2,151 research outputs found
A quasi-elastic regime for vibrated granular gases
Using simple scaling arguments and two-dimensional numerical simulations of a
granular gas excited by vibrating one of the container boundaries, we study a
double limit of small and large , where is the restitution
coefficient and the size of the container. We show that if the particle
density and where is the particle diameter, are
kept constant and small enough, the granular temperature, i.e. the mean value
of the kinetic energy per particle, , tends to a constant whereas the
mean dissipated power per particle, , decreases like when
increases, provided that . The relative fluctuations
of , and the power injected by the moving boundary, , have simple
properties in that regime. In addition, the granular temperature can be
determined from the fluctuations of the power injected by the moving
boundary.
Retrofitting housing with lightweight green roof technology in Sydney, Australia, and Rio de Janeiro, Brazil
© 2015 by the authors. The built environment contributes around half of total greenhouse gas emissions and with 87% of residential buildings that we will have by 2050 already built, it is vital to adopt sustainable retrofitting practices. The question is: what are the viable solutions? One answer may be green roof retrofitting. The environmental benefits include reduced operational carbon emissions, reduced urban heat island effect, increased bio-diversity, housing temperature attenuation and reduced stormwater run-off. The economic benefits are the reduced maintenance costs and lower running costs. The social gain is the creation of spaces where people have access to green areas. However, the barriers to retrofitting include the perceptions of structural adequacy, the risk of water damage, high installation and maintenance costs, as well as access and security issues. Many Australian and Brazilian residential buildings have metal sheet roofs, a lightweight material with poor thermal performance. During the summer, temperatures in Sydney and Rio de Janeiro reach 45 degrees Celsius, and in both cities, rainfall patterns are changing, with more intense downpours. Furthermore, many residential buildings are leased, and currently, tenants are restricted by the modifications that they can perform to reduce running costs and carbon emissions. This research reports on an experiment on two small-scale metal roofs in Sydney and Rio de Janeiro to assess the thermal performance of portable small-scale modules. The findings are that considerable variation in temperature was found in both countries, indicating that green roof retrofitting could lower the cooling energy demand considerably
Retrofitted green roofs and walls and improvements in thermal comfort
© 2017 Author(s). Increased urbanization has led to a worsening in the quality of life for many people living in large cities in respect of the urban heat island effect and increases of indoor temperatures in housing and other buildings. A solution may be to retrofit existing environments to their former conditions, with a combination of green infrastructures applied to existing walls and rooftops. Retrofitted green roofs may attenuate housing temperature. However, with tall buildings, facade areas are much larger compared to rooftop areas, the role of green walls in mitigating extreme temperatures is more pronounced. Thus, the combination of green roofs and green walls is expected to promote a better thermal performance in the building envelope. For this purpose, a modular vegetated system is adopted for covering both walls and rooftops. Rather than temperature itself, the heat index, which comprises the combined effect of temperature and relative humidity is used in the evaluation of thermal comfort in small scale experiments performed in Sydney - Australia, where identical timber framed structures prototypes (vegetated and non-vegetated) are compared. The results have shown a different understanding of thermal comfort improvement regarding heat index rather than temperature itself. The combination of green roof and walls has a valid role to play in heat index attenuation
Modelling green roof stormwater response for different soil depths
© 2016 Elsevier B.V. Green roofs have been proposed as a way to mitigate stormwater run-off in urban areas due to the possibility of retrofit to existing buildings. The amount of run-off is influenced by the, humidity, evapotranspiration, as well as soil type and depth. A modelling approach was undertaken to evaluate the response of different soil depths to cumulative rainfall and the efficiency in stormwater flow rate attenuation. The soil hydraulics were modelled using HYDRUS-1D software developed for modelling water flow in variably saturated porous media. Model runs were carried out for three quarterly scenarios to determine run-off peak flow rates and the overall retention, based on evapotranspiration rates of succulent plants and rainfall registers from Auckland, New Zealand. The soil depths modelled ranged from 5 to 160 cm. The efficiencies in peak flow attenuation by the shallowest soil considered were reduced under extreme and longer rainfall events by 3%. Therefore shallow soil or extensive green roofs may, on a wide scale, overcome the performance of deep soils due to their lighter weight which adds limited loads to existing roof structures thereby making them suited to retrofit greater numbers of buildings
A fluidized granular medium as an instance of the Fluctuation Theorem
We study the statistics of the power flux into a collection of inelastic
beads maintained in a fluidized steady-state by external mechanical driving.
The power shows large fluctuations, including frequent large negative
fluctuations, about its average value. The relative probabilities of positive
and negative fluctuations in the power flux are in close accord with the
Fluctuation Theorem of Gallavotti and Cohen, even at time scales shorter than
those required by the theorem. We also compare an effective temperature that
emerges from this analysis to the kinetic granular temperature.Comment: 4 pages, 5 figures, submited to Physical Review Letters; Revised
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