Hiromasa Nakayama

Preliminary Estimation of Release Amounts of 131I and 137Cs Accidentally Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere

(2011). Preliminary Estimation of Release Amounts of 131I and 137Cs Accidentally Discharged from the Fukushima Daiichi Nuclear Power Plant into the Atmosphere. Journal of Nuclear Science and Technology: Vol. 48, No. 7, pp. 1129-1134.

LES Analysis of the Aerodynamic Surface Properties for Turbulent Flows over Building Arrays with Various Geometries

AbstractThis 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 λf and building height variability Vh, 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 λf and Vh, based on this building morphological analysis. Third, aerodynamic roughness parameters such as roughness length z0 and drag coefficient were evaluated for the central Tokyo area from the values of z0 and Vh using the LES results. The values of z0 and as a function of both λf and Vh were comparable to those found in earlier studies. Th...

Development of Local-Scale High-Resolution Atmospheric Dispersion Model Using Large-Eddy Simulation Part 1: Turbulent Flow and Plume Dispersion over a Flat Terrain

We have developed a LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergencies against accidental or intentional release of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume over a flat terrain was successfully simulated. In Part 2, a new scheme to generate a spatially developing turbulent boundary layer flow was proposed. Then, the large-eddy simulation (LES) model for turbulent flow and plume dispersion around an isolated building was validated. In this study, we extend the LES model to turbulent flows and plume dispersion in various building arrays that represent typical urban surface geometries. Concerning the characteristics of flow and dispersion in building arrays, the flow patterns associated with obstacle densities and the distribution patterns of mean and root-mean-square (r.m.s.) concentrations agree well with those of the wind tunnel experiments. It is shown that the LES model successfully simulates the unsteady behaviors of turbulent flows and plume dispersion in urban-type surface geometries.

Development of local-scale high-resolution atmospheric dispersion model using large-eddy simulation. Part 4: turbulent flows and plume dispersion in an actual urban area

We have developed a local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergency situations resulting from accidental or deliberate releases of radioactive materials (e.g., a terrorist attack in an urban area). In Part 1, the unsteady behavior of a plume dispersing over a flat terrain was successfully simulated. In Parts 2 and 3, LESs of turbulent flows and plume dispersion around an isolated building and in building arrays with different obstacle densities were performed, which showed the basic performance comparable to wind tunnel experimental technique. In this study, we apply the LES model to turbulent flows and plume dispersion in an actual urban area. Although some of the turbulence and dispersion characteristics are quantitatively different from the wind tunnel experimental data, the distribution patterns are generally similar to those of the experiments. It is concluded that our LES model simulates reasonably the unsteady behavior of turbulent flows and plume dispersion even for complex heterogeneous urban areas.

Development of Local-Scale High-Resolution Atmospheric Dispersion Model Using Large-Eddy Simulation Part 2: Turbulent Flow and Plume Dispersion around a Cubical Building

We have developed a local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation (LES) for the safety assessment of nuclear facilities and emergency responses against accidental or intentional release of radioactive materials such as a terrorist attack within urban areas. In Part 1, we successfully simulated the unsteady behavior of a plume over a flat terrain. In this study, we propose a new scheme to generate a spatially developing turbulent boundary layer flow in the driver region based on the scheme in Part 1, and we perform LES of the turbulent flow and plume dispersion around a cubical building. Concerning the characteristics of flow and dispersion around a building, the main characteristics of turbulence structures and the distribution patterns of mean and r.m.s. concentrations agree well with those of wind tunnel experiments. Our LES model successfully simulates the unsteady behaviors of turbulent flow and plume dispersion in the near-wake region of a cubic building.

Development of LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation. Part 5: detailed simulation of turbulent flows and plume dispersion in an actual urban area under real meteorological conditions

We have developed a LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES) to assess the safety at nuclear facilities and to respond to emergency situations resulting from accidental or deliberate releases of radioactive materials (e.g., a terrorist attack in an urban area). In parts 1–4, LESs of turbulent flows and plume dispersion over a flat terrain, around an isolated building, within building arrays with different obstacle densities, and within an actual urban area were performed, which showed the basic performance comparable to wind tunnel experimental technique. In this study, we extend the LOHDIM-LES to turbulent flows and plume dispersion in an actual urban area under real meteorological conditions by coupling with a meso-scale meteorological simulation model. The LES results of wind speed, wind direction, and concentration values are generally reproduced well. It is concluded that our coupling approach between LES and meso-scale meteorological models is effective in detailed simulations of turbulent flows and plume dispersion in urban areas under real meteorological conditions.