研究者詳細 - 河野晴彦

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Validity condition for the local sheath impedance boundary condition and a nonlocal generalization 査読有り国際誌

J. R. Myra, H. Kohno

AIP Conference Proceedings ( AIP Publishing ) 2984 ( 1 ) 060002-1 - 060002-6 2023年08月

担当区分：最終著者記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

ICRF sheaths can cause unwanted interactions of high-power RF waves with material surfaces in magnetic fusion devices. In previous work, a local RF sheath impedance boundary condition (BC) was derived for use in ICRF codes together with a microscale (i.e., Debye or sheath width scale) model for obtaining the sheath impedance used in that BC. This local RF sheath BC matches the normal component of current and electrostatic potential across the sheath-plasma interface. Collapsing the matching conditions at the sheath-plasma interface to a BC depends on the assumption of scale separation, which can be violated when conditions along the local radius of curvature of the surface vary sufficiently rapidly. The validity condition is explored in this contribution, with special attention to the case where the magnetic field approaches being tangent to the surface. When the local sheath BC no longer applies, a nonlocal sheath BC is developed under the assumption of a more relaxed scale separation assumption. It is shown that the nonlocal sheath BC reduces to the previous local sheath BC under appropriate conditions. A surface-integrated sheath admittance parameter [H. Kohno and J. R. Myra, this conference] describes the 2D physics in the new BC.

DOI： 10.1063/5.0163291

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Magnetic potential based formulation for linear and non-linear 3D RF sheath simulation 査読有り国際誌

S. Shiraiwa, N. Bertelli, W. Tierens, R. Bilato, J. Hillairet, J. Myra, H. Kohno, M. Poulos, M. Ono

Nuclear Fusion ( IOP Publishing ) 63 ( 2 ) 026024-1 - 026024-12 2023年01月

記述言語：英語掲載種別：研究論文（学術雑誌）

This paper reports a new numerical scheme to simulate the radio-frequency (RF) induced RF sheath, which is suitable for a large 3D simulation. In the RF sheath boundary model, the tangential component of the electric field ( E t ) is given by the gradient of a scalar electric field potential. We introduce two additional scalar potentials for the tangential components of the magnetic field, which effectively impose the normal electric displacement (D n ) on the plasma sheath boundary condition via in-homogeneous Neumann boundary condition and constrain the tangential electric field on the surface as curl-free ( ∇ × E t = 0 ). In our approach, the non-linear sheath impedance is formulated as a natural extension of the large thickness (or asymptotic) sheath limit ( D n = 0 ), allowing for handling both asymptotic and non-linear regimes seamlessly. The new scheme is implemented using the Petra-M finite element method analysis framework and is verified with simulations in the literature. The significance of non-linearity is discussed in various plasma conditions. An application of this scheme to asymptotic RF sheath simulation on the WEST ICRF antenna side limiters is also discussed.

DOI： 10.1088/1741-4326/aca6f9

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交流磁場下の3次元電磁流体流れにおける連成解析手法の検討査読有り

久米川知也，牟禮良晃，河野晴彦

日本計算工学会論文集 ( 日本計算工学会 ) 2022 20220016-1 - 20220016-9 2022年10月

担当区分：最終著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

複数の場の相互作用を伴う3次元連成解析では，一般に弱連成手法または強連成手法が用いられ，解の正確度を低下させることなく，妥当な計算時間で数値解が得られる手法を選択することが重要である．本研究では，電磁流体流れを計算するための弱連成スキームに分離反復型解法を導入した新たなスキーム（IPCスキーム）を構築し，両スキームを用いて得られた数値解の比較を通して，弱連成スキームの信頼性を評価する．流体・固体・気体を含む領域に一様な交流磁場が印加されるモデルを用いて，ハルトマン数が小さく，時間刻み幅が比較的大きい場合，IPCスキームにより得られる解は弱連成スキームにより得られる解よりも信頼性が高いことが示された．一方で，低ハルトマン数の条件においても，時間刻み幅を十分に小さくすると，両スキームにより得られる解はよく一致することも確かめられた．

DOI： 10.11421/jsces.2022.20220016

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ALE有限要素法による直流磁場下における導電性液滴の振動の数値解析

澁田謙太，河野晴彦

日本機械学会2021年度年次大会 ( 日本機械学会 ) 2021 S055-39 2021年09月

担当区分：最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

A numerical scheme to analyze oscillations of a conductive droplet under DC magnetic fields in zero gravity was developed, and the effect of DC magnetic fields on the oscillation of a conductive droplet is investigated. In the present scheme, the Navier-Stokes equations for the liquid-gas two-phase fluid flow are solved by a Galerkin finite element method and the ALE method is used to allow a conductive droplet to oscillate. An elliptical conductive droplet is placed in the center of a two-dimensional rectangular domain and uniform DC magnetic fields are applied in the x direction. It is confirmed that the DC magnetic fields generate Lorentz force in the directions which prevent oscillations of the conductive droplet, resulting in a damping effect of the oscillations. Furthermore, as compared with the case without applying a DC magnetic field, phase delay and phase advance are observed as a result of applying DC magnetic fields.

DOI： 10.1299/jsmemecj.2021.s055-39

その他リンク： https://www.jstage.jst.go.jp/article/jsmemecj/2021/0/2021_S055-39/_pdf

直流磁場下における導電性流体の2次元スロッシング解析

澁田謙太，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2021.74 E44 2021年03月

Convergence improvement of the simultaneous relaxation method used in the finite element analysis of incompressible fluid flows 査読有り国際誌

Haruhiko Kohno

Engineering Computations ( Emerald Publishing ) 37 ( 2 ) 481 - 500 2020年03月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Purpose: This paper aims to present an improved finite element method used for achieving faster convergence in simulations of incompressible fluid flows. For stable computations of incompressible fluid flows, it is important to ensure that the flow field satisfies the equation of continuity in each element of a generally distorted mesh. The study aims to develop a numerical approach that satisfies this requirement based on the highly simplified marker-and-cell (HSMAC) method and increases computational speed by introducing a new algorithm into the simultaneous relaxation of velocity and pressure. Design/methodology/approach: First, the paper shows that the classical HSMAC method is equivalent to a Jacobi-type method in terms of the simultaneous relaxation of velocity and pressure. Then, a Gauss–Seidel or successive over-relaxation (SOR)-type method is introduced in the Newton–Raphson iterations to take into account all the derivative terms in the first-order Taylor series expansion of a nodal-averaged error explicitly. Here, the nine-node quadrilateral (Q2–Q1) elements are used. Findings: The new finite element approach based on the improved HSMAC algorithm is tested on fluid flow problems including the lid-driven square cavity flow and the flow past a circular cylinder. The results show significant improvement of the convergence property with the accuracy of the numerical solutions kept unchanged even on a highly distorted mesh. Originality/value: To the best of the author’s knowledge, the idea of using the Gauss–Seidel or SOR method in the simultaneous relaxation procedure of the HSMAC method has not been proposed elsewhere.

DOI： 10.1108/EC-02-2019-0069

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環状流路内における進行磁場により駆動される電磁流体流れの3次元数値解析

牟禮良晃，河野晴彦

計算力学講演会講演論文集 ( 日本機械学会 ) 2019.32 28 2019年09月

担当区分：最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

Three-dimensional numerical simulation of incompressible, magnetohydrodynamic flows under traveling magnetic fields are carried out, which takes into account the coupling with the electromagnetic field in the solid and gas regions. A numerical scheme is constructed by combining the Galerkin finite element method and the edge-element based finite element method, which are applied to the discretizations of Navier-Stokes equations and the electromagnetic field equations, respectively. The solution algorithm for fluid flow is based on an explicit fractional step approach and the simultaneous relaxation of velocity and pressure to satisfy the continuity equation. The electromagnetic field equations are formulated with the use of the magnetic vector potential which is defined on the edge elements. Numerical simulations of the MHD flows in an annular channel are carried out under traveling magnetic fields. It is confirmed that the magnitude and oscillation amplitude of the z-component of the velocity becomes larger with the increase in the Hartmann number. It is also shown that the frequency of the velocity pulsation is doubled than the frequency of the externally applied traveling magnetic field.

DOI： 10.1299/jsmecmd.2019.32.028

その他リンク： https://ci.nii.ac.jp/naid/130007817209

ALE法を用いた2次元スロッシング解析における安定性向上の検討

小島拓人，河野晴彦

計算力学講演会講演論文集 ( 日本機械学会 ) 2019.32 73 2019年09月

担当区分：最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

In this study, we investigate the stability required for a two-dimensional sloshing analysis with the use of an arbitrary Lagrangian–Eulerian (ALE) method. Here, two well-known upwind methods are employed for the discretization of the kinematic condition on the free surface: SUPG and CIP methods. On the other hand, the Navier-Stokes equations are discretized by a finite element scheme without any upwind method to see whether the stability is retained solely by introducing an appropriate upwind method into the calculation on the free surface. It is shown in both primary and tertiary modes that stable results are obtained when the CIP method is employed, while unstable grid-scale oscillations are observed at some point when the SUPG method is used. This suggests that a semi-Lagrangian scheme is essential for stable computation based on the ALE formulation.

DOI： 10.1299/jsmecmd.2019.32.073

その他リンク： https://ci.nii.ac.jp/naid/130007817141

正方形ダクト内における交流磁場下の3次元電磁流体流れ解析

坂井駿介，牟禮良晃，河野晴彦

計算力学講演会講演論文集 ( 日本機械学会 ) 2019.32 103 2019年09月

担当区分：最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

In our previous work, a three-dimensional numerical scheme for analyzing magnetohydrodynamic (MHD) flows under alternating-current magnetic fields was constructed, which takes into account the coupling with the electromagnetic fields in the solid and gas regions. In this study, a periodic boundary condition and an external traveling-wave magnetic field are introduced into the numerical scheme to investigate the dependences of the MHD flow in a square duct on the Hartmann number and the phase velocity of the external magnetic field. It is shown that the flow in the axial direction is encouraged with the increase in the Hartmann number or the angular frequency, while an opposite behavior is observed in terms of the oscillation amplitude of the fluid flow.

DOI： 10.1299/jsmecmd.2019.32.103

その他リンク： https://ci.nii.ac.jp/naid/130007817254

Calculation of RF sheath properties from surface wave-fields: a post-processing method 査読有り国際誌

J. R. Myra, H. Kohno

Plasma Physics and Controlled Fusion ( IOP Publishing ) 61 ( 9 ) 095003-1 - 095003-10 2019年07月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

In ion cyclotron range of frequency (ICRF) experiments in fusion research devices, radio frequency (RF) sheaths form where plasma, strong RF wave fields and material surfaces coexist. These RF sheaths affect plasma material interactions such as sputtering and localized power deposition, as well as the global RF wave fields themselves. RF sheaths may be modeled by employing a sheath boundary condition (BC) in place of the more customary conducting wall (CW) BC; however, there are still many ICRF computer codes that do not implement the sheath BC. In this paper we present a method for post-processing results obtained with the CW-BC. The post-processing method produces results that are equivalent to those that would have been obtained with the RF sheath BC, under certain assumptions. The post-processing method is also useful for verification of sheath BC implementations and as a guide to interpretation and understanding of the role of RF sheaths and their interactions with the waves that drive them.

DOI： 10.1088/1361-6587/ab2f41

Kyutacar

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Radio frequency wave interactions with a plasma sheath: The role of wave and plasma sheath impedances 査読有り国際誌

J. R. Myra, H. Kohno

Physics of Plasmas ( AIP Publishing ) 26 ( 5 ) 052503-1 - 052503-14 2019年05月

担当区分：最終著者記述言語：英語掲載種別：研究論文（学術雑誌）

Radio frequency (RF) sheaths form near surfaces where plasma and strong RF fields coexist. The effect of these RF sheaths on wave propagation near the boundary can be characterized by an effective sheath impedance that includes both resistive and capacitive contributions describing RF sheath rectification and RF power absorption in the sheath [J. R. Myra and D. A. D'Ippolito, Phys. Plasmas 22, 062507 (2015)]. Here, we define a dimensionless parameter, the ratio of incoming wave impedance to the sheath impedance, which determines the characteristics of the interaction, ranging from quasi-conducting to quasi-insulating, or in the case of matched impedances, to either perfect absorption or a sheath-plasma resonance. A semi-analytical analysis is carried out for electrostatic slow waves in the ion cyclotron range of frequencies. For the propagating slow wave case, where the incident wave is partially reflected, the fraction of power dissipated in the sheath is calculated. For the evanescent slow wave case, which admits a sheath-plasma resonance, an amplification factor is calculated. Using the impedance ratio approach, RF sheath interactions are characterized for a range of RF wave and plasma parameters including plasma density, magnetic field angle with respect to the surface, wave frequency, and wave-vector components tangent to the surface. For a particularly interesting example case, results are compared with the rfSOL code [H. Kohno and J. R. Myra, Comput. Phys. Commun. 220, 129 (2017)]. Finally, electromagnetic effects, absent from the semi-analytical analysis, are assessed.

DOI： 10.1063/1.5088343

Kyutacar

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Radio-frequency wave interactions with a plasma sheath in oblique-angle magnetic fields using a sheath impedance model 査読有り国際誌

H. Kohno, J. R. Myra

Physics of Plasmas ( AIP Publishing ) 26 ( 2 ) 022507-1 - 022507-15 2019年02月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

The physics of interactions between waves in plasmas and sheaths for background magnetic fields which make oblique angles with sheath surfaces is studied with the use of the self-consistent finite element code rfSOL incorporating the recently developed sheath impedance model [J. R. Myra, Phys. Plasmas 24, 072507 (2017)]. The calculation based on this model employs the generalized sheath boundary condition (sheath BC), which surpasses the previously used capacitive sheath BC in reliability by taking into account the contributions of the ion and electron currents in the sheath and the displacement current. A series of numerical simulations is carried out in two-dimensional slab geometry with a flat or curved sheath surface as part of the boundary. It is shown that the sheath-plasma wave appears when the equilibrium magnetic field line angle with respect to the sheath surface is small, the absolute value of the radio-frequency (RF) sheath voltage is large, and the plasma density is slightly higher than the lower hybrid resonance density (LHR density), all of which bring the sheath property closer to being capacitive. It is also shown that the sharp variation of the magnetic field line angle along the sheath surface can sensitively affect the maximum absolute value of the RF sheath voltage at a plasma density slightly lower than the LHR density.

DOI： 10.1063/1.5054920

Kyutacar

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交流磁場下における導体壁に囲まれた電磁流体流れの3次元数値解析査読有り

牟禮良晃，河野晴彦

日本機械学会論文集 ( 日本機械学会 ) 84 ( 863 ) 17-00589-1 - 17-00589-16 2018年07月

担当区分：最終著者,　責任著者記述言語：日本語掲載種別：研究論文（学術雑誌）

Three-dimensional (3D) numerical simulation of incompressible, magnetohydrodynamic (MHD) flows under alternating-current (AC) magnetic fields are carried out, which takes into account the coupling with the electromagnetic fields in the solid and gas regions. A numerical scheme is constructed by combining the Galerkin finite element method and the edge-element based finite element method, which are applied to the discretizations of the Navier–Stokes equations and the electromagnetic field equations, respectively. The solution algorithm for fluid flow is based on an explicit fractional step approach and the simultaneous relaxation of velocity and pressure to satisfy the continuity equation. The electromagnetic field equations are formulated with the use of the magnetic vector potential which is defined on the edge elements. In the proposed numerical scheme, the advection term in the induction equation is not neglected, because the scheme needs to deal with the condition where the advection term is the same order with the diffusion term in that equation. The validity of the numerical scheme is verified through the analysis of the electromagnetic field under a direct-current magnetic field, and numerical simulations of the MHD flows under spatially uniform AC magnetic fields are carried out. It is confirmed that the spatio-temporal mean Lorentz force in the conducting fluid becomes weaker with the increase in the dimensionless frequency due to the skin effect. It is also shown that the flow pattern in a hexahedral closed domain is largely changed when the frequency is getting higher, which is associated with the change in the Lorentz force profile.

DOI： 10.1299/transjsme.17-00589

その他リンク： https://ci.nii.ac.jp/naid/130007419065

A finite element procedure for radio-frequency sheath-plasma interactions based on a sheath impedance model 査読有り国際誌

H. Kohno, J. R. Myra

Computer Physics Communications ( Elsevier B.V. ) 220 129 - 142 2017年11月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A finite element code that solves self-consistent radio-frequency (RF) sheath-plasma interaction problems is improved by incorporating a generalized sheath boundary condition in the macroscopic solution scheme. This sheath boundary condition makes use of a complex sheath impedance including both the sheath capacitance and resistance, which enables evaluation of not only the RF voltage across the sheath but also the power dissipation in the sheath. The newly developed finite element procedure is applied to cases where the background magnetic field is perpendicular to the sheath surface in one- and two-dimensional domains filled by uniform low- and high-density plasmas. The numerical results are compared with those obtained by employing the previous capacitive sheath model at a typical frequency for ion cyclotron heating used in fusion experiments. It is shown that for sheaths on the order of 100 V in a high-density plasma, localized RF power deposition can reach a level which causes material damage. It is also shown that the sheath-plasma wave resonances predicted by the capacitive sheath model do not occur when parameters are such that the generalized sheath impedance model substantially modifies the capacitive character of the sheath. Possible explanations for the difference in the maximum RF sheath voltage depending on the plasma density are also discussed.

DOI： 10.1016/j.cpc.2017.06.025

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A post-processing method to simulate the generalized RF sheath boundary condition 査読有り国際誌

James R. Myra, Haruhiko Kohno

EPJ Web of Conferences ( EDP Sciences ) 157 03037-1 - 03037-4 2017年10月

担当区分：最終著者記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

For applications of ICRF power in fusion devices, control of RF sheath interactions is of great importance. A sheath boundary condition (SBC) was previously developed to provide an effective surface impedance for the interaction of the RF sheath with the waves. The SBC enables the surface power flux and rectified potential energy available for sputtering to be calculated. For legacy codes which cannot easily implement the SBC, or to speed convergence in codes which do implement it, we consider here an approximate method to simulate SBCs by post-processing results obtained using other, e.g. conducting wall, boundary conditions. The basic approximation is that the modifications resulting from the generalized SBC are driven by a fixed incoming wave which could be either a fast wave or a slow wave. The method is illustrated in slab geometry and compared with exact numerical solutions; it is shown to work very well.

DOI： 10.1051/epjconf/201715703037

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A mixed-interpolation finite element method for incompressible thermal flows of electrically conducting fluids 査読有り国際誌

Haruhiko Kohno

International Journal for Numerical Methods in Fluids ( John Wiley & Sons ) 83 ( 11 ) 813 - 840 2017年04月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A new mixed-interpolation finite element method is presented for the two-dimensional numerical simulation of incompressible magnetohydrodynamic (MHD) flows which involve convective heat transfer. The proposed method applies the nodal shape functions, which are locally defined in nine-node elements, for the discretization of the Navier–Stokes and energy equations, and the vector shape functions, which are locally defined in four-node elements, for the discretization of the electromagnetic field equations. The use of the vector shape functions allows the solenoidal condition on the magnetic field to be automatically satisfied in each four-node element. In addition, efficient approximation procedures for the calculation of the integrals in the discretized equations are adopted to achieve high-speed computation. With the use of the proposed numerical scheme, MHD channel flow and MHD natural convection under a constant applied magnetic field are simulated at different Hartmann numbers. The accuracy and robustness of the method are verified through these numerical tests in which both undistorted and distorted meshes are employed for comparison of numerical solutions. Furthermore, it is shown that the calculation speed for the proposed scheme is much higher compared with that for a conventional numerical integration scheme under the condition of almost the same memory consumption.

DOI： 10.1002/fld.4292

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ベクトル有限要素法を用いた交流磁場下における3次元電磁流体解析

牟禮良晃，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2017.70 257 - 258 2017年01月

MPIおよびCUDAによる熱流体解析の高速化

宇都信一，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2017.70 255 - 256 2017年01月

2次元移流問題におけるCIP法に基づく離散化手法の比較

河野雄太，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2017.70 259 - 260 2017年01月

An improved rf-sheath boundary condition and implications for ICRF modeling 査読有り国際誌

J. R. Myra, H. Kohno

26th IAEA Fusion Energy Conference, Kyoto, Japan, 17-22 October 2016, paper IAEA-CN-234/TH/P4-31 2016年10月

Erratum: "Numerical investigation of fast-wave propagation and radio-frequency sheath interaction with a shaped tokamak wall" [Phys. Plasmas 22, 072504 (2015)] 査読有り国際誌

H. Kohno, J. R. Myra, D. A. D'Ippolito

Physics of Plasmas ( AIP Publishing ) 23 ( 8 ) 089901-1 - 089901-4 2016年08月

流れの有限要素法による自由表面の変形を有する電磁流体の数値解析

梶山祐甫，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2016.69 351 - 352 2016年01月

CUDAによる電磁熱流体解析の高速化

迫田卓，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2016.69 349 - 350 2016年01月

An efficient, high-order finite element method using the nodal averaging technique for incompressible fluid flows 査読有り国際誌

Haruhiko Kohno

Computer Physics Communications ( Elsevier B.V. ) 195 68 - 76 2015年10月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A new finite element method is presented for use of quadrilateral nine-node elements in the solution of the incompressible Navier-Stokes equations. In a conventional predictor-corrector scheme, the method applies the nodal averaging technique to discretize the Poisson equation used for the simultaneous relaxation of velocity and pressure. Additionally, efficient approximation procedures are devised to increase the speed of computation without deteriorating solution accuracy. The proposed numerical schemes are evaluated on two-dimensional test problems including a classical lid-driven cavity flow and a flow over a backward-facing step in a flow channel. The results show good accuracy even when distorted elements are used for calculation.

DOI： 10.1016/j.cpc.2015.05.002

Numerical investigation of fast-wave propagation and radio-frequency sheath interaction with a shaped tokamak wall 査読有り

H. Kohno, J. R. Myra, D. A. D'Ippolito

Physics of Plasmas ( AIP Publishing ) 22 ( 7 ) 072504-1 - 072504-12 2015年07月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Interactions between propagating fast waves and radio-frequency (RF) sheaths in the ion cyclotron range of frequencies are numerically investigated based on a cold fluid plasma model coupled with a sheath boundary condition. In this two-dimensional study, the capability of the finite element code rfSOL, which was developed in previous numerical work, is extended to analyze self-consistent RF sheath-plasma interaction problems in a tokamak with a non-circular cross-section. It is found that a large sheath voltage is generated near the edges of the limiter-shaped deformation as a result of the conversion from fast to slow waves on the sheaths. The sheath voltage associated with this conversion is particularly significant in the localized region where the contact angle between the magnetic field line and the conducting wall varies rapidly along the curved sheath surface, which is consistent with the results in previous one-dimensional theoretical work. The dependences of the RF sheaths on various parameters in plasma such as the toroidal wavenumber, edge plasma density, and the degree of the RF wave absorption in the core region are also examined in detail.

DOI： 10.1063/1.4926449

直流磁場下における上昇気泡の数値解析

田中宏幸，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2015.68 3 - 4 2015年01月

ハイブリッド並列による電磁熱流体解析の高速化

迫田卓，宇都信一，河野晴彦

日本機械学会九州支部講演論文集 ( 日本機械学会 ) 2015.68 1 - 2 2015年01月

Calculation of contact angles at triple phase boundary in solid oxide fuel cell anode using the level set method 査読有り国際誌

Xiaojun Sun, Yosuke Hasegawa, Haruhiko Kohno, Zhenjun Jiao, Koji Hayakawa, Kohei Okita, Naoki Shikazono

Materials Characterization ( Elsevier B.V. ) 96 100 - 107 2014年10月

記述言語：英語掲載種別：研究論文（学術雑誌）

A level set method is applied to characterize the three dimensional structures of nickel, yttria stabilized zirconia and pore phases in solid oxide fuel cell anode reconstructed by focused ion beam-scanning electron microscope. A numerical algorithm is developed to evaluate the contact angles at the triple phase boundary based on interfacial normal vectors which can be calculated from the signed distance functions defined for each of the three phases. Furthermore, surface tension force is estimated from the contact angles by assuming the interfacial force balance at the triple phase boundary. The average contact angle values of nickel, yttria stabilized zirconia and pore are found to be 143°-156°, 83°-138° and 82°-123°, respectively. The mean contact angles remained nearly unchanged after 100 hour operation. However, the contact angles just after reduction are different for the cells with different sintering temperatures. In addition, standard deviations of the contact angles are very large especially for yttria stabilized zirconia and pore phases. The calculated surface tension forces from mean contact angles were close to the experimental values found in the literature. Slight increase of surface tensions of nickel/pore and nickel/yttria stabilized zirconia were observed after operation. Present data are expected to be used not only for the understanding of the degradation mechanism, but also for the quantitative prediction of the microstructural temporal evolution of solid oxide fuel cell anode.

DOI： 10.1016/j.matchar.2014.07.020

その他リンク： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=84905373556&origin=inward

Role of magnetic field tangency points in ICRF sheath interactions 査読有り国際誌

J. R. Myra, D. A. D'Ippolito, H. Kohno

AIP Conference Proceedings ( AIP Publishing ) 1580 ( 1 ) 322 - 325 2014年02月

担当区分：最終著者記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

ICRF waves can sometimes interact with plasma-facing surfaces in tokamak fusion experiments causing degradation of core heating efficiency, impurity injection and even component damage. While presently available low dimensionality if sheath models are useful in understanding many features of these interactions, more quantitative modeling will require attention to realistic geometrical details of the boundary plasma and surfaces. In this paper, we explore the situation in which there exists a tangency point of the background magnetic field with a surface. We find that the rf interactions are strongly influenced by the generation and propagation of sheath-plasma waves (SPW) along the surface. It is found that these waves preferentially propagate towards, and accumulate at, a convex tangency point. An analytical theory of SPW propagation is developed to understand these features.

DOI： 10.1063/1.4864553

Detailed transport-reaction models for SOFC Ni-YSZ patterned anodes: A critical inquiry 査読有り国際誌

Haruhiko Kohno, Shixue Liu, Teppei Ogura, Takayoshi Ishimoto, Dayadeep S. Monder, Kunal Karan, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 2821 - 2830 2013年10月

担当区分：筆頭著者記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

Understanding the detailed physicochemical mechanisms for the electro-oxidation of fuel at the triple phase boundaries (TPBs) in solid oxide fuel cell (SOFC) anodes is a key step towards improving SOFC performance. Significant efforts have been directed toward this goal via both experimental and computational modeling studies. In particular, patterned Ni anode on a planar YSZ substrate has been examined because of its well-defined two-dimensional geometry that in principle offers the advantages of known TPB length and minimization of mass transport effects. A common formulation for such patterned anodes considers modeling of surface reactions and diffusion on both electrode and electrolyte surfaces, along with charge-transfer reactions at the TPBs based on elementary kinetics. In this study, we review the theoretical approaches proposed by several authors and offer a critique of their methods.

DOI： 10.1149/05701.2821ecst

その他リンク： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=84905040215&origin=inward

Molecular dynamics simulation studies of H diffusion in SOFC anode using reactive force field 査読有り国際誌

Leton C. Saha, Kazuhide Nakao, Haruhiko Kohno, Takayoshi Ishimoto, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 2649 - 2654 2013年10月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

Employing reactive molecular dynamics simulation, we investigated diffusion pathways of H-atoms into the bulk from the Ni(100) surface. We found that surface pathway is dominant compared to bulk pathway for H atoms diffusion. Our simulation suggests that H atoms can diffuse into the bulk Ni. From 973 to 1273 K, H atoms migration into bulk is observed, but number of H atom is influenced by the temperature. At 1273 K, 30 % H atoms move into the bulk region. Diffusion barrier of H into bulk is calculated higher than the surface pathway. Diffusion coefficient of H between bulk and surface pathway is calculated to be one order of magnitude discrepancy.

DOI： 10.1149/05701.2649ecst

Molecular dynamics study for sintering property analysis of Ni-YSZ cermet 査読有り国際誌

Kazuhide Nakao, Haruhiko Kohno, Takayoshi Ishimoto, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 1407 - 1413 2013年10月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

Microstructural change of porous electrodes in SOFC caused by sintering affected by materials and impurities is one of the main factors of the long-term degradation. Since it is difficult to clarify the effects of materials and impurities on sintering by experiments, atomistic scale simulation is important. In this study, we performed molecular dynamics (MD) simulation of porous structure of Ni and analyzed sintering properties of porous structure by employing master sintering curve theory. We have succeeded to obtain the sintering activation energy and its size dependency and to identify the dominant sintering mechanism by comparing sintering activation energy with activation energy of surface diffusion on each facet.

DOI： 10.1149/05701.1407ecst

First-principles calculations of the anodic oxidation reactions of solid oxide fuel cell: Oxygen potential effect on Nickel (111) surface 査読有り国際誌

Shixue Liu, Takayoshi Ishimoto, Haruhiko Kohno, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 2429 - 2436 2013年10月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

The anodic reactions of solid oxide fuel cells are examined by density functional theory method under different oxygen coverage on the Ni(111) surface. The oxidation reactions H, OH and CO are calculated and the reaction barriers are affected by the oxygen potential. The concept of oxygen potential is introduced to discuss the effect of the surface chemical potential to the oxidation reactions. The results are valuable for estimating the reaction rate of anodic reactions under different oxygen potential near the anodic three phase boundary.

DOI： 10.1149/05701.2429ecst

Density functional theory calculation of spin-state transition in LaCoO3 査読有り国際誌

Takayoshi Ishimoto, Yumi Ito, Haruhiko Kohno, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 2655 - 2660 2013年10月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

We analyzed the dependence of X-ray absorption spectra for Co 2p orbital of LaCoO3 on the Hubbard U and lattice parameters by using the density functional theory (DFT) method. The X-ray absorption spectra and partial density of states (PDOS) are studied with the different U parameters ranging from 2.5 to 7.8. The characteristic peak position of X-ray absorption spectra is also investigated for different structures: i.e., the O atom positions assuming the displacement by thermal vibrations, the lattice expansion and the crystalline lattice distortion. The effect of the above parameters on the X-ray absorption spectra are discussed on the basis of theoretical calculation results.

DOI： 10.1149/05701.2655ecst

Detailed kinetic modeling for SOFC Ni pattern anodes fuelled by methane 査読有り国際誌

Teppei Ogura, Haruhiko Kohno, Shixue Liu, Takayoshi Ishimoto, Michihisa Koyama, Hisayoshi Tsukikawa, Masaki Tajima

ECS Transactions ( IOP Publishing ) 57 ( 1 ) 2865 - 2870 2013年10月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

High SOFC operating temperature allows direct internal reforming (DIR) of methane within the SOFC anode. However, carbon formation reduces considerably the performance and durability of the cells. A key step towards improving SOFC performance is to understand the detailed surface kinetics and electrochemistry for DIR of methane at the triple phase boundaries (TPBs) in SOFC anodes. In this study, we have performed kinetic simulations with a numerical model on Ni pattern anode, combined with surface reactions, charge transfer reactions, and diffusions for DIR of methane. The I-V characteristics of SOFC near TPBs are discussed.

DOI： 10.1149/05701.2865ecst

Radio-frequency sheath-plasma interactions with magnetic field tangency points along the sheath surface 査読有り国際誌

H. Kohno, J. R. Myra, D. A. D'Ippolito

Physics of Plasmas ( AIP Publishing ) 20 ( 8 ) 082514-1 - 082514-9 2013年08月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

Computer simulations of radio-frequency (RF) waves propagating across a two-dimensional (2D) magnetic field into a conducting boundary are described. The boundary condition for the RF fields at the metal surface leads to the formation of an RF sheath, which has previously been studied in one-dimensional models. In this 2D study, it is found that rapid variation of conditions along the sheath surface promote coupling of the incident RF branch (either fast or slow wave) to a short-scale-length sheath-plasma wave (SPW). The SPW propagates along the sheath surface in a particular direction dictated by the orientation of the magnetic field with respect to the surface, and the wave energy in the SPW accumulates near places where the background magnetic field is tangent to the surface.

DOI： 10.1063/1.4818991

A new method for the level set equation using a hierarchical-gradient truncation and remapping technique 査読有り国際誌

Haruhiko Kohno, Jean-Christophe Nave

Computer Physics Communications ( Elsevier B.V. ) 184 ( 6 ) 1547 - 1554 2013年06月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

We present a novel numerical method for solving the advection equation for a level set function. The new method uses hierarchical-gradient truncation and remapping (H-GTaR) of the original partial differential equation (PDE). Our strategy reduces the original PDE to a set of decoupled linear ordinary differential equations with constant coefficients. Additionally, we introduce a remapping strategy to periodically guarantee solution accuracy for a deformation problem. The proposed scheme yields nearly an exact solution for a rigid body motion with a smooth function that possesses vanishingly small higher derivatives and calculates the gradient of the advected function in a straightforward way. We will evaluate our method in one- and two-dimensional domains and present results to several classical benchmark problems.

DOI： 10.1016/j.cpc.2013.02.002

Molecular dynamics study for sintering characteristics of solid oxide fuel cell anode 査読有り国際誌

Kazuhide Nakao, Haruhiko Kohno, Takayoshi Ishimoto, Michihisa Koyama

ECS Transactions ( IOP Publishing ) 50 ( 30 ) 1 - 9 2013年05月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

The prediction of long-term durability of SOFC anode is important to understand the sintering phenomena in the anode, which are affected by microstructure, materials, and impurities. To understand the sintering phenomena considering the effects of materials and chemical species, molecular dynamics (MD) simulation is one of the most effective computational methods. In this study, we developed a theoretical methodology based on the master sintering curve to analyze the sintering property. We performed MD simulation for the densification of both Ni and YSZ porous structures. The sintering activation energy obtained from our proposed method was in reasonable agreement with the experimental result.

DOI： 10.1149/05030.0001ecst

Applications of computational chemistry to designing materials and microstructure in fuel cell technologies 査読有り国際誌

Michihisa Koyama, Haruhiko Kohno, Teppei Ogura, Takayoshi Ishimoto

Journal of Computer Chemistry, Japan ( 日本コンピュータ化学会 ) 12 ( 1 ) 1 - 7 2013年03月

A finite element procedure for radio-frequency sheath–plasma interactions in the ion cyclotron range of frequencies 査読有り国際誌

H. Kohno, J. R. Myra, D. A. D'Ippolito

Computer Physics Communications ( Elsevier B.V. ) 183 ( 10 ) 2116 - 2127 2012年10月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A numerical code that solves self-consistent radio-frequency (RF) sheath-plasma interactions in the ion cyclotron range of frequencies is developed based on a nonlinear finite element technique. The present code solves for plasma waves based on the cold plasma model subject to a sheath boundary condition. The finite element procedure has been implemented for one- and two-dimensional analyses using simplified models for the poloidal plane of a tokamak. The results show good accuracy and generalize previous analytical calculations. The present algorithmic approach shows promise for developing a code to predict RF sheath potentials in the scrape-off layer of RF-heated fusion experiments.

DOI： 10.1016/j.cpc.2012.05.014

Numerical analysis of radio-frequency sheath-plasma interactions in the ion cyclotron range of frequencies 査読有り国際誌

H. Kohno, J. R. Myra, D. A. D'Ippolito

Physics of Plasmas ( AIP Publishing ) 19 ( 1 ) 012508-1 - 012508-9 2012年01月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A new finite element numerical scheme for analyzing self-consistent radio-frequency (RF) sheath-plasma interaction problems in the ion cyclotron range of frequencies is applied to various problems represented by simplified models for the tokamak scrape-off layer. The present code incorporates a modified boundary condition, which is called a sheath boundary condition, that couples the radio-frequency waves and sheaths at the material boundaries by treating the sheath as a thin vacuum layer. A series of numerical analyses in one- and two-dimensional domains show several important physical properties, such as the existence of multiple roots, hysteresis effects, presence and characteristics of the sheath-plasma waves, and the phase shift of a reflected slow wave, some of which are newly identified by introducing a spatially varying plasma density and background magnetic field.

DOI： 10.1063/1.3677262

Progress on quantitative modeling of rf sheaths 招待有り査読有り国際誌

D. A. D'Ippolito, J. R. Myra, H. Kohno, J. C. Wright

AIP Conference Proceedings ( AIP Publishing ) 1406 ( 1 ) 195 - 202 2011年12月

記述言語：英語掲載種別：研究論文（国際会議プロシーディングス）

A new quantitative approach for computing the rf sheath potential is described, which incorporates plasma dielectric effects and the relative geometry of the magnetic field and the material boundaries. The new approach uses a modified boundary condition ("rf sheath BC") that couples the rf waves and the sheaths at the boundary. It treats the sheath as a thin vacuum region and matches the fields across the plasma-vacuum boundary. When combined with the Child-Langmuir Law (relating the sheath width and sheath potential), the model permits a self-consistent determination of the sheath parameters and the rf electric field at the sheath-plasma boundary. Semi-analytic models using this BC predict a number of general features, including a sheath voltage threshold, a dimensionless parameter characterizing rf sheath effects, and the existence of sheath plasma waves with an associated resonance. Since the sheath BC is nonlinear and dependent on geometry, computing the sheath potential numerically is a challenging computational problem. Numerical results will be presented from a new parallel-processing finite-element rf wave code for the tokamak scrape-off layer (called "rfSOL"). The code has verified the physics predicted by analytic theory in 1D, and extended the solutions into model 2D geometries. The numerical calculations confirm the existence of multiple roots and hysteresis effects, and parameter studies have been carried out. Areas for future work will be discussed.

DOI： 10.1063/1.3664959

その他リンク： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=84857973915&origin=inward

A finite element procedure for multiscale wave equations with application to plasma waves 査読有り国際誌

Haruhiko Kohno, Klaus-Jürgen Bathe, John C. Wright

Computers & Structures ( Elsevier B.V. ) 88 ( 1-2 ) 87 - 94 2010年01月

担当区分：筆頭著者,　責任著者記述言語：英語掲載種別：研究論文（学術雑誌）

A finite element wave-packet procedure is presented to solve problems of wave propagation in multiscale behavior. The proposed scheme combines the advantages of the finite element and spectral methods. The basic formulation is presented, and the capabilities of the procedure are demonstrated through the solution of some illustrative problems, including a problem that characterizes the mode-conversion behavior in plasmas.

DOI： 10.1016/j.compstruc.2009.05.001