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- Yokoi, Nobumitsu
(author)
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Unappreciated cross-helicity effects in plasma physics : anti-diffusion effects in dynamo and momentum transport
- 2024
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In: REVIEWS OF MODERN PLASMA PHYSICS. - : Springer Nature. - 2367-3192. ; 7:1
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Research review (peer-reviewed)abstract
- The cross-helicity (velocity-magnetic-field correlation) effects in the magnetic-field induction and momentum transport in the magnetohydrodynamic (MHD) turbulence are investigated with the aid of the multiple-scale renormalized perturbation expansion analysis, which is a theoretical framework for strongly non-linear and inhomogeneous turbulence. The outline of the theory is presented with reference to the role of the cross-interaction response functions between the velocity and magnetic field. In this formulation, the expressions of the turbulent fluxes: the turbulent electromotive force (EMF) in the mean induction equation and the Reynolds and turbulent Maxwell stresses in the momentum equation are obtained. Related to the expression of EMF, the physical origin of the cross-helicity effect in dynamos, as well as other dynamo effects, is discussed. Properties of dynamo and momentum transport are determined by the spatiotemporal distribution of turbulence. To understand the actual role of the turbulent cross helicity, its transport equations is considered. Several generation mechanisms of cross helicity are discussed with illustrative examples. On the basis of the cross-helicity production mechanisms, its effect in stellar dynamos is discussed. The role of cross helicity in the momentum transport and global flow generation is also argued. One of the situations where the cross-helicity effects both in magnetic-field induction and global flow generation play an important role is the turbulent magnetic reconnection. Characteristic features of turbulence effects in fast reconnection are reviewed with special emphasis on the role of cross helicity in localizing the effective resistivity. Finally, a remark is addressed on an approach that elucidates the structure generation and sustainment in extremely strong turbulence. An appropriate formulation for the anti-diffusion effect, which acts against the usual diffusion effect, is needed. Turbulence modeling approach based on such an analytical formulation is also argued in comparison with the conventional heuristic modeling. The importance of the self-consistent framework treating the non-linear interaction between the mean field and turbulence is stressed as well.
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| 3. |
- Yokoi, N., et al.
(author)
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Large-scale flow generation by inhomogeneous helicity
- 2016
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In: PHYSICAL REVIEW E. - : American Physical Society. - 2470-0045. ; 93:3
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Journal article (peer-reviewed)abstract
- The effect of kinetic helicity (velocity-vorticity correlation) on turbulent momentum transport is investigated. The turbulent kinetic helicity (pseudoscalar) enters the Reynolds stress (mirror-symmetric tensor) expression in the form of a helicity gradient as the coupling coefficient for the mean vorticity and/or the angular velocity (axial vector), which suggests the possibility of mean-flow generation in the presence of inhomogeneous helicity. This inhomogeneous helicity effect, which was previously confirmed at the level of a turbulence-or closure-model simulation, is examined with the aid of direct numerical simulations of rotating turbulence with nonuniform helicity sustained by an external forcing. The numerical simulations show that the spatial distribution of the Reynolds stress is in agreement with the helicity-related term coupled with the angular velocity, and that a large-scale flow is generated in the direction of angular velocity. Such a large-scale flow is not induced in the case of homogeneous turbulent helicity. This result confirms the validity of the inhomogeneous helicity effect in large-scale flow generation and suggests that a vortex dynamo is possible even in incompressible turbulence where there is no baroclinicity effect.
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| 6. |
- Yokoo, Masaki, et al.
(author)
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Role of the hyaluronan receptor CD44 during porcine oocyte maturation
- 2007
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In: Journal of reproduction and development. - : Society for Reproduction and Development. - 0916-8818 .- 1348-4400. ; 53:2, s. 263-270
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Journal article (peer-reviewed)abstract
- Previous our studies have shown that CD44, the principal receptor for hyaluronan, is present on cumulus cells during oocyte maturation. Although hyaluronan-CD44 interaction has been implicated in cumulus expansion and/or oocyte maturation, the full significance of CD44 remains unknown. The objective of the present study was to further investigate the role of CD44 in cumulus expansion and oocyte maturation in pigs. We demonstrate here in that CD44 has a key role in oocyte maturation but not in cumulus expansion. Previous studies have reported the physiological significance of cumulus expansion in oocyte maturation. However, our results suggest that cumulus expansion is a necessary condition for oocyte maturation, but that it is not sufficient on its own. Furthermore, western blot analysis demonstrated that the CD44 of the in vitro-matured cumulus-oocyte complexes (COCs) had a larger molecular weight and more terminal sialic acid, which has been proven to inhibit the hyaluronan-binding ability of the receptor, than the CD44 of the in vivo-matured COCs, indicating that the hyaluronan-CD44 interactions during in vitro maturation might be insufficient compared with those in vivo. The insufficient interactions of hyaluronan-CD44 during in vitro maturation may cause the inferior capacity of fertilization and development of oocytes matured in vitro.
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