SEISAN KENKYU
Online ISSN : 1881-2058
Print ISSN : 0037-105X
ISSN-L : 0037-105X
Volume 59 , Issue 1
Showing 1-19 articles out of 19 articles from the selected issue
Preface
(TSFD)
Introduction to Special Section
Research Review
  • Shinsuke KATO, Takamasa HASAMA, Ryozo OOKA
    2007 Volume 59 Issue 1 Pages 5-11
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       In order to investigate induced flow properties through a single opening, large-eddy simulations (LES) are performed on ventilated air through the single opening of a room. In this flowfield, the outdoor air flows parallel to the wall and to the opening, and the room airflow is induced by the outdoor flow through the opening. Firstly, the dependency of the induced room airflow to the shape of the opening is investigated (by changing the shape of the opening). Secondly, the distributions of mean and turbulent variables at the boundary plane of the opening are investigated. Thirdly, the calculation using fluctuating wind as the inflow boundary condition is carried out to investigate the dependency of the in- and outflow property of the opening with the turbulence characteristics at the inflow boundary.
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  • Takamasa HASAMA, Shinsuke KATO, Ryozo OOKA
    2007 Volume 59 Issue 1 Pages 12-15
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       In order to examine the applicability of detached-eddy simulations (DES) to building ventilation analysis, DES and large-eddy simulations (LES) were performed on the flow around a building having a single-sided opening. We also examined the applicability of DES to indoor thermal environmental analysis and made simulations of an anisothermal flowfield with a heat source on the same analysis model. Two types of calculation (LES calculation and DES calculation with the same grid) were performed. It is concluded that DES over-estimates the sub-grid scale (SGS) viscosity in the LES region, but the difference in room mean property between both cases is low.
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  • Yoshihiro ISHIDA, Shinsuke KATO, Hong HUANG
    2007 Volume 59 Issue 1 Pages 16-20
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       In this paper, airflow and concentration calculations in void spaces in 3-D densely built-up area models are done for study on relationship between configurations of the models and performances of natural drafts and ventilation of them. In a center block in the first model, there are ten buildings around a road with width of 4m and length of 40m, which has a dead end and another end opening to a road with width of 7m, and the clearances between walls of buildings are 1m. Fifteen almost the same blocks are set around the center block. Another two models are also set with slightly different city configurations. The gases are supplied from four points on the road surface only in the center block in 3 cases. The simulations are done using 3 models and the tendencies of ventilating performances for 3 levels of city density in the models are obtained.
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Research Flash
  • Daisuke KITAZAWA, Michio KUMAGAI
    2007 Volume 59 Issue 1 Pages 21-26
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       The effects of climate change on ecosystem of Lake Biwa in the latest 20 years are studied numerically by three-dimensional hydrodynamic-ecosystem coupled model. In particular, the present study focuses on understanding of the mechanism why the tension of dissolved oxygen recently decreases at the deepest point in Lake Biwa. As a result of numerical simulation, it is revealed that decrease in the tension of dissolved oxygen is attributed to delay of oxygen supply to bottom water from the upper layer, and to further eutrophication of waters in North Basin of Lake Biwa due to global warming in winter. [This abstract is not included in the PDF]
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  • —Report 2: CFD analysis via LES: Case of simplified underbody configurations—
    Kozo KITOH, Makoto TSUBOKURA, Nobuyuki OSHIMA, Makoto YAMAMOTO, ZHANG ...
    2007 Volume 59 Issue 1 Pages 27-30
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       The objective of this study is to find out a way to reduce aerodynamic drag of the vehicle body focusing on the underbody flow. LES computations were performed for a simplified vehicle body using ca. 25 million meshes. It was found from the comparison with the experiment that LES well reproduces mean flow field around the vehicle. Large scale LES computation can be an effective tool to evaluate vehicle aerodynamic characteristics. Comparisons with RANS results were also made. [This abstract is not included in the PDF]
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  • Fujihiro HAMBA, Masayoshi OKAMOTO
    2007 Volume 59 Issue 1 Pages 31-35
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       Weis and Hutter (J. Fluid Mech. vol.476, 2003, p.63 ) proposed a Euclidean-objective weak-equilibrium condition for the algebraic Reynolds stress model (ARSM). However, Gatski and Wallin (J. Fluid Mech. vol.518, 2004, p.147) pointed out that the weak-equilibrium condition proposed is not correct for actual rotating flows and showed that a non-objective weak-equilibrium condition extended to curved and rotating flows should be assumed. In this paper, it is shown that a frame-invariant model expression for the Reynolds stress is desirable as a general model. The extended weak-equilibrium condition of Gatski and Wallin is reexamined to show that it is in fact objective and it does not contradict the frame-invariance of the model expression for the Reynolds stress. [This abstract is not included in the PDF]
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  • Nobumitsu YOKOI, Fujihiro HAMBA
    2007 Volume 59 Issue 1 Pages 36-40
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       A magnetohydrodynamic (MHD) turbulence model incorporating the turbulent residual energy (the energy di.erence between the kinetic and magnetic energies) as well as the turbulent cross helicity (velocity. magnetic-.eld correlation) is applied to the solar-wind turbulence. Its transition from the Alfven-wave-like. uctuation near the Sun in the inner heliosphere to the fully developed MHD turbulence in the outer heliosphere is discussed. The observed Alfven ratio (kinetic energy / magnetic energy) of rA ≅ 0.5 in theouter heliosphere far from the Sun is elucidated fromthe viewpoint of the turbulence model. [This abstract is not included in the PDF]
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  • Shoji KOYAMA
    2007 Volume 59 Issue 1 Pages 41-46
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       To investigate turbulent thermal convection which is driven solely by buoyancy, non-penetrative convection without stable stratification is numerically analyzed. In this convection, the heat flux is imposed at the lower wall whereas the upper wall is insulated. In order to examine how much and what kind of flow patterns contribute to some mean values, we obtain conditional averages of vertical-velocity and temperature fluctuations. The probability distribution functions of mean values are also calculated for details. As a result, it is shown that not only coherent structure such as buoyant flow but also its backgrounds have a great influence on statistics in some vertical position. [This abstract is not included in the PDF]
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  • Hisanori SATO, Fujihiro HAMBA
    2007 Volume 59 Issue 1 Pages 47-50
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       In the previous report, we analyzed MHD turbulence in rotating frames and investigated mainly the expression for the turbulent electromotive force. In this work, the expression for the Reynolds stress in MHD is also analyzed. Then we found the compatibility of the formulae for the case in which Elsasser variables are applied and for the case in which velocity fields and magnetic fields are individually applied. Moreover, we modeled the system of equations separating the energy of velocity fields and that of magnetic fields, and consider the construction of the five-equation model. [This abstract is not included in the PDF]
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  • Masataka TSUCHIYA, Fujihiro HAMBA
    2007 Volume 59 Issue 1 Pages 51-54
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       Dynamo effect, which creates and maintains large-scale magnetic field by fluid motion, plays an important role in maintaining magnetic field of astrophysical objects. To gain a better understanding about magnetohydrodynamic turbulence and its dynamo effect, we conducted large eddy simulation of turbulent channel flow of electrically conducting fluid. It is shown that turbulent electromotive force due to the cross helicity effect builds up large-scale magnetic field, and cross helicity is strengthen by the combination of Reynolds stress and gradient of large-scale magnetic field. [This abstract is not included in the PDF]
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  • Nobuyuki OSHIMA, Eru KURIHARA, Wanyuan SHI, Takuji NAKASHIMA
    2007 Volume 59 Issue 1 Pages 55-58
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       Heat and mass transfer in a polymer electrolyte fuell cell (PEFC) is numerically investigated. H2 and O2 gas transportation in the gas diffusion layer (GDL) is described by the modified Navier-Stokes equations with Darcy's law. The electrochemical reaction at the catalyst layer is governed by Butler-Volmer equations. In this multiphase mixing model, the water transportation is governed by the equation for the liquid water satulation. As the result, in the simple model, transportation of heat and chemical species is qualitatively evaluated and the distribution of electric potential resulting from electrochemical reaction is obtained. In the full PEFC model, the calculation cost of large computation for real PEFC systems are roughly estimated. [This abstract is not included in the PDF]
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  • Akter HOSSAIN, Nobuyuki OSHIMA, Marie OSHIMA
    2007 Volume 59 Issue 1 Pages 59-62
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       In this study, the effect of external heat source on the propagation of premixed flame is investigated in micro and normal gravity by two-dimensional laminar flow simulation under low Mach number approximation. Fundamental results are observed where the flame shapes and their propagation speeds are affected by the variation of gravity level in presence of external heat source. The results are useful not only to investigate the flame instability mechanism, but also to develop an efficient flame model and numerical method to simulate the effect of local temperature variation due to radiation, heat transfer and convection on the propagation of premixed flame in multi-scale physics. [This abstract is not included in the PDF]
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  • Ryo TAKAYAMA, Chisachi KATO, Yoshinobu YAMADE
    2007 Volume 59 Issue 1 Pages 63-66
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       Presented in this paper is a numerical simulation of the aerodynamic sound radiated from a Propeller Fan. It is found that the turbulent boundary layer can be captured to some extent by using grid resolution of about 1 hundred elements per blade and the leading edge separation vortex play major role in the noise production rather than tip vortex. The broadband noise calculated by curle's equation shows relatively good agreement with the measurement though Blade Passing Frequency noise can not be predicted completely because of no consideration of inflow turbulence. [This abstract is not included in the PDF]
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Research Review
  • Hiroshi YOKOYAMA, Yuichi TSUKAMOTO, Chisachi KATO, Akiyoshi IIDA
    2007 Volume 59 Issue 1 Pages 67-72
    Published: 2007
    Released: February 20, 2007
    JOURNAL FREE ACCESS
       Intense tonal sound is radiated when a small forward-backward facing step, which we hereafter call a bump, is located upstream of a backward-facing step (backstep). Numerical simulations are used to investigate the mechanism and the effects of the bump geometry and position on the sound. The acoustic waves are directly simulated by large eddy simulation (LES) with the compressible Navier-Stokes equations. The results showed the acoustic feedback process: the development of vortices from the bump to the backstep, and the radiation of acoustic waves at the backstep, the upstream propagation and the excitation of vortices at the bumps leading edge. And we clarified the effects of the bump geometry and position on the vortex dynamics, and showed that the tonal sound is radiated when the vortices retain the two-dimensional nature and are fully developed from the bump to the backstep.
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Research Flash
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