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Sato, Takuto; Nakayama, Hiromasa; Satoh, Daiki
Journal of Nuclear Science and Technology, 17 Pages, 2025/09
Times Cited Count：0 Percentile：0.00We developed a framework for rapid monitoring of radioactive plumes in the vicinity of nuclear facilities based on a quick and practical high-resolution atmospheric dispersion simulation method that combines a large-eddy simulation (LES) model pre-simulation database (pre-sim DB) of wind conditions and onsite meteorological observation results, as proposed by the previous study. However, this framework was not quantitatively demonstrated using measurement data. In this study, we evaluated the performance of the wind condition reproduction and plume dispersion analysis methods. Air dose rates observed at monitoring posts around the stack were compared with the values reproduced by the method using the pre-sim DB, and the reproducibility of both air dose rate and flow field was discussed. The pre-sim DB-based method successfully captured the temporal variation of air dose rates at the monitoring posts, though it tended to overestimate the peak values. Particularly when the vertical wind shear was pronounced, the method using the pre-sim DB could cause significant errors. This is likely because the method relies on wind conditions from a single observation point, which inherently limits its ability to represent vertical wind shear within the pre-sim DB. Despite these limitations, particularly in reproducing complex wind fields, the method utilizing the pre-sim DB offers a valuable and practical tool for rapid dose rate simulation due to its lower computational cost compared to unsteady simulations using an LES model.
Sato, Takuto; Goger, B.*; Nakayama, Hiromasa
SOLA (Scientific Online Letters on the Atmosphere) (Internet), 21, p.17 - 23, 2025/01
Times Cited Count：0 Percentile：0.00(Meteorology & Atmospheric Sciences)In this study, we conducted large-eddy simulations of turbulent flows and plume dispersion over idealized two-dimensional double steep hills. In the simulations, we investigated the distribution patterns of the mean plume concentrations, considering various distances between the hills and emission sources. Our objective was to provide information on the area of influence of local hilly terrains on plume dispersion from the viewpoint of accuracy, i.e., determining if the conventional Gaussian plume model can accurately predict plume concentrations. The result showed that the clockwise circulation was dominant in the area between the windward and leeward hills (valley) when the valley width was less than 10 times the hill height (
). This circulation makes the flow close to the stack remain in the valley, resulting in the higher concentrations in the valley than in wider-valley (
10 valley) cases. The effect of the leeward hill on the flow field was negligible when the valley width was greater than 10. In the area beyond 20 from the crest of the windward hill, estimated plume spreads for all cases were similar, indicating that the area of influence of the hills was approximately 20.
Sato, Takuto; Hino, Hideitsu*; Kusaka, Hiroyuki*
Atmospheric Science Letters, 25(12), p.e1279_1 - e1279_10, 2024/12
Times Cited Count：0 Percentile：0.00(Geochemistry & Geophysics)This study applies dynamic mode decomposition (DMD) to three-dimensional simulation results of urban heat island circulation (UHIC, which is horizontal circulation) and thermals (vertical convections). The aim of this study is to revisit how these phenomena coexist based on the characteristics of temporal changes in the flow field. We used DMD to obtain the dominant spatial patterns and information on temporal changes. One of the modes of horizontal wind, which does not change temporally (no oscillation or amplification), exhibits a spatial UHIC pattern. The unique feature of this UHIC mode is that there are small-scale striated structures (150-200 m) and large-scale convergence. The other modes are time-varying (oscillating and decaying) and represent smaller spatial-scale phenomena (150-250 m), such as thermals. The frequency of each mode takes various values, some of which are lower than the lifetime of thermals in accordance with the Deardorff convective scale (
10 min). These low-frequency modes showed striated structures similar to that observed in the UHIC modes. These results suggest that UHIC and thermals deform each other through components that vary in long temporal scales.
Kusaka, Hiroyuki*; Ikeda, Ryosaku*; Sato, Takuto; Iizuka, Satoru*; Boku, Taisuke*
Journal of Advances in Modeling Earth Systems (Internet), 16(10), p.e2024MS004367_1 - e2024MS004367_38, 2024/10
Times Cited Count：4 Percentile：70.98(Meteorology & Atmospheric Sciences)To bridge the gaps between meteorological large-eddy simulation (LES) models and computational fluid dynamics (CFD) models for microscale urban climate simulations, the present study has developed a meteorological LES model for urban areas. This model simulates urban climates across both mesoscale (city scale) and microscale (city-block scale). The paper offers an overview of this LES model, which distinguishes itself from standard numerical weather prediction models by resolving buildings and trees at the microscale simulations. It also differs from standard CFD models by accounting for atmospheric stratification and physical processes. Noteworthy features of this model include: (a) the calculation of long- and short-wave radiations in three dimensions, incorporating multiple reflections within urban canopy layers using the radiosity method, and accounting for building and tree shadows in the simulations; (b) the provision of various heat stress indices (Universal Thermal Climate Index, Wet Bulb Globe Temperature, MRT, THI); (c) the assessment of the efficacy of heat stress mitigation measures such as dry-mist spraying, roadside trees, cool pavements, and green/cool roofs strategies; (d) the capability to run on supercomputers, with the code parallelized in a three-dimensional manner, and the model can also run on a graphics processing unit cluster. Following the introduction of this model, the study confirms its basic performance through various numerical experiments, including simulations of thermals in the convective boundary layer, coherent structure of turbulence over urban canopy, and thermal environment and heat stress indices in urban districts. The model developed in this study is intended to serve as a community tool for addressing both fundamental and applied studies in urban climatology.
Terada, Kentaro*; Ninomiya, Kazuhiko*; Sato, Akira*; Tomono, Dai*; Kawashima, Yoshitaka*; Inagaki, Makoto*; Nambu, Akihiro*; Kudo, Takuto*; Osawa, Takahito; Kubo, Kenya*
Journal of Analytical Science and Technology, 15, p.28_1 - 28_7, 2024/05
Times Cited Count：1 Percentile：17.14(Chemistry, Analytical)In Earth and planetary science, Pb isotopic composition is well known to play a key role in deciphering the origin and evolution of materials because they provide unique chronological and/or indigenous regional information as a radiogenic daughter nuclide from U and Th. To determine such an isotopic composition, a mass spectrometer has been widely used over several decades, which requires a destructive treatment such as laser ablation and thermal ionization, so on. Here we first report the non-destructive Pb isotopic measurement of the natural galena (PbS) using an energy-shift of muon-induced characteristic X-rays. The observed Pb isotopic composition of the natural galena is in good agreement with that obtained by conventional mass spectrometry.
Sato, Takuto; Nakayama, Hiromasa
SOLA (Scientific Online Letters on the Atmosphere) (Internet), 20, p.371 - 377, 2024/00
Times Cited Count：0 Percentile：17.46(Meteorology & Atmospheric Sciences)Numerical simulations of convective boundary layers (CBLs) based on the stability categories of the Pasquill-Gifford (PG) chart were conducted using a large-eddy simulation (LES) model. We studied the simulation settings for different combinations of wind speeds and sensible heat fluxes that could quantitatively generate turbulences satisfying the requirement of each stability category. Additionally, we determined appropriate CBL depths for desired turbulence generation and verified the effectiveness of the velocity scale (
) in turbulence generation simulations. This study identified wind speed and sensible heat flux combinations that could generate turbulences under the atmospheric stability categories B (unstable), C (weakly unstable), and D (neutral). However, the results did not follow the scaling law based on in category D when the CBL depth was 600 m. A shallower CBL (300 m depth) should be set in category D to generate turbulence under the scaling law based on . In category B, the transient layer disappeared due to active thermals in the CBL when the depth was 300 m. These results indicated that we should set appropriate CBL depths in addition to horizontal wind speeds and sensible heat fluxes to generate desired and scalable turbulences based on the PG chart.
Sato, Akira*; Arimizu, Takuto*; Yonemura, Hirotaka*; Sawada, Atsushi
Journal of MMIJ, 125(4,5), p.146 - 155, 2009/04
The advection and dispersion of contaminated materials in a rock mass are one of the important factors to evaluate the characteristics of the rock mass as a natural barrier function for a radioactive waste disposal project. In this study, X-ray CT method was applied to evaluate the advection and dispersion of solute in the crack which exists in rock mass. In order to visualize advection and dispersion phenomena, the tracer migration test have been conducted and tracer migration process was visualized by X-ray CT scanner. Here, an indicator, coefficient of tracer density increment, was newly introduced to analyze tracer migration. This is the indicator that represents the density of the tracer in the crack, and the evaluation of the density distribution of the tracer becomes possible. In this study, the influences of the crack aperture to the tracer migration process are discussed.
Sato, Takuto
no journal, ,
no abstracts in English
Sato, Takuto
no journal, ,
no abstracts in English
Kusaka, Hiroyuki*; Nagata, Aya*; Sato, Takuto
no journal, ,
It is well established that the frequency of cumulus cloud formation over the leeward areas of central Tokyo, as well as cumulonimbus development and precipitation, is higher than in surrounding areas. Urban-induced cloud and precipitation generation is generally attributed to two primary mechanisms: thermal and dynamic effects. Here, the dynamic effects involve airflow modification, such as the inhibition of thermals due to obstacles like buildings, which force air parcels to ascend or divert around them. However, the dynamic effect has not been fully investigated using building-resolving models; instead, it has primarily been studied with mesoscale models incorporating urban canopy parameterization schemes. The purpose of this study is to disentangle the thermal and dynamic effects of Tokyo on cloud formation using the building-resolving City-LES model (Kusaka et al., 2024). The study focuses on central Tokyo, with simulations conducted for a clear-sky summer day when a daytime sea breeze reaches the area. The key findings are as follows:- Thermals form and are transported by southerly winds, generating roll convection.- The thermals reach the lifting condensation level (LCL). -The atmospheric boundary layer is well mixed from the surface to the upper planetary boundary layer. Furthermore, sensitivity experiments were conducted to isolate the thermal and dynamic effects of urban areas on cloud formation. The results indicate that - The thermal effect of the city enhances thermal generation, thereby promoting cloud formation. - The local-scale dynamic effect suppresses thermal development and inhibits cloud formation, which may differ from the mesoscale dynamic effect. These findings provide new insights into the mechanisms of urban-induced cloud formation and highlight the importance of resolving both thermal and dynamic processes in high-resolution urban meteorological models.
Sato, Takuto; Nakayama, Hiromasa
no journal, ,
Large-eddy simulation (LES) models can evaluate complex flow fields and atmospheric dispersion process in urban areas with higher accuracy than conventional RANS based models. However, simulation in urban areas requires high spatio-temporal resolution, which causes the heavy computational load. In this study, we aim to build an atmospheric dispersion evaluation system which can utilize the LES model in low computational cost. The proposed system consists of pre-calculated LES output database and atmospheric dispersion model. To build the system, we discuss the method to use the LES database efficiently, including the reduced-order modelling based on the mode decomposition method. We also discuss the relationship between large-scale environment (e.g., wind speed, wind direction and atmospheric stability) and flow field in target area simulated by LES. The reduced-order model can simulate the flow field from certain number of dominant modes and large-scale environment, that makes the computational load much lower than full simulation. In addition, we investigate the accuracy of atmospheric dispersion models, Eular and Lagrangean models, in case of using the LES database as input data. Combining the reduced-order model and atmospheric dispersion model, we propose the atmospheric dispersion evaluation framework which aimed to use widely and easily even in small computational resources.
Nakayama, Hiromasa; Sato, Takuto
no journal, ,
We developed a framework for prediction system of local-scale atmospheric dispersion based on a coupling of large-eddy simulation (LES) database and on-site meteorological data. The LES-database was created by pre-calculating high-resolution turbulent flows in the target site. We provided the meteorological observed data with the LES-database in consideration of building conditions and calculated the three-dimensional distribution of the plume with a Lagrangian dispersion model. First, we conducted LESs on turbulent flows in the vicinity of Rokkasho Reprocessing Plant during the test operations of reprocessing, the nuclear fuel reprocessing plant in Rokkasho, Japan. Then, we compared with the air dose rates obtained from the monitoring posts (MP). The time variation patterns and the magnitude of the fluctuations of the calculated air dose rates were generally similar to the MP data. It is concluded that a quick atmospheric dispersion simulation based on a coupling LES-database with on-site meteorological data is effective.
