5.1
Hybrid method of LES and meso-scale meteorological model for heat island analysis in coastal urban area
PAPER WITHDRAWN
Tetsuro Tamura, Tokyo Institute of Technology, Yokohama, Japan; and Y. Okuda and J. Nagayama
For the mitigation of heat island effects on coastal cities, it is sometimes expected that the sea breeze comes into the inland area of a city, where its cold air mingles with the hotter air over and inside the urban canopies. In the downtown of Tokyo, there exists a coastline at the southeast boundary. But recently several very tall buildings have been constructed at Shiodome near this coast line between the center of Tokyo and the Tokyo bay. We are concerned about that these tall buildings block a passage of sea breeze into the downtown of Tokyo. So, some of the pre-sent authors carried out the LES computation for wind flows over actual roughened ground sur-face at Shiodome in neutral condition[1]. The temperature has been treated as a passive scalar and its convective and diffusive behavior has been examined. Here we introduce the buoyancy effect by heat transfer to the previous LES analysis. Figure 1 illustrates schematic of the com-putational model for hybrid method of LES and meso-scale meteorological model concerning ur-ban heat island. For the horizontal inflow boundary condition of the computational domain 3 at the Shiodome area of 1 km by 1.75 km, turbulent flow data such as wind velocity or temperature are imposed by taking into consideration existence of the sea upstream. The driver region of domain 1 generates turbulent boundary layer by using the re-scaling technique[2] and domain 2 thermally stabilizes the boundary layer based on the sea breeze characteristics[3]. For setup of the boundary condition at ground surface in cities, radiation temperature measured by the aircraft is utilized. Wind and temperature fields with urban scale are simulated by the meso-scale mete-orological model(MM5), while turbulent flow fields over and inside the urban canopy are com-puted by LES that incorporates explicitly the effects of actual shape for buildings and struc-tures[4]. Generally, by LES we cannot obtain the absolute values for wind flow, only relative value to the reference wind can be estimated. So, combining the results by MM5, we have es-timated the wind velocity value and also give a temperature profile for inlet of stable turbulent boundary layer. According to the previous study[1], the field measurement around the Shio-dome area showed that the extreme reduction of wind velocity behind a group of tall buildings. At the present time, LES results for turbulent flows in the roughness layer over a city also show that the flow velocity extremely decreased behind the building blocks(Figure 2). Accordingly we can easily imagine the occurrence of the environmental degradation for the thermal condition due to densely arrayed tall buildings. By the temperature analysis from the heat sources on the surface of the building blocks, it can be recognized that the heat has been convected and diffused largely far away at a higher position. While below the buildings, the heat is not transported so much by the effect of the blocking sea breeze(Figure 3). Also, we can recognize that heat has stagnated in the building group because the cavity region is formed among dense buildings. According to the comparison with field measurement data at Shiodome area, LES results show good agreement for such as a flow pattern of sea breeze going around the arrayed tall buildings and the resulting air penetration with low temperature at both sides of tall buildings. This study suggests that the hybrid method is a powerful tool for predicting of a wind in urban canopy and temperature environment in the urban area.
References 1 T. Tamura, J. Nagayama, K. Ohta, T. Takemi and Y. Okuda, LES estimation of environ-mental degradation at the urban heat island due to densely-arrayed tall buildings, 17th Sympo-sium on Boundary Layers and Turbulence, AMS, 2006. 2 K. Nozawa and T. Tamura, Large eddy simulation of wind flows over large roughness ele-ments, In J. Naprstek and C. Fischer (Ed.), Proc. of EACWE4, Prague, (2005). 3 T. Tamura and K. Mori, LES of spatially-developing stably stratified turbulent boundary lay-ers, DLES6, 2005. 4 T. Takemi, T. Tamura, Y. Takei and Y. Okuda, Microscale analysis of severe winds within the urban canopy during a period of explosive cyclogenesis by coupling large-eddy simula-tion and mesoscale meteorological models, Proc. CWE2006, 2006.
Figure 1 Schematic of the computational model for hybrid method of LES and meso-scale mete-orological model concerning urban heat island. Figure 2 Instantaneous velocity field in the roughness layer over actual city. Figure 3 Temperature field in the roughness layers over actual city.
Session 5, Advanced Multiscale Urban Modeling II
Tuesday, 11 September 2007, 1:30 PM-3:15 PM, Toucan
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