Hirofumi Niiya

• PhD
• Professor (Associate) at Niigata University

Blowing snow, driven by strong winds, significantly reduces visibility and poses a serious threat to winter road safety in snowy regions. To address this issue, this study aimed to investigate the short-term fluctuations in snowdrift flux caused by turbulent wind gusts, which can lead to rapid visibility reductions. We conducted field observations...

Systematic observations were conducted to investigate the spatiotemporal structures of blowing snow. Along a line perpendicular to the dominant wind direction on the lee side of a flat field, 15 snow particle counters (SPCs) and ultrasonic anemometers (USAs) were placed 1.5 m apart. Data were recorded at high frequencies of 100 kHz for SPCs and 1 k...

In the Niseko area in Hokkaido, Japan, a unique set of rules -the “Niseko Rules”- control the opening and closing of gates to allow off-course skiing according to the avalanche danger level. In recent years, the need for scientific evidence that can correctly explain the Niseko Rules and the establishment of human resource training and organization...

Systematic observations were conducted to investigate the spatio-temporal structures of blowing snow. Along a line perpendicular to the dominant wind direction on the leeside of a flat field, fifteen Snow Particle Counters (SPCs) and Ultra Sonic Anemometers (USAs) were placed 1.5 m apart. Data were recorded at high frequencies of 100 kHz for SPCs a...

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Drifting snow is a phenomenon in which snow is transported by wind, and it fluctuates temporally and spatially due to turbulent fluctuations. Therefore, it is important to understand the average characteristics and unsteady characteristics of drifting snow such as the variability associated with turbulent fluctuations. In this study, we conducted f...

Surface shear stresses produced by wind and particle collision play a key role in aerodynamic entrainment and splash processes. The fluid shear stress at the surface during aeolian transport has been researched for decades; however, the equilibrium property reported in the literature, numerical simulations, and experiments is inconsistent. To discu...

To elucidate vertical structures of blowing snow with the snowfall, we acquired blowing snow events, which reached equilibrium, from observational data of two months. Estimation of the snowfall frequency in each event was based on snow particle diameters that were greater than 400μm, which were assumed as snowfall particles in this study. Wind prof...

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...

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 th...

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 s...

Raw data of wet sand collected at the Tsu site. (CSV)

Sand and burrow data collected at the Ishigaki site. (CSV)

Table of experimental conditions and measured τyield and τmax. (CSV)

Raw data of burrow size (diameter Db and depth H) measured at the Tsu site. (CSV)

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 wi...

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 sli...

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 moment...

Abstract 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,...

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 conte...

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 $10^{-5} \hspace{0.5 ex} \mathrm{m}$ to $10 \hspace{0.5 ex} \mathrm{m}$ above the flat surface. The numerical results are s follows. (i) Time-series data of particle transport are divided into...

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 $10^{-5} \hspace{0.5 ex} \mathrm{m}$ to $10 \hspace{0.5 ex} \mathrm{m}$ above the flat surface. The numerical results are as follows. (i) Time-series data of particle transport are divided int...

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 o...

It has been known that the granular flow of polystyrene particles down a slope forms a wavy pattern with many heads at the moving front of the resulting avalanche. In experiments of granular flow using low-density particles, the instability of the moving front and the subsequent head formation are driven by gravity and air drag. To elucidate the in...

A bifurcation analysis of dune shape transition is made. By use of a reduced model of dune morphodynamics, 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 sta...

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...

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...

A coupled ODEs model is proposed to describe the dynamics of barchans and other types of dunes under a few number of assumptions considering the geometrical characters of dunes and the surface flow over them. Using the model, the transition between coalescence and ejection of two colliding barchans depending on their initial sizes, is numerically a...

To analyze theoretically the stability of the shape and the migration process of transverse dunes and barchans, we propose a {\it skeleton model} of 3D dunes described with coupled dynamics of 2D cross-sections. First, 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 dy...

Avalanches, generally taken as a class of massive landslide phenomena, cover gravity currents and density currents, for in-stance, snow avalanches, debris flows, and pyroclastic flows. These flows migrate downward as a mixture of solid and fluid and form common structures, one of which is termed as the head-tail structure. Concretely, at the moving...