arXiv subjects

Hirofumi Niiya

Publications and source records attributed to Hirofumi Niiya.

4 recordsLinked to original sources

arXiv2019

Simple particle model for low-density granular flow interacting with ambient fluid

To understand the process of pattern formation in a low-density granular flow, we propose a simple particle model. This model considers spherical particles moving over an inclined flat surface based on three forces: gravity as the driving force, repulsive force due to particle collision, and drag force as the particle-- interaction through the ambient fluid. Numerical simulations of this model are conducted in two different types of two-dimensional planes, i.e., the monolayer was treated. In the horizontal plane parallel to the slope, 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. Meanwhile, in the vertical plane on the slope, particle aggregation is also found at the moving front of the granular flow. The aggregation resembles a head--tail structure, where the frontal angle against the slope approaches 60 degree from a larger angle as time progresses. Comparing the numerical result by varying the particle size, the qualitative dynamics of the granular flow are independent of size. To elucidate this reason, we perform a nondimensionalization for this model. The result indicates that our model can be simplified to dimensionless equations with one dimensionless parameter that represents the ratio of the gravity term to the excluded volume effect.

cond-mat.soft↗

arXiv2017

Spatiotemporal Structure of Aeolian Particle Transport on Flat Surface

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 into development, relaxation, and equilibrium phases, which are formed by rapid wind response below $10 \hspace{0.5 ex} \mathrm{cm}$ and gradual wind response above $10 \hspace{0.5 ex} \mathrm{cm}$. (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 $100 \hspace{0.5 ex} μ\mathrm{m}$ remains roughly constant and lower than the fluid threshold at equilibrium. (iv) The mean particle speed above $300 \hspace{0.5 ex} μ\mathrm{m}$ is less than the wind speed, whereas that below $300 \hspace{0.5 ex} μ\mathrm{m}$ 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.

physics.geo-ph↗

arXiv2012

Bifurcation analysis of the transition of dune shape under unidirectional wind

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 state from a straight transverse dune to 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.

nlin.PS↗

arXiv2010

3D Dune Skeleton Model as a Coupled Dynamical System of 2D Cross-Sections

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 dynamics of each and 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 transversal dunes and their deformation to barchans depending on the amount of available sand in the dune field, sand flow in parallel and perpendicular to wind direction.

nlin.PS↗