Papers - Kobayashi, Hiromichi

Theoretical analysis of annular laminar flows with MHD interaction driven by rotating co-axial cylinder

Sasaki R., Fujino T., Takana H., Kobayashi H.

IEEJ Transactions on Power and Energy (IEEJ)  141 ( 3 ) 280 - 286 2021.03

Research paper (scientific journal), Joint Work, Accepted,  ISSN  03854213

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Theoretical solution is derived for annular laminar flows with MHD interaction driven by a rotating co-axial cylinder. The effects on the flow field and rotational torque are clarified by changing the radius ratio and the strength of MHD interaction. As increasing the radius ratio, azimuthal flow velocity increases and approaches the plane Couette flow profile for weak MHD interaction. The azimuthal flow velocity, however, decreases as increasing MHD interaction, and the reverse flow emerges in large radial locations. As increasing the MHD interaction, the rotational torque increases. A larger radius ratio leads to the larger sensitivity of the change in rotational torque.

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Role of various scale-similarity models in stabilized mixed subgrid-scale model

Inagaki K., Kobayashi H.

Physics of Fluids (Physics of Fluids)  32 ( 7 )  2020.07

ISSN  10706631

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© 2020 Author(s). We investigate the physical role of various scale-similarity models in the stabilized mixed model [K. Abe, "An improved anisotropy-resolving subgrid-scale model with the aid of a scale-similarity modeling concept,"Int. J. Heat Fluid Flow 39, 42 (2013); M. Inagaki and K. Abe, "An improved anisotropy-resolving subgrid-scale model for flows in laminar-turbulent transition region,"Int. J. Heat Fluid Flow 64, 137 (2017)] and evaluate their performance in turbulent channel flows. Among various models in the present study, the original model combined with the scale-similarity model for the subgrid-scale (SGS)-Reynolds term yields the best prediction for the anisotropy of the grid-scale (GS) velocity fluctuations and the SGS stress, even in coarse grid resolutions. Moreover, it successfully predicts large intensities of the spectra close to the cut-off scale in accordance with the filtered direct numerical simulation, whereas other models predict a rapid decay of the spectra in the low-wavelength region. To investigate the behavior of the models close to the cut-off scale, we analyze the budget equation for the GS Reynolds stress spectrum. The result shows that the scale-similarity model for the SGS-Reynolds term plays a role in the enhancement of the wall-normal velocity fluctuation close to the cut-off scale. Thereby, it activates turbulence close to the cut-off scale, leading to a reproduction of the proper streak structures observed in wall-bounded turbulent flows. The reproduction of velocity fluctuations close to the cut-off scale and turbulent structures is a key element for further development of SGS models.

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Fully Coupled Two-Fluid Dynamics in Superfluid He 4: Anomalous Anisotropic Velocity Fluctuations in Counterflow

Satoshi Yui, Hiromichi Kobayashi, Makoto Tsubota, Wei Guo

Physical Review Letters (American Physical Society)  124 ( 15 )  2020.04

Research paper (scientific journal), Accepted,  ISSN  00319007

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© 2020 American Physical Society. We investigate the thermal counterflow of the superfluid He4 by numerically simulating three-dimensional fully coupled dynamics of the two fluids, namely quantized vortices and a normal fluid. We analyze the velocity fluctuations of the laminar normal fluid arising from the mutual friction with the quantum turbulence of the superfluid component. The streamwise fluctuations exhibit higher intensity and longer-range autocorrelation, as compared to transverse ones. The anomalous fluctuations are consistent with visualization experiments [Mastracci et al., Phys. Rev. Fluids 4, 083305 (2019)PLFHBR2469-990X10.1103/PhysRevFluids.4.083305], and our results confirm their analysis with simple models on the anisotropic fluctuations. This success validates the model of the fully coupled dynamics and paves the way for solving some outstanding problems in this two-fluid system.

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Regularized, parameter free scale similarity type models for Large Eddy Simulation

Klein M., Ketterl S., Engelmann L., Kempf A., Kobayashi H.

International Journal of Heat and Fluid Flow (International Journal of Heat and Fluid Flow)  81 2020.02

ISSN  0142727X

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© 2019 Elsevier Inc. The fidelity of Large Eddy Simulations (LES) depends strongly on the closures of the sub-grid scale (SGS) stress tensor. Although it is well known that the SGS stresses in LES are not aligned with the strain rate tensor, the most widely used models are still of eddy viscosity type, due to their robust behavior in LES and reasonable performance in a posteriori testing. The unstable behavior of more advanced anisotropic models, that is typically found in LES, has been attributed to either the fact that these models provide backscatter or to the fact that they do not provide a sufficient amount of dissipation. Based on recent advances in the field, an alternative modeling strategy is suggested, which can be used to regularize an arbitrary anisotropic (e.g. scale similarity type) model. The resulting model is easy to implement, can be written in compact form and is free of model parameters. The model has been tested a-posteriori and results are presented for a Taylor-Green-Vortex, a free plane jet and a turbulent channel flow of friction Reynolds numbers 395, 590 and 934. The results are compared to well-known eddy viscosity models and when applicable, to simulations without explicit LES model. The new model exhibits good performance for a variety of mesh resolutions and for all configurations. Furthermore, a-priori analysis results in the context of liquid atomization indicate that the model might be suitable as well in more complex physical scenarios. The a-priori analysis performance of the model is found to be nearly equivalent to the underlying structural anisotropic model in terms of its correlation coefficient, but the model is free of backscatter and provides good stability in LES.

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Numerical Study on Entrance Length in Thermal Counterflow of Superfluid ⁴He

Kobayashi H., Yui S., Tsubota M.

Journal of Low Temperature Physics (Journal of Low Temperature Physics)  196 ( 1-2 ) 35 - 41 2019.07

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00222291

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© 2019, Springer Science+Business Media, LLC, part of Springer Nature. Three-dimensional numerical simulations in a square duct were conducted to investigate entrance lengths of normal fluid and superfluid flows in a thermal counterflow of superfluid 4He. The two fluids were coarse-grained by using the Hall–Vinen–Bekharevich–Khalatnikov (HVBK) model and were coupled through mutual friction. We solved the HVBK equations by parameterizing the coefficient of the mutual friction to consider the vortex line density. A uniform mutual friction parameter was assumed in the streamwise direction. Our simulation showed that the entrance length of the normal fluid from a hot end becomes shorter than that of a single normal fluid due to the mutual friction with the parabolically developed superfluid flow near the hot end. As the mutual friction increases, the entrance length decreases. Same as that, the entrance length of the superfluid from a cold end is affected by the strength of the mutual friction due to the parabolically developed normal fluid flow near the cold end. Aside from the entrance effect, the realized condition of a tail-flattened flow is discussed by parameterizing the superfluid turbulent eddy viscosity and the mutual friction.

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格子ボルツマン法による自転車競技の集団走行の大規模LES空力解析

長谷川 雄太，青木 尊之，小林 宏充，白﨑 啓太

日本機械学会論文集 (日本機械学会)  85 ( 870 ) 18-00441 2019.02

Research paper (scientific journal), Joint Work, Accepted

Improvement of the SGS model by using a scale-similarity model based on the analysis of SGS force and SGS energy transfer

Kobayashi H.

International Journal of Heat and Fluid Flow (International Journal of Heat and Fluid Flow)  72   329 - 336 2018.08

Research paper (scientific journal), Single Work, Accepted,  ISSN  0142727X

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© 2018 Elsevier Inc. The subgrid-scale (SGS) model is improved by using a scale-similarity model based on the analysis of the SGS force and SGS energy transfer around an elliptic Burgers vortex. Abe (2013) proposed an anisotropy-resolving SGS model in which the Bardina term of a scale-similarity model is mixed with an eddy viscosity model under a new concept wherein the Bardina term does not affect the SGS energy transfer although it affects the SGS force. By using the concept, we propose a scale-similarity model with the Clark term. The SGS energy transfer is determined by the scale-similarity term and not by the eddy viscosity term while the SGS force is improved by using the SGS kinetic energy (Abe, 2013). It is observed that the Clark term yields higher spatial correlation with the true distributions of the SGS force and SGS energy transfer around the elliptic vortex when compared to the Bardina term. The SGS model based on the Clark term exhibits good performance for turbulent channel flows with respect to Reτ=180 and 590 even in extremely coarse grid resolutions. Specifically, the SGS model with the SGS kinetic energy fairly improves the mean streamwise velocity profile.

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Three-Dimensional Coupled Dynamics of the Two-Fluid Model in Superfluid 4He: Deformed Velocity Profile of Normal Fluid in Thermal Counterflow

Satoshi Yui, Makoto Tsubota, Hiromichi Kobayashi

Physical Review Letters (Physical Review Letters)  120 ( 155301 )  2018.04

Research paper (scientific journal), Joint Work, Accepted,  ISSN  00319007

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© 2018 American Physical Society. The coupled dynamics of the two-fluid model of superfluid He4 is numerically studied for quantum turbulence of the thermal counterflow in a square channel. We combine the vortex filament model of the superfluid and the Navier-Stokes equations of normal fluid. Simulations of the coupled dynamics show that the velocity profile of the normal fluid is deformed significantly by superfluid turbulence as the vortices become dense. This result is consistent with recently performed visualization experiments. We introduce a dimensionless parameter that characterizes the deformation of the velocity profile.

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液体金属MHD発電の最近の進展

小林宏充

日本エネルギー学会機関誌えねるみくす (日本エネルギー学会)  96   193 - 198 2017.03

Research paper (scientific journal), Single Work

Turbulent and Electrical Phenomena in a Liquid Metal MHD Energy Conversion Device under Various Load Conditions

Hiromichi Kobayashi, Yoshihiro Okuno

IEEJ Transactions on Power and Energy 136 ( 10 ) 773 - 778 2016.10

Research paper (scientific journal), Joint Work, Accepted

Analyses on Response of A Liquid Metal MHD Power Generation System to Various External Inputs

Liancheng Hu, Hiromichi Kobayashi, Yoshihiro Okuno

IEEJ Transactions on Electrical and Electronic Engineering 10 ( 3 ) 268 - 273 2015.05

Research paper (scientific journal), Joint Work, Accepted

Cyclone-anticyclone asymmetry in gravity wave radiation from a co-rotating vortex pair in rotating shallow water

Norihiko Sugimoto, Keiichi Ishioka, Hiromichi Kobayashi and Yutaka Shimomura

Journal of Fluid Mechanics (Cambridge University Press)  772   80-106 2015.04

Research paper (scientific journal), Joint Work, Accepted

Influence of Working Fluid Characteristics on the Performance of a Liquid Metal Magnetohydrodynamic Generator

Liancheng Hu, Hiromichi Kobayashi, Yoshihiro Okuno

Electrical Engineering in Japan B 134 ( 12 ) 1-10 2014.12

Research paper (scientific journal), Joint Work, Accepted

Direct Numerical Simulation and Large Eddy Simulation on a Turbulent Wall-Bounded Flow Using Lattice Boltzmann Method and Multiple GPUs

Xian Wang, Yanqin Shangguan, Naoyuki Onodera, Hiromichi Kobayashi, and Takayuki Aoki

Mathematical Problems in Engineering 2014   1-10 2014.04

Research paper (scientific journal), Joint Work, Accepted

A Reynolds-averaged turbulence modeling approach to the maintenance of the Venus superrotation

Akira Yoshizawa, Hiromichi Kobayashi, Norihiko Sugimoto, Nobumitsu Yokoi, Yutaka Shimomura

Geophysical & Astrophysical Fluid Dynamics (Taylor & Francis)  107 ( 6 ) 614-639 2013.08

Research paper (scientific journal), Joint Work, Accepted

大学一年生の自然科学への関心度と知識度2012年度調査 -- 10年前、20年前との比較

加藤万里子，小林宏充，黒田忠広，青木健一郎，杉本憲彦，松浦 壮

慶應義塾大学日吉紀要 (慶應義塾大学出版会)   ( 53 ) 1-22 2013.03

Research paper (bulletin of university, research institution), Joint Work

Turbulent duct flows in a liquid metal magnetohydrodynamic power generator

Hiromichi Kobayashi, Hiroki Shionoya, Yoshihiro Okuno

Journal of Fluid Mechanics (Cambridge University Press)  713   243-270 2012.12

Research paper (scientific journal), Joint Work, Accepted

Large-Eddy Simulation of Turbulent Channel Flows with Conservative IDO Scheme

Naoyuki Onodera, Takayuki Aoki, Hiromichi Kobayashi

Journal of Computational Physics (Elsevier)  230 ( 14 ) 5787-5805 2011.05

Research paper (scientific journal), Joint Work, Accepted

Statistical properties of the local structure of homogeneous isotropic turbulence and turbulent channel flows

Hiromichi Kobayashi, Eiji Matsumoto, Naoya Fukushima, Mamoru Tanahashi, Toshio Miyauchi

Journal of Turbulence (Taylor & Francis)  12 ( 11 ) 1-16 2011.01

Research paper (scientific journal), Joint Work, Accepted

GPUによるラージエディ・シミュレーションの高速化

小野寺直幸，青木尊之，小林宏充

日本流体力学会誌ながれ (日本流体力学会)  29 ( 6 ) 423-426 2010.12

Research paper (scientific journal), Joint Work