OPTIMIZATION OF ORIENTATION AND CANYON ASPECT RATIO FOR A CITY IN WARM HUMID CLIMATE

Abstract

The developing world is going through the process of rapid urbanization. The Urban population of developing countries has surpassed the urban population of developed nations. Being a developing nation, the economic growth and urbanization are going on at fast pace in India. As most of the future cities of India are yet to be built, there is scope to develop future cities with efficient, climate resilient urban design and planning strategies. Urban outdoor spaces, often referred as ‘urban commons’, enrich the quality of urban life and are of immense importance especially in Indian context. Careful design of outdoor spaces around the buildings, can reduce thermal stress of urban dwellers in densely populated cities of warm humid climate of developing world. A large part of urban India, 18 out of India’s 53 million plus cities, falls under the warm-humid climate zone including Kolkata, Chennai and Mumbai. Research on climate responsive urban design for this particular climatic zone can help to reduce outdoor thermal stress, facilitating sustainable living. Designing street canyons is a key to climate responsive urban design as it is an intermediate interface between the urban scale and individual building scale. Street canyon is formed when both the sides of the street is lined by buildings. Air temperature in urban street canyons is increased due to the morphed urban geometry, increased surface area, decreased long wave radiation and evapo-transpiration, different thermo-physical properties of surface materials and anthropogenic heat which results in thermal discomfort. The orientation and aspect ratio of street canyon influences the wind flow and solar gain, both inside and outside of the buildings and are considered as major influencing factors by researchers. This study investigates the impact of different orientations and canyon aspect ratios to improve street-level thermal comfort in a warm humid climate. It also contributes to the development of a methodology to develop design guidelines while planning urban neighbourhoods. The study is carried out by using ENVI_met version 4.0, a prognostic computational fluid dynamics simulation tool, for simulating the urban microclimate and assess the impact on outdoor thermal comfort. Simulations were run for typical residential neighbourhoods in the Rajarhat Newtown, Kolkata, India, characterized by a warm humid climate as per the National Building Code of India, for a typical summer day with a high level of discomfort. i Thesis Report The study aims to provide guidance for orientation of buildings and canyon aspect ratio to minimise outdoor thermal stress in the residential neighbourhoods of warm–humid urban areas through adopting a case of Rajarhat-Newtown, Kolkata, India. In order to achieve the aim, the following objectives have been fulfilled. i. Investigating the influence of urban design elements on urban climate and outdoor thermal comfort in urban areas through state of the art review with a focus on warm humid climate. ii. Developing the research framework to integrate urban climate in urban design process while optimizing orientation and canyon aspect ratio of Rajarhat- Newtown, Kolkata. iii. Recommendation for optimum orientation and canyon aspect ratio by prognostic evaluation of urban microclimate and comfort for residential neighbourhoods of Rajarhat- Newtown, Kolkata. Rajarhat-Newtown is selected as the location of the study based on few practical considerations. The selection represents a city in warm humid climate region with large population base, from a developing nation. The choice is also guided by the practicality of conducting instrumented meteorological measurement considering the availability of human resource, local contacts and most importantly safety and security of the mounted portable weather stations. The present development trend and growth pattern of The Rajarhat- Newtown has met limited success in meeting the main objective of the Newtown i.e. a better living quality. As the development process is still ongoing, there is future scope to integrate climate responsive urban design strategies in the course of development to achieve the envisioned goal of Newtowns, ensuring better living quality. Already built areas can also be retrofitted by corrective measures, so that the township becomes a ‘Newtown’ in true sense. Different alternative combinations of different orientations (5°, 15°, 30° and 45° to North) and different canyon aspect ratios (1, 1.5, 2 and 2.5) are simulated and compared to find out the most suitable design alternative for the study area. While generating different canyon aspect ratio scenarios, the floor area ratio (the ratio of total built up area and site area) is kept constant without changing the proposed or desired density of the area and existing development control regulations are followed to keep the design alternatives realistic. The first set of simulations are carried out by varying orientation and then the best performing orientation option is combined with different aspect ratio options so that the combined impact of both the parameters can be analysed and the scenario with optimum orientation and canyon aspect ratio can be identified. ii Thesis Report Performance analysis of the design alternatives are carried out by comparing two different thermal comfort indices, Physiological Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI), so that the changes in microclimate, incurred by the alternate design options, can be accessed in terms of human thermal comfort on street level. The analysis of simulated scenarios exhibit perceptible change in the air temperature, wind speed and MRT due to different orientations and as a result thermal comfort is affected (which is quantified by comparison of PET and UTCI). Maximum reduction in air temperature is observed in the scenario with orientation angle between 30°-60° to the north. The scenario also shows substantial improvement in wind speed and reduction in average Mean Radiant Temperature (MRT) due to increased shading on streets by the buildings. The shaded zone created on streets provide less stressful outdoor thermal environment. The reduction in thermal stress level can be observed throughout the temporal range, but the thermal stress reduction is more pronounced in the mid and late afternoon (after 2 pm). At 4pm, PET is reduced by 1°C in 10% area, by 2°C in 19% area, by 3°in 18% area and up to 10°C in around 16% area. The combined option of orientation angle between 30° and 60° to the north and a Canyon Aspect Ratio of 2.5 can reduce the PET value by 5 to 9°C throughout most of the study area during a mid-afternoon summer day. Improvement in the thermophysical assessment classes cannot be achieved for the adopted simulation date and time (May 31st, 14:00 hrs). Side by side comparison of spatial distribution of PET and UTCI from the combined simulation results reveals that during the time period of maximum heat stress in a typical summer day, canyon aspect ratio have a substantial impact on street level thermal comfort too. Optimum aspect ratio of the street canyons can improve the thermal comfort by providing greater shading on the streets and by altering the wind speed and flow pattern. It also influences the natural ventilation inside the buildings. The study concludes that orientation angle of 30° to 60° from north and canyon aspect ratio of 2.5, performs more efficiently to reduce the outdoor thermal stress in the Rajarhat Newtown by providing better shading and better wind movement. Higher canyon aspect ratio (over 2.0) with taller buildings, perform considerably better than the lesser canyon aspect ratio scenarios. Urban setup with lower canyon aspect ratios should be avoided as they create more stressful outdoor thermal environment and nocturnal Urban Heat Islands. The research findings directly influences a lot of design considerations like plot size, road width and development control regulations. Both of the parameters, orientation and canyon aspect ratio, have the potential to iii Thesis Report mitigate the pedestrian level heat stress and improve the urban wind flow if they are combined judiciously. Designing the urban areas in warm humid climate is one of the most challenging tasks because of low level of macro wind flow and drastic spatial and temporal variation of wind speed and direction. Given the different geographical location, wind direction and wind speed, different urban areas require different optimum orientation and canyon aspect ratio that is suitable for the specific context in spite of being situated in same climatic zone. To optimise the above said parameters for any warm humid urban area, quantitative assessment through prognostic evaluation is suggested. The design recommendations that are suggested in this study is for the Rajarhat Newtown, Kolkata; but the analytical framework, developed and validated through application in this research, can be replicated for another urban area in warm humid climatic zone, in order to find out the optimum combination of orientation and canyon aspect ratio suitable for that particular context. The developed methodology can be useful for the urban planners, designers and architects to decide the optimum orientation and canyon aspect ratio for the urban areas in warm humid climate.