UDC1: Impact of Urban forms on outdoor ventilation
Natural Ventilation Performance in a High Density Urban Area Based on CFD Numerical Simulations in Dalian
architecture and art school,dalian university of tech, China, People's Republic of
The urban expansion of Dalian city over the past 60 years from approximately 50 sq. km to 500 sq. km has been characterized by a high-density downtown morphological pattern, while the urban wind speed per year has trended downward. To evaluate natural ventilation performance in the high-density downtown area of Dalian, an in situ wind environmental survey was performed, and the results were used as a reference to configure CFD numerical simulation boundary conditions. The results indicate that urban edifices, such as enclosed city blocks, strip apartments in rows and, especially, high-rise buildings with large podium bulk, were unfavorable to natural ventilation and could reduce the mean wind speed by up to 78% relative to the approaching speed. The natural ventilation performance of different building morphologies were further evaluated via CFD simulations, which indicated such strategies as using ventilation paths, hybrid buildings with different heights, building stilts, and increasing building height while decreasing their land coverage could improve the urban ventilation performance. Increasing the building height and reducing land coverage was one of the most efficient strategies and increased wind velocities up to 2.4 times the real case. Green land significantly cooled and humidified its surroundings, particularly the downwind spaces. The corresponding morphology optimization measures were been discussed.
Anthropogenic heat contribution to air temperature increase at pedestrian height in Singapore’s high density Central Business District (CBD)
National University of Singapore, Singapore
Half of the world’s population today live in cities, causing intensification of urban developments (UN, 2008) created from man-made materials that generate and retain heat. This creates an urban climate which is hotter than its surroundings, generally known as the Urban Heat Island (UHI) effect. Human beings are anthropogenic heat sources which contribute to this increased thermal pollution in cities via human metabolism, buildings and vehicles (Sailor and Lu, 2004) apart from the sun and sky which form the natural sources under the Urban Energy Balance inputs (Oke, 1987). In dense cities like Singapore, motor vehicles ownership is double the amount of Hong Kong with almost identical population density as well as built density based on the Height-to-Width (H/W) Ratio. It measures how close buildings are built to each other. It normally increases with the temperature difference between the urban and rural areas (Roth, 2013) and when population in cities increase (Oke, 1973).
The research is into the contribution of anthropogenic heat at pedestrian level in the densest part of Singapore, which is located in the Central Business District (CBD) at microscale to local scale to air temperature increase. This is classified as Local Climate Zone 1 which is compact high rise (Stewart and Oke, 2012). An anthropogenic heat inventory method study was done in Singapore (Quah and Roth, 2012) which shows that the commercial area has bigger contribution that high rise apartments and landed housing with more contribution during weekdays during office hours. The buildings contribute the most, followed by vehicles and human metabolism. However, at pedestrian level, substantial contribution from the vehicles is expected since most heat waste from commercial buildings are released at the podiums or rooftops.
Computational Fluid Dynamics (CFD) is used to study various aspects of the urban environments for the last 50 years (Blocken, 2014). It has the advantage over the energy balance model as it does not decouple temperature and wind flow (Toparlar, et al., 2014). Urban canyon studies are done to understand flow behavior and more recently on the surface convective heat transfer aspect (Magnusson et al., 2014). In terms of contribution from the climate sources, (Bottillo et al., 2014; Nazarian and Kleissl, 2014; Toparlar et al., 2014) have studied it but at lower H/W Ratios of up to 2. Contributions from buildings alone are studied by Huang et al., 2005; Priyadarsini and Wong, 2009; Hashimoto et al., 2014 while Chen et al., 2009 studied it together with vehicles contribution. Chen et al, 2009 showed that the impact from the traffic is the highest at the pedestrian level regardless of whether it is a high rise or low rise district.
A roadside measurement at 1.5m height was done near a bus stop in a deep canyon in the CBD area during the afternoons of the dry month of February 2014. The measurement showed a maximum air temperature difference of 4.5°C during the peak hour between absence and presence of buses with higher temperatures recorded during weekdays.
A 6am-6pm slab and points street canyon of H/W ratio of 2 and 4 with as well as without double decker buses were simulated in the transient CFD environment with perpendicular and parallel flow. The solar radiation was activated with gravity enabled. Measurements at the pedestrian height at the pavements show air temperature increase of close to 3°C between cases with and without buses. Cases with parallel flow regulates air temperature inside the canyon better than perpendicular flow. Higher H/W ratio cases generate better channeling effect for parallel flows while retaining higher air temperature for perpendicular flow because of bigger heat storage.
In conclusion, results show that form and density make the difference for flow and heat transfer in the canyon. For future studies, parametric studies will be done for the worst case scenario where the road is filled with vehicles, mimicking the peak hour and the impact of various urban morphologies.
Urban wind design for Bonifacio’s citadel
1Urbalterre, France; 2CSTB, France
The citadel of Bonifacio is located at the extreme south of Corsica. That small maritime city was built on a peninsula, up to 50 meters above the sea, to protect the commercial route crossing by the strait of Bonifacio between Corsica and Sardegna. By 1996, the army abandoned the city, leaving 30.000 square meters of land and buildings to transform.
The project deals with the needs of the old city, transportation concerns, economical and tourism development, and environmental concern. Around 2 millions tourists visit Bonifacio each summer, and the city would like to welcome visitors all seasons long. The project comprises a museum, a new luxurious hotel, a big parking and some public facilities.
But, even for summer journeys, if you stay in Bonifacio you will run the risk of strong wind events. Indeed, Bonifacio is the windiest city of Mediterranean area, and one of the windiest in the world. A strong wind is blowing up to 155 days a year, and up to 100 km/h for 20 days amongst it.
To ensure pedestrian safety and comfort for the new streets and public spaces of the citadel, we developed an urban design process, combining Computational Fluid Dynamics (CFD) models and wind tunnel testing with PIV measurements. As a result, the design for the new district organises protected areas with a concept of protection belts. The concept uses new buildings and old ones, gardens with progressive heights of trees, to reduce wind speed and turbulent flow.
In the past, the citadel ensured protection for the army to control the strait. Today, the citadel is open and welcomes tourists to visit a place now dedicated to the environmental protection of the strait.
Air Quality in the City of Erzurum: Strategies for Climate Sensitive Urban Design
1Ataturk University, Turkey; 2Ataturk University, Turkey
In this paper, it is claimed that the relationship between urban design and climate conditions enable the emergence of sustainable urban physical environments that will be the places of the world of the future. After serious ecological problems like urban heat island and urban air pollution in this century, researchers began to focus on bioclimatic- and eco-comfort in urban areas. Air circulation or ventilation channels as one the climate and comfort related variables is essential for the mitigation of air pollution, heat temperature, cooling and ventilation.
The main objective of this paper is to explore the relationship between the wind flow, air pollution and physical structure of the built environment in the city of Erzurum, Turkey. It attempts to understand the reasons of the environmental problems and propose climate sensitive urban design approaches by emphasizing green air corridors or wind channels for Erzurum. In this study, it is questioned whether the urban form and geometry (size, density, street orientation and ventilation), street and public space layout, height and shape of the buildings, vegetation, energy flow and water cycle in selected corridors of Erzurum are designed according to the prevailing wind direction and climate conditions. In this direction, by considering the general features of the ventilation channels, orientation and continuity of open spaces, streets and parks, dimensions and shape of open spaces, construction practices, settlement patterns, housing typology, density, building topography, average height, hard-surfaces, distances, property relations, flora and density of green areas are analyzed for the axles extending parallel to the wind direction towards agglomeration areas of air pollution in the city.
The findings show that urban built environment in the Erzurum prevent the ventilation. Existing air pollution, ventilation problems and high temperature differences in the city are the threatening factors for ecologically sustainable urban development. Settlement pattern and urban form of the selected corridors or axles must be reconsidered with ventilation perspective. Creation of green ecological spaces in old areas, new green public spaces, and reasonable range of building density can be accepted as the first stage solutions for the city. Otherwise, central areas of the city and its urbanization process will again be unsustainable in the future and have low quality of life. For ecologically sustainable urban development, climate sensitive urban design approaches and ventilation channels must be developed and designed for Erzurum.
Keywords: Climate, Air Pollution, Ventilation, Built Environment, Urban Design
Preserving Overall Performance of Air Conditioners by Incorporation of Wind- Permeable Floor in Buildings
1AECOM Asia Co. Ltd., Hong Kong S.A.R. (China); 2Division of Environment, The Hong Kong University of Science and Technology, Hong Kong S. A. R. (China); 3Department of Mathematics, The Hong Kong University of Science and Technology, Hong Kong S. A. R. (China); 4Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Hong Kong S. A. R. (China)
Hong Kong is a highly urbanized city with high population density, therefore extensive mechanical ventilations are required to provide space cooling and thermal comfort for occupants during hot and humid summer seasons. Unitary air conditioners are ubiquitously installed at the re-entrants of buildings, operating continuously and concurrently, generating a vast amount of heat. Residual heat ejected by outdoor condensing units is accumulated within semi-enclosed re-entrants of buildings, degrading the energy conversion efficiency of air conditioners due to high working temperature. Wind advection and thermal buoyancy mainly drive the motion of hot air stream, imposing vertical temperature gradients at the re-entrants. Literatures have reported that temperature difference between rooftop and ground level of a typical 30 storey building could reach 7°C; moreover, overall percentage drop in coefficient of performance of air conditioners could reach a maximum of 26% under no-wind condition. In this study, effectiveness of incorporating an open wind-permeable floor, attempting to alleviate thermal stack effect and preserve energy conservation of buildings, is investigated. A typical residential block with a wind-permeable floor at mid-level is compared with the same residential block without one. Temperature profiles and group performance of the air conditioners, indicated by Condenser Group Performance Index, are evaluated for the subjected buildings by steady Reynolds Averaged Navier Stokes simulations under both wind driven and buoyancy driven conditions. After the implementation of an open permeable floor in the residential blocks, elevated air passing through the floor is cooled by natural means. Performance of air conditioners are retained as a result of lower operating temperature, especially for those installed above the wind-permeable floor.