Initial Aspect Ratio Research Articles

Parametric study of buckling and post-buckling behavior for an aluminum hull structure of a high-aspect-ratio twin hull vessel

Metal plates are essential parts of structures such as ship hulls and offshore oil platforms. These plates are typically under compressive axial forces. Hence, one of the main mechanisms for failure and collapse of such structures is buckling of plates. Thus, for safe and secure design, buckling strength of plates should be evaluated. Finite element analysis techniques are perfect tools for this purpose because of the accuracy and flexibility for performing simulations with different variables. In this study, the strength of aluminum plates has been studied using finite element analysis software and it was tried to study the influence of variables such as initial imperfections, plate thickness and aspect ratio on plate strength. It was found that increase in the imperfection magnitude leads to a decrease in the strength of the plate. This is mostly noticed in thin plates because of the plate’s stiffness reduction. Although the plate’s aspect ratio has a slight effect on strength, it has a considerable impact on the failure mechanism. In order to study a combination of plates and stiffeners buckling behavior, part of a high-aspect-ratio twin hull vessel hull structure is assessed under bending moment. It is observed that longitudinal stiffeners at the bottom of the hull in linear analysis undergo the maximum stress, while in non-linear analysis, the maximum stress is spread along with the structure of the deck and bottom, and neutral axis tolerates the minimum stress.

Dynamics of a pair of ellipsoidal microparticles under a uniform magnetic field

Under a uniform magnetic field, magnetic particles tend to form chains, clusters or columns due to particle–particle interactions. Non-spherical magnetic particles dispersed in a liquid medium show different rheological properties. However, there is a lack of knowledge about the fundamental mechanism of particle–particle interactions of non-spherical particles under a uniform magnetic field. In this work, we numerically investigate the particle–particle interactions and relative motions of a pair of paramagnetic elliptical particles by using direct numerical simulations to create two-dimensional models that resolve the magnetic and flow fields around the finite-sized particles. The modeling is based on the finite element method and arbitrary Lagrangian–Eulerian approach with full consideration of particle–fluid–magnetic field interaction. The effects of initial position and aspect ratio of the particles are investigated. The results show that the particles spend much more time under global reorientation than local magneto-orientation. Larger initial relative angles and distances, and larger aspect ratios, tend to require more time to form a stable chain. The particle–particle interactions and relative motion of a pair of elliptical particles in this study provide insights into the particle alignment and chaining processes under uniform magnetic fields, which are closely related to the response of magneto-rheological fluids to magnetic fields.

Análise do comportamento oscilatório de gotas de fluidos de van der Waals sob condições de microgravidade usando o método Smoothed Particle Hydrodynamics

No presente trabalho, o método Smoothed Particle Hydrodynamics (SPH) é utilizado na modelagem do problema bidimensional da formação de uma gota líquida, sem a presença de uma atmosfera externa, considerando um fluido de van der Waals. Além disso, também estuda-se a influência do número de Reynolds, da razão de aspecto inicial e do número de Péclet no comportamento oscilatório da gota, em condições que simulam experimentos no espaço com gravidade zero. Tendo em vista as pesquisas mais recentes na área, utiliza-se um kernel hiperbólico em todas as simulações, garantindo, assim, a formação de gotas de líquido mais uniformes, ou seja, sem aglomeração de partículas. O uso dessa função de suavização reduz a instabilidade de tensão, de modo que não é necessário empregar nenhum tratamento especial quando da aplicação do método numérico.

Empirical investigation of friction weakening of terrestrial and Martian landslides using discrete element models

Understanding what controls the travelling distance of large landslides has been the topic of considerable debate. By combining observation and experimental data with depth-averaged continuum modelling of landslides and generated seismic waves, it was empirically observed that lower effective friction had to be taken into account in the models to reproduce the dynamics and runout distance of larger volume landslides. Moreover, such simulation and observation results are compatible with a friction weakening with velocity as observed in earthquake mechanics. We investigate here as to whether similar empirical reduced friction should be put into discrete element models (DEM) to reproduce observed runout of large landslides on Earth and on Mars. First we show that, in the investigated parameter range and for a given volume, the runout distance simulated by 3D DEM is not much affected by the number (i.e. size) of grains once this number attains ~ 8000. We then calibrate the model on laboratory experiments and simulate other experiments of granular flows on inclined planes, making it possible for the first time to reproduce the observed effect of initial volume and aspect ratio on runout distances. In particular, the normalised runout distance starts to depend on the volume involved only above a critical slope angle > 16–19°, as observed experimentally. Finally, based on field data (volume, topography, deposit), we simulate a series of landslides on simplified inclined topography. The empirical friction coefficient, calibrated to reproduce the observed runout for each landslide, is shown to decrease with increasing landslide volume (or velocity), going down to values as low as 0.1–0.2. No distinguishable difference is observed between the behaviour of terrestrial and Martian landslides.

Rotations of elliptic vortex beams in media with and without anisotropy

We introduce the anisotropy into the evolutions of the vortex, and discuss three different kinds of dynamics: circular shaped vortex in linear anisotropic media, elliptic shaped vortex in linear isotropic media, and elliptic shaped vortex in linear anisotropic media. The evolutionary processes dramatically depend on both the anisotropic parameter δM of the media and the initial aspect ratio ρ0 of the elliptic vortex. Under the critical condition , the elliptic vortex will not rotate, and keeps its aspect ratio invariant during the evolutions. Generally, when , the dynamics of the elliptic vortex can be divided into three stages: nonuniform process, rotational process and uniform process. In the nonuniform and the uniform processes of the vortex, the positions of the outer elliptic peaks and the inner elliptic holes do not change in spite of the nonzero energy flow. Also, the energy distributions become nonuniform in the former process, and uniform in the latter process. In the rotational process, the outer peaks and the inner holes can both rotate, but with the opposite rotational directions.

Shape oscillations of a nonspherical water droplet.

The dynamics of a nonspherical droplet falling in air is investigated via three-dimensional numerical simulations of the Navier-Stokes and continuity equations. The main focus of this work is to study the effects of the relative influence of surface tension force over the inertial force and the orientation of the droplet. The values of the dimensionless parameters considered in this study are similar to those of a typical falling raindrop. Our results obtained from the numerical simulations for low values of Galilei number agree well with the theoretical predictions in the microgravity condition and in the limit of creeping flow. However, in the inertia-dominated region and for high initial aspect ratio, the droplet undergoes predominant nonlinear oscillations and subsequently breaks up. To the best of our knowledge, none of the previous studies have investigated dynamics of a tilted drop and observed the behavior reported in the present study.

Sensitivity of reliability-based fatigue analysis to crack shape development in cracked pipeline

An accurate prediction of crack shape development during fatigue propagation plays a key role in structural reliability assessment. To predict crack shape, aspect ratio variability (a/c; a: crack depth, c: half crack length) is wildly used and different correlations to estimate this later exist in literature. In this paper, reliability index of a pipeline with axially surface crack is evaluated. Initial aspect ratio was varied. Tow dimensional crack growth models based on Paris Law coupled to liner elastic fracture mechanics were used. The objective is to clarify the effect of initial aspect ratio and it variability during propagation on the life time prediction of cracked pipeline. Based on Importance sampling, reliability index is plotted against number of load cycle. Results show an important role of initial aspect ratio and crack growth model in reliability index estimation. Calculation of different sensitivity factors illustrates the effect of uncertainty and correlation of Paris law parameters on the predicted life time of the pipeline.

Mechanistic Elucidation of Si Particle Morphology on Electrode Performance

High specific capacity silicon anodes are limited by immense volumetric expansion upon lithiation making them prone to deleterious capacity fade through particle cracking and disintegration. Nanosized silicon active particles with spherical and rod morphology demonstrate resistance to lithiation induced fracture owing to their smaller size. However, lithiation in silicon nanospheres and nanowires exhibit varying characteristics as a function of the morphology. In this work, we contrast the lithiation impact on diffusive transport and reaction kinetics for silicon nanospheres and nanorods cognizant of the volume evolution using a particle formalism. For the same equivalent volume, nanorods surpass nanospheres in rate performance beyond a threshold initial length to initial radius aspect ratio. Nanorods show faster radial growth as compared to nanospheres for the same initial radius, resulting in exacerbated diffusion limitations below the threshold aspect ratio. The mechanistic insight into the performance dependence on morphology is elucidated. Diffusivity and exchange current density values for the silicon anode are calculated as well through synergy between experimental dataset and simulation results.

Non-linear models for assessing the fatigue crack behaviour under cyclic biaxial loading in a cruciform specimen

This work addresses FEM-based numerical simulations of fatigue crack propagation in a cruciform specimen under biaxial loading and small scale yielding (SSY) conditions. Three non-linear models are used as fatigue crack propagation laws. The cruciform specimens are made of aluminium alloy D16T and modelled with non-linear material properties. In the experimental tests, starting from a surface flow mechanically created in the centre of the cruciform specimen, biaxial loading conditions are applied to create various pre-cracked configurations that correspond to different values of initial crack aspect ratios. In the numerical simulations, the material behaviour is modelled with a low hardening bilinear law. Each simulation is carried out with one of the established non-linear laws. At the end of the fatigue crack propagation simulations, several crack aspect ratios are obtained. Finally, these aspect ratios are compared with the corresponding experimental tests available in the literature.

A fate-alternating transitional regime in contracting liquid filaments

The fate of a contracting liquid filament depends on the Ohnesorge number ($Oh$), the initial aspect ratio ($\unicode[STIX]{x1D6E4}$) and surface perturbation. Generally, it is believed that there exists a critical aspect ratio $\unicode[STIX]{x1D6E4}_{c}(Oh)$ such that longer filaments break up and shorter ones recoil into a single drop. Through computational and experimental studies, we report a transitional regime for filaments with a broad range of intermediate aspect ratios, where there exist multiple $\unicode[STIX]{x1D6E4}_{c}$ thresholds at which a novel breakup mode alternates with no-break mode. We develop a simple model considering the superposition of capillary waves, which can predict the complicated new phase diagram. In this model, the breakup results from constructive interference between the capillary waves that originate from the ends of the filament.

Collapse of submerged granular columns in loose packing: Experiment and two-phase flow simulation

This study examines, both experimentally and numerically, the collapse process of a submerged granular column in loose packing, with an emphasis on the effects of the grain size and the initial aspect ratio of the granular column. A two-phase flow continuum model, developed based on the rheological characteristics of solid-water mixtures, is used to perform the numerical simulations. Two particle sizes representing very fine particles and coarse particles are examined in this study. The two-phase flow model is validated and verified by comparing with the measured characteristics of the collapse process. Compared to the collapse of a granular column of coarse particles, the runout distance of a granular column with very fine particles is longer and the spread velocity is higher. Based on the two-phase flow simulation results supported by the experimental results, critical initial aspect ratios are identified to classify the types of the initial collapse process and the final deposit morphology, and the empirical expressions are provided to describe the changes of the runout distance, collapse duration, final deposition height, and spread velocity with the initial aspect ratio. A phenomenon similar to the so-called Kelvin-Helmholtz instability can be observed in both the experiment and the simulation at the later stage of the collapse, which causes the formation of several vortexes in the two-phase flow during the collapse process. The presence of these vortexes is responsible for the pressure drop on the bed after the passage of the flow front and the wavy feature on the final deposit morphology.

Erosion-deposition regime formation in granular column collapse over an erodible surface

In this paper, the collapse of a granular column over an erodible bed is investigated experimentally and numerically. Four series of experiments are performed using different materials in order to understand the occurrence of the erosion and its evolution. Three different erosion-deposition regimes are observed in experiments, which primarily depend on the difference in material properties between the column and erodible bed but vary little with the initial aspect ratio of the column. Once the erosion process occurs, the collapsing flow has both longer initial acceleration and final deceleration phases but a shorter intermediate period of constant-velocity flow, a phenomenon that is more prominent when the flow front of collapsing material eventually penetrates into bed material. However, the overall flowing duration satisfies a proportional relationship with the free-fall time of granular column ${\ensuremath{\tau}}_{c}$ for both erosion regimes, almost identical to that observed on a rigid substrate. Furthermore, the mechanism of the erosion is discussed based on the evolution of velocity field and the migration of the interface between static and flowing materials. The proposed equation of motion for the flow front shows that the particle density ratio of the column and the erodible bed plays a crucial role in the formation of different erosion-deposition regimes. In addition to reproducing various deposit morphologies observed in our experiments, our numerical simulation results also confirm the existence of critical particle density ratios that correspond to regime transitions and are almost independent of the initial aspect ratio of the column.

Electrodeformation of Vesicles Suspended in a Liquid Medium.

Deformation of flexible vesicles suspended in a fluid medium due to an applied electric field can provide valuable insight into deformation dynamics at a very small scale. In an electric field, the response of the vesicle membrane is strongly influenced by the conductivity of surrounding fluid, vesicle size and shape, and the magnitude of applied field. We studied the electrodeformation of vesicles immersed in a fluid media under a DC electric field. An immersed interface method is used to solve the electric field over the domain with conductive or non-conductive vesicles while an immersed boundary method is employed to solve fluid flow, fluid-solid interaction, membrane mechanics and vesicle deformation. Initial force analysis on the membrane surface reveals almost linear influence of vesicle size, but the vesicle size does not affect the long-term deformation which is consistent with experimental evidence. Highly nonlinear effect of the applied field as well as the conductivity ratios inside and outside of the vesicle are observed. Results also point towards an early linear deformation regime followed by an equilibrium stage for the membranes. Modeling results suggest that electrodeforming vesicles can create unique external flows for different conductivity ratios. Moreover, significant influence of the initial aspect ratio of the vesicle on the force distribution is observed across a range of conductivity ratios.

Nitrate ion effect on gold nanorods plasmon resonance behavior: A potential chemical sensor

High sensitivity of surface plasmon resonances (SPR) in noble metals to the surrounding refractive index variations in the environment is a powerful bio-sensing technique. Here, we report the development of a simple colorimetric detection method for nitrate ion in water using gold nanorods (Au-NRs). Au-NRs suspension with the average aspect ratio of 3.5 was prepared by a modified seed growth method. According to the safety limit standard of nitrate amount in drinking water, different volumes of 10 mg/l nitrate solution were added to the Au-NRs suspension. Adding the nitrate group to the suspension resulted in turning to the nanorods with lower aspect ratios and finally spherical nanoparticles. The concentration of nitrate in water can be determined by monitoring with the naked eye and a UV–Vis spectrometer. It was shown that lower volumes of nitrate additive (25 μl) was a better identifier than those with higher volumes (50–150 μl). The color of Au-NRs suspension turned somehow from faint pink to purple after two days of adding 25 μl of nitrate solution. In addition, the UV–Vis absorption peak corresponding to longitudinal SPR (LSPR) blue-shifted from 814 nm to 712 nm after two days while the peak corresponding to transverse SPR (TSPR) showed a slight shift from 519 nm to 521 nm. Much more effect of nitrate group on LSPR than TSPR showed that nanorods are better identifier and biosensors than spherical nanoparticles. Decomposing and converting the Au NRs to sphere particles and lowering the initial aspect ratio of NDs in presence of nitrate ion is reported for the first time, but more works needed for talking about its mechanism.

Dynamics and scaling laws of underwater granular collapse with varying aspect ratios

We perform coupled fluid-particle simulations to understand the granular collapse in an ambient fluid (in particular, water) with a wide range of initial aspect ratios. We observe both similar and distinct features in underwater collapses compared to their dry counterparts. As aspect ratio $a$ increases, the normalized runout distance follows a piecewise power-law growth, transitioning at $a=2.5$. We associate this transition with the different growth rates of kinetic energy (with $a$) in vertical and horizontal directions. The ability of utilizing available energy for horizontal motion becomes limited when $ag2.5$. Moreover, the front propagation during underwater collapses can be well scaled by using the initial column height as length scale and considering a reduced gravity (due to buoyancy) in timescale. Under the reduced gravity, the initial fall of tall columns is found to be ballistic, consistent with dry collapses. On the other hand, underwater collapses (especially for large $a$) exhibit unique dynamics due to the presence of water. The eddies generated in water, which may carry considerable fluid inertia, tend to erode the surface of the granular layer, thus modifying the deposit morphology. The energy conversion is also affected by the ambient fluid. While water obviously consumes energy from the granular phase through fluid-particle interactions, it actually increases the efficiency of energy conversion from vertical to horizontal directions. The latter effect compensates the difference of runout distance between underwater and dry collapses.

Experimental Investigation on the Motion of Particle Cloud in Viscous Fluids

Laboratory experiments were conducted to study the dynamics of particle clouds in viscous fluids. Different shapes of frontal head and trailing stems were observed, and particle clouds were classified using data mining methodology. The stability of the frontal head of particle clouds was found to be correlated with the nozzle diameter and mass of sand particles in the form of an initial aspect ratio. The formation of particle clusters into a torus and the split of the frontal head into two or three clusters were investigated in detail. The cluster of particles flow through viscous fluid experienced partial separation due to the release of air bubbles from the rear of frontal head. It was observed that the time and location of major particle separation increase linearly with the aspect ratio. The oscillatory motion of the frontal head, caused by an uneven release of air bubbles from the rear of the frontal head, was found to be correlated with the initial aspect ratio. Both amplitude and wavelength exhibited a linear relationship with nondimensional time. The average drag coefficient of particle clouds Cd in viscous fluids was calculated for different aspect ratios, and the results were compared with the drag coefficient of individual particles. It was found that the averaged drag coefficients of particle clouds were smaller than the drag coefficient of individual particles, and Cd slightly increases with the increasing initial aspect ratio.

Simulation of quasi-static axisymmetric collapse of granular columns using smoothed particle hydrodynamics and discrete element methods

Flow of granular materials under different conditions poses significant challenges to engineers as their behavior can vary from solid-like to fluid-like. The quasi-static axisymmetric collapse of granular columns is an interesting case of granular flow relevant to the active deformation of retaining structures. This study aims to shed light on this type of collapse using numerical simulations including the smoothed particle hydrodynamics (SPH) method in the continuum framework and the discrete element method (DEM) in the discrete framework. Three-dimensional SPH and DEM models are developed to investigate the quasi-static axisymmetric collapse of granular columns, which is initiated by slowly expanding the cylindrical wall of the column. The Drucker–Prager constitutive model with non-associated flow rule is implemented into SPH formulations to model elastoplastic behavior of granular material. The linear contact model is used in DEM simulations with calibrated contact stiffness parameters. The effects of particle shape and hysteretic contact behavior in the DEM model are indirectly considered through reducing particle rotational velocities by a calibrated constant factor at every time step. Using the developed models, the final deposit profile (e.g., height, runout distance, repose angle), non-deformed region, collapse pattern and energy dissipation for different initial aspect ratios are investigated. In addition, final height, runout distance and energy dissipation are theoretically derived and are found to be in good agreement with SPH and DEM simulations. Results show that the quasi-static axisymmetric collapse can be modeled using both the continuum and discrete frameworks as long as appropriate constitutive models or contact behaviors are implemented. However, DEM simulations are more capable of capturing particle-level behaviors, such as the sharp edges of the free surface and flow front. Results demonstrate that quasi-static granular collapse is qualitatively similar to the dynamic collapse, but results in smaller runout distances for all aspect ratios.

Flow of buoyant granular materials along a free surface

We study experimentally the flow of light granular material along the free surface of a liquid of greater density. Despite a rich set of related geophysical and environmental phenomena, such as the spreading of calved ice, volcanic ash, debris and industrial wastes, there are few previous studies on this topic. We conduct a series of lock-release experiments of buoyant spherical beads into a rectangular tank initially filled with either fresh or salt water, and record the time evolution of the interface shape and the front location of the current of beads. We find that following the release of the lock the front location obeys a power-law behaviour during an intermediate time period before the nose of beads reaches a maximum runout distance within a finite time. We investigate the dependence of the scaling exponent and runout distance on the total amount of beads, the initial lock length, and the properties of the liquid that fills the tank in the experiments. Scaling arguments are provided to collapse the experimental data into universal curves, which can be used to describe the front dynamics of buoyant granular flows with different size and buoyancy effects and initial lock aspect ratios.

Surface tension-pressure superposition principle for anisotropic shrinkage of an ellipsoidal pore in viscous sintering

The shrinkage of a spherical pore is a classical model of the final stage of sintering; however, closed pores observed in real sintering have irregular shapes, which can be approximated as ellipsoids in many cases. In this study, we used the finite element simulation to evaluate the anisotropic shrinkage of a non-spherical pore. The shape evolution was expressed as the superposition of the deformation of a pore driven by surface tension and the deformation of a void driven by remote pressure in the absence of surface tension. By using this superposition principle, we can predict the shape evolution of any spheroidal pore, which is affected by the initial aspect ratio, the remote pressure, and the gas pressure inside the pore. We analyzed the role of sintering stress tensor on the anisotropic shrinkage also.

Impact of granular slugs on rigid targets: Effect of grain shape and fracture

Abstract The effect of grain shape and fracture on the interaction of high velocity granular slugs with rigid stationary targets is analysed for targets in normal and inclined orientations. The granular slugs comprise spherical, rod-shaped or cubic grains and are constructed by connecting together spherical sub-particles with either rigid or beam connectors . The case when grain fracture is suppressed (rigid connectors between sub-particles) is first analysed. With increasing grain aspect ratio, the grains tend to slide rather than roll on the target surface and this increases frictional interactions with the target surface. However, these enhanced frictional forces do not affect the momentum transmitted into normally oriented targets due to the symmetry of the problem. By contrast, the break in the symmetry for inclined targets results in the transmitted momentum increasing with grain aspect ratio. Fracture of the grains (as modelled by the fracture of the beam connectors between sub-particles) is shown to affect the momentum transmitted into the inclined targets. This is a consequence of fracture resulting in a change in grain shape. In this case the simulations show that the transmitted momentum is a function of the initial grain shape, the fracture properties of the grains and the impact velocity. In fact, grain fracture results in an enhanced transmitted momentum for initially cubic grains but fracture of grains with a high initial aspect ratio results in a reduction in transmitted momentum as these grains fragment into more spherically shaped grains.