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Modeling outdoor thermal comfort along cycling routes at varying levels of physical accuracy to predict bike ridership in Cambridge, MA
Abstract
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.
... For instance, Liu et al. (2016) combined measured thermal parameters with simulated wind speed to realize outdoor thermal comfort prediction, Mackey (2017) proposed a 'hybrid' outdoor thermal comfort simulation method, and Kastner and Dogan (2017) carried out simulations and validation using a similar approach. Young et al. (2022) further explored the effect of different physical input accuracies on UTCI simulation results. In terms of thermal comfort design for outdoor spaces, Kastner and Dogan (2020) showed how space use can be estimated from simulations and used to determine intervention areas for space design, while Wang et al. (2021) optimized the layout of HRB through a multiobjective optimization approach that considers additional indoor and outdoor physical indicators. ...
... In order to maximize the ability of the wind deflector to change the local wind field in a hot subtropical region during the preliminary design and to produce better benefits for most of the year, this study chose to use the perennial dominant wind direction of Shenzhen as the initial wind field study conditions. This paper is closer in spatial dimensions to the study conducted by Young et al. (2022) applying Eddy3D (OpenFOAM) to a neighbourhood environment, referring to some of its simulation settings in Tables 2 and 3. For numerical settings, the Reynolds Averaged Navier Stokes (RANS) equations are applied, and the k-ε model is selected as the turbulence model. ...
This study focuses on microclimate adjustment for small-scale outdoor spaces in high-rise residential blocks. The study explores the form design of landscape-integrated wind deflectors to investigate and optimize the local outdoor space comfort of high-rise residential blocks in a subtropical climate in Shenzhen, China and to analyze and summarize effective form parameters for environmental enhancement. A workflow is built on the Grasshopper platform and related software to coordinate the data interaction between CAD models and multiple simulation engines to achieve wind field and thermal comfort simulations. For the narrow-shaped deflector, this study found that the level of enclosure and orientation of openings were the most critical parameters in modulating wind fields and thermal comfort in surrounding areas, resulting in a 65% improvement in the average wind speed and a 1.5-3°C increase in the UTCI thermal comfort index in these areas. Finally, the study suggests how this workflow and results can inspire similar design and optimization issues.
... Butterfly, Swift, Eddy3D, ixCube CFD, and ProceduralCS on the Grasshopper platform are all plug-ins for CFD simulations that use the CFD solver OpenFOAM and have been validated for high reliability [24]. Given the greater operability and better reproduction of the simultaneous action of multiple wind directions in the airflow simulation [25,26], eddy3D was selected as the CFD solver for this study. Meanwhile, the thermal environment simulation was performed in Rhino and Grasshopper using ladybug [12]. ...
... Thus, the area ratio of the static wind zone is less impacted (see Table 7). The influence of morphological indicators on the static wind ratio can be accurately depicted using the following equation (Equations: (25)): ...
Workers' villages in East China represent a typical form of residential government-built settlements constructed between the 1950s and the 1980s to address the housing shortage. Recent emphasis has been paid to optimizing wind and thermal comfort in older neighborhoods, following the urban renewal trend. This paper collected the geometries of 150 workers' villages. Pedestrian-level wind and Universal Thermal Climate Index (UTCI) were calculated for workers' villages using validated simulation software. Seven machine learning (ML) algorithms were compared for modeling the nonlinear relationship between the building morphology and the outdoor environment of the workers' villages. The ensemble model, especially the Adaboost model, performs best when predicting static wind ratio and UTCI with R² values of 0.89 and 0.99. The trained models were applied to estimate the outdoor environment of 1118 workers' villages in East China. The result shows most workers' villages have static wind ratios over 0.7. Workers' villages in Jiangsu endure more extreme summer heat, whereas workers' villages in Zhejiang have a higher static wind ratio in winter and summer. The use of ML offers a quicker estimation of outdoor wind and thermal comfort in large-scale workers’ villages than numerical simulations, therefore shedding light on the targeting of urban renewal.
... Shade can be categorized into natural shade, provided by trees, and built shade, provided by buildings. The shade from buildings and the combined shading and cooling effects from trees substantially influence thermal comfort on cycling routes (Young et al., 2022). High-rise buildings can offer considerable shaded areas, while tree canopies can mitigate shortwave solar radiation penetration, thereby decreasing surface and air temperatures in shaded areas (Roy et al., 2012). ...
... This study quantifies heat stress using the UTCI, measured in • C, which has been used widely to indicate human heat stress levels (Young et al 2021). UTCI values higher than 32 • C is generally considered strong heat stress levels (figure 2(c)). ...
Many cities are experiencing increases in extreme heat because of global temperature rise combined with the urban heat island effect. The heterogeneity of urban morphology also leads to fine-scale variability in potential for heat exposure. Yet, how this rise in temperature and local variability together impacts urban residents differently at exposure-relevant scales is still not clear. Here we map the Universal Thermal Climate Index, a more complete indicator of human heat stress at an unprecedentedly fine spatial resolution (1 m), for 14 major cities in the United States using urban microclimate modeling. We examined the different heat exposure levels across different socioeconomic and racial/ethnic groups in these cities, finding that income level is most consistently associated with heat stress. We further conducted scenario simulations for a hypothetical 1 °C increase of air temperature in all cities. Results show that a 1 °C increase would have a substantial impact on human heat stress, with impacts that differ across cities. The results of this study can help us better evaluate the impact of extreme heat on urban residents at decision-relevant scales.
... In this regard, new studies are investigating spatial and accuracy-adaptive workflows to estimate variations in prediction accuracy and their applicability at the large urban scale. Variations in outdoor thermal comfort assessments along cycling routes in Boston, US, were investigated on the basis of the varying levels of physical and spatial accuracy of: weather data; wind scaling; building shading; simulated urban surface temperatures; and wind Preprint simulations [188]. Besides demonstrating the incremental accuracy of the approaches, the research also showed that the variations were not uniform in different urban areas, thus showing that different spatial and accuracy workflows can be used according to the urban area investigated, with a potential reduction in simulation times. ...
It is widely recognized that the sustainability of the built environment is one of the key strategies to tackle global crises such as climate change, resource depletion, and human health. An environmentally driven approach to architectural design and urban planning through simulations has been around for many decades. However, architects and planners haven't fully taken advantage of its potential yet due to the complexity of the tasks and the fragmentation of tools and methods in different fields of expertise. This paper aims to demonstrate how the nascent field of environmental architectural design exploration is a viable answer to complex tasks and challenges through the integration of sustainable design, environmental simulations, building performance analysis, and computational procedures leveraging parametric design environments. After defining the context that favored its development, the ten questions answered present the environmental metrics and design parameters, propose a taxonomy of simulation tools and methods, present optimization processes, identify the required data, and introduce a classification of co-simulation and design exploration workflows, emphasizing their potential for the design of sustainable buildings and districts. Then focus is given to the scales and domains of application and to the potential of machine learning methods. The paper concludes by presenting applications in practice and discussing challenges and future opportunities. Strategies and operational procedures emerge from the ten questions. They emphasize the use of holistic approaches and the selection of balanced design solutions allowed by environmental architectural design exploration for the development of a sustainable built environment.
... For instance, Alcorn and Jiao (2023) argue that more comfortable cycling facilities exhibit a positive correlation with the use of shared bikes, a finding that our results corroborate, as depicted in Figure 7(c). Moreover, Young et al. (2022) contend that the comfort of the cycling environment not only encourages greater cycling activity but also extends the distance. Our study aligns with this perspective. ...
Extending cycling distances is crucial for sustainable urban transport development and plays a role in encouraging the shift from motorized vehicles to public transport. However, there is a lack of research examining the combined impacts of both objective and perceived aspects of the cycling environment on cycling distance, and the existence of threshold effects remains unclear. This study uses 2019 cycling data from Shenzhen, China, employing the XGBoost algorithm to uncover the relative importance and thresholds of objective and perceived factors in the cycling environment. The results indicate that population density (24.8%), road network density (15.2%), the proportion of recreational facilities (9.1%), perceived accessibility (8.0%), and comfort (8.6%) hold high relative importance in predicting cycling distance. Also, maintaining road network density between 3 to 6 km/km2 and increasing the population density to exceed 22,000 people/km2 proves effective in extending cycling distances. Land use demonstrates a threshold effect, with cycling distances increasing when the recreational facilities share exceeds 8%, transport facilities share remains below 25%, and commercial facilities share stays below 30%. Perceived metrics exhibit a clear threshold effect. The study identifies that perceived safety indicates a psychological bottleneck in increasing cycling distance. Perceived accessibility is positively correlated with cycling distance when accessibility is at a low level, while comfort shows a positive correlation with cycling distance when comfort is at a high level. These findings can contribute to refining land planning and prioritizing resource allocation for organizations aiming to promote non-motorized travel and design bicycle-friendly environments.
... where subscripts lw, diff, and dir stand for the contribution from longwave, diffuse shortwave, and direct shortwave radiation, respectively (Walikewitz et al., 2015;Young et al., 2022): ...
... [12][13][14] The choice of an indicator that has been used in previous peer-reviewed studies also allows for comparisons between studies. The UTCI has also been applied to a range of sporting events 15 , and physical activities 16 . Despite the limitations of the UTCI, the vision of the larger research community and published peer-reviewed articles directed our use of the index. ...
We value comments on our research paper in a Commentary in this issue (Simpson, S Afr J Sci. 2024;120(1/2), Art. #16445). Acknowledging the Universal Thermal Comfort Index (UTCI)'s limitations in capturing individual physiological responses remains important; however, we argue for its appropriateness based on recent thermophysiology and heat exchange advancements during its development and broader alignment with standardised indexing efforts. Our original research paper set out with these considerations in mind, and our conclusions remain valid. We further argue for refinement of the UTCI for specific activities instead of using the RET. Finally, future efforts should focus on monitoring data in real-world scenarios to validate and improve thermal indices
... The UTCI, distinguished for its comprehensiveness, surpasses other thermal comfort models. It is versatile enough to be utilized across diverse climatic conditions globally and is applicable at various research scales [21]. ...
Microclimate is a reflection of the climatic conditions within the scope of human daily activities, so it is closely related to human activities. This paper uses Qingshui Village in Hancheng as the research object and the purpose of this paper was to study the influence mechanism of the traditional street spatial form on microclimate and thermal comfort, emphasizing the use of parameterized design platforms in technical methods to construct performance simulations and obtain relatively optimal solutions that are suitable for the spatial form of rural streets and alleys in the region. We select the Universal Thermal Climate Evaluation Index (UTCI) as the evaluation index for microclimate comfort and construct a performance-driven automatic optimization method for street and alley spaces. The results showed that: (1) When the street is in the northeast-southwest direction, the width is taken in the range of 6.5 m–7.3 m, the height of the building on the north side of the street is about 6.0 m, and the height of the building on the south side of the street is about 5.7 m, the comfort rate can reach up to 33.8%. (2) As for street and alley intersections, the focus remains on retaining their original forms while primarily controlling the scale changes. Within streets, the height of the building in the east-west direction is controlled at 5.6 m–6.1 m, the building in the north-south direction is controlled at 7 m–7.4 m, and street widths are controlled between 5.4 m and 6.3 m, resulting in a comfort level of 32.0%; (3) In alleys, east-west building heights are kept between 4.2 m and 5.5 m, and north-south building heights range from 4.5 m to 5.3 m, with widths at around 4.5 m, resulting in a similar comfort level of 32.0%. The research outcomes offer a scientific foundation for the design, creation, and enhancement of the physical environment of local village streets and alleys.
... Sun exposed building structures especially in the urban context store a high amount of solar energy through solar absorption and radiation (Rizwan, Dennis and Liu, 2008). Studies have already revealed that passive cooling strategies that provide shading for exposed (Molter and Chokhachian, 2021;Young et al., 2021). However, the large vertical surfaces of building envelopes also provide a vast design scope for the deployment of passive solar protection. ...
With the IBM Research Center in La Gaude, France (1960-1962), the architect Marcel Breuer pioneered a novel industrial approach towards modular construction using precast façade elements for on-site assembly, combining load-bearing and solar control functions in their configuration. This industrial production method involved a high level of standardization, which was a practical response to the need for rapid and cost-effective construction systems. However, this standardization limited the ability to create custom elements to meet specific local requirements, such as variations in solar exposure. To overcome this limitation, new methods of Additive Manufacturing in Construction (AMC) could enhance design flexibility, allowing for bespoke designs while still maintaining industrialisable production processes. This paper draws inspiration from Breuer's building design with the aim to expand the concept of element prefabrication by incorporating performance-based and locally customized design approaches supported by AMC technology. As such, the authors present the method and results of an experimental case study for multi-scale-differentiation of building envelope elements, which design was informed by solar radiation simulations and AMC-related boundary conditions. The research describes an algorithmic based design-to-production workflow combining computational design and simulation methods using geometry-based graphical methods for solar control and solar radiation simulations for form-based changes, leveraging the potential of Selective Cement Activation (SCA) as an AMC technology. The workflow was tested and evaluated on behalf of the design and additive manufacturing of a building envelope element at full building scale.
... Human heat stress is a comprehensive index in uenced by the air temperature, solar radiation, humidity, and wind speed, along with physiological factors and possibly other environmental considerations. This study quanti es potential heat stress using the UTCI, which has units of °C, has been used widely to indicate the human heat stress level (Young et al., 2021). UTCI higher than 32°C is generally de ned as strong heat stress level (Fig. 2c). ...
Many cities are experiencing more frequent extreme heat events during summer because of the global temperature rise and the urban heat island effects. Yet, how the rising temperature impacts urban residents differently is still not clear. Here we leveraged urban microclimate modeling to map the Universal Thermal Climate Index (UTCI), a more complete indicator of human heat stress at an unprecedentedly fine spatial resolution (1 m) for 14 major cities in the United States. We also examined the sensitivity and vulnerability to summer heat across different socioeconomic and racial/ethnic groups in these cities, finding that income level is most consistently associated with heat stress. Results show that a hypothetical 1 ºC increase in air temperature would have a substantial impact on human heat stress, with impacts that differ across cities. The results of this study can help us better evaluate the impact of extreme heat on urban residents.
... The former uses fully integrated microclimate modelling engines that produce highly accurate maps that account for fluid and thermodynamic interactions and consider air flow, surface heat exchange, water, and vegetation. Hybrid approaches combine independently simulated variables to map their collective impact while ignoring interactions between parameters [17], [18], [27]. Both approaches can take up to a month to run for an annual study of an urban block [17], making them largely unusable for design integration. ...
... Moreover, human thermal comfort indices such as heat index (Ming et al., 2018); effective temperature (Wu and Liao, 2020) and comprehensive comfort index (CCI) , are also considered in the modeling of travel demand. Young et al. (2022) have found a bellshaped effect of temperature on travel demand of active modes. For example, the temperature is positively associated with cycling ridership while it turns into a negative effect when it is higher than a specific value, such as 28℃ (Miranda-Moreno and Nosal, 2011). ...
... Hence, the scope of this study was narrowed to assessing the data quality by low-speed sampling. We tested a thermal-biking campaign [6] in the Esposizione Universale Roma (EUR) district in Rome, Italy. The data quality was assessed by finding the time lag of sampling nuanced Ta and Rh variations below the driving speeds (< 6 m/s). ...
Crowdsourced weather data has been increasingly used to provide the input data for the boundary conditions in the microclimate models as well as to validate the modelling outputs. MeteoTracker (MT) is an Internet of Things (IoT) application for domestic vehicles that can contribute to the intra-urban air temperature database for meteorology enthusiasts as well as research and design communities. To promote green and healthy mobility, we have validated the MT device on bicycles and evaluated its monitoring performance with 2 other Kestrel heat stress trackers (KT) at low travelling/wind speed. The pre-planned cycling route cuts through the EUR district in Rome, Italy in September 2021. The dual-sensor design and the radiation error correct system (RECS) in the MT devices demonstrate good abilities in probing air temperature and humidity nuances when passing through different urban areas, as the three devices remain highly synchronised (Pearson r > 0.86). However, the MT device may overshoot the response time (> 41 s) if the cycling speed is less than 3 m/s. We have suggested some strategies for improvement in reducing the response time of probing the ambience under nuanced thermal variability.
... Therefore, in recent years, several studies have focused on the role of urban layouts and furniture [3][4][5], urban canyon geometries [6][7][8][9], materials applied in cities [10,11], and vegetations [12][13][14] on the airflow in cities and consequently natural ventilation of buildings and the space between them. Furthermore several studies analysed different urban configurations and provided design recommendations for achieving outdoor thermal comfort in outdoor spaces [15][16][17][18][19]. ...
In recent decades, the pedestrian's outdoor thermal comfort has been critical for designing sustainable cities, which can be affected by wind flow in urban spaces. Urban forms influence wind speed, changing the urban air quality, and causing pedestrians' thermal comfort or discomfort. Therefore, it is required to design the urban spaces in such a way as to regulate the wind speed in order to provide thermal comfort situations for pedestrians based on their climate conditions. This study aimed to provide a framework for analysing the outdoor thermal comfort of various building layouts and their relation to wind speed in a site. This paper also proposed new indexes for improvements to the Physiological Equivalent Temperature (PET) index enabling the calculation of outdoor thermal comfort based on the likelihood of people being outside for a whole day. In-situ measurements were carried out in Tehran, Iran, to validate the presented framework. Simulations were conducted for 15 critical points in nine different building layouts for the windiest day on the site. ENVI-met was used to simulate the wind speed, outdoor air temperature, and mean radiant temperature, while PET was calculated by means of Leonardo and RayMan. The results demonstrated that pedestrians' outdoor thermal comfort for in-site and out-site points improved by 2.8% and 2.3%, respectively, compared to the current site situation. Also, the framework produced a suitable urban form to provide outdoor thermal comfort with evaluating different urban forms.
... The transient nature of thermal comfort has been widely discussed in indoor studies (Vellei et al. 2021). However, such understandings of outdoor environments are generally lacking (Young et al. 2022). ...
Cities are composed of a multitude of interconnected interactive layers and systems. The contemporary urban discourse has seen the utilization of Open data in decoding and understanding complex urban patterns that have eluded researchers for decades. Different layers of raw data from historical city cores up to the atmospheric climate have become more accessible, opening new horizons for multidisciplinary research. The rising complexity of cities calls for emerging approaches that can address the relationship between different layers of data—existing or emerging. In this regard, the current chapter is introducing and applying a methodology to use historical, spatial, and temporal datasets from Open Street Map (OSM) processed by Space Syntax superimposed on simulated urban microclimate dataset to find correlating patterns on how urban morphology has shaped the cities and the microenvironments over time. The outcomes for the case of Munich, illustrate the typologies that can be utilized in planning and developing design strategies to address micro-climate and accessibility in cities.KeywordsUrban morphologyUrban climateSpace syntaxMicroclimateGIS
The thermal environment and thermal comfort of an outdoor space have large spatial and temporal variations. To provide an overall picture, outdoor thermal comfort (OTC) should be analyzed on a yearly basic with high temporal-spatial resolution. The difficulty of annual OTC evaluation lies in the huge computational cost of wind simulations. Therefore, our study proposed a method to accelerate wind simulations through the use of Fast Fluid Dynamics (FFD), Proper Orthogonal Decomposition (POD) and Reynolds Number Independence (Re-independence). A case study of an actual urban building complex was employed to validate our study by comparing the integrated index Universal Thermal Climate Index (UTCI) results by our method with those by fully-resolved simulations. The average difference of UTCI was just 0.06 ∘C, indicating that the accuracy of our method is guaranteed. Besides, it only took 8 hours to complete the OTC assessment of this site with an area of 125,600 m2. The framework proposed in this study can be used to reveal the complete picture of OTC with affordable computational cost and reasonable accuracy.
Cycling is a healthy and sustainable means of urban travel. In this research, cycling tests and questionnaires were used to evaluate the impact of vibration intensity on the cycling experience. The standard deviation of vibration acceleration ( AStd) generated in cycling test was collected and vibration comfort and vibration perception were scored. Then, the simulation method was used to quantitatively analyze the influence of the macrostructure of the pavement on the cycling vibration. Moreover, through the simulation analysis of the most unfavorable situation of high-speed riding, the minimum British pendulum number (BPN) value that meets the skid resistance requirements is obtained. According to the results of regression analysis, the prediction equations of pavement structure parameters and AStd/BPN are obtained. After comprehensively considering cycling safety and comfort, the evaluation process of bicycle-specific pavement is given. This research hopes to provide a reference for the sustainable development of urban green travel.
There is a hypothesis that on severe hot weather days, pedestrians may choose routes based on the presence of solar radiation. However, this hypothesis is yet to be verified. Therefore, the present study experimentally clarified the effect of solar radiation in streets on pedestrian route choice. In the experiments, nineteen participants walked freely from the starting point to the designated destination on a sunny summer day. The area for the experiment is a commercial area around the Tokyo Station, which is the central area of Tokyo, Japan. In the experiment, participants were asked to verbalize the reasons for their route choices at every diverging point during their walk. The results are as follows: Most participants chose shaded routes in front of a large commercial facility. The most common reason given was “because there is shade” (28.2%), indicating that the presence or absence of shade is a major factor in route choice under severe thermal environment. Furthermore, in the questionnaire conducted after the walk, the most common reason participants provided for choosing shade was to protect their health, such as preventing heat stroke and sunburn. In summary, participant responses indicated their awareness of the dangers of solar radiation and their defensive motivation to avoid it, which might have led them to prioritize shade over other reasons in their route choice. The clarification of the effect of solar radiation on pedestrian route choice and the reasons for choosing shaded areas is very useful for future consideration in building design and urban planning.
This research is concerned with understanding the degree of human thermal (dis)comfort in connection with the various microclimates present in the vicinity of bike trails in Balneário Camboriú/SC, Brazil, throughout the summer. Local Climate Zones were determined using the Sky View Factor and were identified along research routes and schedules at 9:00 a.m. and 4:00 p.m. on a subtropical summer day (14 January 2022). Data were collected with weather devices attached to the bicycle, measuring air temperature, relative humidity, and globe temperature, from which the mean radiant temperature was calculated. The PET and UTCI indices were used to assess outdoor thermal comfort in the summer. The findings revealed that at 9:00 a.m., the eastern half of the city had a higher tendency for thermal discomfort; however, at 4:00 p.m., this same location had thermal comfort for users along bike routes. At 4:00 p.m., the PET index indicated that 24% of the bike paths were pleasant, and the UTCI index indicated that 100% of them were in thermal comfort. At 9:00 a.m., the majority of the city was under discomfort conditions. The index values reflect the morning time, and the study shows that there is now a negative correlation between the SVF and the indexes, which means that the greater the SVF computations, the lower the values are. The PET and UTCI indices revealed a positive association in the afternoon period: the greater the SVF values, the higher the PET and UTCI indices. Further research should be conducted in the future because many parameters, such as construction, position, and urban (im)permeability, as well as sea breeze and solar radiation, can have a significant impact on outdoor human thermal comfort in Balneário Camboriú, and when combined with the type of LCZ and the SVF, it is possible to understand how all of these active systems interact and form microclimates that are beneficial to bike path users.
Global warming and the urban heat island effect have led to the deterioration of the outdoor thermal environment and the rise of building cooling demand, calling for urban design with high environmental quality. However, little has been changed in the practice of urban street design. This paper bridges this gap by developing a framework of parametric simulation for design guidance of urban streets with an advanced urban canopy model that can simulate the interactive indoor-outdoor environment. Case studies are conducted at the neighborhood scale for six cities to improve outdoor thermal comfort and building energy efficiency in summer. The influence of street and building design parameters has been investigated based on 31104 simulations. Results reveal the conflict between optimizing outdoor thermal comfort and building energy efficiency. In the majority of the investigated cities, window-to-wall ratio, window type, and street orientation are the most influential parameters. Strategies by adjusting these parameters can significantly improve outdoor thermal comfort, as quantified by thermal stress hours, at a cost of increasing the building cooling load. Planting bigger and denser roadside trees always moderately reduces the thermal stress hours and building cooling load. These results demonstrate the potential of this framework to identify the design trade-offs between thermal comfort and building energy efficiency. The framework can be extended to help designers and policymakers at the early design stage for various cities.
Employing urban planning and design to promote active travel modes, such as walking and biking, are important for decarbonizing urban mobility. This paper proposes a modeling framework that investigates the interdependencies between the built environment and travel behaviors through the lens of urban microclimates. We combine travel data, built environment data, and Universal Thermal Climate Index (UTCI) calculations for New York City and train a predictive model for intra-city mobility patterns. Then, the impacts of UTCI features are studied through a sensitivity analysis and a spatial heterogeneity analysis. Results show that impacts of UTCI features can account for up to 4% change in the choice of active travel mode in dense urban areas. Also, impacts of microclimate vary across different travel contexts regarding season, time of day, activity, built environment, and traveler type. Our methodology and findings can inform future decision-making of microclimate-oriented spatial planning and design interventions.
In the context of dense urban environments and climate change, pedestrians’ thermal experience plays an increasingly significant role in people’s health and well-being. In this research, the authors combine the fields of architecture, climate-responsive design, and robotic fabrication with the goal of investigating strategies to improve outdoor thermal comfort for pedestrians in cities with frequent extreme heat events. Based on a case study in the city of Munich, this paper presents findings into the technological approaches and methods for location-specific climate-resilient brick facades using robotic assembly. To achieve this goal, different bricklaying patterns were investigated to create a self-shading effect and thus reduce solar radiation and ultimately achieve an improved thermal condition for pedestrians moving along urban facades at street level. Using computer-aided microclimate simulation, generic self-shading brick pattern designs were tailored to highly location-specific microclimate requirements. Robotic assembly technology was used to produce such tailored, non-standard brickwork facades. The results of this research led to a data-informed design process with a demonstrator object being realized at 1:1 scale with a height of 2 m and a length of 3 m using a collaborative robot on site. Thermal measurements on the built demonstrator provided indications of reduced surface temperatures despite high solar radiation and thus validated the location-specific self-shading effects according to solar radiation simulation.
PANDO is a numerical process-based simulation tool that enables the user to model individual trees and canopies in Rhino Grasshopper interface. This plugin simulates radiation fluxes in three different wavebands of photosynthetic (PAR), near infrared (NIR) and thermal. It also counts for photosynthesis and transpiration by tree canopies. PANDO can be used also to estimate root water uptake, drainage, canopy interception and cooling potential of trees by coupling with outdoor thermal comfort models. The tool enables the user to plan and design urban greenery more efficiently in cities considering the benefits and future paybacks.
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.
Urban areas are characterized by a plurality of microclimates given by the diversity of morphologies, optical and radiative properties, solar access and air circulation in different street canyons. For these reasons, beyond strategies at urban scale, mitigation needs to be addressed at district or even at urban canyon scale, just like the design of buildings. Among different options, canopy shading has been proposed in several urban contexts, and to assess the impact of this mitigation technique on air temperature and outdoor thermal comfort conditions we performed numerical simulations for a north-south oriented urban canyon with high solar access (height to width ratio equal to 0.18) in the climate context of Milano (Italy).
The Universal Thermal Climate Index (UTCI) aimed for a one-dimensional quantity adequately reflecting the human physiological reaction to the multi-dimensionally defined actual outdoor thermal environment. The human reaction was simulated by the UTCI-Fiala multi-node model of human thermoregulation, which was integrated with an adaptive clothing model. Following the concept of an equivalent temperature, UTCI for a given combination of wind speed, radiation, humidity and air temperature was defined as the air temperature of the reference environment, which according to the model produces an equivalent dynamic physiological response. Operationalising this concept involved (1) the definition of a reference environment with 50% relative humidity (but vapour pressure capped at 20 hPa), with calm air and radiant temperature equalling air temperature and (2) the development of a one-dimensional representation of the multivariate model output at different exposure times. The latter was achieved by principal component analyses showing that the linear combination of 7 parameters of thermophysiological strain (core, mean and facial skin temperatures, sweat production, skin wettedness, skin blood flow, shivering) after 30 and 120 min exposure time accounted for two-thirds of the total variation in the multi-dimensional dynamic physiological response. The operational procedure was completed by a scale categorising UTCI equivalent temperature values in terms of thermal stress, and by providing simplified routines for fast but sufficiently accurate calculation, which included look-up tables of pre-calculated UTCI values for a grid of all relevant combinations of climate parameters and polynomial regression equations predicting UTCI over the same grid. The analyses of the sensitivity of UTCI to humidity, radiation and wind speed showed plausible reactions in the heat as well as in the cold, and indicate that UTCI may in this regard be universally useable in the major areas of research and application in human biometeorology.
Over the past century more than 100 indices have been developed and used to assess bioclimatic conditions for human beings. The majority of these indices are used sporadically or for specific purposes. Some are based on generalized results of measurements (wind chill, cooling power, wet bulb temperature) and some on the empirically observed reactions of the human body to thermal stress (physiological strain, effective temperature). Those indices that are based on human heat balance considerations are referred to as "rational indices". Several simple human heat balance models are known and are used in research and practice. This paper presents a comparative analysis of the newly developed Universal Thermal Climate Index (UTCI), and some of the more prevalent thermal indices. The analysis is based on three groups of data: global data-set, synoptic datasets from Europe, and local scale data from special measurement campaigns of COST Action 730. We found the present indices to express bioclimatic conditions reasonably only under specific meteorological situations, while the UTCI represents specific climates, weather, and locations much better. Furthermore, similar to the human body, the UTCI is very sensitive to changes in ambient stimuli: temperature, solar radiation, wind and humidity. UTCI depicts temporal variability of thermal conditions better than other indices. The UTCI scale is able to express even slight differences in the intensity of meteorological stimuli.
Outdoor thermal comfort simulation simulations rely on the mean radiant temperature (MRT) seen by pedestrians as an important input that remains difficult to compute. Especially for large urban models, computing relevant surface temperatures and radiation fluxes that make up the MRT is a daunting task in terms of simulation setup and the computational overhead. We propose a new algorithm to estimate exterior surface temperatures of building facades, roofs, and ground surfaces in an arbitrary urban 3D model. The algorithm discretizes all model surfaces and clusters them by material properties and sky and sun exposure to reduce computational complexity. The model setup is fully automated, and the algorithm is implemented in the popular Rhino3d CAD environment. We demonstrate the accuracy of the algorithm by comparing both the resulting external surface temperatures against a high-fidelity simulation and the final MRT against real-world measurements. We report an RMSE of 1.8 °C and 2.0 °C, respectively, while reducing simulation times by a factor of ∼80. Envisioned applications of the algorithm range from rapid microclimate simulations in fast-paced urban design processes to large scale urban comfort evaluation of existing cities.
There has been no study on the impacts of street tree interval on the street-level radiant environment, although it has been a key factor in the design of thermally comfortable streets. In this study, the variation in pedestrian mean radiant temperature (MRT) is investigated as a function of the tree interval using a newly developed multilayer MRT model. Tree size and street size are considered in the model experiments. The results show that MRT was most effectively reduced by large trees, indicating that they may be planted at wider intervals compared to smaller trees to achieve comparable MRT reductions. Furthermore, as the tree interval decreases, MRT reduction was increased exponentially by small trees, while MRT reduction was increased linearly by large trees. Therefore, urban planners can reduce pedestrian exposure to radiation by planting either larger trees or closely spaced smaller trees. These results provide insight into optimal configurations of street trees for maximum reduction of MRT in a variety of urban canyon configurations.
This study presents a novel approach to validate the capability of biometeorological indices to predict the likelihood of urban dwellers to be outside during midday. Over a period of ten months three Wi-Fi scanners were used in a public courtyard in Cambridge, MA, to record outside dwelling patterns. Based on encrypted MacIDs courtyard attendees could be divided into 16,000 regulars and 676,000 visitors. Universal Thermal Climate Index (UTCI) predictions based on a combination of measured microclimatic conditions and mean radiant temperature simulations using ENVI-met were shown to strongly correlate with the number of regulars present during lunchtime with coefficients of determination (R2) of 92% during spring and 70% during summer/fall, respectively. Lunchtime attendance peaked for UTCI values in the thermal comfort and moderate heat stress ranges. In parallel, the probability for regulars to have lunch outside more than doubled during those UTCI conditions and the median lunchbreak length increased from 8 min to 12 min. These findings suggest that UTCI can be used as a reliable environmental performance metric to support the design and preservation of comfortable outdoor spaces. The reported use of public Wi-Fi data can help city governments to better understand – and potentially improve – the use of outdoor spaces while maintaining the privacy of their constituents.
The study encompasses the validation of the dynamic, RADIANCE-based daylight simulation method DAYSIM, which uses the concept of daylight coefficients and the Perez sky model to predict the short-time-step development of indoor illuminances. Measured and simulated illuminances have been compared under 10,097 sky conditions in a full-scale test office with a double glazing and external venetian blinds. The additional planning effort for the designer compared to a conventional daylight simulation is addressed. It has been found that the treatment of direct sunlight strongly influences the accuracy of the daylight coefficient method. Three different simulation modes for the direct sunlight are investigated. The simulation results prove that indoor illuminances can be modeled with comparable accuracy for various blind settings under arbitrary sky conditions. Daylight autonomies are predicted with an accuracy below 2% points, where simulation errors stem with roughly equal parts from the raytracing and the sky model.
A numerical simulation method is developed for predicting the effective radiation area and the projected area of a human body for any posture. This method is based on the solar heat gain simulation for buildings. To confirm the validity of the present method, predicted effective radiation area factors and projected area factors for both standing and seated persons are compared with those by the measurements. It was found that predicted values agree quite well with those by the subjective experiments within 10% accuracy. The effective radiation area and the diagrams of the projected area factors for a person sitting on the floor are illustrated. Moreover, the angle factors between a standing person and rectangular planes are calculated and compared with the results by Fanger.
The relative humidity (RH) and the dewpoint temperature (td) are two widely used indicators of the amount of moisture in air. The exact conversion from RH to td, as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%) which does not appear to be widely known by the meteorological community: td decreases by about 1°C for every 5% decrease in RH (starting at td = t, the dry bulb temperature, when RH = 100%). This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as zLCL (20 + t/5) (100 - RH), where zLCL is in meters when t is in degrees Celcius and RH in percent. Finally, a historical perspective is given with anecdotes about some of the early work in this field.
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