Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

An introduction to heat transfer

Urban greening to cool towns and cities: a systematic review of the empirical evidence.

Short- and long-wave radiation flux densities absorbed by people have a significant influence on their energy balance. The heat effect of the absorbed radiation flux densities is parameterised by the mean radiant temperature. This paper presents the physical basis of the RayMan model, which simulates the short- and long-wave radiation flux densities from the three-dimensional surroundings in simple and complex environments. RayMan has the character of a freely available radiation and human-bioclimate model. The aim of the RayMan model is to calculate radiation flux densities, sunshine duration, shadow spaces and thermo-physiologically relevant assessment indices using only a limited number of meteorological and other input data. A comparison between measured and simulated values for global radiation and mean radiant temperature shows that the simulated data closely resemble measured data.

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Modelling radiation fluxes in simple and complex environments—application of the RayMan model

Influence of urban geometry on outdoor thermal comfort in a hot dry climate: A study in Fez, Morocco

The outdoor environment is deteriorating in many tropical cities due to rapid urbanization. This leads to a number of problems related to health and well-being of humans and also negatively affects social and commercial outdoor activities. The creation of thermally comfortable microclimates in urban environments is therefore very important. This paper discusses the influence of street-canyon geometry on outdoor thermal comfort in Colombo, Sri Lanka. Five sites with different urban geometry, ground cover, and distance from the sea were studied during the warmest season. The environmental parameters affecting thermal comfort, viz. air temperature, humidity, wind speed, and solar radiation, were measured, and the thermal comfort was estimated by calculating the physiologically equivalent temperature (PET). The thermal comfort is far above the assumed comfort zone due to the combination of intense solar radiation, high temperatures, and low wind speeds, especially on clear days. The worst conditions were found in wide streets with low-rise buildings and no shade trees. The most comfortable conditions were found in narrow streets with tall buildings, especially if shade trees were present, as well as in areas near the coast where the sea breeze had a positive effect. In order to improve the outdoor comfort in Colombo, it is suggested to allow a more compact urban form with deeper street canyons and to provide additional shade through the use of trees, covered walkways, pedestrian arcades, etc. The opening up of the city's coastal strip would allow the sea breeze to penetrate further into the city.

The influence of urban design on outdoor thermal comfort in the hot, humid city of Colombo, Sri Lanka

In this paper we review instruments and methods used to assess outdoor thermal comfort and subjective thermal perception in 26 studies reported in the literature during the last decade, covering a wide range of climates and geographical contexts. We found a great variety of instruments and methods used to measure meteorological variables, especially with respect to the mean radiant temperature and wind speed. Moreover, many different subjective judgement scales were used to assess subjective thermal perception, thermal neutrality and thermal preference and a multitude of thermal indices were used to quantify the combined effect of meteorological variables on thermal perception. The use of a variety of methods makes it difficult to compare results of the different studies. There is thus a need for standardization and to give guidance regarding how to conduct field surveys in outdoor environments. Such standards and guidelines should give advice regarding the choice of measurement sites, type and positioning of instruments, appropriate methods to determine the mean radiant temperature, questionnaire design and suitable thermal comfort indices. These guidelines should also include advice on reporting.

Instruments and methods in outdoor thermal comfort studies – The need for standardization

Recent urban microclimate studies in Colombo, Sri Lanka, indicate that the maximum daily temperature within street canyons decreases with increasing height to width (H/W) ratio, but higher H/W ratio negatively affects street-level wind flow. There is also evidence pointing to the cooling effect of sea breeze. The nocturnal heat island is small in contrast to daytime urban-rural differences. In this paper, we use the software ENVI-met to simulate the effect of different urban design options on air and surface temperatures, as well as on outdoor thermal comfort. The latter is expressed as the physiologically equivalent temperature (PET), an index based on air and radiant temperatures as well as wind and humidity. It is found that high albedo at street level gives the lowest air temperature during daytime, although the reduction is only about 1 degrees C. The lowest daytime mean radiant temperatures result from high H/W ratios of streets. This has a positive effect on thermal comfort; the increase of H/W ratio from about 1 to 3 leads to a decrease in PET by about 10 degrees C. Differences in air and surface temperatures, as well as PET, are small during the night. The results show that strategies that lead to better air temperature mitigation may not necessarily lead to better thermal comfort. However, shade enhancement through increased H/W ratios is clearly capable of significant reductions in PET, and thus, improved outdoor thermal comfort. Consequently, a critical urban design task in the humid tropics will be to guide the rapid urban growth towards efficient 'shade growth'.

Urban shading—a design option for the tropics? A study in Colombo, Sri Lanka

Urbanisation leads to increased thermal stress in hot-humid climates due to increased surface and air temperatures and reduced wind speed We examined the influence of urban morphology and sea breeze on the microclimate of Colombo, Sri Lanka Air and surface temperatures, humidity and wind speed were measured at 1 rural and 5 urban sites during the warmest season The urban sites differed in their height to width (H/W) ratio, ground cover and distance to the sea Intra-urban air temperature differences were greatest during the daytime A maximum intra-urban difference of 7 K was recorded on clear days Maximum temperatures tended to decrease with increasing H/W ratio and proximity to the sea All urban sites experienced a nocturnal urban heat island (UHI) when the sky was clear or partly cloudy The temperature differences between sunlit and shaded urban surfaces reached 20 K, which shows the importance of shade in urban canyons (reducing long-wave radiation from surfaces) Within the urban areas, the vapour pressure was high (> 30 hPa) and showed little diurnal variation Wind speeds were low (< 2 m s(-1)) and tended to decrease with increasing H/W ratio Shading is proposed as the main strategy for lowering air and radiant temperatures; this can be achieved by deeper canyons, covered walkways and shade trees It is also suggested to open up wind corridors perpendicular to the sea to facilitate deeper sea breeze penetration (Less)

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Erik Johansson

Papers

Influence of urban geometry on outdoor thermal comfort in a hot dry climate: A study in Fez, Morocco

The influence of urban design on outdoor thermal comfort in the hot, humid city of Colombo, Sri Lanka

Instruments and methods in outdoor thermal comfort studies – The need for standardization

Urban shading—a design option for the tropics? A study in Colombo, Sri Lanka

Influence of urban morphology and sea breeze on hot humid microclimate: the case of Colombo, Sri Lanka