Research on urban environment modification and storm disaster mitigation
- Capture and touch the nature of wind flows in atmospheric boundary layer by CFD simulation as well as full-scale observation
When the surrounding air moves, we are feeling something of a breeze with warmth or coldness, also catching sound or odor. Transport motion of air plays an important role for the environmental formulation in our life. In the case of vigorous motion, it causes high wind disaster, which sometimes threatens the entire human society. Our laboratory focuses on elucidating the transport phenomena of atmosphere and seeking various problems such as effects of thermal instability on urban environment, atmospheric dispersion of hazardous gases, severe storm impact on a city and the aero- and aero-elastic behavior of buildings and structures. Our final goal is to solve the urban environmental problems related with the atmospheric flows or to establish the wind disaster mitigation.
(1) Full-scale measurement, experiment and CFD of urban and rural turbulent boundary layer
Buildings and vegetation on urban ground surface bring a special aspect on turbulent boundary layer. By usage of the supercomputers Earth Simulator 2(ES2), the high resolved large scale computation is realized for the urban or rural model correctly reproducing details of ground-surface aspects. We provide the data base of various types of urban boundary layer which can formulate the universal fundamental characteristics of turbulent flows. Also based on the complementary use of both the observational data by helicopter or lidar (remote sensor) at the upper atmospheric boundary layer (cooperative research with JAXA) and the wind tunnel experimental data of urban wind (cooperative research with MRI), we elucidate the coherent structures of heat and flow which determine the urban environment. On the other hand, we provide new valid information for safety against the gust wind, which irregularly and randomly occurs among dense buildings. Also, from a field measurement and LES of wind over a complicated terrain covered with vegetation, the wind characteristics near the surface are predicted and adaptability of the wind energy technology is estimated in the wind farm.
Figure 1: Remote observation for turbulent structures in the urban canopy
Figure 2: Wind flow analysis over complicated terrain
(2) Construction of wind-flow models representing the actual surface shape of a real city
and the problem-mitigation technology for the environment and disaster due to wind flows
Under the condition of actual city, CFD simulation with high resolution of unsteady wind flow system has been performed above and within the densely arrayed buildings and vegetation on the urban ground surface. Using the 3D spatial GIS data with 2m resolution, we have constructed the numerical model where each building shape or scattered vegetation covering the urban surface is directly represented. Accuracy for the results by turbulent simulation with high resolution has been guaranteed by comparing them with the observational data of wind at high altitude obtained by the Doppler Lidar. Risk assessment is performed based on maximum of the estimated instantaneous wind velocity. In view of urban environment, current states of degradation of air quality in a city or enhancement of urban heat island stemming from the stagnation of atmospheric flows such as a compacted wind flow among densely arrayed buildings have been captured
Figure3: Wind flow characteristics and urban heat island among the densely arrayed tall buildings
(3) Atmospheric dispersion of the hazardous gases predicted by LES
Thus far mean concentration or exposure estimation has been emphasized on analysis of the atmospheric dispersion and their environment load has been estimated significantly. However, when the dispersed material has toxic consequences, the behavior of concentration fluctuation or statistical evaluation of the peak concentration determines the life and death. Then the unsteady computation such as LES is strongly required to be upgraded as an analytical method. If convective and stagnant behavior has been elucidated in advance within the near-ground region of the complicated terrain (valley or hill) and the urban canopies, the hazardous gas can be intentionally inducted to the proper location in order to obtain the appropriate evacuation route. We aim to make the strong living environment against the toxic and hazardous materials and to establish the urban system which mitigates the damage due to the undesired material by appropriately estimating the atmospheric dispersion.