618 research outputs found
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Influence of ground surface characteristics on the mean radiant temperature in urban areas
The effect of variations in land cover on mean radiant surface temperature (Tmrt) is explored through a simple scheme developed within the radiation model SOLWEIG. Outgoing longwave radiation is parameterised using surface temperature observations on a grass and an asphalt surface, whereas outgoing shortwave radiation is modelled through variations in albedo for the different surfaces. The influence of surface materials on Tmrt is small compared to the effects of shadowing. Nevertheless, altering ground surface materials could contribute to a reduction on Tmrt to reduce the radiant load during heat-wave episodes in locations where shadowing is not an option. Evaluation of the new scheme suggests that despite its simplicity it can simulate the outgoing fluxes well, especially during sunny conditions. However, it underestimates at night and in shadowed locations. One grass surface used to develop the parameterisation, with very different characteristics compared to an evaluation grass site, caused Tmrt to be underestimated. The implications of using high resolution (e.g. 15 minutes) temporal forcing data under partly cloudy conditions are demonstrated even for fairly proximal sites
Assessment of predicted versus measured thermal comfort and optimal comfort ranges in the outdoor environment in the temperate climate of Glasgow, UK
In a warming world, the risk of overheating is significant in temperate climate areas such as Glasgow, UK where adaptation to overheating is low. An easy-to-use thermal comfort evaluation is therefore a necessary first step towards developing effective coping mechanisms. In this study, we explore the effectiveness of Predicted Mean Vote, Predicted Percentage of Dissatisfied and Physiologically Equivalent Temperature, together with air temperature in mimicking actual thermal sensation votes of street users obtained in 2011 in Glasgow City Centre. The Predicted Mean Vote/Predicted Percentage of Dissatisfied indices developed for controlled indoors show a surprising similarity to actual thermal sensation votes derived from outdoor surveys, than the Physiologically Equivalent Temperature developed specifically for the outdoors. The method of calculation of mean radiant temperature is the key to improved performance of Physiologically Equivalent Temperature, with fish-eye lens photographs improving its performance. The results also show air temperature alone has nearly equal predictive power of the actual thermal sensation. A preliminary comfort range for Glasgow is also derived and its limitations are explored. Practical application: The strong relation between thermal sensation votes and air temperature (Ta) enables future thermal comfort studies to predict the thermal comfort using easy-to-access Ta only. A current thermal comfort study in Glasgow aiming at developing a link between urban morphology and Ta is already using this strong relation to predict outdoor thermal comfort in the city centre. This helps to establish a correlation between these three factors. </jats:p
The impact of urban geometry on the radiant environment in outdoor spaces
Urban geometry, namely the quantitative relationship of building volumes and open spaces (i.e. built density) and their spatial configuration (i.e. urban layout), is a major modifier of urban microclimate. This paper presents the results of an ongoing research which explores the impact of urban geometry on the radiant environment in outdoor spaces, with direct implications for urban microclimate and outdoor thermal comfort. In particular, the research investigates the relationship between a set of urban geometric indicators (such as Built Density, Site Coverage, Mean building Height and Frontal Area Density) and Mean Radiant Temperature (Tmrt) at the pedestrian level, in different areas of London.
Three representative areas of London were selected to be studied; in central, west and north London which are of high, medium and low built density, respectively. Each area was divided into squares of 500m x 500m size, with a total of 84 urban squares included in the study. The methodology comprises three stages: (i) A set of simple geometric indicators have been computed for all urban squares using special algorithms written and executed in Matlab software. (ii) Radiation simulations have been performed for 10 days of a typical year in London, with the use of SOLWEIG software. SOLWEIG simulates hourly, 3-D radiation fluxes, incoming to / outgoing from the ground, spatial variations of Tmrt, Ground View Factor (GVF) as well as Sky View Factor (SVF). Sunny and cloudy days have been considered, evenly distributed in the year in order for the effect of solar angles to be examined. (iii) Statistical tests have been conducted for investigating the correlation between urban geometry, as expressed by the geometric variables, and hourly, average values of Mean Radiant Temperature in the outdoor spaces of the urban squares.
The simulation results show that at night-time and in fully overcast conditions, the outdoor spaces of central London’s urban squares are warmer than those of west and north London, due to greater longwave radiation emitted and reflected by building volumes. In contrast, on sunny days, average daytime Tmrt values have been found to be higher in North London’s urban squares due to the larger insolation of their outdoor spaces. Additionally, the statistical analysis has shown that in the absence of direct solar radiation, the correlation between the geometrical variables and average values of Tmrt is very high with an almost perfect linear relationship between the geometrical variables and average SVF values (r2= 0.980). In the presence of direct solar radiation, the strength of the correlation varies with the sun altitude angle; the higher the sun altitude angle, the higher the correlation. In particular, a threshold altitude angle of 20 degrees has been identified, above which the correlation of average Tmrt values with urban geometry approximates that of night-time / cloudy hours. Finally, further statistical tests showed that site coverage (built area over site area) and frontal area density (façades’ total area over site area) are the strongest indicators among those considered in the analysis
LAI based trees selection for mid latitude urban developments: A microclimatic study in Cairo, Egypt
To study the leaf area index, LAI, based thermal performance in distinguishing trees for Cairo's urban developments, ENVI-met plants database was used as platform for a foliage modeling parameter, the leaf area density, LAD. Two Egyptian trees: Ficus elastica. and Peltophorum pterocarpum were simulated in 2 urban sites with one having no trees, whilst the second is having Ficus nitida trees. Trees LAD values were calculated using flat leaves' trees LAI definition to produce maximum ground solid shadow at peak time. An empirical value of 1 for LAI is applied to numerically introduce LAD values for ENVI-met.
Basically, different meteorological records showed improvements for pedestrian comfort and ambient microclimate of the building using E elastica. About 40-50% interception of direct radiation, reductions in surfaces' fluxes around trees and in radiant temperature T-mrt in comparison to base cases gave preferability to E elastica. The lack of soil water prevented evapotranspiration to take place effectively and the reduced wind speeds concluded negligible air temperature differences from both base cases except slightly appeared with the F elastica. Results show that a flat leaves tree if does not validate LAI of 1, the ground shading would not fulfill about 50% direct radiation interception and this value can be used as a reference for urban trees selection.
Further simulations were held to investigate LAI value of maximum direct radiation interception.
Performing additional simulations, F elastica of LAI of 3 intercepted almost 84% of direct radiation and revealed implications about urban trees in practice and its actual LAI. (C) 2009 Elsevier Ltd. All rights reserved
Thermal comfort based fuzzy logic control
Most heating, ventilation and air conditioning (HVAC) control systems are considered as temperature control problems. In this work, the predicted mean vote (PMV) is used to control the indoor temperature of a space by setting it at a point where the PMV index becomes zero and the predicted percentage of persons dissatisfied (PPD) achieves a maximum threshold of 5%. This is achieved through the use of a fuzzy logic controller that takes into account a range of human comfort criteria in the formulation of the control action that should be applied to the heating system to bring the space to comfort conditions. The resulting controller is free of the set up and tuning problems that hinder conventional HVAC controllers. Simulation results show that the proposed control strategy makes it possible to maximize the indoor thermal comfort and, correspondingly, a reduction in energy use of 20% was obtained for a typical 7-day winter period when compared with conventional control
A GIS-based assessment method for mean radiant temperature in dense urban areas
The mean radiant temperature (Tmrt) is among the most important factors affecting thermal comfort. Its assessment in dense cities has been complicated due to the presence of buildings, pavings, and infrastructure. This paper introduced the RAMUM model, a GIS based software method developed to simulate outdoor mean radiant temperature at microscale. The advantages of this method lie in its efficiency and resolution that supports the design of buildings, streets, and public open spaces. The model is evaluated using field measurements under cold and warm weather in Boston. This study is sponsored by the EFRI-1038264 award from the National Science Foundation (NSF), Division of Emerging Frontiers in Research and Innovation (EFRI).postprin
Assessment and mitigation strategies to counteract overheating in urban historical areas in Rome
As urban overheating is increasing, there is a strong public interest towards mitigation strategies to enhance comfortable urban spaces, for their role in supporting urban metabolism and social life. The study presents an assessment of the existing thermal comfort and usage of San Silvestro Square in Rome during the summer, and performs the simulation of cooling strategies scenarios, to understand their mitigation potential for renovation projects. The first stage concerns a field analysis of the thermal and radiative environment on the 1st and 2nd of August 2014, including meteorological measurements and unobtrusive observations, to understand how people experience and respond to extreme microclimate conditions. In the second stage, the research proposes scenario simulations on the same day to examine the influence of cool colored materials, trees and vegetative surfaces on thermal comfort. The thermal comfort assessment was based on Physiologically Equivalent Temperature (PET), whereas microclimatic simulations were conducted with CFD calculations (ENVImet v.4.3.1). The first stage shows a strong relationship between lower PET values and attendance rate, depending on daily shading patterns. The second stage shows a relevant improvement of thermal comfort, with PET values of 12 °C comparing to the no-intervention scenario, associated with a combination of cool materials and trees
Estimation of mean radiant temperature in cities using an urban parameterization and building energy model within a mesoscale atmospheric model
During daylight hours, the mean radiant temperature Tmrt is one of the most important meteorological parameters to analyse heat stress for humans. This study conducts a spatio-temporal analysis of Tmrt for a summer period in 2018 for the city of Berlin, Germany. To this end, the mesoscale climate model COSMOCLM (CCLM) is coupled with the urban Double Canyon Effect Parameterization scheme with a building energy model (DCEP–BEM) to derive Tmrt. This coupled model system CCLM/DCEP–BEM enables a dynamic calculation of Tmrt for the microscale urban street canyons using a mesoscale model. To bring a more accurate comparison, a two-step approach is applied to assess the radiative fluxes and Tmrt from CCLM/DCEP–BEM. The radiation model SOLWEIG is first validated against measurement and then used to evaluate the DCEP–BEM model. Overall good agreement in Tmrt is found between CCLM/DCEP–BEM and SOLWEIG (R2 = 0.96). Nighttime Tmrt simulated with CCLM/DCEP–BEM is higher than that with SOLWEIG (MBE = 2.9K), yet closer to measurements. Tmrt during the afternoon hours modeled with CCLM/DCEP–BEM is underestimated compared to SOLWEIG (MBE = −3.1K). Further, excluding vegetation, higher values for nighttime Tmrt are found in the densely built-up city center than in the suburbs with more open structures, while the city center has lower values for Tmrt during midday. This study provides a reliable representation of Tmrt in a mesoscale model and would be beneficial for future implementation of human-biometeorological variables such as the Universal Thermal Climate Index or Physiological Equivalent Temperature. These quantities are calculated using Tmrt.Peer Reviewe
The most problematic variable in the course of human-biometeorological comfort assessment - The mean radiant temperature
This paper gives a review on the topic of the mean radiant temperature T mrt, the most important parameter influencing outdoor thermal comfort during sunny conditions. T mrt summarizes all short wave and long wave radiation fluxes reaching the human body, which can be very complex (variable in spatial and also in temporal manner) in urban settings. Thermal comfort researchers and urban planners need easy and sound methodological approaches to assess T mrt . After the basics of the T mrt calculation some of the methods suitable for obtaining T mrt also in urban environments will be presented.. Two of the discussed methods are based on instruments which measure the radiation fluxes integral (globe thermometer, pyranometer-pyrgeometer combination), and three of the methods are based on modelling the radiation environment with PC software (RayMan, ENVI-met and SOLWEIG). © 2011 Versita Warsaw and Springer-Verlag Wien
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