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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

Spatial characteristics of turbulent organized structures within the roughness sublayer over idealized urban surface with obstacle-height variability

Yoshida, Toshiya; Takemi, Tetsuya*

Environmental Fluid Mechanics, 21(1), p.129 - 154, 2021/02

 Times Cited Count:0 Percentile:0.01(Environmental Sciences)

The effects of obstacle-height variability on spatial characteristics of turbulent organized structures were investigated with the use of a large-eddy simulation technique for airflows over roughness obstacles. Two-types simulation cases were considered: one is uniform-height case in which uniform-height obstacles are aligned in streamwise direction, the other is height-variability case with staggered higher-height obstacles. Streaky structures were observed above the roughness sublayer (RSL) regardless of obstacle-height variability. When obstacles are uniform, flow fields within the RSL contain low- and high-speed regions along the streamwise streets. When obstacle heights vary, airflow within the RSL collides with the front-facing surfaces of taller obstacles. The statistical features of low- and high-speed structures were examined using the spatial correlations of flow fields centering on strong ejection and sweep, respectively. The ejection- and sweep-center spatial correlations extend forward and backward in the streamwise direction, respectively. Length scales were obtained from the ejection-center and sweep-center spatial correlations. The streamwise lengths vary significantly below the canopy height when obstacles are uniform. In contrast, the streamwise length scales remain nearly constant when obstacle heights vary. The horizontal aspect ratios below the canopy heights indicate that turbulent organized structures over obstacles with variable heights are more isotropic than those over uniform obstacles. The inclination angles of the organized structures were also deduced using the spatial correlations. The angles of sweep-center structures are steeper than those of the ejection-center structures. The angles of the ejection-center structures at the RSL heights become larger with obstacle-height variability.

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.

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

Large-Eddy Simulation of urban boundary-layer flows by generating turbulent inflows from mesoscale meteorological simulations

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

Atmospheric Science Letters, 13(3), p.180 - 186, 2012/07

 Times Cited Count:45 Percentile:81.72(Geochemistry & Geophysics)

We propose an approach to generate turbulent flows by using mesoscale meteorological simulations in order to conduct building-resolving Large-Eddy Simulation (LESs) of boundary-layer flows over urban areas under realistic meteorological conditions. The urban surface geometry was explicitly represented in the LES model. This approach was applied for a strong wind event in Tokyo owing to the landfall of a major typhoon whose intensity and track was well reproduced in the meteorological simulation. The observed ranges of wind fluctuations and gust factors, and significant decelerations of wind speeds within the urban canopy layer were successfully represented in the LES.

Journal Articles

LES analysis of the aerodynamic surface properties for turbulent flows over building arrays with various geometries

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

Journal of Applied Meteorology and Climatology, 50(8), p.1692 - 1712, 2011/08

 Times Cited Count:28 Percentile:63.86(Meteorology & Atmospheric Sciences)

This paper describes the applicability of roughness-length and drag-force approaches of mesoscale meteorological models to actual urban areas. We first investigated building morphological characteristics. Then, we performed large-eddy simulations (LESs) of turbulent flows over building arrays with various surface geometries that are characterized by a wide range of values for not only roughness density and but also building height variability. Then, we evaluated the aerodynamic roughness parameters such as roughness length and drag coefficient for central Tokyo area using the LES results. The values of roughness length and drag coefficient as a function of both roughness length and building height variability is comparable to those in the previous studies. The roughness aerodynamic parameters as a function of roughness length and drag coefficient obtained from our LES results are useful to incorporate the urban effects into weather forecasting models.

Journal Articles

Coupling of WRF and building-resolving urban CFD models for analysis of strong winds over an urban area

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

Proceedings of 14th Conference on Mesoscale Processes (Internet), 7 Pages, 2011/00

Coupling of the NWP (Numerical Weather Prediction) model with the CFD (Computational Fluid Dynamics) model using LES (Large-Eddy Simulation) can be effective means to predict small-scale wind fluctuations over urban areas under real meteorological conditions. In this study, we performed a numerical simulation for strong winds over the central Tokyo during the passage of Typhoon Melor (2009). In the LES model, the urban surface geometry is explicitly resolved. At the inlet boundary of the LES computational region, the wind profile obtained from the NWP model is imposed and turbulent fluctuations are generated by the existing turbulent inflow generation method (Kataoka et al, 2002). Compared to Mesoscale Analysis data by Japan Meteorological Agency (JMA), the simulation results show that the variability of wind velocity is similar to that observed at a surface site of JMA.

Journal Articles

Fine-scale analysis of atmospheric flow by merging meteorological and CFD models

Takemi, Tetsuya*; Nakayama, Hiromasa

Nagare, 28(1), p.13 - 20, 2009/02

We explain one theme "Meteorological model of a local scale atmospheric flow and uniting analysis of the CFD model" of feature project "Development and application of the next generation weather model who centers on WRF" of academic society magazine "Flow" by the request of the Japanese fluid mechanics association. In this text, the problem and the view that should be examined when a local scale atmospheric flow by the both models' uniting is simulated about LES analysis of the turbulent boundary layer flow that develops under a realistic rough degree form by reproducibility and CFD model of the wind speed fluctuation that originates in the microindentation of the weather disturbance by making of the meteorological model a high resolution are described.

Journal Articles

LES analysis of turbulent boundary-layer flow over urban-like building arrays with various spatial arrangement and height distribution

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

Proceedings of 89th American Meteorological Society Annual Meeting (Internet), 6 Pages, 2009/01

Our objective is to develop LES atmospheric dispersion models and introduce into SPEEDI-MP. In this study, in order to validate a numerical simulation for urban turbulence flow, first, we conducted LES for turbulent flow over regular arrays of cubic structures. As the results, it is found that LES data for mean wind velocity and turbulent statistic are good agreement with wind tunnel experimental data. Next, we examined the building morphological characteristics such as roughness density, the mean and standard deviation of building heights in actual urban area and propose a model that represents realistic urban surface geometries. Then, we performed LES on turbulent flows over the above-mentioned building arrays. As the results, we have succeeded in simulating turbulent flows corresponding to the surface geometries.

Oral presentation

LES analysis of plume dispersion through urban-like building arrays

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

no journal, , 

So far, many researchers have studied plume dispersion in regular arrays of cubic models. For example, Macdonald (1997) conducted field experiments of plume dispersion through regular arrays of cubic models and investigate the effects of the plan area density on dispersion characteristics. However, the geometries of obstacles employed in these studies are too simplified to directly apply their results to real urban settings. On the other hand, the shape of city surfaces is complex and the building heights are highly variable. In this study, firstly, we first examine the building morphological characteristics and propose a model that represents realistic urban surface geometries. Next, we perform LES on plume dispersion through the above-mentioned building arrays and investigate the relationship between the dispersion characteristics and the building morphological parameters.

Oral presentation

LES analysis of the aerodynamic surface properties for turbulent flows over building arrays with various geometries and its application to actual urban area

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

no journal, , 

There are important issues of treatment of urban ground surfaces for both the roughness and drag force approaches in mesoscale meteorological models. Thus, for using the roughness approach, the geometry type of urban surface where the roughness length is appropriately estimated should be considered. On the other hand, for using the drag force approach, the drag coefficient values also should be evaluated by considering the complex geometry of urban surface. Therefore, in this study, we first examine the building morphological characteristics in some actual urban areas and set up building arrays that represent realistic urban surface geometries. Then, we perform LES of turbulent flows over various urban-type surfaces of building arrays and investigate the relationship between the aerodynamic roughness parameters and the building morphological characteristics.

Oral presentation

Fine-scale analysis of atmospheric flow by merging meteorological and LES models

Takemi, Tetsuya*; Nakayama, Hiromasa

no journal, , 

The performance of two-way nesting for large eddy simulation (LES) of PBL turbulence is investigated using the Weather Research and Forecasting model framework. A pair of LES-within-LES experiments are performed where a finer-grid LES covering a smaller horizontal domain is nested inside a coarser-grid LES covering a larger horizontal domain. In this study, we report the time series of wind velocity in comparison with the observed data.

Oral presentation

An Approach for quantitative forecasting of turbulent flow over an urban area by coupling numerical weather prediction and Large-Eddy Simulation models

Takemi, Tetsuya*; Nakayama, Hiromasa

no journal, , 

Atmospheric flows in urban areas are highly turbulent owing not only to meteorological disturbances but also to complex and complicated distribution of roughness obstacles (i.e., buildings). Numerical modeling is an important research for this purpose. In order to numerically examine flows over such highly rough urban surfaces, a large-eddy simulation (LES) approach is promising. In order to ensure the turbulent nature of flows in urban areas, an LES approach with a proper turbulence-generation technique is necessary. The present study proposes an approach to quantitatively simulate turbulent flows in a densely-built environment with the mixture of high-rise and low-rise buildings by coupling the WRF model and an LES model.

Oral presentation

Coupling of WRF and LES models using turbulent inflow generation method for analysis of strong winds over urban areas

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

no journal, , 

Due to the rapid growth of computer resources, recently, the boundaries between mesoscale meteorological models and computational fluid dynamics (CFD) models are overlapping; namely, there are now some examples of very high-resolution mesoscale meteorological simulations and some examples of CFD simulations that deal with very large computational domains. As the next phase of such simulation studies, attempts are being made to merge mesoscale and CFD models. In this study, we attempt to merge the Weather Research and Forecasting (WRF) model and a Large-eddy simulation (LES) model using turbulent inflow generation method for analysis of strong winds over urban areas.

Oral presentation

Turbulent analysis of strong wind variability by merging numerical weather prediction and large-Eddy simulation models

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

no journal, , 

The purpose of this study is to propose a numerical approach for quantitative analyses of wind flows in real urban areas. We conduct numerical simulations for strong winds and gusts over the central business district of Tokyo during the passage of Typhoon Melor (2009). The WRF model is used for the meteorological simulation; Mesoscale Analysis data by JMA are used as the initial and boundary conditions. The nesting technique is used to resolve the area including central Tokyo at 60-m grid. The high-resolution temporal and spatial data from the WRF model are then used as the initial and boundary conditions for an LES model. The lower boundary of the LES model is determined by a building-height dataset for the analysis area. A turbulent simulation of wind flows in the realistic urban area is conducted with the LES model. The simulation successfully captures the wind variability and peaks observed at a surface site of JMA.

37 (Records 1-20 displayed on this page)