For the risk estimation due to strong wind on buildings and structures in urban area, wind characteristics in the boundary layer developing over densely arranged buildings should be investigated. Thus far the recommendations for wind loads on buildings have been proposed and published in various countries in the world, where roughness aspect of ground surface are usually classified into several categories to prescribe the wind flow profile for providing the design wind velocity. Although this classification should be performed by the field observation data, almost cases were intentionally determined only by surveying of building arrangement on the ground. Especially the validation of vertical profiles of mean wind velocity and turbulence intensity related with categorized ground surface cannot be confirmed due to the lack of fundamental data of wind flows generally obtained by full scale measurement. Here we adopt the numerical technique for the prediction of wind profile over complex roughened ground surface in actual urban area. Accordingly the present study aims to establish the numerical technique which can predict strong wind and provide the data for wind-resistant design of buildings in any urban area.

In order to get the data for building shapes in a local area, the electronic mapping information is utilized. The present study uses height data of surface roughness and expresses directly surface shapes of urban area for the numerical simulation. The previous numerical simulation of the boundary layers usually has employed the wall layer model for the treatment of the ground surface condition. It means that the integral quantities are utilized for representing total boundary effects, but not local effects. Recent data for building height determining the surface roughness have a resolution with about 2m, so it might be enough to simulate roughness shapes placed on the ground surface in urban areas. Further, in case of the numerical simulations for winds in urban canopies, which need to deal with details of complex flows throughout spaces between buildings, so the RANS (Reynolds averaged Navier-Stokes) technique might have advantage to compute the flow field highly resolved by the grid from standpoints for computer capability. However the complete RANS modeling of turbulence has not been developed yet for such a complex flow. Also, in the field of strong wind hazard, the numerical method, which can predict time sequential turbulence, is appropriate, because the gust and peak of wind velocity determine the scale of hazard. Hence this study employs the LES (Large eddy simulation) technique is applied to the wind flow over actual roughened ground surface in Tokyo. According to the previous study, one of the authors carried out LES for flow over uniformly arrayed roughness blocks and showed the accuracy of numerical. We compare the LES results between the actual urban model and conventional simple roughness block model. Especially we bring into focus the turbulence structures based on the spectral density functions and spatial coherence in the near region of the roughened surface. Also, we discuss how to determine the wind profiles in the actual urban area.