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Nakayama, Hiromasa; Takemi, Tetsuya*
Atmospheric Science Letters, 25(4), p.e1204_1 - e1204_9, 2024/04
Times Cited Count：1 Percentile：20.99(Geochemistry & Geophysics)We proposed a dynamically controlled recycling method. The magnitude of turbulent fluctuations is dynamically controlled to match with the target turbulence statistics using a turbulence enhancement coefficient based on the ratio of the target to the computed values. When compared to the recommended data of Engineering Science Data Unit (ESDU) 85020, the turbulence characteristics were quantitatively generated well. Furthermore, the spanwise and vertical plume spreads were also reproduced well. It is concluded that the dynamically controlled recycling method successfully simulates plume dispersion in a neutral turbulent boundary layer.
Nakayama, Hiromasa; Takemi, Tetsuya*; Yoshida, Toshiya
Atmosphere (Internet), 12(7), p.889_1 - 889_15, 2021/07
Times Cited Count：3 Percentile：15.14(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.
Yoshida, Toshiya; Takemi, Tetsuya*
Environmental Fluid Mechanics, 21(1), p.129 - 154, 2021/02
Times Cited Count：5 Percentile：27.92(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.
Nakayama, Hiromasa; Takemi, Tetsuya*
Journal of Advances in Modeling Earth Systems (Internet), 12(8), p.e2019MS001872_1 - e2019MS001872_18, 2020/08
Times Cited Count：3 Percentile：11.72(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.
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：18 Percentile：82.94(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; Takemi, Tetsuya*; Nagai, Haruyasu
Procedia Earth and Planetary Science, 15, p.560 - 565, 2015/09
Times Cited Count：0 Percentile：0.00(Geosciences, Multidisciplinary)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.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Advances in Science & Research (Internet), 12(1), p.127 - 133, 2015/00
Times Cited Count：8 Percentile：91.55(Multidisciplinary Sciences)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.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Advances in Science & Research (Internet), 11, p.75 - 81, 2014/07
Times Cited Count：10 Percentile：97.60(Multidisciplinary Sciences)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.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Atmospheric Science Letters, 13(3), p.180 - 186, 2012/07
Times Cited Count：55 Percentile：80.09(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.
Nakayama, Hiromasa; Takemi, Tetsuya*; Nagai, Haruyasu
Journal of Applied Meteorology and Climatology, 50(8), p.1692 - 1712, 2011/08
Times Cited Count：38 Percentile：66.80(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.
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.
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.
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.
Nakayama, Hiromasa; Takemi, Tetsuya*
no journal, ,
In using Large-Eddy Simulation (LES) in study fields of computational fluid engineering, boundary layer meteorology, and atmospheric dispersion, one of difficult problems is a treatment of turbulent inflow boundary. The variability of atmospheric flow is induced by meteorological disturbances, terrains, and surface roughness elements. Therefore, wind velocities are also always changed. In conducting LESs, time-dependent turbulent inflow data have to be imposed at the inlet boundary depending on the atmospheric conditions. In this study, from a practical application perspective, we propose a generation method for thermally-stratified boundary layer flows by a short fetch and discuss the effectiveness of the approach in comparison to the existing wind tunnel experimental data.
Nakayama, Hiromasa; Takemi, Tetsuya*
no journal, ,
The model validation of an large-eddy simulation (LES)-based computational fluid dynamics (CFD) model coupled to a numerical weather prediction (NWP) is carried out. The Joint Urban 2003 field experimental data of gas concentration released as puff and 30-minute continuous releases is used to evaluate the performance of the coupling approach. In the LES model, wind velocity and potential temperature data of the WRF are imposed at lateral boundaries. The LES results of vertical profiles of wind speed, wind direction, and potential temperature are in good agreement with the WRF data. The time-averaged concentrations of the LES are different from the experimental data at the several points especially for the puff case. However, the LES generally show reasonable performance in comparison to the experimental data through the continuous and puff release cases. It is concluded that our approach is physically reasonable.
Takemi, Tetsuya*; Nakayama, Hiromasa
no journal, ,
This study investigated turbulent flow and dispersion in complex surfaces under real meteorological settings by the Weather Research and Forecasting (WRF) model and a large-eddy simulation (LES) model. The turbulence and dispersion measured in Oklahoma City during the field experiment were simulated by the WRF/LES modeling. Overall the simulated results well reproduced the boundary-layer flow. The simulated dispersion fields agree well with the field measurements in some locations; however, in other cases the simulation disagrees with the measurements, because even a slight departure of the simulated winds to the measurements may result in a large discrepancy of the simulations from the observations. The coupling simulation is extended to the flows over the urban area of Kyoto City. In this paper, we introduce our studies on the coupling simulations of turbulent flows over the central districts of Oklahoma City and Kyoto, and discuss about the issue and prospects.
Nakayama, Hiromasa; Takemi, Tetsuya*
no journal, ,
Large-eddy simulation (LES)-based computational fluid dynamics (CFD) model coupled to a numerical weather prediction (NWP) was carried out to reproduce a diurnal cycle of atmospheric turbulent boundary layer flows under real meteorological conditions. In evaluating the coupling approach, the Joint Urban 2003 field experimental data were used. In the LES model, wind velocity and potential temperature data of the WRF were imposed at lateral boundaries. The surface heat fluxes were calculated using the surface potential temperature obtained by the WRF model. The LES model generally simulated the spatial distribution patterns of wind velocities, wind directions, and potential temperature from the morning to the evening in comparison to the experimental data. It is concluded that our coupling approach is physically reasonable.
