Advance

MICHIOKA Takenobu

    Department of Mechanical Engineering　Professor
Last Updated :2024/04/25

Researcher Information

Research funding number

  • 20371370

J-Global ID

Research Interests

  • 数値シミュレーション   乱流構造   都市防災   都市キャニオン   風洞実験   ガス拡散   大気拡散   

Research Areas

  • Social infrastructure (civil Engineering, architecture, disaster prevention) / Construction environment and equipment
  • Manufacturing technology (mechanical, electrical/electronic, chemical engineering) / Fluid engineering

Academic & Professional Experience

  • 2015/04 - Today  Kindai UniversityFaculty of Science and Engineering, Department of Mechanical Engineering
  • 2001/04 - 2015/03  Central Research Institute of Electric Power Industry
  • 2012/04 - 2013/03  Kyushu UniversitySchool of Engineering
  • 2006/04 - 2011/03  東京工業大学大学院環境理工学創造専攻連携准教授
  • 2006/10 - 2007/09  カリフォルニア大学バークレイ校客員研究員

Education

  • 1998/04 - 2001/03  京都大学大学院  工学研究科 機械工学専攻  博士後期課程
  • 1996/04 - 1998/03  九州大学大学院  工学研究科 化学機械工学専攻  修士課程
  • 1993/04 - 1996/03  Kyushu University  School of Engineering  化学機械工学科

Association Memberships

  • JAPAN SOCIETY OF CIVIL ENGINEERS   ARCHITECTURAL INSTITUTE OF JAPAN   THE SOCIETY OF CHEMICAL ENGINEERS, JAPAN   JAPAN SOCIETY FOR ATMOSPHERIC ENVIRONMENT   THE JAPAN SOCIETY OF FLUID MECHANICS   THE JAPAN SOCIETY OF MECHANICAL ENGINEERS   

Published Papers

  • Takenobu Michioka; Ryo Funaki; Takumi Kawai
    BOUNDARY-LAYER METEOROLOGY SPRINGER 0006-8314 2023/01 [Refereed]
     
    Large-scale turbulent motions at a streamwise street coupled with spanwise motion in a canyon have been observed within an idealized urban canopy, but the effects of building array geometry on large-scale turbulent motions within the canopy remain unresolved. To address this issue, large eddy simulation is performed for the canyon width of 0.5 H-1.5 H (H = cube height) and the street width of H-1.5 H. In the case of a large canyon with (1.5 H) and a large street width (1.5 H), large-scale turbulent motions are observed at the streamwise street. However, large-scale turbulent motions are not generated in the case of a small canyon width (0.5 H). The size of the canyon or street width affects the generation of the large-scale turbulent motions. When a pair of the high- and low-momentum motions are present at the adjacent intersection pathways under the generation of the large-scale turbulent motions, large-scale streamwise vortices are occasionally generated. These streamwise vortices maintain the generation of large-scale turbulent motions.
  • Takenobu Michioka
    Environmental Fluid Mechanics Springer Science and Business Media LLC 1567-7419 2022/06 [Refereed]
  • Takumi Sato; Yusei Kobayashi; Takenobu Michioka; Noriyoshi Arai
    SOFT MATTER ROYAL SOC CHEMISTRY 17 (15) 4047 - 4058 1744-683X 2021/04 [Refereed]
     
    In this study, a coarse-grained molecular simulation was performed to investigate the morphologies and phase diagrams of self-assembled polymer-tethered nanoparticles (NPs) confined in nanotubes (NTs). Unlike ordinary NPs, polymer-tethered NPs have two distinct characteristic lengths, which are key factors that determine their self-assembly. Herein, two distinct types of NT walls and three types of polymer-tethered NPs were considered: hydrophilic and hydrophobic walls, and hydrophilic, hydrophobic, and Janus surfaces. First, the qualitative phase diagrams of the axial pressure, P-z, versus the ratio of the NT radius to the NP radius, L, were derived. The results revealed that diverse self-assembled morphologies, which are not formed in non-tethered NPs, were observed in the polymer-tethered NPs. For example, three types of ordered structures with different structural characteristic lengths, depending on P-z, were obtained. In addition, the effect of the chemical nature of the polymer-tethered NP surface on the self-assembled morphology confined in NTs was investigated. Clusters of water molecules were formed, particularly in the hydrophobic polymer-tethered NPs, and these clusters caused the structural distortion of the NP. Moreover, in the polymer-tethered NPs with the Janus amphiphilic surface, the hydrophobic and hydrophilic polymer tethered NPs assembled in the axial direction to form an ordered structure, and a double-helix structure was formed at L = 3.0 in the hydrophobic NT. The results of these simulations indicate that the self-assembly behaviours of polymer-tethered NPs can be qualitatively predicted based on the chemical nature of the NT walls and the surface design of the polymer-tethered NP.
  • Hayashi Masashi; Michioka Takenobu
    Journal of Japan Society for Atmospheric Environment / Taiki Kankyo Gakkaishi Japan Society for Atmospheric Environment 55 (3) 116 - 124 1341-4178 2020 [Refereed]
     

    Thermal power plant was recently built near an existing wind farm. The pollutant emitted from the thermal power plant is possibly affected by the rotating wind turbine at the wind farm, and the high concentration of the pollutant potentially reaches at the ground. However, the effect of the rotating wind turbine on the pollutant dispersion is less understood. In the present study, a large-eddy simulation was implemented to investigate the effect of the rotating wind turbine on the pollutant dispersion emitted from a point source. The arbitrary mesh interface technique was applied to represent the rotation of the wind turbine in the computational domain. The point sources were located upwind or downwind of the wind turbine. In the case of the upwind source location, the rotating wind turbine strongly affected the pollutant dispersion and increased the pollutant concentration at the ground level when the source height was equal to the wind turbine hub height. As the source height increased, the rotating wind turbine had less influence on the pollutant dispersion. In the case of the downwind sources where the velocity defect mostly vanishes, the pollutant dispersion was not strongly affected by the rotating wind turbine irrespective of the source height.

  • Suzuki Naoya; Michioka Takenobu; Konda Masanori; Kutsuwada Kunio; Masuda Yuji
    Journal of Advanced Marine Science and Technology Society 海洋理工学会 25 (1) 1 - 14 1341-2752 2019 [Refereed]
     
    <p>Generally, the air-sea momentum, heat, and CO<sub>2</sub> gas flux are calculated in field observation using the eddy correlation method, which simultaneously measures the horizontal and vertical wind components. Measuring the components of wind velocity is usually limited to fixed installations owing to the minor effect of the platform on the wind flow. Occasionally, numerical simulations are sometimes used for shipboard observations. However, the details of the effect of the wind flow around an observation ship are scarcely investigated using numerical simulations, although ships affect the wind flow and wind direction by wind distortion. In this study, we investigated the effect of ships on the wind flow around the Hakuho–Maru of the Japan Agency for Marine-Earth Science and Technology using a numerical simulation. As a result of the flow visualization, measuring the horizontal and vertical components of wind velocity was very difficult because the wind distortion (upward and downward gusts, among others) was very large owing to the effect of the ship. This showed that the effect of the ship was significant for the measurements of air-sea momentum, heat, and CO<sub>2</sub> gas flux.</p>
  • Takenobu Michioka; Hiroshi Takimoto; Hiroki Ono; Ayumu Sato
    Boundary-Layer Meteorology 0006-8314 2019/01 [Refereed]
     
    © 2019, Springer Nature B.V. Large-eddy simulation of turbulent flow and gas dispersion in a cubical canopy is used to investigate the effect of wind-direction fluctuations on gas dispersion. Square blocks are set at regular intervals on the bottom surface, with line sources placed within the first, second, third, fifth and seventh rows. Large-eddy simulation without wind-direction fluctuations produces a good prediction of the mean streamwise velocity component, and the standard deviations of the fluctuations in the streamwise and spanwise velocity components, obtained from a wind-tunnel experiment. Wind-direction fluctuations marginally affect the mean streamwise velocity component above the canopy in the first row, and do not significantly affect the component beyond the third row. The standard deviations of the fluctuations in the streamwise and spanwise velocity components above the canopy are also affected by wind-direction fluctuations, but within the canopy the components are less sensitive to the fluctuations beyond the third row. The spatially-averaged concentrations within the canyon with wind-direction fluctuations before the third row are marginally greater than concentrations without the fluctuations, but they are essentially identical beyond the fifth row. The low-frequency turbulent flow that passes through the canyon is generated with and without wind-direction fluctuations.
  • TAKIMOTO Hiroshi; ONO Hiroki; SATO Ayumu; MICHIOKA Takenobu
    Journal of Wind Engineering Japan Association for Wind Engineering 44 (3) 59 - 67 1349-3507 2019 [Refereed]
     
    When a stack in industrial facilities is not sufficiently high compared to the surrounding buildings, the dispersion of discharged pollutants can be affected by the buildings; this is commonly termed as building downwash. To enable more precise prediction of the building downwash, we have developed a computational fluid dynamics model that can reproduce momentum plume-rise process. We confirmed that the most of the surface concentrations predicted using the proposed model are within a factor of two of the wind-tunnel experimental results. Our study suggests that the mesh types around a stack can affects the prediction of surface concentration in high wind speed conditions.
  • Takenobu Michioka
    International Journal of Heat and Mass Transfer Elsevier Ltd 125 569 - 579 0017-9310 2018/10 [Refereed]
     
    Large-eddy simulation is implemented for turbulent flow and gas dispersion over wavy walls across a wide range of the wave amplitude to wavelength ratio (α = 1.0, 1.5, and 2.0). Two tracer gases are emitted from point sources located at a single crest and trough of the wavy wall. Because the values of α increase under the Reynolds number based on the bulk velocity and the wavelength is 6.67×104, the flow separates behind the crest and the standard deviations of the fluctuation in streamwise and spanwise velocity components become relatively large especially at the upslope of the wavy wall. The increase are related to the vortices linked to the Görtler instability mechanism. The change in the flow pattern with respect to the value of α significantly affects the gas dispersion within the valley. As the Reynolds number increases under the wavy wall with α = 2.0, the flow tends to follow the bottom surface of the wavy wall and the number of the vortices increases at the upslope, but the vortices do not significantly affect the gas dispersion over the wavy wall.
  • Takenobu Michioka; Hiroshi Takimoto; Hiroki Ono; Ayumu Sato
    Boundary-Layer Meteorology Springer Netherlands 168 1 - 21 1573-1472 2018/03 [Refereed]
     
    The effects of fetch on turbulent flow and pollutant dispersion within a canopy formed by regularly-spaced cubical objects is investigated using large-eddy simulation. Six tracer gases are simultaneously released from a ground-level continuous pollutant line source placed parallel to the spanwise axis at the first, second, third, fifth, seventh and tenth rows. Beyond the seventh row, the standard deviations of the fluctuations in the velocity components and the Reynolds shear stresses reach nearly equivalent states. Low-frequency turbulent flow is generated near the bottom surface around the first row and develops as the fetch increases. The turbulent flow eventually passes through the canopy at a near-constant interval. The mean concentration within the canopy reaches a near-constant value beyond the seventh row. In the first and second rows, narrow coherent structures frequently affect the pollutant escape from the top of the canopy. These structures increase in width as the fetch increases, and they mainly affect the removal of pollutants from the canopy.
  • T. Michioka; H. Takimoto; H. Ono; A. Sato
    Asian Journal of Atmospheric Environment Korean Society for Atmospheric Environment 11 (4) 270 - 282 2287-1160 2017/12 [Refereed]
     
    To evaluate the deposition amount on a ground surface, mesoscale numerical models coupled with atmospheric chemistry are widely used for larger horizontal domains ranging from a few to several hundreds of kilometers however, these models are rarely applied to high-resolution simulations. In this study, the performance of a dry and wet deposition model is investigated to estimate the amount of deposition via computational fluid dynamics (CFD) models with high grid resolution. Reynolds-averaged Navier-Stokes (RANS) simulations are implemented for a cone and a two-dimensional ridge to estimate the dry deposition rate, and a constant deposition velocity is used to obtain the dry deposition flux. The results show that the dry deposition rate of RANS generally corresponds to that observed in wind-tunnel experiments. For the wet deposition model, the transport equation of a new scalar concentration scavenged by rain droplets is developed and used instead of the scalar concentration scavenged by raindrops falling to the ground surface just below the scavenging point, which is normally used in mesoscale numerical models. A sensitivity analysis of the proposed wet deposition procedure is implemented. The result indicates the applicability of RANS for high-resolution grids considering the effect of terrains on the wet deposition.
  • Takenobu Michioka; Hiroshi Takimoto; Hiroki Ono; Ayumu Sato
    BOUNDARY-LAYER METEOROLOGY SPRINGER 164 (3) 401 - 418 0006-8314 2017/09 [Refereed]
     
    Large-eddy simulation is used to investigate the Reynolds-number dependence of gas dispersion over a wavy wall, because the Reynolds-number dependence is important for reproducing normal flow and gas dispersion in a wind tunnel. The ratio of amplitude to wavelength of the wavy surface is set to 0.1, and the Reynolds number based on the bulk velocity and the channel height is varied from to . Two tracer gases are emitted from point sources located at a single crest and trough of the wavy wall. For the lowest Reynolds number, the flow over the wavy wall separates behind the crest and reattaches to the upslope. A recirculation zone is observed near the trough, and the gas emitted from the trough is transported upwind by the recirculating reverse flow. Some gas is discharged from the valley by intermittent velocity bursts that originate in the recirculation zone. As the Reynolds number is increased, the recirculation zone shrinks and the flow increasingly follows the wavy wall. The gas generally disperses in the forward direction and is discharged by the advective flow. As for the gas emitted from the crest, this disperses with the separating flow, while some gas is trapped within the recirculation zone at the lower Reynolds number. As the Reynolds number is increased, the gas advection increasingly follows the wavy wall and the height of the peak concentration approaches the wavy wall. In addition, the accumulated concentration within the valley in both sources depends strongly on the Reynolds number.
  • 小野浩己; 瀧本浩史; 佐藤歩; 道岡武信; 道岡武信; 佐田幸一
    大気環境学会誌 Japan Society for Atmospheric Environment 52 (1) 19‐29 - 29 1341-4178 2017/01 [Refereed]
     

    A numerical model for the environmental impact assessment of geothermal power plants was developed. The model was based on the Large Eddy Simulation (LES) that accurately takes into account the effects of plume rise, surrounding buildings and geophysical features. A new grid generation program for the LES was also included in the model. We carried out wind tunnel experiments to validate our numerical model. Based on the validation results, our numerical model represented the characteristics of the surface concentration obtained by the wind tunnel experiments. We concluded that our numerical model was applicable for the environmental impact assessment of geothermal power plants as an alternative to wind tunnel experiments.

  • 鈴木直弥; 早稲田卓爾; 道岡武信; 辰巳公亮; 竹本剛志
    海洋理工学会誌(Web) 23 (1) 11‐19(J‐STAGE)  2188-3262 2017 [Refereed]
  • Takenobu Michioka; Hiroshi Takimoto; Hiroki Ono; Ayumu Sato
    BOUNDARY-LAYER METEOROLOGY SPRINGER 160 (1) 185 - 199 0006-8314 2016/07 [Refereed]
     
    Large-eddy simulation is conducted to investigate the effect of fetch on the pollutant-removal mechanism from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. The line sources were placed within the first, second, third, fifth, seventh and tenth canyons, and the six tracer gases are simultaneously released by a ground-level continuous pollutant line source placed parallel to the spanwise axis at the canyons. The mean concentration and the deviation of the concentration fluctuation within the canyon roughly reach a near-constant value downwind of the seventh canyon, which is similar to the behaviour of the turbulent intensities. In the first canyon, pollutant removal is affected by both advective flow and turbulent flow; however, the turbulent motions mainly affect pollutant removal from the top of the canyon as the fetch increases. In the first and third canyons, the low-momentum fluid does not always affect pollutant removal, but does so gradually as the fetch increases.
  • 小野浩己; 瀧本浩史; 道岡武信; 佐藤歩; 佐田幸一
    大気環境学会誌 Japan Society for Atmospheric Environment 51 (3) 161 - 173 1341-4178 2016/05 [Refereed]
     

    A number of automatic grid generators for numerical simulations has been developed in recent years. These utilities and CFD code using an unstructured grid enabled us to calculate the flow and dispersion over complex terrains with buildings. However, no reports are available regarding the large eddy simulations (LES) of contaminant dispersion using these utilities. This study was made to clarify the applicability of these grids to the simulation of flow and contaminant dispersion over a two-dimensional hill compared to the legacy orthogonal grids. We found that the characteristics of the separated flow were affected by the grid types near the terrain. Consequently, the contaminant dispersion was also strongly affected by the grid types due to the differences in the separated flow. We concluded that the grids near terrains should be parallel to the flow direction for the accurate predictions of contaminant dispersion.

  • Kazutaka Takata; Takenobu Michioka; Ryoichi Kurose
    ATMOSPHERE MDPI AG 7 (4) 2073-4433 2016/04 [Refereed]
     
    Heated moist air from a cooling tower forms a visible plume and needs to be predicted, not only for the performance design of the cooling tower, but also for environmental impact assessments. In this study, a computational fluid dynamics analysis is conducted to predict the scale of a visible plume rising from a cross flow cooling tower with mechanical draft (provided by a rotating fan). The results of computational fluid dynamics analysis are verified by comparing predictions with an actual observed plume. The results show that the predicted visible plume represents the observed plume in an error range of 15%-20%, which is permissible for designing a cooling tower. Additionally, the mixing condition of heated dry air and moist air under dry and wet combined operation is examined, and the condition is thought to affect the scale of the visible plume. It is found that, in the case of a mechanical-draft cooling tower, the fan has a mixing function which performs the complete mixing of wet and dry air, and this suggests that the generation of the plume can be determined by the intersection of the operation line and saturation line. Additionally, the effect of external wind on the scale of the visible plume is large, especially for dry and wet combined operation.
  • Takenobu Michioka; Koichi Sada; Kazuki Okabayashi
    ATMOSPHERE MDPI AG 7 (4) 2073-4433 2016/04 [Refereed]
     
    Wind-tunnel experiments were conducted to evaluate the effect of additional structure (building, sea wall and banking) on the effective stack height, which is usually used in safety analyses of nuclear power facilities in Japan. The effective stack heights were estimated with and without the additional structure in addition to the reactor building while varying several conditions such as the source height, the height of additional structure and the distance between the source position and the additional structure. When the source height is equivalent to the reactor building height, the additional structure enhances both the vertical and horizontal gas dispersion widths and decreases the ground gas concentration, and it means that the additional structure does not decrease the effective stack height. When the source height is larger than the reactor height, the additional structures might affect the effective stack height. As the distance between the source and the additional structure decreases, or as the height of the additional structure increases, the structure has a larger effect on the effective stack height.
  • 瀧本浩史; 小野浩己; 佐藤歩; 道岡武信; 佐田幸一
    大気環境学会誌 50 (5) 226 - 232 1341-4178 2015/09 [Refereed]
  • Hiroki Ono; Hiroshi Takimoto; Takenobu Michioka; Ayumu Sato
    Journal of Environmental Engineering (Japan) Architectural Institute of Japan 80 (718) 1143 - 1151 1881-817X 2015 [Refereed]
     
    Validation of Large Eddy Simulation based on a Finite Volume Method was undertaken using data from our previous experiment. Four schemes for convection-term discretization were tested on a momentum transport equation, a passive scalar transport equation and an active scalar transport equation. The results support the use of a low-dissipation scheme for the momentum transport equation to avoid overestimating circulation on a rooftop. Such overestimation of circulation can lead to an underestimation of concentrations behind the building. Conversely, a bounded scheme, even in cases that resulted in excess numerical dissipation, is preferred for each scalar transport equation to maintain a profile and peak concentration value.
  • Hiroki Ono; Hiroshi Takimoto; Takenobu Michioka; Auymu Sato
    Journal of Environmental Engineering (Japan) Architectural Institute of Japan 79 (701) 607 - 613 1881-817X 2014/07 [Refereed]
     
    Wind tunnel experiments were carried on the concentration fields with rooftop exhaust under neutral and stably stratified flow. Mean velocity distribution behind the building was not affected by atmospheric stability, nevertheless turbulence intensity and contaminant distribution became quite different. Partial spread width was estimated with assuming Gaussian distribution therefore it was found that strength of thermal stratification behind the building was attenuated locally by wake from the building.
  • Takenobu Michioka; Hiroshi Takimoto; Ayumu Sato
    BOUNDARY-LAYER METEOROLOGY SPRINGER 150 (2) 259 - 275 0006-8314 2014/02 [Refereed]
     
    Large-eddy simulations were conducted to investigate the mechanism of pollutant removal from a three-dimensional street canyon. Five block configurations with aspect ratios (building height to length) of 1, 2, 4, 8 and were used to create an urban-like array. A pollutant was released from a ground-level line source at the centre of the target canyon floor. For smaller aspect ratios, the relative contribution of the turbulent mass flux to net mass flux at the roof level, which was spatially averaged along the roof-level ventilation area, was closer to unity, indicating that turbulent motions mainly affected pollutant removal from the top of the canyon. As aspect ratio increased, the relative contribution became smaller, owing to strong upwind motions. However, the relative contribution again reached near unity for the infinite aspect ratio (i.e. a two-dimensional street canyon) because of lowered lateral flow convergence. At least 75 % of total emissions from the three-dimensional street canyon were attributable to turbulent motions. Pollutant removal by turbulent motions was related to the coherent structures of low-momentum fluid above the canyons. Though the coherent structure size of the low-momentum fluid differed, the positions of low-momentum fluid largely corresponded to instantaneous high concentrations of pollutant above the target canyon, irrespective of canyon geometry.
  • TANNO Kenji; NODA Naoki; YAMAMOTO Toru; KUROSE Ryoichi; MICHIOKA Takenobu; MAKINO Hisao
    粉体工学会誌 The Society of Powder Technology, Japan 50 (10) 733 - 740 0386-6157 2013/10 [Refereed]
     
    In coal-fired power plants, selective catalytic de-NOx systems are widely used and offer a very efficient method to reduce NOx emissions from such plants. However, de-NOx catalyst degrades in long-term operation and catalysts must be periodically exchanged. Exchange of catalysts costs a great deal, hence there is a strong economic incentive to elucidate the degradation mechanism of catalysts and optimize the operation conditions of de-NOx equipment. In this study, fluorescent X-ray analysis is performed to investigate the elemental distribution on the catalyst surface. Also, X-ray absorption fine structure (XAFS) analysis is applied to determine the chemical form of Vanadium in the catalyst. The results show that elements found in fly ash are detectable in the upstream region near the inlet, whereas such elements are not detected in the downstream region near the outlet. This tendency is in good qualitative agreement with the author's previous numerical result. Vanadium in the fresh catalyst is mainly in its 3+ valence state, whereas that in used catalyst is mainly in the 4+ or 5+ valence states. The ratio of vanadium in 5+ valence state is larger in the inlet region than in the downstream region. This is due to turbulent eddy mixing which promotes vanadium oxidation in the inlet region.
  • Kenji Tanno; Ryoichi Kurose; Takenobu Michioka; Hisao Makino; Satoru Komori
    ADVANCED POWDER TECHNOLOGY ELSEVIER SCIENCE BV 24 (5) 879 - 885 0921-8831 2013/09 [Refereed]
     
    Selective catalytic reaction is a very efficient method to reduce NOx emissions from thermal power plants and is widely used in Japan. To develop a higher performance de-NOx system and optimize its maintenance schedule, it is important to understand the NOx reduction mechanism in the honeycomb channel which supports the de-NOx catalysts. In this study, the effects of duct channel flow behavior on the de-NOx, reaction at the catalyst surface were investigated using a direct numerical simulation (DNS). The DNS computations were performed for three inflow conditions, one laminar and two turbulent. The results show that although the flow transitions from turbulent to laminar flow as the flow moves downstream for the turbulent inflow conditions, de-NOx reaction rates for the turbulent inflow conditions are higher than that for the laminar inflow condition even in the downstream region. This is because of the remaining cross-sectional fluid motions caused by the inflow turbulence. As a result, de-NOx efficiencies for the turbulent conditions are higher than that for the laminar case. For both laminar and turbulent inflow conditions, de-NOx reaction is suppressed in the corner regions due to the flow stagnation. (C) 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
  • T. Michioka; A. Sato; K. Sada
    ATMOSPHERIC ENVIRONMENT PERGAMON-ELSEVIER SCIENCE LTD 75 153 - 162 1352-2310 2013/08 [Refereed]
     
    A microscale large-eddy simulation (LES) model coupled to a mesoscale LES model is implemented to estimate a ground concentration considering the meteorological influence in an actual urban district. The microscale LES model is based on a finite volume method with an unstructured grid system to resolve the flow structure in a complex geometry. The Advanced Regional Prediction System (ARPS) is used for mesoscale meteorological simulation. To evaluate the performance of the LES model, 1-h averaged concentrations are compared with those obtained by field measurements, which were conducted for tracer gas dispersion from a point source on the roof of a tall building in Tokyo. The concentrations obtained by the LES model without combing the mesoscale LES model are in quite good agreement with the wind-tunnel experimental data, but overestimates the 1-h averaged ground concentration in the field measurements. On the other hand, the ground concentrations using the microscale LES model coupled to the mesoscale LES are widely distributed owing to large-scale turbulent motions generated by the mesoscale LES, and the concentrations are nearly equal to the concentrations from the field measurements. (C) 2013 Elsevier Ltd. All rights reserved.
  • Hiroshi Takimoto; Atsushi Inagaki; Manabu Kanda; Ayumu Sato; Takenobu Michioka
    Boundary-Layer Meteorology 147 (2) 217 - 236 0006-8314 2013/05 [Refereed]
     
    We examine the similarity of turbulent organized structures over smooth and very rough wall flows. Turbulent flow fields in horizontal cross-sections were measured using particle image velocimetry, and the characteristics of turbulent organized structures over four types of surfaces were investigated. Measurements were conducted at several measurement heights across the internal boundary layer. The length and width of turbulence structures were quantified using a two-point correlation method. We selected two thresholds of two-point correlation coefficients to consider both large-scale and small-scale structures the validity of these choices was examined through the analyses using proper orthogonal decomposition. For large-scale structures, the length and aspect ratios (streamwise length/spanwise width) of structures were highly correlated with the velocity gradient for each measurement height and boundary-layer thickness. This relationship was also examined in the results of previous studies, and the scaling of the aspect ratio with the non-dimensional velocity gradient again showed the importance of the velocity gradient, with slight differences found between smooth and rough surfaces. In contrast, the small-scale structures exhibited weak dependency on the velocity gradient and boundary-layer thickness. Instantaneous snapshots of turbulent organized structures at the same shear level also displayed differences in small-scale structures, but the structures of the organized motions resembled each other, as in the results of the two-point correlation method. © 2012 Springer Science+Business Media Dordrecht.
  • TANNO Kenji; KUROSE Ryoichi; MICHIOKA Takenobu; MAKINO Hisao; KOMORI Satoru
    粉体工学会誌 The Society of Powder Technology, Japan 50 (3) 204 - 211 0386-6157 2013/03 [Refereed]
     
    In coal-fired thermal power plants, selective catalytic de-NOx system is widely used and very efficient method to reduce NOx emission from the plant. However, ash particles laden in flue gas adhere on a de-NOx catalyst surface, and ash coverage causes the severe degradation of de-NOx catalyst. Authors numerically investigated the effect of flow behavior in a honeycomb channel on the adhesion characteristics of particles to the wall in the previous study. However, previous study was performed under the assumption that all particles, which reach the wall, adhere. Therefore, in this study, the effect of particle collision and rebound behavior on the adhesion characteristics on the wall of honeycomb shaped catalyst was investigated by applying a direct numerical simulation (DNS). In order to take account of particle collision and rebound behavior, the probability coefficient of adhesion was introduced. The results show that although particle adheres on the wall only in the upstream region, in which flow condition is turbulent, irrespective of adhesion probability coefficient, the amount of adhered particles decreases with decreasing probability coefficient. The results also indicate that particle re-adhesion also occurs only in the upstream region even though some particle rebound and return to the flow from the wall.
  • Takenobu Michioka; Ayumu Sato
    BOUNDARY-LAYER METEOROLOGY SPRINGER 145 (3) 469 - 484 0006-8314 2012/12 [Refereed]
     
    Large-eddy simulations are conducted to investigate the effects of the incoming turbulent structure of the flow on pollutant removal from an ideal canyon. The target canyon is a two-dimensional street canyon with an aspect ratio of 1.0 (building height to street width). Three turbulent flows upwind of the street canyon are generated by using different block configurations, and a tracer gas is released as a ground-level line source at the centre of the canyon floor. Mean velocity profiles for the three flows are similar, except near the roof. However, the root-mean-square values of the velocity fluctuations and the Reynolds shear stress increase with the friction velocity of the incoming turbulent flow. The spatially-averaged concentration within the canyon decreases with increasing friction velocity. Coherent structures of low-momentum fluid, generated above the upwind block configurations, contribute to pollutant removal, and the amount of pollutant removal is directly related to the size of the coherent structure.
  • SADA Koichi; SATO Ayumu; MICHIOKA Takenobu; ICHIKAWA Yoichi
    大気環境学会誌 = Journal of Japan Society for Atmospheric Environment 大気環境学会 47 (6) 261 - 269 1341-4178 2012/11 [Refereed]
     
    A three-dimensional computational fluid model is applied to the atmospheric diffusion evaluation of stack gas emitted from thermal power plants under complex terrain conditions for environmental assessment in Japan. Recently, the construction of many low stacks has been attempted instead of high stacks, due to the improvements in the thermal power plants facilities and the decrease in pollutant gas levels with the selection of high-quality fuels. In this study, a numerical simulation model of the diffusion of gas emitted from low stacks near the ground surface was developed using the nonhydrostatic formulation for flow, which takes into consideration the effects of terrain near the ground surface. Prior to the application of the developed numerical model to low stacks near the ground surface, the diffusion of gas emitted from high stacks, such as those of conventional thermal power stations, was simulated in this study. The surface concentrations of the stack gas were simulated and compared to those obtained in wind tunnel experiments. Furthermore, the ratios of the maximum concentration and its observed distance to those obtained under flat-plate conditions were also compared with other calculation results (Ichikawa and Sada, 2002) and wind tunnel experimental results. It was apparent that almost the same ratios could be obtained using the numerical simulation model developed in this study.
  • TANNO Kenji; KUROSE Ryoichi; MICHIOKA Takenobu; MAKINO Hisao; KOMORI Satoru
    Journal of the Research Association of Powder Technology, Japan The Society of Powder Technology, Japan 49 (10) 738 - 744 0386-6157 2012/10 [Refereed]
     
    In a coal-fired thermal power plant, ash particles laden in flue gas adhere on de-NOx catalyst surface, and ash coverage causes the degradation of de-NOx catalyst. Therefore, in this study, effect of the flow in a honeycomb rectangular channel, which represents a single channel of de-NOx catalyst, on the adhesion characteristics of particles to the wall was investigated by applying a direct numerical simulation (DNS). Results show that the adhesion of particles is strongly affected by the flow. The particle adhesion is enhanced by turbulence near the inlet of channel, whereas suppressed due to the attenuation of turbulence in the downstream region. The particle adhesion also depends on particle diameter. Small particles adhere on the wall only in the upstream region, whereas large particles adhere all the way down to outlet, nevertheless flow transients to laminar. This is due to the fact that large particles keep the inertial force even in the downstream region. Moreover, the cross sectional position of particle adhesion also depends on both flow stagnation near the corner and a secondary flow which is driven on the plane normal to the streamwise direction.
  • MICHIOKA Takenobu; SATO Ayumu
    大気環境学会誌 = Journal of Japan Society for Atmospheric Environment 大気環境学会 47 (3) 119 - 126 1341-4178 2012/05 [Refereed]
     
    This study focuses on the performance of a Reynolds-averaged Navier-Stokes simulation (RANS) for air flow and gas dispersion in urban districts. The present numerical simulation is based on a finite volume method with an unstructured grid system to resolve the flow structure in a complex geometry. To evaluate the performance of RANS, the results are compared to those obtained by wind-tunnel experiments. The wind-tunnel experiments are conducted for gas dispersion from point sources in a complex residential area. The results show that RANS tends to underestimate the mean velocity and the turbulent intensities behind the relatively-high buildings. In addition, the mean concentration is strongly sensitive to the turbulent Schmidt number, which is the selective parameter appearing in the model of turbulent scalar fluxes, and RANS can be used to predict the concentration only when the optimum empirical value of the turbulent Schmidt number is selected.
  • Ryoichi Kurose; Naohisa Takagaki; Takenobu Michioka; Naoki Kohno; Satoru Komori
    AICHE JOURNAL WILEY-BLACKWELL 58 (2) 377 - 384 0001-1541 2012/02 [Refereed]
     
    The subgrid scale (SGS) variance for a high-Schmidt-number passive scalar of Sc >> 1 is measured using a high-resolution planar laser-induced fluorescence technique in a grid-generated turbulent liquid flow, and the values of the model coefficients in the scale-similarity model and the scalar-gradient model used for estimating the SGS scalar variance are experimentally evaluated. The results show that for both models, the measured values are much larger than the well-known values obtained in the previous studies done for non-high-Sc scalars of Sc congruent to 1. Similarly, the measured value of the model coefficient in the scalar-gradient model tends to be larger than the value estimated by the dynamic procedure. The increases in the measured values of the model coefficients for the high-Sc scalar can be explained by the presence of the viscous-convective range showing a nearly (-1)-slope in the high-wavenumber range of the power spectrum of concentration fluctuation. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 377-384, 2012
  • MICHIOKA Takenobu; SATO Ayumu
    PROCEEDINGS OF THE JAPANESE CONFERENCE ON HYDRAULICS Japan Society of Civil Engineers 68 (4) I_1795 - I_1800 2185-467X 2012 [Refereed]
     
    Large-eddy simulation was implemented for airflow and thermal diffusion in an urban canyon. To evaluate the performance of the LES, the mean velocities, standard deviations of turbulent fluctuations and mean temperature estimated by the LES are compared with those obtained by wind-tunnel experiments. The results show that the LES can capture the flow and temperature within the canyon, and accurately represent the effect of the wall heating on flow within the canyon. Though the previous studies only focused on the vertical cross section, it is found that two axisymmetrical vortexes are formed in the horizontal cross section with a large clockwise vortex in vertical cross section irrespective of the wall heating conditions. The horizontal vortexes are enlarged by wall heating, and horizontal turbulent fluctuations are increased. Thus, the thermal environment within the canyon is affected not only by the vertical large vortex, but also by the horizontal turbulent flow.
  • Takenobu Michioka; Ayumu Sato; Koichi Sada
    BOUNDARY-LAYER METEOROLOGY SPRINGER 141 (1) 35 - 51 0006-8314 2011/10 [Refereed]
     
    Large-scale turbulent motions enhancing horizontal gas spread in an atmospheric boundary layer are simulated in a wind-tunnel experiment. The large-scale turbulent motions can be generated using an active grid installed at the front of the test section in the wind tunnel, when appropriate parameters for the angular deflection and the rotation speed are chosen. The power spectra of vertical velocity fluctuations are unchanged with and without the active grid because they are strongly affected by the surface. The power spectra of both streamwise and lateral velocity fluctuations with the active grid increase in the low frequency region, and are closer to the empirical relations inferred from field observations. The large-scale turbulent motions do not affect the Reynolds shear stress, but change the balance of the processes involved. The relative contributions of ejections to sweeps are suppressed by large-scale turbulent motions, indicating that the motions behave as sweep events. The lateral gas spread is enhanced by the lateral large-scale turbulent motions generated by the active grid. The large-scale motions, however, do not affect the vertical velocity fluctuations near the surface, resulting in their having a minimal effect on the vertical gas spread. The peak concentration normalized using the root-mean-squared value of concentration fluctuation is remarkably constant over most regions of the plume irrespective of the operation of the active grid.
  • Ryoichi Kurose; Takenobu Michioka; Naoki Kohno; Satoru Komori; Yuya Baba
    AICHE JOURNAL WILEY-BLACKWELL 57 (4) 911 - 917 0001-1541 2011/04 [Refereed]
     
    Large-eddy simulations using the flamelet models are applied to turbulent reacting liquid flows and validated by comparing with the experiments. The computations are performed for two reaction conditions, namely a rapid reaction and a moderately fast reaction in a grid-generated turbulent flow. For the flamelet models, both the steady flamelet model and the unsteady Lagrangian flamelet model are tested. A second-order, irreversible, and isothermal reaction is considered. The results show that the flamelet models inherently developed for turbulent combustion are applicable to turbulent reacting liquid flows, provided that the model coefficient in evaluating the subgrid scale variance of mixture fraction in the scale-similarity model is set to be 5.0. The rapid reaction can be adequately predicted by both the steady and unsteady Lagrangian flamelet models, whereas the moderately fast reaction can be predicted only by the unsteady Lagrangian flamelet model which is capable to take slow chemical processes into account. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 911-917, 2011
  • Takenobu Michioka; Ayumu Sato; Hiroshi Takimoto; Manabu Kanda
    BOUNDARY-LAYER METEOROLOGY SPRINGER 138 (2) 195 - 213 0006-8314 2011/02 [Refereed]
     
    Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.
  • A. Sato; T. Michioka; H. Takimoto
    INTERNATIONAL JOURNAL OF ENVIRONMENT AND POLLUTION INDERSCIENCE ENTERPRISES LTD 47 (1-4) 184 - 192 0957-4352 2011 [Refereed]
     
    Field experiments of air flow and pollution dispersion within a street canyon in the Comprehensive Outdoor Scale Model (COSMO) facility were conducted to investigate the effect of the mean and turbulent flow above the canyon on pollutant dispersion inside the canyon. Tracer gas concentration was measured using fast-response photo ionisation detectors. Three-dimensional wind velocity components were also simultaneously measured using ultrasonic anemometers within and above the same canyon. Using both tracer gas concentration and velocity fluctuation data, we investigated the effects of atmospheric turbulence above the height of the buildings on pollution dispersion within the street canyon.
  • Takenobu Michioka; Ayumu Sato; Yasuo Hattori
    Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B Japan Society of Mechanical Engineers 76 (765) 823 - 829 0387-5016 2010 [Refereed]
     
    A wind tunnel experiment was conducted to simulate large-scale turbulent motions in an atmospheric boundary layer (ABL). The large-scale turbulent motions originating from outer-layer disturbance were generated using an active grid installed at the front of the test section and the winglets of the active grid were randomly rotated. The large-scale horizontal motions in the ABL were generated in the wind tunnel using the active grid when appropriate values of the angle and rotation speed were chosen. The validity of the present method using the active grid was confirmed by comparing the integral scale and energy spectra obtained by the experiments with empirical formulas inferred from field observations. This indicates that the present method of using the active grid accurately simulates the turbulent motions in a nearly neutral ABL.
  • MICHIOKA Takenobu; SATO Ayumu; SADA Koichi
    Transactions of the Japan Society of Mechanical Engineers. B The Japan Society of Mechanical Engineers 75 (749) 93 - 100 0387-5016 2009/01 [Refereed]
     
    We developed a method for a wind tunnel experiment to predict a visible plume region from a mechanical-draft cooling tower. The diffusions of water vapor and temperature emitted from the cooling tower in the wind tunnel experiment are tracked using tracer gas. It is assumed that the plume-induced fog is generated whenever the instantaneous specific humidity predicted from the concentration of the tracer gas at measuring points is larger than the inferred saturation specific humidity. To estimate the accuracy of the present method, the measurements are compared with the observations. The results show that the visible plume length and height are in good agreement with the observations. Furthermore, it is found that the visible length and height is strongly affected by wind direction to the cooling tower, temperature difference between atmosphere and exit of the cooling tower and atmospheric humidity, and hence the present wind tunnel method considered these conditions is valid to predict the visible plume.
  • 河野 直紀; 黒瀬 良一; 小森 悟; 道岡 武信; 馬場 雄也
    化学工学会 研究発表講演要旨集 公益社団法人 化学工学会 2009 118 - 118 2009
  • Koichi Sada; Sumito Komiyama; Takenobu Michioka; Yoichi Ichikawa
    Transactions of the Atomic Energy Society of Japan Atomic Energy Society of Japan 8 (2) 184 - 196 1347-2879 2009 [Refereed]
     
    A numerical simulation method has been developed to predict atmospheric flow and stack gas diffusion, considering the buildings and complex terrain located near and relatively far from a stack, respectively. The turbulence closure technique was used for flow calculation, some calculation grids on the ground near a stack were treated as buildings, and stack gas diffusion was predicted using the Lagrangian particle model. The calculated flow and stack gas diffusion results were compared with those obtained by wind tunnel experiments under actual terrain containing buildings. Effective stack height was estimated by comparing the surface concentration along the plume axis with those under a flat-plate condition, and it was apparent that the effective stack heights estimated by calculations were almost the same as those obtained by the wind tunnel experiment. Then, the effective dose and relative concentration of stack gas were calculated using the effective stack heights obtained by a numerical model. Almost the same effective dose and relative concentration were obtained when compared with those using the effective stack height obtained by wind tunnel experiment. © lAtomic Energy Society of Japan.
  • Takenobu Michioka; Ayumu Sato; Koichi Sada
    Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B Japan Society of Mechanical Engineers 75 (752) 604 - 611 0387-5016 2009 [Refereed]
     
    A Large eddy simulation (LES) was developed to predict a visible plume region from a mechanical -draft cooling tower. This numerical model solves the equations for conservation of mass, momentum, energy, vapor and liquid water. Itis assumed that the visible plume is generated whenever the instantaneous liquid water mixing ratio is existed. To estimate the accuracy of the present numerical model, the predictions of visible plume length and height are compared with observations. The results show the visible plume length and height are in good agreement with the observations. Furthermore, we investigate the basic behavior of the visible plume and temperature emitted from the cooling tower under severe metrological condition. It is found that whether the visible plume reaches the ground or not mostly depends on the wind angle to the cooling tower, and temperature emitted from the cooling tower has little influence on atmosphere environment on the ground.
  • Shimota Akiro; Michioka Takenobu; Sada Koichi; Sato Ayumu; Kanzaki Takao; Wada Koji; Ichikawa Yoichi
    Journal of Japan Society of Air Pollution Japan Society for Atmospheric Environment 44 (5) 255 - 261 1341-4178 2009 [Refereed]
     
    Recently, power plant's cooling towers equipped with instruments for preventing plume to visualize, which is a mechanical-draft cooling tower, tend to be planned and be constructed in Japan. Traditional ways of environmental assessment for a cooling tower has adopted a visible plume prediction model based on a natural-draft cooling tower. The requirement of developing new visible plume model having capability of prediction of visible plume, temperature, moisture and droplet from a mechanical-draft cooling tower is rising up. A field campaign to obtain data from a power plant with a mechanical-draft cooling tower for evaluation and validation of a new visible plume model and some conventional models is conducted. Measurements of surface temperature and humidity distribution around the plant does not show a clear effect of visible plume. Over 100 pairs of visible plume height and length are obtained by using video cameras and thermographyies Furthermore, droplet from the cooling tower shows very few amounts at a ground because of adopting to it a eliminator for preventing droplet scattering.
  • Michioka Takenobu; Sato Ayumu; Shimota Akiro; Sada Koichi; Ichikawa Yoichi; Okura Arata
    Journal of Japan Society of Air Pollution Japan Society for Atmospheric Environment 44 (5) 227 - 235 1341-4178 2009 [Refereed]
     
    A new visible plume model is developed to predict visible plume, temperature, moisture and droplet from a mechanical-draft cooling tower. The developed model is a normal Gaussian plume model for moisture and temperature, considering fan configuration, wet-dry combined cooling tower system, vertical atmospheric temperature and atmospheric stability. The accuracy of previous models (FOG, SACTI) and the developed model is investigated to compare the model results with observation conducted in Japan. The results show that the previous models can not accurately predict the visible plume length and height because they have mainly developed for a natural-drift cooling tower, which is different type from a mechanical-draft cooling tower. On the other hand, the visible plume length and height predicted by the developed model are in good agreement with the observation, and statistical data analysis also shows that the model provides better performance than the previous models. Furthermore, the model predicts not only the visible plume region but also temperature, moisture and droplet from the cooling tower.
  • Michioka Takenobu; Sato Ayumu; Sada Koichi; Shimota Akiro; Ichikawa Yoichi
    Journal of Japan Society of Air Pollution Japan Society for Atmospheric Environment 44 (3) 147 - 154 1341-4178 2009 [Refereed]
     
    We developed new plume rise and diffusion models for gas dispersion from a mechanical-draft cooling tower. Wind tunnel experiments were conducted to investigate effects of fan location and cooling tower configuration on gas dispersion from the cooling towers. The results show that cooling tower configuration does not affect the plume rise and diffusion, but the fan location and numbers strongly affect them. The plume rise increases with the fun number in case of the fans located in downwind direction, but it is comparable level to the single fan in case of orthogonally-aligned fans to the wind direction. In addition, the vertical and horizontal diffusion becomes large as the fan number increases. These effects are not considered by the previous Gaussian type model (FOG model), generating the large difference of the plume rise and dispersion between the model and the wind tunnel experiment. On the other hand, the developed models considering the fan location and number can accurately estimate the plume rise and diffusion from the mechanical-draft cooling towers.
  • Takenobu Michioka; Fotini Katopodes Chow
    Journal of Applied Meteorology and Climatology AMER METEOROLOGICAL SOC 47 (12) 3150 - 3169 1558-8424 2008/12 [Refereed]
     
    This paper presents high-resolution numerical simulations of the atmospheric flow and concentration fields accompanying scalar transport and diffusion from a point source in complex terrain. Scalar dispersion is affected not only by mean flow, but also by turbulent fluxes that affect scalar mixing, meaning that predictions of scalar transport require greater attention to the choice of numerical simulation parameters than is typically needed for mean wind field predictions. Large-eddy simulation is used in a mesoscale setting, providing modeling advantages through the use of robust turbulence models combined with the influence of synoptic flow forcing and heterogeneous land surface forcing. An Eulerian model for scalar transport and diffusion is implemented in the Advanced Regional Prediction System mesoscale code to compare scalar concentrations with data collected during field experiments conducted at Mount Tsukuba, Japan, in 1989. The simulations use horizontal grid resolution as fine as 25 m with up to eight grid nesting levels to incorporate time-dependent meteorological forcing. The results show that simulated ground concentration values contain significant errors relative to measured values because the mesoscale wind typically contains a wind direction bias of a few dozen degrees. Comparisons of simulation results with observations of arc maximum concentrations, however, lie within acceptable error bounds. In addition, this paper investigates the effects on scalar dispersion of computational mixing and lateral boundary conditions, which have received little attention in the literature-in particular, for high-resolution applications. The choice of lateral boundary condition update interval is found not to affect time-averaged quantities but to affect the scalar transport strongly. More frequent updates improve the simulated ground concentration values. In addition, results show that the computational mixing coefficient must be set to as small a value as possible to improve scalar dispersion predictions. The predicted concentration fields are compared as the horizontal grid resolution is increased from 190 m to as fine as 25 m. The difference observed in the results at these levels of grid refinement is found to be small relative to the effects of computational mixing and lateral boundary updates.
  • MICHIOKA Takenobu; KUROSE Ryoichi
    JFST The Japan Society of Mechanical Engineers 3 (5) 610 - 621 1880-5558 2008 
    Large-eddy simulations (LES) are applied to particle-laden swirling jets, and effect of the swirl on particle dispersion is investigated. The trajectories of all particles are individually pursued with a Lagrangian method. The particles with different diameters are uniformly injected into a non-swirling flow or swirling flows with different swirling numbers. The results show that the trajectories of the particles largely differ depending on their diameters. In swirling jets, the peak of particle number density for small particle is located on the central axis, whereas that for larger particles is shifted outward by the centrifugal force. However, the larger particles, which migrate outward in the upstream region, tend to gradually migrate inward toward downstream, and this trend is remarkable as the particle diameter decreases. This is due to the fact that the direction of the particle migration in the downstream region is dominated by the turbulent motions, which act to transport the smaller particles inward in wide range of the swirling jets.
  • Koichi Sada; Takenobu Michioka; Yoichi Ichikawa
    Asian Journal of Atmospheric Environment Korean Society for Atmospheric Environment 2 (1) 1 - 13 2287-1160 2008 [Refereed]
     
    A numerical simulation method has been developed to predict atmospheric flow and stack gas diffusion using a calculation domain of several km around a stack under complex terrain conditions containing buildings. The turbulence closure technique using a modified k-ε-type model under a nonhydrostatic assumption was used for the flow calculation, and some of the calculation grids near the ground were treated as buildings using a terrain-following coordinate system. Stack gas diffusion was predicted using the Lagrangian particle model, that is, the stack gas was represented by the trajectories of released particles. The numerical model was applied separately to the flow and stack gas diffusion around a cubical building and to a two-dimensional ridge in this study, before being applied to an actual terrain containing buildings in our next study. The calculated flow and stack gas diffusion results were compared with those obtained by wind tunnel experiments, and the features of flow and stack gas diffusion, such as the increase in turbulent kinetic energy and the plume spreads of the stack gas behind the building and ridge, were reproduced by both calculations and wind tunnel experiments. Furthermore, the calculated profiles of the mean velocity, turbulent kinetic energy and concentration of the stack gas around the cubical building and the ridge showed good agreement with those of wind tunnel experiments.
  • SATO AYUMU; TAKIMOTO HIROSHI; MICHIOKA TAKENOBU
    PROCEEDINGS OF NATIONAL SYMPOSIUM ON WIND ENGINEERING Japan Association for Wind Engineering 20 (0) 21 - 21 2008 [Refereed]
     
    A wind tunnel experiment was conducted to investigate the effects of unstable stratification on cavity flow behind a cubical building. Mean and fluctuation velocities were measured by both laser Doppler anemometry and particle image velocimetry in the thermally stratified wind tunnel. Although both mean and fluctuation velocity components showed similar profiles in the near wake of the building under neutral and unstable conditions, turbulent intensities showed higher values in the far wake in unstable stratified flow owing to strong vertical mixing. In particular, as the unstable stratification became stronger, the circular vortex behind the trailing edge of the building tended not to be formed because separation vortices shed from the roof were greatly disturbed by buoyancy.
  • Takenobu Michloka; Ayumu Sato; Takao Kanzaki; Kouichl Sada
    JOURNAL OF WIND ENGINEERING AND INDUSTRIAL AERODYNAMICS ELSEVIER SCIENCE BV 95 (8) 741 - 754 0167-6105 2007/08 [Refereed]
     
    We developed a new method for a wind tunnel experiment to predict a visible plume region from a wet cooling tower. The diffusion of water vapor and beat emitted from a cooling tower in a wind tunnel is estimated using a tracer gas. The instantaneous concentration of the tracer gas is measured using high-response flame ionization detectors. A moist plume-induced fog is assumed to be generated whenever the instantaneous specific humidity predicted from the concentration of the tracer gas at measured points is larger than the inferred saturation specific humidity. To confirm the validity of the present method, the results in the wind tunnel experiments are roughly compared with the observations obtained at the mechanical-draft cooling tower of the Benning Road plant. The results show that the visible plume length and height are nearly in agreement with observations and the present method has the capability to predict the visible plume region from the cooling tower. (c) 2007 Elsevier Ltd. All rights reserved.
  • MICHIOKA Takenobu; KUROSE Ryoichi
    Transactions of the Japan Society of Mechanical Engineers Series C The Japan Society of Mechanical Engineers 73 (725) 30 - 37 0387-5016 2007 [Refereed]
     
    Three-dimensional large-eddy simulation (LES) is applied to a particle-laden swirling jet, and the effect of swirl on particle diffusion is investigated. The trajectories of all particles are individually pursued with a Lagrangian method. The particles with different diameters are uniformly injected into the non-swirling and swirling flows with different Swirling numbers. The result shows that the trajectories of the particles with different diameters are quite different. The smaller particles have the peak of the particle number densities at central axis, and diffuse across swirling jet. On the other hand, the larger particles at the exit of the nozzle migrate outward by the centrifugal force, but the particles gradually migrate inward with the axial distance increases. The inward movement of the particle in the downstream region is dominated by turbulent motions, which transport the particles inward near central axis. Furthermore, above behavior is shifted upstream with the particle diameter decrease because the smaller particles are easily transferred by smaller inward fluid motions existing in the wide range of the swirling jet.
  • Kurose Ryoichi; Michioka Takenobu; Makino Hisao; Komori Satoru
    Journal of the Research Association of Powder Technology, Japan The Society of Powder Technology, Japan 44 (2) 107 - 112 0386-6157 2007 [Refereed]
     
    Effect of flow behavior in a honeycomb rectangular channel of a de-NOX catalyst on the adhesion of particles onto the wall is studied using a direct numerical simulation (DNS). The results show that the adhesion of particles is markedly affected by the turbulent-laminar-transition flow and is enhanced by the turbulence in the upstream region near the inlet. This agrees well with the previous experimental observation, in which the degradation of catalyst appeared in the region near the inlet. The adhesion of particles is found to be enhanced by the turbulence especially for larger particles. In the downstream region, the adhesion on the wall near the corners of rectangular channels is suppressed since the turbulent-laminar transition proceeds from these regions.
  • Koichi Sada; Takenobu Michioka; Yoichi Ichikawa
    JSME International Journal, Series B: Fluids and Thermal Engineering 49 (1) 48 - 59 1340-8054 2006/08 
    A numerical simulation method has been developed to predict atmospheric flow and stack gas diffusion under a neutral stratification condition, considering the buildings and complex terrain located near and relatively far from a stack, respectively. The turbulence closure technique was used for flow calculation, some calculation grids on the ground within a plant area were treated as buildings, and stack gas diffusion was predicted using the Lagrangian particle model. The calculated flow and stack gas diffusion results were compared with those obtained by wind tunnel experiments under a neutral stratification condition, and the features of surface concentration, such as the occurrence of downdraft phenomena behind the buildings, were reproduced by both calculations and wind tunnel experiments. Furthermore, effective stack heights were estimated by the comparison of the surface concentration along the plume axis with those under a flat-plate condition, and it was apparent that the effective stack heights estimated by calculations were almost the same as those obtained by wind tunnel experiments. Copyright © 2006 by The Japan Society of Mechanical Engineers.
  • T Michioka; R Kurose; K Sada; H Makino
    INTERNATIONAL JOURNAL OF MULTIPHASE FLOW PERGAMON-ELSEVIER SCIENCE LTD 31 (7) 843 - 866 0301-9322 2005/07 [Refereed]
     
    A direct numerical simulation (DNS) is applied to a particle-laden turbulent mixing layer with a chemical reaction, and the effects of particles on turbulence and chemical species' diffusion and reaction in both zero and finite gravity cases are investigated. Unreactive particles, whose response time, tau(P), is smaller than the Kolmogorov time scale, tau(K) [tau(P)/tau(K) = O(10(-1))], are uniformly injected into the high-speed side of the mixing layer. Two reactive chemical species are separately introduced through different sides. The results show that although laden particles generally depress turbulent intensities, they begin to enhance turbulent intensities downstream as the particle size decreases provided that the inlet particle volume fraction is fixed. This is because that the small particles with small particle response time easily accumulate at the circumference of coherent vortices and act to suppress the growth of primitive small-scale coherent vortices upstream but enhance that of relatively developing large-scale ones downstream. Also, since the small-scale turbulence, which promotes the chemical reaction, is suppressed by the laden particles in the entire region, chemical product decreases overall. Furthermore, the presence of finite gravity on the particles acts to depress the turbulent intensities, but its effects on chemical species' diffusion and reaction are quite small. (c) 2005 Elsevier Ltd. All rights reserved.
  • T Michioka; S Komori
    AICHE JOURNAL JOHN WILEY & SONS INC 50 (11) 2705 - 2720 0001-1541 2004/11 [Refereed]
     
    A subgrid-scale (SGS) model for the filtered reaction source term is presented to develop the large-eddy simulation (LES) of a nonpremixed, turbulent liquidflow with a moderately fast reaction. The SGS model is based on the SGS probability density function (PDF) and SGS conditional expectation. The SGS probability density function (SGS-PDF) is assumed to follow a beta distribution and a simple algebraic model for the SGS conditional expectation is developed using the filtered data obtained from the direct numerical simulation (DNS) of stationary isotropic liquid turbulence with a second-order chemical reaction. For a rapid reaction, the SGS-PDF model based on the conserved scalar is used as the SGS model. The LES based on these SGS models is applied to a liquid mixing layer flow downstream of a turbulence-generating grid with a chemical reaction, and the LES predictions of the mean concentration and concentration variance are directly compared with the previous measurements for both moderately fast and rapid reactions to examine the proposed SGS models. The results show that the predictions by the LES are in good agreement with the measurements and the present LES can well describe the diffusive-reactive process in a turbulent reacting liquidflow. (C) 2004 American Institute of Chemical Engineers.
  • ONISHI Ryo; MICHIOKA Takenobu; NAGATA Koji; KOMORI Satoru
    TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series B The Japan Society of Mechanical Engineers 69 (679) 636 - 643 0387-5016 2003 [Refereed]
     
    The effects of thermal stratifications on turbulent mixing and chemical reaction in the liquid mixing layer were investigated by both laboratory experiments and numerical simulations. The experiments were conducted in neutrally, unstably and stably stratified water flows with a rapid chemical reaction. Instantaneous velocities and concentration were simultaneously measured using a combined technique with a two component laser-Doppler velocimeter (LDV) and a later-induced fluorescence (LIF) method. Turbulence quantities such as the mean concentration of chemical product and turbulent mass fluxes were estimated and the effects of thermal stratifications on the quantities were clarified. Further, the large-eddy simulation (LES) using the large-eddy probabil- ity-density function (LEPDF) model as a SGS model was applied to the thermally-stratified reacting liquid flows. The results show that the effects of the stratifications on the turbulent mixing and chemical reaction can be well explained by the LES.
  • R Kurose; H Makino; T Michioka; S Komori
    COMBUSTION AND FLAME ELSEVIER SCIENCE INC 127 (3) 2157 - 2163 0010-2180 2001/11 [Refereed]
  • Takenobu Michioka; Kouji Nagata; Atsushi Ida; Satoru Komori
    Nihon Kikai Gakkai Ronbunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B Japan Society of Mechanical Engineers 66 (651) 2815 - 2822 0387-5016 2000 [Refereed]
     
    A large eddy simulation (LES) based on a finite volume method was applied to a liquid mixing layer flow downstream of a turbulence-generating grid with chemical reactions. The large eddy probability density function (LEPDF) model and joint LEPDF model were used as a SGS model for a rapid reaction and for a moderately fast reaction, respectively. The results of the LES were compared with the measurements to examine the proposed subgrid-scale (SGS) models. Furthermore, the correlation coefficient in the scale similarity model between the variance of SGS concentration and that of test-filtered concentration was determined using the data of the direct numerical simulation (DNS) of liquid isotropic turbulence. The results show that the correlation coefficient for a liquid flow is 5 times bigger than that for a gas flow. When the appropriate correlation coefficent is given, the LES results are in good agreement with the measurements.
  • 道岡武信; 小森悟
    日本機械学会論文集(B編) 65 (32) 839 - 846 1999 [Refereed]

MISC

Awards & Honors

  • 2018/04 日本原子力学会 関東・甲越支部 技術貢献賞
     安全解析のための原子力発電所排ガス拡散予測の数値モデルの開発 
    受賞者: 佐田幸一;道岡武信
  • 2012 粉体工学会 技術賞
     数値シミュレーションによるハニカム形脱硝触媒の劣化特性評価 
    受賞者: 丹野賢二;道岡武信;牧野尚夫;黒瀬良一;小森悟
  • 2010 日本原子力学会 保健物理・環境科学部会 論文賞
     安全解析のための数値モデルによる排ガス大気拡散予測および被ばく線量評価－風洞実験と数値モデルによる放出源の有効高さ・被ばく線量－ 
    受賞者: 佐田幸一;込山有人;道岡武信;市川陽一
  • 2010 大気環境学会 論文賞（原著論文）
     機械通風式冷却塔からの白煙予測手法（その１）―プルーム上昇・拡散モデルの開発― 
    受賞者: 道岡武信;佐藤歩;佐田幸一;下田昭朗;市川陽一
  • 2005 日本機械学会 日本機械学会
     化学反応を伴う乱流場に対するLarge-Eddy Simulationの開発 
    受賞者: 道岡武信

Research Grants & Projects

  • 日本学術振興会：科学研究費助成事業 基盤研究(C)
    Date (from‐to) : 2022/04 -2025/03 
    Author : 道岡 武信
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2018/04 -2022/03 
    Author : Michioka Takenobu
     
    The structure and mechanism of large-scale turbulent motions (wind fluctuations) were investigated by numerical simulation in an idealized urban canyon. First, a numerical simulation was performed for a turbulent flow in an idealized urban canyon with the homogeneous laminar flow at the inlet boundary, and it was found that a large-scale turbulent structure was generated within the urban canopy. Second, the results of the different wind-direction fluctuations show that effects of them on large-scale turbulent structures and gas dispersion within the canopy are quite small. Large-scale turbulent structures within the canopy are mainly generated by block.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2015/04 -2018/03 
    Author : MICHIOKA Takenobu
     
    In environmental impact assessment in Japan, over 1 hour averaged concentration is generally used. To estimate the concentration in an urban canyon, a numerical simulation considering a wind directional fluctuation may be needed, but the effect of wind directional fluctuation on a gas dispersion within an urban canyon is unknown. In the present study, large-eddy simulations were conducted for a gas dispersion within an idealized urban canopy considering wind directional fluctuation. The results show that the effect of wind directional fluctuation on gas dispersion within the urban canopy is small.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2013/04 -2015/03 
    Author : MICHIOKA Takenobu
     
    Large-eddy simulations were conducted to investigate the effect of the turbulent flows outside surface layer on the gas removal from the two-dimensional and three-dimensional street canyon. The results show that the mean concentration within the canyon increases as the fetch increases, and roughly reach a near constant value after fifth to seventh canyon. In addition, at the first and third canyons the low-momentum fluid does not always affect pollutant removal, but as the fetch increases the low-momentum fluid gradually affects pollutant removal.
  • Ministry of Education, Culture, Sports, Science and Technology：Grants-in-Aid for Scientific Research(若手研究(B))
    Date (from‐to) : 2011/04 -2013/03 
    Author : Takenobu MICHIOKA
     
    Large-eddy simulations were conducted to investigate mechanismof pollutant gas removal form urban canyons under neutral, stable and unstable thermalstratification. The result shows that the gas removal from the canyon is strongly relatedto the size of the low-momentum fluid, defined as a region in the flow where instantaneousstreamwise velocity is lower than the local mean velocity. Irrespective of the thermalstratification, the ejection which transports low-momentum fluid upward has influenceon pollutant removal from the canyon.

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