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Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa
Dai-36-Kai Suchi Ryutai Rikigaku Shimpojiumu Koen Rombunshu (Internet), 3 Pages, 2022/12
We have developed a wind simulation code named CityLBM to realize wind digital twins. Mesoscale wind conditions are given as boundary conditions in CityLBM by using a nudging data assimilation method. It is found that conventional approaches with constant nudging coefficients fail to reproduce turbulent intensity in long time simulations, where atmospheric stability conditions change significantly. We propose a dynamic parameter optimization method for the nudging coefficient based on an ensemble Kalman filter. CityLBM was validated against plume dispersion experiments in the complex urban environment of Oklahoma City. The nudging coefficient was updated to reduce the error of the turbulent intensity between the simulation and the observation. The mean error of velocity variance is reduced by 10% compared to the conventional nudging method with a constant nudging coefficient.
Nakayama, Hiromasa; Onodera, Naoyuki; Satoh, Daiki; Nagai, Haruyasu; Hasegawa, Yuta; Idomura, Yasuhiro
Journal of Nuclear Science and Technology, 59(10), p.1314 - 1329, 2022/10
Times Cited Count:2 Percentile:71.47(Nuclear Science & Technology)We developed a local-scale high-resolution atmospheric dispersion and dose assessment system (LHADDAS) for safety and consequence assessment of nuclear facilities and emergency response to nuclear accidents or deliberate releases of radioactive materials in built-up urban areas. This system is composed of pre-processing of input files, main calculation by local-scale high-resolution atmospheric dispersion model using large-eddy simulation (LOHDIM-LES) and real-time urban dispersion simulation model based on a lattice Boltzmann method (CityLBM), and post-processing of dose-calculation by simulation code powered by lattice dose-response functions (SIBYL). LHADDAS has a broad utility and offers superior performance in (1) simulating turbulent flows, plume dispersion, and dry deposition under realistic meteorological conditions, (2) performing real-time tracer dispersion simulations using a locally mesh-refined lattice Boltzmann method, and (3) estimating air dose rates of radionuclides from air concentrations and surface deposition in consideration of the influence of individual buildings and structures. This system is promising for safety assessment of nuclear facilities as an alternative to wind tunnel experiments, detailed pre/post-analyses of a local-scale radioactive plume dispersion in case of nuclear accidents, and quick response to emergency situations resulting from deliberate release of radioactive materials by a terrorist attack in an urban central district area.
Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa
Dai-35-Kai Suchi Ryutai Rikigaku Shimpojiumu Koen Rombunshu (Internet), 3 Pages, 2021/12
A detailed wind simulation is very important for designing smart cities. Since a lot of tall buildings and complex structures make the air flow turbulent in urban cities, large-scale CFD simulations are needed. We develop a GPU-based CFD code based on a Lattice Boltzmann Method (LBM) with a block-based Adaptive Mesh Refinement (AMR) method. In order to reproduce real wind conditions, the wind condition and ground temperature of the mesoscale weather forecasting model are given as boundary conditions. In this research, a surface heat flux model based on the Monin-Obukhov similarity theory was introduced to improve the calculation accuracy. We conducted a detailed wind simulation in Oklahoma City. By executing this computation, wind conditions in the urban area were reproduced with good accuracy.
Nakayama, Hiromasa; Satoh, Daiki; Nagai, Haruyasu; Terada, Hiroaki
Journal of Nuclear Science and Technology, 58(9), p.949 - 969, 2021/09
Times Cited Count:5 Percentile:78.24(Nuclear Science & Technology)We introduced a detailed dose calculation method considering building shielding effects into LOcal-scale High-resolution atmospheric DIspersion Model using LES (LOHDIM-LES). To estimate quickly and accurately dose distributions considering shielding effects of buildings, we employed the calculation method using dose-response matrices which were evaluated by photon transport simulations with Particle and Heavy-Ion Transport code System (PHITS). Compared to the air dose rate data obtained from monitoring posts in an actual nuclear facility, it was shown that the calculated dose rate is reasonably simulated well. It is concluded that LOHDIM-LES equipped with the calculation method using dose-response matrices can reasonably estimate the air dose rates considering shielding effects of individual buildings and structures.
Nakayama, Hiromasa; Takemi, Tetsuya*; Yoshida, Toshiya
Atmosphere (Internet), 12(7), p.889_1 - 889_15, 2021/07
Times Cited Count:2 Percentile:32.98(Environmental Sciences)Contaminant gas dispersion within urban area resulting from accidental or intentional release is of great concern to public health and social security. When estimating plume dispersion in a built-up urban area under real meteorological conditions by computational fluid dynamics (CFD), a crucial issue is how to prescribe the model input conditions. There are typically two approaches: using the outputs of a meso-scale meteorological simulation (MMS) model and meteorological observations (OBS). However, the influence of the different approaches on the simulation results have not been fully demonstrated. In this study, we conducted large-eddy simulations (LESs) of plume dispersion in the urban central district under real meteorological conditions by coupling with a MMS model and OBS obtained at a single stationary point, and evaluated the two different coupling simulations in comparison with the field experimental data. The LES-MMS coupling showed better performance than the LES-OBS one. However, the latter one also showed a reasonable performance comparable to the acceptance criteria on the model prediction within a factor of two of the experimental data. These facts indicate that the approach of using observations at a single stationary point still has enough potential to drive CFD models for plume dispersion under real meteorological conditions.
Nakayama, Hiromasa; Yoshida, Toshiya; Terada, Hiroaki; Kadowaki, Masanao
Atmosphere (Internet), 12(7), p.899_1 - 899_16, 2021/07
Times Cited Count:0 Percentile:0.01(Environmental Sciences)In this study, first, we conducted meteorological observations by a Doppler LiDAR and simple plume release experiments by a mist spraying system at the site of Japan Atomic Energy Agency. Then, 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 observation. The LES-database was also created by pre-calculating high-resolution turbulent flows in the target site at mean wind directions of class interval. We provided the meteorological observed data with the LES database in consideration of building conditions and calculated three-dimensional distribution of the plume by a Lagrangian dispersion model. Compared to the instantaneous shot of the plume taken by a digital camera, it was shown that the mist plume transport direction was accurately simulated. It was concluded that our proposed framework for prediction system based on a coupling of LES-database and on-site meteorological observation is effective.
Onodera, Naoyuki; Idomura, Yasuhiro; Hasegawa, Yuta; Nakayama, Hiromasa; Shimokawabe, Takashi*; Aoki, Takayuki*
Boundary-Layer Meteorology, 179(2), p.187 - 208, 2021/05
Times Cited Count:8 Percentile:83.12(Meteorology & Atmospheric Sciences)A plume dispersion simulation code named CityLBM enables a real time simulation for several km by applying adaptive mesh refinement (AMR) method on GPU supercomputers. We assess plume dispersion problems in the complex urban environment of Oklahoma City (JU2003). Realistic mesoscale wind boundary conditions of JU2003 produced by a Weather Research and Forecasting Model (WRF), building structures, and a plant canopy model are introduced to CityLBM. Ensemble calculations are performed to reduce turbulence uncertainties. The statistics of the plume dispersion field, mean and max concentrations show that ensemble calculations improve the accuracy of the estimation, and the ensemble-averaged concentration values in the simulations over 4 km areas with 2-m resolution satisfied factor 2 agreements for 70% of 24 target measurement points and periods in JU2003.
Satoh, Daiki; Nakayama, Hiromasa; Furuta, Takuya; Yoshihiro, Tamotsu*; Sakamoto, Kensaku
PLOS ONE (Internet), 16(1), p.e0245932_1 - e0245932_26, 2021/01
Times Cited Count:2 Percentile:53.86(Multidisciplinary Sciences)In this study, we developed a simulation code named SIBYL, which estimates external gamma-ray doses at ground level from radionuclides distributed nonuniformly in atmosphere and on ground. SIBYL can combine with the local-scale atmospheric dispersion model LOHDIM-LES, and calculate the dose distributions according to the map of the activity concentrations simulated by LOHDIM-LES. To apply the SIBYL code to emergency responses of nuclear accidents, the time-consuming three-dimensional radiation transport simulations were performed in advance using the general-purpose Monte Carlo code PHITS, and then the results were compiled to the database for the SIBYL's dose calculations. Moreover, SIBYL can consider the dose attenuation by obstacles and the changes of terrain elevations. To examine the accuracy of SIBYL, typical five cases including Kr emission from a ventilation shaft and
Cs dispersion inside urban area were investigated. The results of SIBYL agreed within 10% with those of PHITS at the most of target locations. Furthermore, the calculation speed was approximately 100 times faster than that of PHITS.
Nakayama, Hiromasa; Takemi, Tetsuya*
Journal of Advances in Modeling Earth Systems (Internet), 12(8), p.e2019MS001872_1 - e2019MS001872_18, 2020/08
Times Cited Count:1 Percentile:7.24(Meteorology & Atmospheric Sciences)We proposed a data assimilation method using a vibration equation which can incorporate turbulence winds toward target mean winds while maintaining small-scale turbulent fluctuations. First, we conducted test simulations in which nudging is applied in a basic turbulent boundary layer (TBL) flow toward a target one. It was shown that the basic TBL flow can be reasonably nudged toward the target one while maintaining the turbulent fluctuations well, when prescribing the natural frequency in the vibration equation smaller than the spectral peak frequency in the TBL flow. Then, we applied the proposed nudging method by incorporating data obtained from meteorological observations located in the southern part of the actual city of Kyoto. The mean wind velocity profiles are reasonably nudged toward the target observed profile and the turbulence statistics are also favourably maintained. It is concluded that the LES data assimilation method successfully nudges toward the turbulence winds well.
Yoshida, Toshiya; Nakayama, Hiromasa
Nihon Keisan Kogakkai Rombunshu (Internet), 2020, p.20200013_1 - 20200013_9, 2020/07
To quickly and accurately predict the dispersion of hazardous materials released over urban areas, we propose a combined method in which dispersion fields are simulated using a Reynolds-averaged Navier-Stokes model with pre-calculated flow fields from a large-eddy simulation (LES) model. First, the combined model is conducted for dispersion in a simple street canyon. The results of the combined model are compared with those of a wind-tunnel experiment to adjust empirical parameters in the turbulent scalar flux. The horizontal dispersion fields predicted in the combined model with the best parameters are well consistent with those calculated from our LES model. We then apply the combined model to predict the scalar dispersion over a real urban area. The combined model well predicts the results obtained from the LES model with less calculation time. Therefore, we find that the combined model is potentially effective for emergency response to hazardous-material release over urban areas.
Nakayama, Hiromasa; Takemi, Tetsuya*
Proceedings of 19th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes (HARMO-19) (USB Flash Drive), 5 Pages, 2019/06
We proposed the data assimilation method using the vibration equation suitable for LES models. First, we generated the basic turbulent boundary layer (TBL) flow with a power law exponent of 0.14 at the upstream area and then attempted to nudge toward the TBL flow with a power law exponent of 0.25 by our proposed data assimilation. For a case that the natural frequency set in the vibration equation was larger than the peak frequency of the basic TBL flow, the turbulent fluctuations were rapidly damped although the mean velocity profile matched with the target one. On the other hand, for a case that the natural frequency was smaller than the peak frequency, the mean wind velocity profile was reasonably nudged toward the target profile while maintaining the turbulent fluctuations. It is concluded that our proposed data assimilation method using the vibration equation successfully nudges toward the target mean winds with small-scale turbulent fluctuations.
Nakayama, Hiromasa; Takemi, Tetsuya*
International Journal of Environment and Pollution, 64(1/3), p.125 - 144, 2018/00
We have attempted to develop a practical and quick local-scale atmospheric dispersion calculation method using an overlapping technique for plume concentration distributions in an emergency response to nuclear accidents. In order to evaluate the overlapping approach, we performed LESs of turbulent flows and plume dispersion under varying mean wind directions using the meteorological data as the model input. When compared with the simulated results under the real meteorological condition, it is shown that the concentration distribution patterns are reasonably simulated by the overlapping method. It can be concluded that the atmospheric dispersion calculation method using the overlapping technique has potential performance for emergency responses to nuclear accidents.
Nakayama, Hiromasa; Takemi, Tetsuya*
Proceedings of 18th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes (HARMO-18) (USB Flash Drive), p.843 - 847, 2017/10
We have attempted to develop a practical and quick local-scale atmospheric dispersion calculation method using an overlapping technique for plume concentration distributions in an emergency response to nuclear accidents. In order to evaluate the overlapping approach, we performed LESs of turbulent flows and plume dispersion under varying mean wind directions using the meteorological data as the model input. When compared with the simulated results under the real meteorological condition, it is shown that the concentration distribution patterns are reasonably simulated by the overlapping method. It can be concluded that the atmospheric dispersion calculation method using the overlapping technique has potential performance for emergency responses to nuclear accidents.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Journal of Nuclear Science and Technology, 53(6), p.887 - 908, 2016/06
Times Cited Count:15 Percentile:84.46(Nuclear Science & Technology)We extend the local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation (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 to simulate turbulent flows and plume dispersion in urban areas under real meteorological conditions.
Nakayama, Hiromasa; Nagai, Haruyasu
JAEA-Data/Code 2015-026, 37 Pages, 2016/03
We developed LOcal-scale High-resolution atmospheric DIspersion Model using Large-Eddy Simulation (LOHDIM-LES). This dispersion model is designed based on LES which is effective to reproduce unsteady behaviors of turbulent flows and plume dispersion. The basic equations are the continuity equation, the Navier-Stokes equation, and the scalar conservation equation. Buildings and local terrain variability are resolved by high-resolution grids with of a few meters and these turbulent effects are represented by immersed boundary method. In simulating atmospheric turbulence, boundary layer flows are generated by a recycling turbulent inflow technique in a driver region set up at the upstream of the main analysis region. This turbulent inflow data are imposed at the inlet of the main analysis region. By this approach, the LOHDIM-LES can provide detailed information on wind velocities and plume concentration in the investigated area.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Procedia Earth and Planetary Science, 15, p.560 - 565, 2015/09
Times Cited Count:0 Percentile:0.02The local-scale high-resolution atmospheric dispersion model can simulate turbulent flows considering the effects of not only local terrain variability and buildings but also atmospheric stability conditions, and has been validated for representative meteorological condition cases. In this study, first, we conducted numerical simulations of plume dispersion under various thermally-stratified boundary layer flows by changing the stability conditions and compared with the wind tunnel experimental data. Then, we categorized the patterns of vertical profiles of mean concentrations based on the ratio of the convective velocity scale to the friction velocity scale. It is found that a plume quickly begins to touch the ground for a strongly unstable case with the ratio value less than 0.34 while the plume touchdown is observed at the downstream positions located away from the point source for a weak condition case with the ratio value greater than 0.34.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Advances in Science & Research (Internet), 12(1), p.127 - 133, 2015/00
Times Cited Count:8 Percentile:93.12One of the important issues to be solved to couple LES and MM models is to impose time-dependent turbulent inflow data for LESs from the MM model outputs, because the MM models are not able to reproduce high-frequency turbulent fluctuations appropriate to drive LES models. In this study, we apply the recycling technique to couple the CFD and MM models. We conduct an LES of turbulent winds during the Fukushima Daiichi Nuclear Power Plant accident and report the usefulness of our approach by comparing the simulated results with the observations.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Advances in Science & Research (Internet), 11, p.75 - 81, 2014/07
Times Cited Count:9 Percentile:97.6We have developed a local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation and tried to introduce it into SPEEDI-MP. In this study, our objectives are to first create a numerical model for LES on atmospheric dispersion in spatially-developing stable and unstable boundary layer flows which and compare to the existing wind tunnel experimental data. Based on the comparison, we discuss the basic performance of the LES model.
Nakayama, Hiromasa; Leitl, B.*; Harms, F.*; Nagai, Haruyasu
Journal of Nuclear Science and Technology, 51(5), p.626 - 638, 2014/02
Times Cited Count:17 Percentile:81.17(Nuclear Science & Technology)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.
Nakayama, Hiromasa; Nagai, Haruyasu
Nihon Kaze Kogakkai-Shi, 38(4), p.396 - 403, 2013/10
The Fukushima Daiichi Nuclear Power Plant accident in Japan triggered by a magnitude 9.0 earthquake and resulting tsunami on March 11, 2011 caused the month-long discharge of radioactive materials into the atmosphere. It was urgent to assess the radiological dose to the public resulting from this release, using both environmental monitoring data and computer simulation based on atmospheric dispersion modeling of radioactive materials. This paper describes the outline of atmospheric dispersion system developed at Japan Atomic Energy Agency and its application to the Fukushima Daiichi Nuclear Power Plant accident.