Published papers

Turbulence phenomena caused by the interaction between superfluid and normal-fluid in superfluid helium are investigated by using two-way coupled numerical simulation via mutual friction. In homogeneous isotropic turbulence, quantized vortices winded by the normal-fluid vortex tubes are accumulated inside and around the vortex tubes, which corresponds to quasi-classical turbulence. In contrast, in a thermal counterflow, two large normal-fluid tornadoes with dense quantized vortices are induced in the streamwise direction, which corresponds to ultra-quantum turbulence. Those interactions between quantum turbulence and normal-fluid turbulence are revealed.

Recent developments in professional baseball have introduced the “Sweeper” pitch, a variation of the traditional slider that exhibits pronounced lateral movement and minimal vertical drop. Despite its increasing use, the aerodynamic mechanisms behind its distinctive flight remain unclear. This study investigates the unsteady aerodynamic forces acting on the Sweeper, with a focus on the effects of seam geometry and spin efficiency. Using high-resolution computational fluid dynamics simulations based on the lattice Boltzmann method and a 3D-scanned model of an official Major League Baseball, we analyze the airflow and wake behavior around the ball. Our results show that asymmetric seam placement leads to asymmetric boundary layer separation on the upper and lower surfaces, resulting in a diagonally deflected wake and a longitudinal lift force not explained by the traditional Magnus effect. Simulated flight trajectories closely match MLB tracking data when the spin efficiency is between (Formula presented) and (Formula presented). These findings highlight the critical role of seam-induced flow structures in shaping pitch behavior and provide new insights into the aerodynamic design and control of breaking pitches in baseball.

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.

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.

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.

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.

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.

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.

The coupled dynamics of quantum turbulence (QT) and normal-fluid turbulence (NFT) have been a central challenge in quantum hydrodynamics. We numerically studied the coupled dynamics of the two turbulences in thermal counterflow. NFT is driven by external forces to control its turbulent intensity, and the fast multipole method accelerates the calculation of QT. We show that NFT enhances QT via mutual friction. The vortex line density L of the QT satisfies the statistical law L1/2 ≈ γVns with the counterflow velocity Vns. It is found that the response coefficient γ is amplified by NFT. We propose a theoretical insight into the relation between the two turbulences.

We study the influence of two mutual friction models on quantized vortices and normal fluid using two-way coupled simulations of superfluid He4. The normal fluid is affected by quantized vortices via mutual friction. A previous study [Y. Tang, Nat. Commun. 14, 2941 (2023)2041-172310.1038/s41467-023-38787-w] compared the time evolutions of the vortex ring radius and determined that the self-consistent two-way coupled mutual friction (S2W) model yielded better agreement with the experimental results than the two-way coupled mutual friction (2W) model whose model parameters were determined through experiments with rotating superfluid helium. In this study, we compare the two models in more detail in terms of the quantized vortex ring propagation, reconnection, and thermal counterflow. We show that the S2W model exhibits better results than the 2W model on the microscopic scale near a quantized vortex, such as during quantized vortex ring propagation and reconnection, although the S2W model requires a higher spatial resolution. For complex flows such as a thermal counterflow, the 2W model can be applied even to a low-resolution flow while maintaining the anisotropic normal fluid velocity fluctuations. In contrast, the 2W model predicts lower normal fluid velocity fluctuations than the S2W model. The two models show probability density functions with-3 power-law tails for the normal fluid velocity fluctuations.

We investigate the drag reduction effect of the streamwise travelling wave-like wall deformation in a high-Reynolds-number turbulent channel flow by large-eddy simulation (LES). First, we assess the validity of subgrid-scale models in uncontrolled and controlled flows. For friction Reynolds numbers and, the Smagorinsky and wall-adapting local eddy-viscosity (WALE) models with a damping function can reproduce well the mean velocity profile obtained by direct numerical simulation (DNS) in both the uncontrolled and controlled flows, leading to a small difference in drag reduction rate between LES and DNS. The LES with finer grid resolution can reproduce well the key structures observed in the DNS of the controlled flow. These results show that the high-fidelity LES is valid for appropriately predicting the drag reduction effect. In addition, a small computational domain is sufficient for reproducing the turbulence statistics, key structures and drag reduction rate obtained by DNS. Subsequently, to investigate the trend of drag reduction rate at higher Reynolds numbers, we utilize the WALE model with the damping function to investigate the control effect at higher Reynolds numbers up to. According to the analyses of turbulence statistics and instantaneous flow fields, the drag reduction at higher Reynolds numbers occurs basically through the same mechanism as that at lower Reynolds numbers. In addition, the drag reduction rate obtained by the present LES approaches that predicted using the semi-empirical formula (Nabae et al., Intl J. Heat Fluid Flow, vol. 82, 2020, 108550) as the friction Reynolds number increases, which supports the high predictability of the semi-empirical formula at significantly high Reynolds numbers.

Turbulent Taylor-Couette flow applied in the axial magnetic field with upper and lower walls is numerically investigated by large-eddy simulation. As the Hartmann number increases proportional to the magnetic field, turbulent fluctuations are suppressed, and the flow is laminarized. The alignment of vortex structures is drastically changed from the azimuthal direction to the axial direction. The fine streaks near the inner wall are suppressed, and the high-speed triplet streaks emerge near the inner wall. The electric current streamlines reveal the feature of the difference between open and short circuit conditions.

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.

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.

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.

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.

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.

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.