No abstract

No abstract

No abstract

We propose an approach to generate turbulent flows by using mesoscale meteorological simulations in order to conduct building-resolving large-eddy simulations (LESs) of boundary-layer flows over urban areas under realistic meteorological conditions. The urban surface geometry was explicitly represented in the LES model. This approach was applied for a strong wind event in Tokyo owing to the landfall of a major typhoon whose intensity and track were well reproduced in the meteorological simulation. The observed ranges of wind fluctuations and gust factors and significant decelerations of wind speeds within the urban canopy layer were successfully represented in the LES.

This paper describes aerodynamic roughness properties for turbulent flows over various building arrays that represent realistic urban surface geometries. First, building morphological characteristics such as roughness density l f and building height variability V h , defined respectively as the ratio of total frontal area of roughness elements to the total surface area and the ratio of standard deviation in building height to the average building height of the study site, were investigated. Next, large-eddy simulations (LESs) of turbulent flows over building arrays were performed with various surface geometries characterized by a wide range of values for both l f and V h , based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z 0 and drag coefficient Cd were evaluated for the central Tokyo area from the values of l f and V h using the LES results. The values of z 0 and Cd as a function of both l f and V h were comparable to those found in earlier studies. The values of z 0 and Cd evaluated by a conventional method using only l f were underestimated, particularly for densely built-up areas. This indicates that the present approach to estimating aerodynamic roughness parameters, taking account of both roughness density and building height variability, is more appropriate than conventional approaches when applied to actual urban areas. The roughness aerodynamic parameters as a function of l f and V h obtained from the LES results will be useful in incorporating urban effects into weather forecasting models.