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Journal Articles

Development of a surface heat flux model for urban wind simulation using locally mesh-refined lattice Boltzmann method

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.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation, 6; Introduction of detailed dose calculation method

Nakayama, Hiromasa; Satoh, Daiki; Nagai, Haruyasu; Terada, Hiroaki

Journal of Nuclear Science and Technology, 58(9), p.949 - 969, 2021/09

 Times Cited Count:1 Percentile:81.22(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.

Journal Articles

Large-eddy simulation of plume dispersion in the central district of Oklahoma City by coupling with a mesoscale meteorological simulation model and observation

Nakayama, Hiromasa; Takemi, Tetsuya*; Yoshida, Toshiya

Atmosphere (Internet), 12(7), p.889_1 - 889_15, 2021/07

 Times Cited Count:0 Percentile:0.01(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.

Journal Articles

Toward development of a framework for prediction system of local-scale atmospheric dispersion based on a coupling of LES-database and on-site meteorological observation

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.

Journal Articles

Real-time tracer dispersion simulations in Oklahoma City using the locally mesh-refined lattice Boltzmann method

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:2 Percentile:87.32(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.

Journal Articles

Simulation code for estimating external gamma-ray doses from a radioactive plume and contaminated ground using a local-scale atmospheric dispersion model

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:1 Percentile:81.22(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 $$^{85}$$Kr emission from a ventilation shaft and $$^{137}$$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.

Journal Articles

Development of a combined LES/RANS model to predict atmospheric dispersion over urban areas

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.

Journal Articles

Development of the data assimilation method suitable for large-eddy simulation model using the vibration equation

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.

Journal Articles

Large-eddy simulation studies for predicting plume concentrations around nuclear facilities using an overlapping technique

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.

Journal Articles

LES studies for predicting plume concentrations around nuclear facilities using an overlapping technique

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.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation. 5; Detailed simulation of turbulent flows and plume dispersion in an actual urban area under real meteorological conditions

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Journal of Nuclear Science and Technology, 53(6), p.887 - 908, 2016/06

 Times Cited Count:13 Percentile:86.01(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.

JAEA Reports

Local-scale high-resolution atmospheric dispersion model using large-eddy simulation; LOHDIM-LES

Nakayama, Hiromasa; Nagai, Haruyasu

JAEA-Data/Code 2015-026, 37 Pages, 2016/03

JAEA-Data-Code-2015-026.pdf:2.48MB

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.

Journal Articles

The Numerical analysis of the capping inversion effect in a convective boundary layer flow on the contaminant gas dispersion

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Procedia Earth and Planetary Science, 15, p.560 - 565, 2015/09

 Times Cited Count:0 Percentile:0.02

The 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.

Journal Articles

Large-Eddy Simulation of turbulent winds during the Fukushima Daiichi Nuclear Power Plant accident by coupling with a meso-scale meteorological simulation model

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Advances in Science & Research (Internet), 12(1), p.127 - 133, 2015/00

 Times Cited Count:8 Percentile:94.11

One 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.

Journal Articles

Large-Eddy Simulation of plume dispersion under various thermally stratified boundary layers

Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu

Advances in Science & Research (Internet), 11, p.75 - 81, 2014/07

 Times Cited Count:8 Percentile:97.2

We 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.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation, 4; Turbulent flows and plume dispersion in an actual urban area

Nakayama, Hiromasa; Leitl, B.*; Harms, F.*; Nagai, Haruyasu

Journal of Nuclear Science and Technology, 51(5), p.626 - 638, 2014/02

 Times Cited Count:15 Percentile:81.82(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.

Journal Articles

Numerical prediction of atmospheric dispersion of radionuclides during the Fukushima Dai-ichi Nuclear Power Plant accident by WSPEEDI

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.

Journal Articles

Development of local-scale high-resolution atmospheric dispersion model using Large-Eddy Simulation, 3; Turbulent flow and plume dispersion in building arrays

Nakayama, Hiromasa; Jurcakova, K.*; Nagai, Haruyasu

Journal of Nuclear Science and Technology, 50(5), p.503 - 519, 2013/05

 Times Cited Count:11 Percentile:70.63(Nuclear Science & Technology)

There is a potential problem that hazardous materials are accidentally or intentionally released within populated urban areas. In this study, we perform LESs of plume dispersion in idealized urban canopies where the surface geometries are represented by regularly arrayed obstacle arrays with various obstacle densities. In this paper, we examine the basic performance of the LES model by comparing to wind tunnel experiments.

Journal Articles

Reconstruction of the atmospheric releases of $$^{131}$$I and $$^{137}$$Cs resulting from the Fukushima Daiichi Nuclear Power Plant accident

Chino, Masamichi; Terada, Hiroaki; Katata, Genki; Nagai, Haruyasu; Nakayama, Hiromasa; Yamazawa, Hiromi*; Hirao, Shigekazu*; Ohara, Toshimasa*; Takigawa, Masayuki*; Hayami, Hiroshi*; et al.

NIRS-M-252, p.127 - 135, 2013/03

We estimated the release rates and total amounts of $$^{131}$$I and $$^{137}$$Cs discharged into the atmosphere from March 12 to April 5, 2011. The applied method is a reverse estimation by coupling environmental monitoring data with atmospheric dispersion simulations under the assumption of unit release rate (1 Bq/h). It calculates release rates of radionuclides (Bq/h) by dividing measured air concentrations of $$^{131}$$I and $$^{137}$$Cs into calculated ones at sampling points. The estimated temporal variation of releases indicates that the significant release, over 10$$^{15}$$ Bq/h of $$^{131}$$I, occurred on March 15, following to relatively small releases, 10$$^{13}$$ $$sim$$ 10$$^{14}$$ Bq/h, but the release rates from March 16 are estimated to be rather constant on the order 10$$^{14}$$ Bq/h until March 24. The release rates have decreased with small day-to-day variations to the order of 10$$^{11}$$ $$sim$$ 10$$^{12}$$ Bq/h of $$^{131}$$I on the beginning of April. The estimated source term was examined on the point of the time trend, total releases and the ground depositions of $$^{137}$$Cs by using different atmospheric dispersion models with above source term and compared them with observed $$^{137}$$Cs deposition distribution. These examinations showed that the estimated source term was reasonably accurate during the period when the plume flowed over land in Japan.

Journal Articles

Large-Eddy Simulation of plume dispersion within various actual urban areas

Nakayama, Hiromasa; Jurcakova, K.*; Nagai, Haruyasu

Advances in Science & Research (Internet), 10, p.33 - 41, 2013/02

 Times Cited Count:7 Percentile:96.83

There is a potential problem that hazardous and flammable materials are accidentally or intentionally released within urban areas. Our objective is to compare our LES results for distribution patterns of mean and fluctuating concentrations in various actual urban areas with the existing LES and experimental results for those in regularly arrayed obstacles, and investigate the influence of actual urban surface geometries on the characteristics of mean and fluctuating concentrations.

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