation potential of classrooms for BRAC University. These include specific placement based on the wind direction and either relocation or shape optimization of adjacent staircases. Additionally, we suggest the use of operable windows to accommodate temporal fluctuations throughout the year. Finally, passive scalar transport methods, which derive metrics like the age of air or CO2 concentrations, provide additional valuable information that should not be overlooked when evaluating design principles. In the future, it may be possible to employ tools based on the implemented ventilation
metrics to automate the search for optimized building geometries for maximizing the natural ventilation potential.
Urban physics is the science and engineering of physical processes in urban areas. It basically refers to the transfer of heat and mass in the outdoor and indoor urban environment, and its interaction with humans, fauna, flora and materials. Urban physics is a rapidly increasing focus area as it is key to understanding and addressing the grand societal challenges climate change, energy, health, security, transport and aging. The main assessment tools in urban physics are field measurements, full-scale and reduced-scale laboratory measurements and numerical simulation methods including Computational Fluid Dynamics (CFD). In the past 50 years, CFD has undergone a successful transition from an emerging field into an increasingly established field in urban physics research, practice and design. This review and position paper consists of two parts. In the first part, the importance of urban physics related to the grand societal challenges is described, after which the spatial and temporal scales in urban physics and the associated model categories are outlined. In the second part, based on a brief theoretical background, some views on CFD are provided. Possibilities and limitations are discussed, and in particular, ten tip and tricks towards accurate and reliable CFD simulations are presented. These tips and tricks are certainly not intended to be complete, rather they are intended to complement existing CFD best practice guidelines on ten particular aspects. Finally, an outlook to the future of CFD for urban physics is given.
The objective of this article is to show the potential of natural ventilation as a passive cooling method within the residential sector of countries which are located in warm conditions using Mexico as a case study. The method is proposed as performing, with a simplified ventilation model, thermal–airflow simulations of 27 common cases of dwellings (considered as one thermal zone) based on the combination of specific features of the building design, occupancy and climate conditions. The energy saving potential is assessed then by the use of a new assessment method suitable for large-scale scenarios using the actual number of air-conditioned dwellings distributed among the 27 cases. Thereby, the energy saving is presented as the difference in the cooling demand of the dwelling during one year without and with natural ventilation, respectively. Results indicate that for hot-dry conditions, buildings with high heat capacity combined with natural ventilation achieve the lowest indoor temperature, whereas under hot-humid conditions, night ventilation combined with low heat capacity buildings present the best results. Thereafter, an average aggregated saving potential of 4.2 TW h for 2008 is estimated, corresponding to 54.4% of the Mexican electric cooling demand for the same year. The practical implications of the study are that the results contribute to an assessment of the economic and environmental benefits for using natural ventilation rather than an active method such as air conditioning. Thereby, the average economic saving is estimated at US$ 900 M and the environmental benefit at an annual average mitigation of 2 Mt CO2eq, both for 2008.
This paper proposes the use of a Grid Convergence Index (GCI) for the uniform reporting of grid refinement studies in Computational Fluid Dynamics. The method provides an objective asymptotic approach to quantification of uncertainty of grid convergence. The basic idea is to approximately relate the results from any grid refinement test to the expected results from a grid doubling using a second-order method. The GCI is based upon a grid refinement error estimator derived from the theory of generalized Richardson Extrapolation. It is recommended for use whether or not Richardson Extrapolation is actually used to improve the accuracy, and in some cases even if the conditions for the theory do not strictly hold. A different form of the GCI applies to reporting coarse grid solutions when the GCI is evaluated from a ''nearby'' problem. The simple formulas may be applied a posterior by editors and reviewers, even if authors are reluctant to do so.
This report describes the theoretical development of work done within task group “wind pressure distribution” of the COMIS Workshop. The paper is divided into three Sections with an introductory part on the physical fundamentals. The first Section entails the objectives and the meaning of modelling wind pressure distribution as an integrated part of multizone airflow modelling. A literature review is presented, on calculation techniques and wind tunnel tests, and a description of the evaluation of an existing numerical model. The second Section is related to the development of the Cp calculation model. Objectives, characteristics and methodology of the parametrical approach chosen for the analysis are depicted, together with a description of the reference data, the regression technique, the algorithm, and the structure of the calculation model. The third Section is a detailed report of the results of the regression analysis. The curve-fitting process is explained with reference to the main factors affecting the wind pressure distribution on a building envelope: terrain roughness, surrounding buildings, aspect ratios, and wind direction. Figures of the curves are shown. In the Appendix, equations and relevant coefficients of the curve-fitting are presented.
Although natural ventilation is one of the major mechanisms that controls the greenhouse climate, our understanding of the underlying processes remains insufficient to allow accurate prediction of the rates of such exchanges.This paper deals with the physical mechanisms involved in natural ventilation of a greenhouse equipped with continuous lateral windows, and uses the following experimental procedures: •• air exchange rate measurements, using tracer gas or heat and water balance techniques;•• direct determination of the air and heat flows through an opening, using an eddy correlation system, comprising a sonic anemometer and a fine wire thermocouple;•• measurements of mean and turbulent pressure differences at ground level between inside and outside.The methods employed allow the prediction of greenhouse air exchange rates as well as the characterization of its components: a steady effect resulting from the combination of both mean wind-related and stack effects and a turbulent effect linked to wind speed fluctuations.Local estimations of total, mean and turbulent flows are provided: a wind parallel to the greenhouse axis produces an inflow at the leeward half and an outflow at the windward half. The mean flow of sensible heat is estimated between 55% and 80% of the total flux so that the turbulent flow does not exceed 45% of the total.Local estimations of total, mean and turbulent flows are compared with air exchange rate measurements using the decay rate method and a good agreement between both approaches is demonstrated.
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.
Modelling of buildings with natural or hybrid ventilation systems requires the coupling of a thermal and an air flow model because of the strong mutual impact of the thermal and the air flow behaviour. The newly developed tool TRNFlow is the complete integration of the multizone air flow and pollutant transport model COMIS (Dorer 2001) into the thermal multizone building module of the building and system simulation program TRNSYS (Klein 2000). An internal solver algorithm using successive substitution with automatic adapted relaxation finds consistent solutions of the two models. The existing TRNSYS user interface for the building module has been updated for the easy and user friendly input of the additional air flow and pollutant transport model data.
Natural ventilation, which is in line with the concepts of sustainability and green energy, is widely acknowledged nowadays. Prevailing winds in urban areas are unavoidably modified by the increasing number of closely placed high-rise buildings that significantly modify the natural ventilation behaviour. This paper explores the effects of building interference on natural ventilation using computational fluid dynamics (CFD) techniques. The cross-ventilation rate (temporal-average volumetric airflow rate) of hypothetical apartments in a building cluster under isothermal conditions was examined using the standard two-equation k−ɛ turbulence model. The sensitivity of ventilation rate to wind direction, building separation and building disposition (building shift) was studied. Placing buildings farther away from one another substantially promoted the ventilation rate, cancelling the unfavourable interference eventually when the building separation was about five times the building width (the optimum separation). The characteristic flow pattern leading to this behaviour was revealed. With the adoption of building disposition, the optimum separation could be reduced to three times the building width. In addition, the airflow rates could be doubled with suitable shifts. Building disposition is therefore one of the feasible solutions to improve the natural ventilation performance in our crowded environment.
In summer natural ventilation is the most effective passive cooling system of the Mediterranean area. The correct exposure of the buildings and of the urban morphology to prevailing winds allows reducing the cooling loads also in non-bioclimatic buildings, without any cost. This paper points out the cooling capacity and the possibilities of energy saving offered by a correct natural ventilation by means of the simulation of a non-bioclimatic building, that is, a building having common characteristics as for construction materials and technologies.
Significant improvements of computer facilities and computational fluid dynamics (CFD) software in recent years have enabled prediction and assessment of the pedestrian wind environment around buildings in the design stage. Therefore, guidelines are required that summarize important points in using the CFD technique for this purpose. This paper describes guidelines proposed by the Working Group of the Architectural Institute of Japan (AIJ). The feature of these guidelines is that they are based on cross-comparison between CFD predictions, wind tunnel test results and field measurements for seven test cases used to investigate the influence of many kinds of computational conditions for various flow fields.
Natural ventilation in buildings can create a comfortable and healthy indoor environment, and can save energy compared to mechanical ventilation systems. In building design the prediction of ventilation can be difficult; cases of wind-driven single-sided ventilation, where the effects of turbulence dominate, are particularly problematic to simulate. In order to investigate the mechanism of natural ventilation driven by wind force, large-eddy simulation (LES) is used. In the meanwhile, detailed airflow fields, such as mean and fluctuating velocity and pressure distribution inside and around building-like models were measured by wind tunnel tests and compared to LES results for model validation. Three ventilation cases, single-sided ventilation with an opening in windward wall, single-sided ventilation with an opening in leeward wall, and cross ventilation, are studied. In the wind tunnel, a laser Doppler anemometry was used to provide accurate and detailed velocity data. In LES calculations, two subgrid-scale (SS) models, a Smagorinsky SS model and a filtered dynamic SS model, were used. The numerical results from LES are in good agreement with the experimental data, in particular with the predicted airflow patterns and velocities around and within, and the surface pressures over, the models. This is considered to establish confidence in the application of the LES methods to the calculation of ventilation in buildings, in particular for single-sided ventilation cases.
Linking the COMIS Multizone Airflow Model with the EnergyPlus
- J Huang
- F Winkelmann
- F Buhl
Huang, J., F. Winkelmann, and F. Buhl. 1999. Linking the COMIS Multizone
Airflow Model with the EnergyPlus.