SEISAN KENKYU
Online ISSN : 1881-2058
Print ISSN : 0037-105X
ISSN-L : 0037-105X
Volume 68 , Issue 1
Showing 1-17 articles out of 17 articles from the selected issue
Preface
Introduction to Special Section
Research Flash
  • Fujihiro HAMBA
    2016 Volume 68 Issue 1 Pages 5-9
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    On the basis of the two-point velocity correlation, the energy density in the scale space was introduced to better understand inhomogeneous turbulence. The transport equation for the energy density was derived for inhomogeneous turbulence. Direct numerical simulation data of turbulent channel flow was used to evaluate the energy transport in the physical and scale spaces. The energy flux from the large to the small scales was observed for the three components of the turbulent energy. The pressure strain correlation suggests that for large scales the energy redistribution occurs in the opposite direction because of the wall effect.
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Research Review
  • Nobumitsu YOKOI
    2016 Volume 68 Issue 1 Pages 11-19
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Vigorous nonlinearity represented by huge Reynolds numbers and strong inhomogeneities of large-scale fields such as velocity shear, non-uniform magnetic field, density variation, etc. are very ubiquitous properties of astrophysical turbulent flows. Turbulence modeling approach provides a very strong tool for analyzing such nonlinear and inhomogeneous flows. In addition to the mutual interaction between the mean and fluctuation, as is present in the usual hydrodynamic turbulent flows, there are some features in the astrophysical ones. A prominent feature may be the dynamics between the turbulent transport enhancement and suppression due to symmetry breakage arising from rotation, magnetic field, etc. In order to treat astrophysical flows properly, such features should be incorporated into the astrophysical turbulence models. Some illustrative applications of turbulence models to the astrophysical phenomena are discussed, which include solar winds, magnetic reconnection, and solar-activity cycle.
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Research Flash
  • Shoji KOYAMA
    2016 Volume 68 Issue 1 Pages 21-27
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Numerical simulation using a turbulence model for an axially rotating pipe flow is conducted. To reproduce the property of such flow through an eddy-viscosity model (kε type two-equation model), it is well knownthat the non-linear cubic turbulence models are effective. In this study, we carefully examine how each model term works in the computation with a cubic model. In addition, we propose a modification on the model, and consider the physical meaning of the cubic terms.
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  • Kazuhiro INAGAKI, Nobumitsu YOKOI, Fujihiro HAMBA
    2016 Volume 68 Issue 1 Pages 29-33
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    An algebraic Reynolds stress model with the turbulent helicity gives a good result in predicting a decaying swirling pipe flow. In this model, gradient of turbulent helicity coupled with the absolute vorticity plays a key role in conuterbalancing the eddy-viscosity effect, which suggests that the inhomogeneous turbulent helicity may generate the mean flow in a rotating system on the analogy of the turbulent magnetic dynamo in magnetohy- drodynamics. In order to study the effects of inhomogeneous turbulent helicity in the rotating system, a large eddy simulation( LES) of turbulence driven by an inhomogeneous external force is performed. In the simulation, the mean flow directed to the rotation axis is generated when the inhomogeneous turbulent helicity exists. This phenomenon cannot be explained only by the eddy viscosity effect. In the Reynolds stress transport equation, the terms leading to this phenomenon are discussed and the algebraic Reynolds stress model with the turbulent helicity coupled with the absolute vorticity is reproduced by modeling the pressure gradient diffusion term.
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Research Review
  • Keigo NAKAJIMA, Ryozo OOKA, Hideki KIKUMOTO
    2016 Volume 68 Issue 1 Pages 35-41
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Computational fluid dynamics (CFD) analysis using large-eddy simulation (LES) and standard k-ε modelThe results of LES and k-ε were compared with the wind tunnel experiment to investigate the prediction accuracy of CFD analysis. LES agreed with the wind tunnel experiment for the mean wind velocity and mean temperature. k-ε failed to reproduce the spatial distribution of the mean wind velocity and mean temperature within the urban canyon in unstable case. LES and k-ε agreed well with the wind tunnel experiment for the turbulent kinetic energy in stable case, although they underestimated it within the urban canyon in unstable case. k-ε also underestimated the Reynolds stress <u1'u3'> and turbulent heat flux <u3'T'> within the urban canyon in unstable and stable cases. It is thought that the prediction accuracy of k-ε decreases because k-εunderestimates the exchange of momentum and heat within and above the urban canyon compared with LES and the wind tunnel experiment.
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  • Hideki KIKUMOTO , Ryozo OOKA
    2016 Volume 68 Issue 1 Pages 43-49
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    We conducted an experiment and large-eddy simulations (LESs) of turbulent dispersion of a passive tracer in a cavity space. LESs were run on simple orthogonal numerical grids that had a grid spacing of H/200 and H/400 (where H is the cavity height). Although the grid spacing of H/200 in the cavity was sufficient to allow prediction of mean and variance of concentration, it underestimated concentration fluctuation at high frequency, as compared to the experiment. However, LES with a grid spacing of H/400 successfully provided probability density functions and power spectra of concentration that agreed very well with those in the experiment.
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Research Flash
  • Daisuke KAWAHARA, Shinsuke KATO
    2016 Volume 68 Issue 1 Pages 51-53
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Thermal insulation performance of buildings is important. We focused on dynamic insulation system (DI)applied to windows which is conventionally poor thermal insulation performance. DI can decrease heat loss from building in winter, on the other hand it decreases indoor surface temperature because of venting cold outer air and it deteriorates indoor thermal comfort. To evaluate indoor thermal environment, we used Contribution Ratio of Indoor climate (CRI) which represents how each heat source in a room contributes to form local air temperature. According to numerical analysis results, CRI in a occupied area is small.
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  • Hyokeun HWANG, Shinsuke KATO
    2016 Volume 68 Issue 1 Pages 55-58
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    The air conditioning system maintaining a uniform temperature distribution is required for indoor thermal comfort in office. Therefore, we propose the direct processing system of internal heat load, which is capable of thermal diffusion control into the room due to install heat recovery units near internal heat sources. In this paper, it reviews using experimental and numerical analysis of thermal manikin about the influence of human thermal comfort by the proposed system. As a result of analysis, a thermal resistance of skin-surface varies at the upper body because heat load devices and heat recovery units to affect. In addition, the thermal influence changes depending on the thermal properties of heat sources and the position of heat recovery units.
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  • Li WANG, Shinsuke KATO, Hyokeun HWANG
    2016 Volume 68 Issue 1 Pages 59-61
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    With the aim of evaluating the indoor thermal environment in a model office room with active chilled beam (ACB) and liquid cooling air-conditioning system (LCAC), the thermal influence around human body 0.2m by heat sources are examined with contribution ratio of indoor climate (CRI). The results showed that the thermal environment of the interior zoon was mainly influenced by ACB, compared with DFCU. The heat sources that the sensible heat load was dealt with by LCAC influenced the thermal environment around human uniformly, but the heat from walls can only affect the environment within the scope of narrow area obviously.
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  • Olivier PACOT, Chisachi KATO, Yang GUO, Yoshinobu YAMADE
    2016 Volume 68 Issue 1 Pages 63-68
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    The cavitating vortex rope developing downstream a Francis runner has been numerically studied for two off-design conditions. The first condition is a part load condition and the second a full load condition. The numerical approach selected is the Large Eddy Simulation using the FrontFlow/blue (FFB) software. To compute cavitating flows, the Okita cavitation model is implemented in FFB. The computations were performed on K supercomputer using 1,024 nodes. The mesh size features approximately 120 million elements. For both off-design conditions, the cavitating vortex ropes were qualitatively well reproduced. The shape of the vortex rope and the flow distribution within the diffuser are shown for each flow condition.
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  • Yoshinobu YAMADE, Chisachi KATO, Takahide NAGAHARA, Jun MATSUI
    2016 Volume 68 Issue 1 Pages 69-73
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Unsteady submerged vortices in the test pump sump with 100 mm height and 100 mm diameter of the inlet pipe are investigated by performing large eddy simulations (LES). Width of the pump sump is 300 mm and the inlet velocity is 0.37 m/s. Computational grids with 0.255 mm horizontal resolution resolve turbulent boundary layer on the bottom and side wall of the pump sump. Unsteady flows with different conditions for boundary layer on the wall of the pump sump were computed to investigate the relationship between the boundary layer and vortices generation. In these flow computations, we found that the origins of the submerged vortices are shear of the boundary layer. However we underpredicted the pressure drop of a vortex core because the grid resolution is not enough to present vortex cores. In order to take finer grid resolution, we used a simplified computational model where the computational domain is localized to vortex core. Its horizontal grid resolution is 0.03 mm. A pressure drop of a vortex core computed by the simplified computational model is 40% for the atmospheric pressure, while one computed by LES for the pump sump model is 5%.
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  • Yang GUO, Chisachi KATO
    2016 Volume 68 Issue 1 Pages 75-78
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Benchmark test of acoustic solver using finite element method (FEM) in the time domain was done. The wave equation of sound pressure was solved by finite element method in the time domain. The central difference scheme is used for second-order derivative in time and the standard Galerkin method is used for the spatial derivatives. One dimensional cases and a three dimensional case were tested. With the current numerical schemes, the points per wave length (PPW) of the computational mesh should be at least 20 if wall reflection is important. The phase error of the scheme has important effect on the solution.
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  • Junbo ZHANG, Daisuke KITAZAWA
    2016 Volume 68 Issue 1 Pages 79-82
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    A three dimensional Marine Environmental Committee (MEC) model was run to predict the environmental impacts of marine current turbines (MCTs). The removal of momentum due to the rotation of turbine blades should be taken into account in addition to the drag of the blades and their supporting structures. Numerical simulation was carried out assuming that several MCTs are installed in the simple strait. As a result, the range of the reduction in current velocity is depending on the number of installation of MCTs.
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  • Yosuke HASEGAWA, Maurizio QUADRIO, Bettina FROHNAPFEL
    2016 Volume 68 Issue 1 Pages 83-86
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    We propose a new flow condition for evaluating flow control techniques through DNS of internal flows. Instead of keeping either a flow rate or a pressure gradient, the new simulation approach is to maintain the total power input, i.e., the sum of pumping and control power inputs. This new condition is the most suitable approach for energetic analyses of flows with/without control in order to investigate how the applied pumping energy is eventually dissipated by the fluid viscosity. We first implement the constant power input condition in DNS, and reevaluate existing flow control techniques in the new framework.
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Research Paper
Research Flash
  • Toshiharu MORIMOTO, Jun YANAGIMOTO
    2016 Volume 68 Issue 1 Pages 87-90
    Published: January 01, 2016
    Released: January 29, 2016
    JOURNAL FREE ACCESS
    Interstitial free steels, which have a preferential orientation, have different draw-ability in the rolling and width direction even if the yield stresses in the rolling direction and width directions is the same. This phenomenon causes plastic anisotropy. In this study, an ODF calculation method was developed using measured orientation data of a polycrystalline metal. The ODF is an approximation to the orientation of a polycrystalline metal using generalized Legendre polynomials. Using ODF calculation method, plastic anisotropy can be determined, which improve the accuracy of FEM simulation.
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