Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The formation of aggregates consisting of snow, water, and tephra has been reported in small-scale experiments on three-phase flows containing tephra, water, and snow, representing lahars triggered by snowmelt. Such aggregates reduce the mobility of mud flow. However, the formation mechanism of such aggregates under various conditions has not been investigated. To elucidate the formation conditions and mechanical properties of the aggregates, we performed mixing experiments with materials on a rotating table and compression tests on the resulting aggregates with a universal testing machine in a low-temperature room at 0 degrees C. From experiments with varying component ratios of the mixture and tephra diameter, the following results were obtained: (i) the aggregate grew rapidly and reached maturity after a mixing time of 5 min; (ii) the mass of aggregates increased with snow concentration, exhibiting an approximately linear relationship; (iii) single aggregates with large mass formed at lower and higher tephra concentrations, whereas multiple aggregates with smaller mass were observed at intermediate concentrations; (iv) the shape of the aggregate satisfied the similarity law for an ellipsoid; (v) the compressive mechanical behavior could be modeled by an empirical nonlinear model. The obtained mechanical properties of the aggregates were independent of the experimental conditions; (vi) scaling analysis based on the Reynolds number and the strength of the aggregates showed that the aggregates cannot form in ice-slurry lahars. Our findings suggest that low-speed lahars containing snow and ice are likely to generate aggregates, but snow and ice in the ice-slurry lahars are dispersed without such aggregates.

DOI： 10.1186/s40623-020-01289-w

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Other Link： http://link.springer.com/article/10.1186/s40623-020-01289-w/fulltext.html

Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：MDPI  

To understand the time evolutions of frontal speed and shape in a low-density granular flow, we propose a simple particle model. This model solves the equation of motion for each particle and simulates the time evolution of low-density granular flow. Spherical particles constituting a low-density granular flow slide on a slope at a steeper angle than the angle of repose. The particle motion is determined based on three forces: gravity as the driving force, repulsive force due to particle collision, and drag force due to the particle interaction through the ambient fluid. Two-dimensional numerical simulations of this model are conducted on the slope: the x-y plane parallel to the slope and the x-z plane perpendicular to the slope. In the x-y plane, particles aggregate at the moving front of the granular flow, and subsequently, flow instability occurs as a wavy pattern. This flow pattern is caused by the interparticle interaction arising from the drag force. Additionally, a vortex convection of particles is formed inside the aggregations. Simultaneously, particle aggregation is also found at the moving front of the granular flow in the x-z plane. The aggregation resembles a head-tail structure, where the frontal angle against the slope approaches 60 circle from a larger angle as time progresses. Comparing the numerical result by varying the particle size reveals that the qualitative dynamics of the granular flow are independent of particle size. Although the model is not realistic, our study presents a new particle-based approach that elucidates the dynamics of low-density granular flow.

DOI： 10.3390/geosciences10020069

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PUBLIC LIBRARY SCIENCE  

The diameter and vertical depth of sand crab tunnels in sandy beaches are usually restricted to a few centimeters scale and several tens of centimeters, respectively. We designed a study to determine what physical factors restrict tunnel diameter and predict the maximum attainable tunnel diameter and depth. We collected field data on the size and spatial distributions of ghost crab (Ocypode spp.) burrows on two sandy beaches (Kawage Beach in Tsu, Mie Prefecture, Japan and Sakieda Beach in Ishigaki, Okinawa Prefecture, Japan), where O. ceratophthalma dominants the ghost crab fauna. We measured burrow depths and distance from shoreline in concert with water content of sandy beaches. To explain our observed distributions of crab burrows in the field, we performed experiments in a lab microcosm, comprising a horizontal tunnel through wet sand. We measured the static stability of tunnel structures in relation to water content and two strengths computed from loading force exerted on the sand overlying the tunnels. By comparing field and experimental data, we found that crabs construct their burrows in appropriately wet zones (wet enough to provide sufficient cohesion of the sand grains in tunnel walls to prevent collapse) and that tunnel diameters and depths are sufficiently small to prevent deformation and collapse of their tunnels.

DOI： 10.1371/journal.pone.0215743

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Society of Powder Technology, Japan  

© 2019 Society of Powder Technology. All rights reserved. Empirical forms for the strengths characterizing stability of a horizontal tunnel in wet granular matter are experimentally derived. The motivation of this study comes from the observation of crab burrows on sandy beach. To understand the mechanical constraints for constructing the stable burrow structures, we perform a set of simple experiments with a horizontal tunnel structure made in a wet granular layer. Then, the tunnel is compressed from the top of the layer using a universal testing machine. During the compression, the shrinking tunnel shape is captured by a camera. Using the measured compression force, stroke, and shrinking tunnel images, we define and measure the mechanical strengths of wet granular matter with a horizontal tunnel. By systematically varying the experimental conditions, we obtained the empirical forms for estimating strength values. Using the measured results, we briefly discuss the mechanical conditions of sandy crab burrows.

DOI： 10.4164/sptj.56.194

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Resistance force acting on a rod vertically withdrawn from a granular layer is studied by experiments and numerical simulations. The initial packing fraction of the granular layer is controlled to evaluate the packing dependence of the resistance force. In both experiments and numerical simulations, the frictional resistance force in the steady slipping regime increases as the initial packing fraction is increased. According to the numerical results, the principal reason for the increase of frictional resistance is the increase of normal force acting on the rod. This large normal force stems from the effective solidification of the granular layer which is caused by the shearing induced by the withdrawn rod. Based on this shear-induced solidification, the experimental data are analyzed using a simple 'cylindrical shear-jammed zone' model. Then, the critical divergence of the size of solidified (shear-jammed) zone is observed by increasing the initial packing fraction towards the jamming packing fraction.

DOI： 10.1088/1367-2630/ab00c8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The collapse of an inclined cohesive granular layer triggered by a certain perturbation can be a model for not only landslides on Earth but also relaxations of asteroidal surface terrains. To understand such terrain dynamics, we conduct a series of experiments of a solid-projectile impact onto an inclined wet granular layer with various water contents and inclination angles. As a result, we find two types of outcomes: "crater formation" and "collapse". The "collapse" phase is observed when the inclination angle is close to the maximum stable angle and the impact-induced vibration at the bottom of wet granular layer is sufficiently strong. To explain the collapse condition, we propose a simple block model considering the maximum stable angle, inclination angle, and impact-induced vibrational acceleration. Additionally, the attenuating propagation of the impact-induced vibrational acceleration is estimated on the basis of three-dimensional numerical simulations with discrete element method using dry particles. By combining wet-granular experiments and dry-granular simulations, we find that the impact-induced acceleration attenuates anisotropically in space. With a help of this attenuation form, the physical conditions to induce the collapse can be estimated using the block model. (C) 2018 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physd.2018.08.002

DOI： 10.48550/arXiv.1808.05055

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER  

The aim of this study is to develop a large-eddy simulation (LES)-coupled Lagrangian snow transport model. This model consists of an LES for turbulent flow fields, and a particle transport simulation model based on Newton's equations of motion. Particles are treated as point masses for turbulent flow fields in this study. The interactions of momentum exchange between a fluid and particles are considered. The particle transport simulation includes three physical sub-processes on the snow surface: aerodynamic entrainment, rebound, and splash. Then, we simulated particle transport within a turbulent boundary layer under the low-wind condition in which saltation intermittently occurs. This simulation was calculated with high frequency and high resolution to capture the fine turbulent structures. The initial stage of saltation mainly related to sweeps, and particles were ejected through the process of aerodynamic entrainment. These ejected particle induced more saltating particles, and the number of saltating particles transported through the processes of splash and rebound increased and became dominant as time proceeds. In such processes of splash and rebound under low-wind conditions, the particle transport might be unaffected by turbulent structures.

DOI： 10.1016/j.jweia.2018.09.027

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Cohesive granular matter can support stable void structures, which can universally be found in various scenes from everyday lives to space. To quantitatively characterize the stability and strength of a void structure in cohesive granular matter, we perform a simple tunnel-compression experiment with wet granular matter. In the experiment, a horizontal tunnel in a wet granular layer is vertically compressed with a slow compression rate. The experimental result suggests that the tunnel deformation can be classified into the following three types: (i) shrink, (ii) shrink with collapse, and (iii) subsidence by collapse. Using the experimental result, we estimate the stable limit of various void structures in a cohesive granular layer from crab burrows on a sandy beach to the pits observed on cometary surfaces.

DOI： 10.1038/s41598-018-33978-8

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We conduct numerical simulations based on a model of blowing snow to reveal the long-term properties and equilibrium state of aeolian particle transport from 10(-5) to 10m above the flat surface. The numerical results are as follows. (i) Time-series data of particle transport are divided into development, relaxation, and equilibrium phases, which are formed by rapid wind response below 10 cm and gradual wind response above 10 cm. (ii) The particle transport rate at equilibrium is expressed as a power function of friction velocity, and the index of 2.35 implies that most particles are transported by saltation. (iii) The friction velocity below 100 mu m remains roughly constant and lower than the fluid threshold at equilibrium. (iv) The mean particle speed above 300 mu m is less than the wind speed, whereas that below 300 mu m exceeds the wind speed because of descending particles. (v) The particle diameter increases with height in the saltation layer, and the relationship is expressed as a power function. Through comparisons with the previously reported random-flight model, we find a crucial problem that empirical splash functions cannot reproduce particle dynamics at a relatively high wind speed.

DOI： 10.7566/JPSJ.86.054402

DOI： 10.48550/arXiv.1609.02328

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Authorship：Lead author   Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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Language：English   Publishing type：Doctoral thesis  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

The dune skeleton model is a reduced model to describe the formation process and dynamics of characteristic types of dunes emerging under unidirectional steady wind. Using this model, we study the dependency of the morphodynamics of transverse dunes on the initial random perturbations and the lateral field size. It was found that (i) an increase of the lateral field size destabilizes the transverse dune to cause deformation of a barchan, (ii) the initial random perturbations decay with time by the power function until a certain time; thereafter, the dune shapes change into three phases according to the amount of sand and sand diffusion coefficient, and (iii) the duration time, until the transverse dune is broken, increases exponentially with increasing the amount of sand and sand diffusion coefficient. Moreover, under the condition without the sand supply from windward ground, the destabilization of transverse dune in this model qualitatively corresponds to the subaqueous dunes in water tank experiments. (C) 2012 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.aeolia.2012.08.008

DOI： 10.48550/arXiv.1201.0864

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We develop a reduced complexity model for three-dimensional sand dunes, based on a simplified description of the longitudinal and lateral sand transport. The spatiotemporal evolution of a dune migrating over a nonerodible bed under unidirectional wind is reduced to the dynamics of its crest line, providing a simple framework for the investigation of three-dimensional dunes, such as barchan and transverse dunes. Within this model, we derive analytical solutions for barchan dunes and investigate the stability of a rectilinear transverse dune against lateral fluctuations. We show, in particular, that the latter is unstable only if the lateral transport on the dune slip face prevails over that on the upwind face. We also predict the wavelength and the characteristic time that control the subsequent evolution of an unstable transverse dune into a wavy ridge and the ultimate fragmentation into barchan dunes. © 2013 American Physical Society.

DOI： 10.1103/PhysRevE.87.052206

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

A bifurcation analysis of dune shape transition is made. By use of a reduced model of dune morphodynamics, the Dune Skeleton model, we elucidate the transition mechanism between different shapes of dunes under unidirectional wind. It was found that the decrease in the total amount of sand in the system and/or the lateral sand flow shifts the stable state from a straight transverse dune to a wavy transverse dune through a pitchfork bifurcation. A further decrease causes wavy transverse dunes to shift into barchans through a Hopf bifurcation. These bifurcation structures reveal the transition mechanism of dune shapes under unidirectional wind.

DOI： 10.1103/PhysRevLett.108.158001

DOI： 10.48550/arXiv.1110.4748

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

DOI： 10.1143/JPSJ.80.078001

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

To theoretically analyze the stability of the shape and the migration process of transverse dunes and barchans, we propose a skeleton model of three-dimensional (3D) dunes described by the coupled dynamics of 2D cross sections. First, the 2D cross sections of a 3D dune parallel to the wind direction are extracted as elements of a skeleton of the 3D dune; hence, the dynamics of each cross section and the interaction between them is considered. This model simply describes the essential dynamics of 3D dunes as a system of coupled ordinary differential equations. Using the model, we study the stability of the shape of 3D transverse dunes and their deformation to barchans depending on the amount of available sand in the dune field, and on sand flow parallel and perpendicular to wind direction.

DOI： 10.1143/JPSJ.79.063002

DOI： 10.48550/arXiv.1003.3563

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

DOI： 10.1063/1.3435418

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Mathematical Society of Traffic Flow  

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