Papers - Kobayashi, Hiromichi

Highly Reproducible Synthesis of Hollow Zirconia Particles via Atmospheric-Pressure Plasma Processing with Inkjet Droplets

Nitta K., Sakai T., Muneoka H., Shimizu Y., Kobayashi H., Terashima K., Ito T.

Plasma Chemistry and Plasma Processing (Plasma Chemistry and Plasma Processing)  44 ( 1 ) 289 - 303 2024.01

ISSN  02724324

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Hollow particles have attracted considerable attention owing to their unique properties. In this study, hollow monoclinic zirconia particles were directly synthesized from inkjet droplets of a zirconyl hydroxychloride aqueous solution via atmospheric-pressure plasma processing. Hollow structures with craggy surfaces were obtained in the plasma at gas temperatures above 1000 K. The steep solvent evaporation rate induced by the localized high-energy reaction field of the atmospheric-pressure plasma may have induced solute condensation near the droplet surface and contributed to the formation of hollow particles. The average diameter of the synthesized particles was ~ 3 μm, while their size distribution was narrow (coefficient of variation: 0.06–0.10). The high reproducibility of the synthesized particles was attributed to the small variations in inkjet droplet size. The proposed method enables the rapid synthesis of hollow particles of various inorganic materials, while controlling their number and composition.

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Imaging quantized vortex rings in superfluid helium to evaluate quantum dissipation

Tang Y., Guo W., Kobayashi H., Yui S., Tsubota M., Kanai T.

Nature Communications (Nature Communications)  14 ( 1 )  2023.12

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The motion of quantized vortices is responsible for many intriguing phenomena in diverse quantum-fluid systems. Having a theoretical model to reliably predict the vortex motion therefore promises a broad significance. But a grand challenge in developing such a model is to evaluate the dissipative force caused by thermal quasiparticles in the quantum fluids scattering off the vortex cores. Various models have been proposed, but it remains unclear which model describes reality due to the lack of comparative experimental data. Here we report a visualization study of quantized vortex rings propagating in superfluid helium. By examining how the vortex rings spontaneously decay, we provide decisive data to identify the model that best reproduces observations. This study helps to eliminate ambiguities about the dissipative force acting on vortices, which could have implications for research in various quantum-fluid systems that also involve similar forces, such as superfluid neutron stars and gravity-mapped holographic superfluids.

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Analysis of anisotropic subgrid-scale stress for coarse large-eddy simulation

Inagaki K., Kobayashi H.

Physical Review Fluids (Physical Review Fluids)  8 ( 10 )  2023.10

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This study discusses the necessity of anisotropic subgrid-scale (SGS) stress in large-eddy simulations (LESs) of turbulent shear flows using a coarse grid resolution. We decompose the SGS stress into two parts to observe the role of SGS stress in turbulent shear flows in addition to the energy transfer between grid-scale (GS or resolved scale) and SGS. One is the isotropic eddy-viscosity term, which contributes to energy transfer, and the other is the residual anisotropic term, which is separated from the energy transfer. We investigate the budget equation for GS Reynolds stress in turbulent channel flows accompanied by the SGS stress decomposition. In addition, we examine the medium and coarse filter length cases; the conventional eddy-viscosity models can fairly predict the mean velocity profile for the medium filter case and fails for the coarse filter case. The budget for GS turbulent kinetic energy shows that the anisotropic SGS stress has a negligible contribution to energy transfer. In contrast, the anisotropic stress has a large and nondissipative contribution to the streamwise and spanwise components of GS Reynolds stress when the filter size is large. Even for the medium-size filter case, the anisotropic stress contributes positively to the budget for the spanwise GS Reynolds stress. Spectral analysis of the budget reveals that the positive contribution is prominent at a scale consistent with the spacing of streaks in the near-wall region. Therefore, we infer that anisotropic stress contributes to the generation mechanism of coherent structures. Predicting the positive contribution of the anisotropic stress to the budget is key to further improving SGS models.

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Experimental and theoretical analyses on power generation characteristics of co-axial MHD energy conversion device

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

Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi) (Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi))  216 ( 3 )  2023.09

ISSN  04247760

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Experiments on power generation were conducted under the open-circuit condition to validate theoretical analyses on the power generation characteristics of a co-axial MHD energy conversion device. Considering distribution of externally applied magnetic field and the electromotive force induced by the rotation of the conductive inner cylinder, the results show that the experimental open-circuit voltage was lower than the theoretical one for the wide channel width, whereas the experimental open-circuit voltage was higher than the theoretical one for the narrow channel width. The inclination of the inner cylinder might cause the decrease in effective radial ratio.

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Evaporation behavior of liquid microdroplets in atmospheric-pressure nonequilibrium plasma

Nitta K., Muneoka H., Shimizu Y., Kobayashi H., Terashima K., Ito T.

Plasma Sources Science and Technology (Plasma Sources Science and Technology)  32 ( 5 )  2023.05

ISSN  09630252

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In recent years, atmospheric-pressure nonequilibrium plasma processing using microdroplets has attracted significant attention. To improve the controllability of this process, an understanding of the evaporation behavior of droplets in plasma is highly desirable. In this study, we examine the evaporation behavior of well-controlled inkjet droplets in atmospheric-pressure nonequilibrium argon plasma through both experiments and modeling. A comparison of the droplet evaporation model based on energy balance considering gas temperature, electron and ion collisions, and recombination reactions with experimental evaporation behavior suggests that droplet evaporation is enhanced in high-density plasma environments with electron and ion densities exceeding 1019 m−3 when compared with that in non-ionized gaseous environments at a gas temperature below 1000 K.

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Experimental and Theoretical Analyses on Power Generation Characteristics of Co-axial MHD Energy Conversion Device

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

IEEJ Transactions on Power and Energy (IEEJ Transactions on Power and Energy)  143 ( 7 ) 454 - 460 2023

ISSN  03854213

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Experiments on power generation were conducted under the open-circuit condition to validate theoretical analyses on the power generation characteristics of a co-axial MHD energy conversion device. Considering distribution of externally applied magnetic field and the electromotive force induced by the rotation of the conductive inner cylinder, the results show that the experimental open-circuit voltage was lower than the theoretical one for the wide channel width, whereas the experimental open-circuit voltage was higher than the theoretical one for the narrow channel width. The inclination of the inner cylinder might cause the decrease in effective radial ratio.

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Universal Anomalous Diffusion of Quantized Vortices in Ultraquantum Turbulence

Satoshi Yui, Yuan Tang, Wei Guo, Hiromichi Kobayashi, Makoto Tsubota

Physical Review Letters (American Physical Society)  129 ( 2 ) 025301  2022.07

Research paper (scientific journal), Accepted,  ISSN  00319007

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In classical viscous fluids, turbulent eddies are known to be responsible for the rapid spreading of embedded particles. However, in inviscid quantum fluids where the turbulence is induced by a chaotic tangle of quantized vortices, dispersion of the particles can be achieved via a nonclassical mechanism, i.e., their binding to the evolving vortices. However, knowledge on how the vortices diffuse and spread in quantum-fluid turbulence is very limited, especially for the so-called ultraquantum turbulence (UQT) generated by a random tangle of vortices. Here we report a systematic numerical study of the apparent diffusion of vortices in UQT in superfluid helium-4 using the full Biot-Savart simulation. We reveal that the vortices in the superfluid exhibit a universal anomalous diffusion (superdiffusion) at small times, which transits to normal diffusion at large times. This behavior is found to be the result of a generic scaling property of the vortex velocity. Our simulation at finite temperatures also nicely reproduces recent experimental observations. The knowledge obtained from this study may form the base for understanding turbulent transport and universal vortex dynamics in various quantum fluids.

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Transport and modeling of subgrid-scale turbulent kinetic energy in channel flows

Kazuhiro Inagaki, Hiromichi Kobayashi

AIP Advances (AIP Advances)  12 ( 4 ) 045222 2022.04

Research paper (scientific journal), Joint Work, Accepted

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To develop a more convenient subgrid-scale (SGS) model that performs well even in coarse grid cases, we investigate the transport and modeling of SGS turbulent kinetic energy (hereafter SGS energy) in turbulent channel flows based on the stabilized mixed model (SMM). In this paper, we try to increase the convenience of the SMM by replacing the modeled transport equation for the SGS energy with an algebraic model. The SMM quantitatively adequately predicts the total turbulent kinetic energy of the direct numerical simulation (DNS) even in coarse grid cases. For both the filtered DNS (fDNS) and large-eddy simulation (LES), the statistically averaged production term balances with the dissipation in the region away from the wall in the SGS energy transport equation. In contrast, we reveal that the correlation coefficient between the production and dissipation terms is high for the modeled transport equation in LES, whereas that for the fDNS is low. Based on the high correlation or local equilibrium between the production and dissipation observed in the LES, we demonstrate the reduction of the SMM into a zero-equation SMM (ZE-SMM). We construct a new damping function based on the grid-scale Kolmogorov length to reproduce the near-wall behavior of the algebraic model for the SGS energy. The ZE-SMM provides quantitatively the same performance as the original SMM that employs the SGS energy transport model. This result suggests that the local equilibrium model for the SGS energy provides the equivalent performance as the transport model in wall-bounded turbulent flows even in coarse grid cases.

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Quasi-Two-Dimensional Numerical Analysis of Influence of Taylor Vortex and Eddy Current on Generation Characteristics of Co-axial MHD Energy Conversion Device under Laminar Flow

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

IEEJ Transactions on Power and Energy (IEEJ Transactions on Power and Energy)  142 ( 5 ) 268 - 274 2022

ISSN  03854213

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Quasi-two-dimensional magnetohydrodynamic (MHD) numerical simulation was conducted to clarify the power generation characteristics of a co-axial MHD energy conversion device under laminar flow. The numerical results were compared with one-dimensional theoretical results in the radial direction. For a fixed load resistance, the dependence of output power and efficiency on Hartmann number, which is proportional to the magnetic flux density, was investigated. We found the decrease in mean velocity and the increase in viscous wall shear stress owing to upper and lower walls, the increase in viscous wall shear stress owing to Taylor vortex cells, and the increase in Joule heating owing to eddy current in the MHD numerical simulation compared to those in the one-dimensional theoretical analysis.

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LES OF MHD TURBULENT TAYLOR-COUETTE FLOW IN AXIAL MAGNETIC FIELD

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

12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022 (12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022)   2022

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Large eddy simulation (LES) of magnetohydrodynamic (MHD) turbulent Taylor-Couette (TC) flows is conducted to reveal the effect of the Hartmann number proportional to the axial magnetic field strength on the flow and electric current fields. The upper-lower walls are insulated and the inner-outer sidewalls are set to insulating walls (open condition) or perfectly conducting walls (short condition). As increasing the Hartmann number, turbulent fluctuations are suppressed, and the turbulent flow distributions approach the MHD laminar flow distributions. The orientations of turbulent vortices change from the azimuthal flow direction to the axial magnetic field direction even in the TC flow. The triplet high-speed streaky structures are uncovered near the inner wall.

INVESTIGATION OF SUBGRID-SCALE TURBULENT KINETIC ENERGY IN CHANNEL FLOWS

Inagaki K., Kobayashi H.

12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022 (12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022)   2022

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We investigate the subgrid-scale (SGS) turbulent kinetic energy (hereafter referred to as SGS energy) and its transport equation by employing the stabilized mixed model (SMM) developed by Abe (2013). The prediction of the budget for the modeled transport equation is qualitatively well compared with the filtered DNS via the Gaussian filter. In addition, the production term balances with the dissipation rate in the outer region, which suggests that the transport equation can be reduced by assuming the equilibrium between them. We investigate five algebraic expressions for the SGS energy. Among them, the equilibrium form with a newly introduced damping function yields the best compared with the SMM. The zero-equation SMM (ZE-SMM) that employs the equilibrium form instead of the transport equation for the SGS energy provides quantitatively equivalent results as the original SMM. This study paves the way for constructing a convenient anisotropic SGS model that is robust even in coarse grid resolutions.

Tree Cutting Approach for Domain Partitioning on Forest-of-Octrees-based Block-structured Static Adaptive Mesh Refinement with Lattice Boltzmann Method

Yuta Hasegawa, Takayuki Aoki, Hiromichi Kobayashi, Yasuhiro Idomura, Naoyuki Onodera

Parallel Computing (Parallel Computing)  108   102851 2021.12

Research paper (scientific journal), Accepted,  ISSN  01678191

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The aerodynamics simulation code based on the lattice Boltzmann method (LBM) using forest-of-octrees-based block-structured adaptive mesh refinement (AMR) with temporary-fixed refinement was implemented, and its performance was evaluated on GPU-based supercomputers. Although the Space-Filling-Curve-based (SFC) domain partitioning algorithm for the octree-based AMR has been widely used on conventional CPU-based supercomputers, accelerated computation on GPU-based supercomputers revealed a bottleneck due to costly halo data communication. Our new tree cutting approach adopts a hybrid domain partitioning with the coarse structured block decomposition and the SFC partitioning in each block. This hybrid approach improved the locality and the topology of the partitioned sub-domains and reduced the amount of the halo communication to one-third of the original SFC approach. In the strong scaling test, the code achieved maximum ×1.82 speedup at the performance of 2207 MLUPS (mega-lattice update per second) on 128 GPUs (NVIDIA® Tesla® V100). In the weak scaling test, the code achieved 9620 MLUPS at 128 GPUs with 4.473 billion grid points, while keeping the parallel efficiency of 93.4% from 8 to 128 GPUs.

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Theoretical Analysis on Power Generation Characteristics for Co-axial MHD Energy Conversion Device

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

IEEJ Transactions on Power and Energy (IEEJ)  141 ( 10 ) 642 - 648 2021.10

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

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Theoretical solution is derived for power generation efficiency and electrical output power of the co-axial MHD energy conversion device using liquid metal. Although the electrical output power is proportional to the square of Reynolds number, the power generation efficiency is independent of Reynolds number. The energy input to the liquid metal is converted into electrical output power, viscous and Joule dissipation. For obtaining high power generation efficiency, viscous and Joule dissipation should be minimized. The combination of Hartmann number and electrical resistance ratio exists for the maximum power generation efficiency under the condition of the weak MHD interaction rather than that of the maximum electrical output power. The theoretical analysis clearly shows that the maximum power generation efficiency of the co-axial MHD energy conversion device is 1/3 ≈ 33.3% at the highest.

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Aerodynamic Study on Rotating Baseball with Low Spin Rates (Drop Mechanism of “Forkball” or “Splitter”)

Ryoga Ohashi, Takayuki Aoki, Seiya Watanabe, Hiromichi Kobayashi

日本流体力学会誌ながれ (日本流体力学会)  40 ( 5 ) 343 - 355 2021.10

Research paper (scientific journal), Joint Work, Accepted

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

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

Electronics and Communications in Japan (Willy)  104 ( 2 )  2021.06

Research paper (scientific journal), Joint Work, Accepted,  ISSN  19429533

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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 the radius ratio increases, azimuthal flow velocity increases and approaches the plane Couette flow profile for weak MHD interaction. The azimuthal flow velocity, however, decreases as the MHD interaction increases, and the reverse flow emerges in large radial locations. As the MHD interaction increases, the rotational torque increases. A larger radius ratio leads to the larger sensitivity of the change in rotational torque.

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Quantum Turbulence Coupled with Externally Driven Normal-Fluid Turbulence in Superfluid 4He

Satoshi Yui, Hiromichi Kobayashi, Makoto Tsubota, Rio Yokota

arXiv  2021.03

Research paper (scientific journal), Joint Work

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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