Building-Resolving Large-Eddy Simulations of Turbulent Flows and Gusty Winds in Densely Built Urban Districts Under Typhoon Conditions

A gusty wind by typhoons is one of the major natural hazards and has been the most threatening windstorm in urban districts. In recent year some major cities in Japan have experienced extreme winds during typhoon landfalls. For example, Typhoon Jebi (2018) caused extreme wind gusts in Osaka and Kyoto, while Typhoon Faxai (2019) and Hagibis (2019) produced high winds in Tokyo and neighboring cities. Urban roughness obstacles exert significant influences on the magnitude of wind gustiness. With the growing urbanization globally, the quantification of turbulent winds in densely built, urban districts is important to the assessment and prediction of risks of wind damages and the understanding of the underlying physical mechanisms. Influences of densely built urban environments on the occurrence of wind gusts in urban districts during the typhoon landfalls are studied by merging mesoscale meteorological and building-resolving large-eddy simulations (LES), which allows an explicit representation of the complicated building structures while retaining the strong mesoscale perturbations from the typhoon. The actual building data of Osaka, Kyoto, and Tokyo are used in the building-resolving LES computational domains. With the successful reproduction of the track and intensity of the typhoon in meteorological simulations, the simulated winds at the simulated boundary-layer top are used to quantify the wind gusts in the urban district. The maximum wind gust in the analysis area of Osaka during the landfall of Typhoon Jebi is found to exceed 60 m/s, which is comparable to the wind speed at the height of about 300 m. Such wind gusts are generated by the instantaneous downward momentum transfer in areas, where buildings of great height variability are clustered. The instantaneous wind gusts are found to be the strongest for moderate building packing density. The results suggest that the risks of wind damages are mostly likely to be maximized in urban districts of high building-height variability and moderate packing density.