Vu Thanh Ca

Heat storage of pavement and its effect on the lower atmosphere

Abstract Heat flux at the air/ground interface was observed and analyzed for various pavement materials on summer days. The surface temperature, heat storage and its subsequent emission to the atmosphere were significantly greater for asphalt than for concrete or bare soil. At the maximum, asphalt pavement emitted an additional 150 W m−2 in infrared radiation and 200 W m−2 in sensible transport compared to a bare soil surface. Analyses based on a parallel layers model of the atmosphere indicated that most of the infrared radiation from the ground was absorbed within 200 m of the lower atmosphere, affecting air temperature near the ground. With large difference between air and ground surface temperature at noon, the rate of infrared absorption by the lower atmosphere over asphalt pavement was greater by 60 W m−2 than that over the soil surfaces: or concrete pavement, a figure comparable to the absorption by turbulent transport.

Reductions in air conditioning energy caused by a nearby park

Abstract Field observations were carried out to determine the influence of a park on the urban summer climate in the nearby areas. The possibilities of reduction in air conditioning energy were investigated. Air temperature, relative humidity and other meteorological factors were measured at many locations inside a park and in the surrounding areas in the Tama New Town, a city in the west of the Tokyo Metropolitan Area, Japan. The observations indicated that vegetation could significantly alter the climate in the town. At noon, the highest temperature of the ground surface of the grass field in the park was 40.3 °C, which was 19 °C lower than that of the asphalt surface or 15 °C lower than that of the concrete surface in the parking or commercial areas. At the same time, air temperature measured at 1.2 m above the ground at the grass field inside the park was more than 2 °C lower than that measured at the same height in the surrounding commercial and parking areas. Soon after sunset, the temperature of the ground surface at the grass field in the park became lower than that of the air, and the park became a cool island whereas paved asphalt or concrete surfaces in the town remained hotter than the overlying air even late at night. With a size of about 0.6 km 2 , at noon, the park can reduce by up to 1.5 °C the air temperature in a busy commercial area 1 km downwind. This can lead to a significant decrease of in air conditioning energy in the commercial area.

Building canopy model for the analysis of urban climate

Abstract For the purpose of practical application of CFD for urban climate planning, a building canopy model coupled with CFD was developed, and the effects of building planting were discussed for the decrease of urban heat island phenomena and energy for cooling. In this paper, radiate heat exchange in the building canopy, the drag force model of building walls, the effects of heat released from air conditioning systems are presented. Numerical results show that building planting has possibilities for the reduction of air temperature by 0.4–1.3°C and energy for cooling by 3–25%.

THE SUBSURFACE TRANSPORT OF HEAT AND MOISTURE AND ITS EFFECT ON THE ENVIRONMENT: A NUMERICAL MODEL

A numerical model was developed to study the transport of heat and vapor under the surface of bare soil and soil covered by some materials such as asphalt and concrete under no rainfall conditions. The computational results provide a good match with the experimental data. The results show that the transport of water vapor inside the soil has an important effect on the subsurface distribution of temperature, especially for bare soil. Because of evaporation, the temperature of bare soil is much lower than that under covered surfaces throughout the day and the temperature of the surface covered by asphalt is extremely high-higher than the atmospheric temperature even at night. An increase of thickness of the covering material further increases the temperature and heat stored under surfaces. The stored heat is released to the atmosphere at night, contributing to environmental effects such as the urban heat island.

A k-*epsiv Turbulence Closure Model For The Atmospheric Boundary Layer Including Urban Canopy

A numerical model for the computation of the wind field,air temperature and humidity in the atmospheric boundary layer (ABL) including the urbancanopy was developed for urban climate simulation. The governing equations of the modelare derived by applying ensemble and spatial averages to the Navier–Stokes equation, continuityequation and equations for heat and water vapour transfer in the air. With the spatial averagingprocedure, effects of buildings and other urban structures in the urban canopy can be accounted for byintroducing an effective volume function, defined as the ratio between the volume of air in acomputational mesh over the total volume of the mesh. The improved k - ε model accounts for the anisotropyof the turbulence field under density stratification. In the improved k - ε model, the transportof momentum and heat in the vertical direction under density stratification is evaluated based onthe assumption of a near-equilibrium shear flow where transport effects on the stresses andheat fluxes are negligible. The heating processes at surfaces of buildings and ground are alsomodelled. The comparison of the computational results obtained with the present modeland existing observational data and numerical models shows that the present model is capableof predicting the structure of turbulence in the urban canopy layer under density stratification.Numerical experiments with the new model show that the flow behaviour of the air in the urbancanopy layer is strongly affected by the existence of buildings and density stratification.

Development of a numerical model for the evaluation of the urban thermal environment

Abstract A numerical model was developed for the computation of the wind field, air temperature and humidity in the urban canopy layer and in the atmospheric boundary layer above urban areas. The model is of k – e type. The ensemble-spatial averaged three-dimensional Reynolds equations, equation of continuity, turbulent kinetic energy equation ( k -equation), and equation for dissipation rate of turbulent energy ( e -equation) are solved together with equations of heat and moisture transfer in the air. Inside the urban canopy layer, volumes of buildings and other urban structures are accounted for by a spatial averaging procedure. With given average building height and building width for each grid mesh, effects of buildings on the momentum transfer are modelled by introducing a form drag force. Temperatures of the ground surface, building walls or roof are computed by the solution of the heat conduction equation in the ground or walls, roof. Evaporation at the ground surface is evaluated using a Bowen ratio. The exhausted heat by building air conditioning is evaluated by employing a building air conditioning model. This heat together with traffic-induced artificial heat are accounted for in the model as heat sources. A numerical model for the momentum, heat and moisture transfer in the plant canopy is also coupled to the model to investigate the effects of vegetation on the urban climate. Verification of the model against observational data in the Tokyo Metropolitan area, Japan, reveals that the model is capable of simulating the momentum, heat and mass transfer in the urban boundary layer. Especially, the model can compute air temperature, humidity and wind velocity at the street level, which cannot be computed by a general above city atmospheric circulation model.

Characteristics of wind field in a street canyon

Characteristics of the wind field in a North-South oriented street canyon were studied by a numerical model to couple the heat and mass transfer processes. The heat fluxes were traced in and out of the street canyon, and the feedback of the heating processes inside the street canyon to the atmosphere was simulated by simultaneous solutions of the two-dimensional Navier-Stokes equations and heat transfer equation inside the street canyon, using a large eddy simulation (LES) model, coupling with the solutions of the heat conduction equations for roof, walls and road. It was found that with the existence of a wind blowing over the roof of the buildings on both sides of the street, large eddies were formed in the street canyon and transported the sensible heat out of the street canyon. The number of large eddies formed inside the street canyon depends on the aspect ratio, that is the ratio between the height of the buildings and the width of the street canyon. With an aspect ratio smaller than unity, only one large eddy was formed, while for aspect ratios larger than 2, two or more large eddies were formed. The time from the beginning of the computation until the statistically steady solutions of the flow and air temperature fields were achieved, was different for different street canyons, increasing with the street canyon heights, but was usually less than 100 s. With small aspect ratio and strong outside wind, the thermal convection due to heating of the road and wall surfaces was much less than the mechanically generated turbulence. However, with a weak outside wind or large aspect ratio, the thermal convection patterns could be clearly seen.

A Simulation Study on the Heat and Vapor Transfer at the Ground Surface

A numerical model is developed for the simulation of heat and moisture transfer above and below the ground surface. In the soil, the equations coupling the processes of heat and moisture transfer are solved to get the temperature and the moisture content at the ground surface, which are necessary for the computation of the heat and moisture fluxes. The sensible heat flux and evaporation rate at the ground surface are evaluated based on the bulk type formulas and by an improved k-c: model for the transport of momentum, heat and water vapor in the atmospheric surface layer. The accuracy of the numerical model was verified using field experimental data on the temperature and water content at the soil surface and different depths under the ground surface. Based on the numerical model, the accuracy of the flux-gradient method for the prediction of evaporation from the ground surface is investigated.

A Case Study on the Effects of Vegetation on the Climate in the Urban Area

The purpose of this study is to give a detailed investigation into the effects of vegetated areas on the thermal climate of a new city in the Tokyo Metropolitan area — the Tama New Town — through field observations of the ground surface temperature, atmospheric temperature and other atmospheric conditions during hot summer days.

Spatial Structure of Zooplankton inside Lakes

This paper describes spatial distribution of zooplankton inside lakes based on observational results. Lake A partially has vegetated bank, while Pond B is characterized by two different parts, an artificial shore and a naturally vagetated basin. Zooplankton was sampled at several points including near the vegetated bank and non-vegetated bank, together with phytoplankton concentration and other water quality parameters. It was found that the zooplankton concentration has a close relationship with vegetation and phytoplankton concetration.