Initial Aspect Ratio Research Articles

Axisymmetric column collapses of bi-frictional granular mixtures

The behaviour of granular column collapses is associated with the dynamics of geohazards, such as debris flows, landslides and pyroclastic flows, yet their underlying physics is still not well understood. In this paper, we explore granular column collapses using the sphero-polyhedral discrete element method, where the system contains two types of particles with different frictional properties. We impose three different mixing ratios and multiple different particle frictional coefficients, which lead to different run-out distances and deposition heights. Based on our previous work and a simple mixture theory, we propose a new effective initial aspect ratio for the bi-frictional granular mixture, which helps unify the description of the relative run-out distances. We analyse the kinematics of bi-frictional granular column collapses and find that deviations from classical power-law scaling in both the dimensionless terminal time and the dimensionless time when the system reaches the maximum kinetic energy may result from differences in the initial solid fraction and initial structures. To clarify the influence of initial states, we further decrease the initial solid fraction of granular column collapses, and propose a trial function to quantitatively describe its influence. Due to the utilization of a simple mixture theory of contact occurrence probability, this study can be associated with the friction-dependent rheology of granular systems and friction-induced granular segregations, and further generalized to applications with multiple species of particles in various natural and engineering mixtures.

Analytical and Numerical Solutions for Three-Dimensional Granular Collapses

This research paper presents a comprehensive approach to investigating dry granular collapses in three dimensions, by combining analytical, numerical, and experimental methods. The experimental investigation utilised a novel apparatus to study granular collapses in the laboratory. It is demonstrated that a quasistatic understanding of granular collapses can accurately predict the final normalised run-out distances for dynamic granular collapses. Our approach involved establishing a correlation between the angle of repose and the initial aspect ratio of the granular column. It is also shown that the material point method (MPM) is suitable for modelling granular collapses in three dimensions. Our in-house solver was further validated using experimental evidence under an explicit formulation, resulting in good agreement between the numerical and experimental results. The findings demonstrate the effectiveness of our in-house solver for three-dimensional granular collapse modelling.

Granular column collapse: Analysis of inter-particle friction effects

The purpose of this paper is to investigate the effects of inter-particle friction on the granular column collapses via the discrete element method (DEM) while eliminating the particle-size effects. As the inter-particle friction increases, the runout distance decreases, the deposit height increases, the force chains tend to be closer to the vertical upward direction, the percentage of the strong force chains become greater and the granular assembly is less susceptible to the external gravity field, thus making the collapse tend to be toppling-dominant flow. The correlation between the initial height and runout distance of particles at the final deposits becomes stronger when the collapse tends to be more toppling-dominant. The motion patterns of the collapse are governed by the inter-particle friction and initial aspect ratio through dimensional analysis and the effective aspect ratio parameter is proposed to classify granular column collapse into three regimes (sliding-dominant, transition, and toppling-dominant collapse).

Regular Pore Network Model to Study Dynamic Permeability Evolution in Hydrate-Bearing Sediments

Permeability is a key parameter to characterize fluid flow in hydrate-bearing sediments. Figuring out dynamic permeability evolution is of great importance for the effective development of hydrate-bearing deposits. In this paper, a grain-coating hydrate-bearing regular pore network model with complex pore throat cross-sections is first constructed. Afterward, the dynamic permeability evolution regularity is calculated. After the validation, the effects of initial aspect ratio, coordination number, and pore throat cross-sections on dynamic permeability evolution are investigated. The results show that hydrate narrows the effective flow space, which results in the exponential decrease of dynamic permeability with the increased hydrate saturation. The larger initial aspect ratio aggravates the heterogeneity of the pore network, resulting in a faster permeability decline rate. However, hydrate weakens the effect of initial aspect ratio on dynamic permeability evolution since the physical hydrate thickness in large pore bodies and throats is larger. The high initial coordination number reduces the dynamic permeability decline rate with the increased hydrate saturation since the higher coordination number increases the topology of the network, while hydrate compresses or blocks the effective pore throat space. Pore throat cross-sections have nothing to do with dynamic permeability evolution, but they dramatically influence the absolute permeability values. This study provides a novel insight into dynamic permeability evolution in hydrate-bearing sediments.

Investigation on the erosion and deposition process of granular collapse flow on an erodible inclined plane

The occurrence of slope disasters mainly depends on the terrain slope, size and amount of soil and rock particles, and the presence of heavy rain. If particles erode and entrain soil and rock in the bottom bed as they flow after the slope collapse, a massive avalanche can occur, threatening human life and causing substantial property damage. We employed an experimental approach to investigate the effects of the particle size of granular beds (including column and erodible beds) and the initial aspect ratio on their erosion and deposition behavior in granular collapse flows. We found that the erosion area increases when the particle size of the column bed increases, whereas the particle size of the erodible bed remains the same. The final erosion area increases slightly when the initial aspect ratio increases, and subsequently decreases as the initial aspect ratio increases in the presence of a bigger particle size of the erodible bed. However, when the particle size of the erodible bed is smaller than that of the column bed, the final erosion area increases significantly as the aspect ratio increases. Furthermore, we explored the flow trends of the column and erodible bed particles over time at different locations. For different particle sizes of the erodible bed, the potential energy loss of the column bed may be transformed into erosion of the erodible bed, or may be lost due to the friction and collisions between particles, and thus cannot cause significant variation in erosion behavior.

Micromechanical simulations of the collapse of a submerged granular column

In this work, we present a numerical study of a granular collapse in a viscous fluid adopting a DEM-PFV (Pore Finite Volume) coupled model. We analyze the time evolution of the collapse and the final morphology of the deposit (triangular or trapezoidal) as a function of the initial solid fraction and column aspect ratio. Finally, we discuss the role of the initial packing fraction of the granular material on the time evolution of the pore pressure. The numerical results are consistent with experimental observation [3] showing the capability of the DEM-PFV coupled model to simulate saturated granular media from triggering to flowing and stopping conditions.

APS-Net: An Adaptive Point Set Network for Optical Remote-Sensing Object Detection

Oriented object detection in optical remote-sensing images has been a challenging task due to arbitrary orientations and densely packed distribution of objects. Specifically, most existing methods lack adaptivity when regressing objects with different shapes and orientations. Although the point set representation is relatively flexible, the initial distribution of the point set is fixed in advance. In addition, some models based on the point set cannot get high location precision of points, affecting the bounding box generation. In this letter, we propose an Adaptive Point Set Network (APS-Net) for optical remote-sensing object detection, including three improvements. First, we propose the initial distribution learner (IDL) to learn the optimal initial aspect ratio, which helps the point set fit the object’s shape well. Second, we design the uncertainty measurement module (UMM), which considers the uncertainty of point location to improve location precision. Third, we introduce the local outlier factor (LOF) in the loss to punish outlier points more reasonably. Extensive experiments demonstrate that our proposed model achieves state-of-the-art performance on three commonly used datasets (i.e., DOTA-v1.0, UCAS-AOD, and HRSC2016) in the remote-sensing field.

Simulations of Bubble Formation and Oscillation Behavior in Micro-Leakage Measurement System

In the process of measuring micro-leakage based on the bubble method, the physical parameters of the liquid phase have a great impact on the formation and movement of bubbles, thus affecting the measurement accuracy of the system. Therefore, it is of great significance to study the physical parameters of the liquid phase. At present, it is difficult to independently study the parameters of the liquid phase by experimental methods. In this paper, based on the Cahn-Hilliard phase field model, a gas-liquid two-phase flow model for the expression of leakage is established to independently study the influence of different physical parameters on the formation and movement of bubbles in the measuring system. Firstly, the effects of liquid medium density, dynamic viscosity coefficient and surface tension on the initial shape (aspect ratio), the generation period and the shape oscillation of bubbles after they leave the nozzle are analyzed. Then, an experimental platform for visualization of bubbles in two-phase flow based on machine vision is built, through which the morphological parameters of bubbles in the liquid phase can be obtained in real time. Finally, the results of visualization experiment and numerical simulation are compared. The results show that the initial aspect ratio error of bubbles obtained by visual experiment and numerical simulation is 1.8%, and the average oscillation period error is 12.3%. The results are basically consistent, which proves the rationality and correctness of the mathematical model established. It is found that the liquid with larger dynamic viscosity coefficient μ and density ρ and smaller surface tension σ can be selected as the liquid medium of the measurement system. It is more conducive to the accurate expression of micro-leakage in the measurement system, and the bubble shape can quickly reach a stable state. It provides theoretical support for improving the accuracy and stability of the micro-leakage measurement system.

Dynamical motion of an oblate shaped particle exposed to an acoustic standing wave in a microchannel

We study the dynamics of an oblate shape micro-particle exposed to standing bulk acoustic waves in a microchannel. A parametric study examines the effects of initial orientation, aspect ratio, size, and initial location of the particle on the translational and rotational motion. We find that the particle undergoes rotation to minimize the acoustic radiation torque potential. The direction of rotation of the particle was found to change from anticlockwise to clockwise beyond a critical aspect ratio and larger particles are found to rotate faster and closer to the nodal plane. These results can be relevant to the dynamics of an elongated microorganism or biological cells in an acoustic field.

Growth of random polyhedral void in structural steel based on micromechanical RVE simulations

This paper investigates growth of random polyhedral voids (RPVs) in structural steel, which is a critical stage for ductile fracture of steel. An in-situ high-resolution μXCT experiment shows that most micro-voids in structural steels can be characterized by RPVs. In this study, a micromechanical representative volume element (RVE) model containing an RPV was established with periodic boundary conditions. Comparison of RPV growths with spherical and elliptical void growths was conducted for model validation. On this basis, effects of geometric parameters of the RPV on void growth were analyzed through Python-based parametric modeling in ABAQUS with respect to the aspect ratio, orientation, sphericity, initial void volume fraction, respectively. The results found that growth of the RPV decreases with an increasing initial aspect ratio of the RPV. Growth of the RPV increases as the initial sphericity of RPV decreases. When the initial orientation of RPV tends to be perpendicular to the main tensile direction, a greater void growth will be obtained. The initial volume fraction of RPV has a minor effect on void growth compared with the other geometric parameters. For engineering application, accurate and simplified formulae were proposed to evaluate growth of microscopic RPVs, respectively.

Numerical simulations of a toroidal droplet breakup in viscous oils

Toroidal droplets are inherently unstable in viscous oils; they either shrink to a single drop or break into several spherical droplets due to Rayleigh–Plateau instability. In this paper, the breakup dynamics of toroidal droplets in immiscible viscous oils have been numerically investigated. A two-dimensional model combined level-set method is proposed. Numerical results reveal that the initial aspect ratios, interfacial tensions, and outer liquid viscosities play important roles in determining the breakup dynamics of toroidal droplets. The initial aspect ratios dominate the number of split droplets, which is consistent with a linearly scaling law n = 0.57 R0/a0. By considering key factors of interfacial tension in this process, it is found that interfacial tension is crucial in the initial morphology of the toroidal droplet and helps to retard the unstable breakup dynamics. Interestingly, reducing the interfacial tension stabilizes it against breakup. We further study the effect of viscosity on the breakup dynamics. The surrounding viscous oils contribute to stabilizing the interfacial-tension-driven instabilities and extending the breakup time. Thus, for a toroidal droplet in high viscosity oils and a sufficiently low interfacial tension system, the unstable breakup dynamics could be delayed. Our findings provide a novel fundamental understanding of toroidal droplets and are beneficial to applications involving the manipulation of toroidal droplets.

Shape dynamics and rheology of dilute suspensions of elastic and viscoelastic particles

This paper examines the shape dynamics of deformable elastic and viscoelastic particles in an ambient Newtonian fluid subjected to simple shear. The particles are allowed to undergo large deformation, with the elastic stress determined using the neo-Hookean constitutive relation. We first present a method to determine the shape dynamics of initially ellipsoidal particles that is an extension of the method of Roscoe (J. Fluid Mech., vol. 28, issue 2, 1967, pp. 273–293), originally used to determine the shape at steady state of an initially spherical particle. We show that our method recovers earlier results for the in-plane trembling and tumbling dynamics of initially prolate spheroids in simple shear flow, obtained by a different approach. We then examine the in-plane dynamics of oblate spheroids and triaxial ellipsoids in simple shear flow, and show that they too, like prolate spheroids, exhibit time-periodic tumbling or trembling dynamics, depending on the initial aspect ratios of the particle and the elastic capillary number $G \equiv \mu \dot {\gamma }/\eta$ , where $\mu$ is the viscosity of the fluid, $\eta$ is the elastic shear modulus of the particle and $\dot {\gamma }$ is the shear rate. In addition, we find a novel state wherein the particle extends indefinitely in time and asymptotically aligns with the flow axis. We demarcate all the dynamical regimes in the parameter space comprising $G$ and the initial particle aspect ratios. When the particles are viscoelastic, damped oscillatory dynamics is observed for initially spherical particles, and the tumbling–trembling boundary is altered for initially prolate spheroids so as to favour tumbling.

Numerical evaluation on fatigue crack growth and life predictions of an FPSO mooring system

This study aims to systematically study the fatigue crack propagation characteristics of the mooring system for a floating production storage and offloading vessel (FPSO) through the fracture mechanics-based approach. The remaining fatigue life of the mooring lines is predicted by a self-integrated program through the simulation of the crack growth behavior. Additionally, parametric studies are performed to investigate the influence of initial crack status (location/size/shape) on mooring crack propagation characteristics. Results illustrate that the remaining fatigue life at the uppermost position of mooring lines at the wave-ward side is similar, which is approximately 178 years longer than that of mooring lines at the wave-back side. Fatigue issues are the severest in the crown region, where the critical fatigue life is about 49% of that in the bend region and 19.9% of that in the straight region. With the increase of the initial crack size and aspect ratio, the remaining fatigue life experiences a remarkable decrease with amplitudes up to 47.8% and 30.1% respectively. The integrated evaluation program is a promising tool for the damage tolerance design of mooring system due to the excellent capability in predicting the remaining fatigue life and charactering the crack growth behavior.

Fire resistance design of austenitic SUS316 stainless columns subjected to axial compression

To explore the effect of cold-working on the properties of austenitic stainless steel, a material mechanical property test of austenitic SUS316 stainless steel coupons extracted from cold-formed RHS columns at elevated temperatures was conducted. Based on the fire test of 5 austenitic stainless SHS columns subjected to axial compression, a refined finite element model was established. A single parametric analysis of different parameters, cross-sectional area, height-thickness ratio, initial overall imperfection, eccentricity, slenderness ratio, aspect ratio and load ratio was conducted, mainly investigating the effects on the critical temperature and maximum axial displacement. As the effect of the load ratio was the most significant, a multiparameter analysis of the load ratio with other parameters was further conducted. Presently, the design methods in EN 1993–1-2 for carbon steel columns are also applied to stainless steel columns, where section classification is needed. Based on numerous parametric analyses, the design methods in EN 1993–1-2 were assessed and modified. EN 1993–1-2 underestimated the bearing capacity of stainless steel SHS and RHS columns, and the modified method improved the utilization rate of stainless steel. A new method for evaluating the bearing capacity of stainless SHS and RHS columns using the maximum axial displacement was proposed, for which the section classification was not needed. As a preliminary study, the new method was only applicable to stainless SHS and RHS columns with load ratios between 0.2 and 0.6, which was also the most commonly applied scope in engineering.

Motion of a neutrally buoyant elliptical particle in a double-lid-driven square cavity

Understanding, predicting and controlling the motion of the solid particles became active research topics, like debris flows, sand storms, transport of volcanic ash, coal combustion, drug delivery and steel making. Thus, the motion of a neutrally buoyant elliptical particle in a double-lid-driven square cavity is studied with the lattice Boltzmann method, where the effects of the initial position, Reynolds number and aspect ratio are studied. The square cavity is divided into two parts by the two primary vortexes, and the motion of the elliptical particle is determined by the interaction of the two primary vortexes. The limit cycle is sensitive to the initial position of the elliptical particle, namely, placing the elliptical particle at different positions initially, the motion of the elliptical particle exhibits different modes. Especially, the elliptical particle is apt to cross the horizontal centerline of the square cavity at moderate Reynolds numbers. The effect of the Reynolds number on the motion of the elliptical particle is significant, with the increase of the Reynolds number, the effects of the two primary vortexes on the motion of the elliptical particle are different, which affects the trajectory of the elliptical particle fundamentally. With the increase of the aspect ratio, the elliptical particle becomes flatter, the hydrodynamic force and torque acting on the elliptical particle become unbalanced and the elliptical particle no longer moves along the 8-like limit cycle.

Unchannelized collapse of wet granular columns in the pendular state: Dynamics and morphology scaling

Different regimes are observed in the unchannelized collapse of wet granular columns. The initial aspect ratio of the column and the dimensionless macroscopic cohesion, which contains the particle size and the water content, are two relevant variables in the formation of different regimes and in the collapse dynamics. Generalized scaling laws are developed to characterize the deposit of wet collapsing material for which morphological quantities may be conveniently expressed by adding variations caused by the cohesion effect to the results of dry granular material.

Estimation of the FST-Layering Time for Shock Ignition ICF Targets

The challenge in the field of inertial confinement fusion (ICF) research is related to the study of alternative schemes for fuel ignition on laser systems of medium and megajoule scales. At the moment, it is considered promising to use the method of shock ignition of fuel in a pre-compressed cryogenic target using a focused shock wave (shock- or self-ignition (SI) mode). To confirm the applicability of this scheme to ICF, it is necessary to develop technologies for mass-fabrication of the corresponding targets with a spherically symmetric cryogenic layer (hereinafter referred to as SI-targets). These targets have a low initial aspect ratio Acl (Acl = 3 and Acl = 5) because they are expected to be more hydrodynamically stable during implosion. The paper discusses the preparation of SI-targets for laser experiments using the free-standing target (FST) layering method developed at the Lebedev Physical Institute (LPI). It is shown that, based on FST, it is possible to build a prototype layering module for in-line production of free-standing SI-targets, and the layering time, τform, does not exceed 30 s both for deuterium and deuterium-tritium fuel. Very short values of τform make it possible to obtain layers with a stable isotropic fuel structure to meet the requirements of implosion physics.

Premixed syngas/air combustion in closed ducts with varied aspect ratios and initial pressures

Syngas is a promising renewable energy carrier, and its combustion can result in a significant reduction of pollutant emissions. However, syngas has various production sources, its compositions are diverse accordingly. That brings some challenges in its practical application. Therefore, in order to deeply investigate the premixed stoichiometric syngas/air combustion in confined spaces, experiments were conducted in a closed duct with varied aspect ratios (12, 26 and 40) and initial pressures from 0.5 atm to 1.5 atm. Flame shape evolutions in varied cases were captured through high-speed photography, and four flame speed oscillation regimes were distinguished qualitatively. The pressure buildup was recorded by sensors and scrutinized by dimensionless method. The results denoted the propagation dynamics became more violent with higher initial pressure or aspect ratio. Further, the combustion instability was examined, and it indicated the acoustic effect and the Rayleigh-Taylor/Richtmyer-Meshkov instability would be stronger in cases with large aspect ratio, resulting in the deformation of elongated tulip flame. Besides, the Bychkov's theory considering gas compression effect was validated by the experimental data. The discrepancy between them was interpreted with a comprehensive view of enlarged laminar burning speed, non-adiabatic sidewall and finite flame front thickness.

Influence of cross-section shape on granular column collapses

We investigate granular column collapses with different column cross-sections and associate the cross-section shape influence with a finite-size analysis. Previous research, confined to initially circular configurations, reviewed the importance of granular column collapse studies and concluded that the run-out distance scales with the initial aspect ratio. In this work, granular columns with three different types of initial column cross-sections (square, equilateral triangular, and rectangular cross-sections) are simulated using the discrete element method (DEM). We explore how non-circular cross-sections lead to different run-out distances. Based on the previously obtained finite-size analysis, we further link the initial radius in different directions to the relative size of a column and perform the finite-size analysis to explain the cross-section influence. In the end, a universal relationship, which includes frictional properties, relative sizes, and the cross-section influence, is proposed. Our results are shown to have direct relevance to various natural and engineering systems.

Fracture mechanics and hot spot stress‐based fatigue life calculation: Case study for a crane runway girder

AbstractThis work presents the case study of a welded overhead crane runway girder. A full‐shell global finite element model was developed and validated using strain measurements. The global model drives solid submodels of different scales. Using load spectra based on real operational data, hot spot stress and XFEM‐based fracture mechanics simulations are performed to assess the fatigue properties of two critical joints. For the fracture mechanics‐based approach, it is shown that small cracks quickly converge to the same aspect ratio. Hence, the choice of the initial crack aspect ratio was found to be inconsequential. But the aspect ratio must not be assumed to be constant during the fatigue crack growth simulations. Finally, it is shown that the size at which a crack is transferred from one submodel to a subsequent is critical for the accuracy of the fatigue crack growth simulations.