GRAIN NON-SPHERICITY EFFECTS ON THE ANGLE OF REPOSE OF GRANULAR MATERIAL

1993 ◽  
Vol 07 (09n10) ◽  
pp. 2037-2046 ◽  
Author(s):  
JASON A.C. GALLAS ◽  
STEFAN SOKOLOWSKI
Keyword(s):  
Granular Material ◽  
Granular Media ◽  
Angle Of Repose ◽  
Site Model ◽  
Specific Effects ◽  
A Site ◽  
Spherical Grains

We use a site-site model to describe non-sphericity of particles composing a granular media. Specific effects of grain non-sphericity 011 the angle of repose are investigated. We report evidence indicating the possible existence of a shape-roughness threshold for grains: below it angles of repose are essentially the same as those obtained for spherical grains; above it there are pronounced changes 011 the angle of repose and it is possible to find rather large piles of grains.

Avalanche Stratification - Experimental Tests of the “Metastable Wedge” and “Continuous Flow” Models

2000 ◽  
Vol 627 ◽  
Author(s):  
M. E. Swanson ◽  
M. Landreman ◽  
J. Michel ◽  
J. Kakalios
Keyword(s):  
Continuous Flow ◽  
Granular Media ◽  
Experimental Tests ◽  
Coarse Sand ◽  
Angle Of Repose ◽  
Flow Models ◽  
Maximum Angle ◽  
Stratification Effect ◽  
Upward Moving ◽  
Moving Layer

ABSTRACTWhen an initially homogeneous binary mixture of granular media such as fine and coarse sand is poured near the closed edge of a “quasi-two-dimensional” Hele-Shaw cell consisting of two vertical transparent plates held a narrow distance apart, the mixture spontaneously forms alternating segregated layers. Experimental measurements of this stratification effect are reported in order to determine which model, one which suggests that segregation only occurs when the granular material contained within a metastable heap between the critical and maximum angle of repose avalanches down the free surface, or one for which the segregation results from smaller particles becoming trapped in the top surface and being removed from the moving layer during continuous flow. The result reported here indicate that the Metastable Wedge model provides a natural explanation for the initial mixed zone which precedes the formation of the layers, while the Continuous Flow model explains the observed upward moving kink of segregated material for higher granular flux rates, and that both mechansims are necessary in order to understand the observed pairing of segregated layersfor intermediate flow rates and cell separations.

Stress fields in granular material and implications for performance of robot locomotion over granular media

2015 ◽  
Vol 8 (1) ◽  
pp. 2005-2009
Author(s):  
Diandong Ren ◽  
Lance M. Leslie ◽  
Congbin Fu
Keyword(s):  
Mechanical Properties ◽  
Granular Material ◽  
Granular Media ◽  
Rotation Frequency ◽  
Legged Locomotion ◽  
Working Environment ◽  
Navier Stokes ◽  
Maximum Speed ◽  
Sigmoid Curve ◽  
Multi Body

 Legged locomotion of robots has advantages in reducing payload in contexts such as travel over deserts or in planet surfaces. A recent study (Li et al. 2013) partially addresses this issue by examining legged locomotion over granular media (GM). However, they miss one extremely significant fact. When the robot’s wheels (legs) run over GM, the granules are set into motion. Hence, unlike the study of Li et al. (2013), the viscosity of the GM must be included to simulate the kinematic energy loss in striking and passing through the GM. Here the locomotion in their experiments is re-examined using an advanced Navier-Stokes framework with a parameterized granular viscosity. It is found that the performance efficiency of a robot, measured by the maximum speed attainable, follows a six-parameter sigmoid curve when plotted against rotating frequency. A correct scaling for the turning point of the sigmoid curve involves the footprint size, rotation frequency and weight of the robot. Our proposed granular response to a load, or the ‘influencing domain’ concept points out that there is no hydrostatic balance within granular material. The balance is a synergic action of multi-body solids. A solid (of whatever density) may stay in equilibrium at an arbitrary depth inside the GM. It is shown that there exists only a minimum set-in depth and there is no maximum or optimal depth. The set-in depth of a moving robot is a combination of its weight, footprint, thrusting/stroking frequency, surface property of the legs against GM with which it has direct contact, and internal mechanical properties of the GM. If the vehicle’s working environment is known, the wheel-granular interaction and the granular mechanical properties can be grouped together. The unitless combination of the other three can form invariants to scale the performance of various designs of wheels/legs. Wider wheel/leg widths increase the maximum achievable speed if all other parameters are unchanged.

A Phenotype–Genotype Codon Model for Detecting Adaptive Evolution

2019 ◽  
Vol 69 (4) ◽  
pp. 722-738 ◽  
Author(s):  
Christopher T Jones ◽  
Noor Youssef ◽  
Edward Susko ◽  
Joseph P Bielawski
Keyword(s):  
Positive Selection ◽  
Adaptive Evolution ◽  
Evolutionary History ◽  
Life History Trait ◽  
Molecular Adaptation ◽  
Simulation Studies ◽  
Site Model ◽  
Branch Site ◽  
A Site ◽  
Post Hoc

Abstract A central objective in biology is to link adaptive evolution in a gene to structural and/or functional phenotypic novelties. Yet most analytic methods make inferences mainly from either phenotypic data or genetic data alone. A small number of models have been developed to infer correlations between the rate of molecular evolution and changes in a discrete or continuous life history trait. But such correlations are not necessarily evidence of adaptation. Here, we present a novel approach called the phenotype–genotype branch-site model (PG-BSM) designed to detect evidence of adaptive codon evolution associated with discrete-state phenotype evolution. An episode of adaptation is inferred under standard codon substitution models when there is evidence of positive selection in the form of an elevation in the nonsynonymous-to-synonymous rate ratio $\omega$ to a value $\omega > 1$. As it is becoming increasingly clear that $\omega > 1$ can occur without adaptation, the PG-BSM was formulated to infer an instance of adaptive evolution without appealing to evidence of positive selection. The null model makes use of a covarion-like component to account for general heterotachy (i.e., random changes in the evolutionary rate at a site over time). The alternative model employs samples of the phenotypic evolutionary history to test for phenomenological patterns of heterotachy consistent with specific mechanisms of molecular adaptation. These include 1) a persistent increase/decrease in $\omega$ at a site following a change in phenotype (the pattern) consistent with an increase/decrease in the functional importance of the site (the mechanism); and 2) a transient increase in $\omega$ at a site along a branch over which the phenotype changed (the pattern) consistent with a change in the site’s optimal amino acid (the mechanism). Rejection of the null is followed by post hoc analyses to identify sites with strongest evidence for adaptation in association with changes in the phenotype as well as the most likely evolutionary history of the phenotype. Simulation studies based on a novel method for generating mechanistically realistic signatures of molecular adaptation show that the PG-BSM has good statistical properties. Analyses of real alignments show that site patterns identified post hoc are consistent with the specific mechanisms of adaptation included in the alternate model. Further simulation studies show that the covarion-like component of the PG-BSM plays a crucial role in mitigating recently discovered statistical pathologies associated with confounding by accounting for heterotachy-by-any-cause. [Adaptive evolution; branch-site model; confounding; mutation-selection; phenotype–genotype.]

The regulated production of mu m and mu s mRNA is dependent on the relative efficiencies of mu s poly(A) site usage and the c mu 4-to-M1 splice

1989 ◽  
Vol 9 (2) ◽  
pp. 726-738
Author(s):  
M L Peterson ◽  
R P Perry
Keyword(s):  
B Cells ◽  
Consensus Sequence ◽  
Splice Junction ◽  
Detectable Effect ◽  
Small Nuclear Rna ◽  
Site Model ◽  
Proximal Site ◽  
Precursor Rna ◽  
Immunoglobulin Mu ◽  
A Site

The relative abundance of the mRNAs encoding the membrane (mu m) and secreted (mu s) forms of immunoglobulin mu heavy chain is regulated during B-cell maturation by a change in the mode of RNA processing. Current models to explain this regulation involve either competition between cleavage-polyadenylation at the proximal (mu s) poly(A) site and cleavage-polyadenylation at the distal (mu m) poly(A) site [poly(A) site model] or competition between cleavage-polyadenylation at the mu s poly(A) site and splicing of the C mu 4 and M1 exons, which eliminates the mu s site (mu s site-splice model). To test certain predictions of these models and to determine whether there is a unique structural feature of the mu s poly(A) site that is essential for regulation, we constructed modified mu genes in which the mu s or mu m poly(A) site was replaced by other poly(A) sites and then studied the transient expression of these genes in cells representative of both early- and late-stage lymphocytes. Substitutions at the mu s site dramatically altered the relative usage of this site and caused corresponding reciprocal changes in the usage of the mu m site. Despite these changes, use of the proximal site was still usually higher in plasmacytomas than in pre-B cells, indicating that regulation does not depend on a unique feature of the mu s poly(A) site. Replacement of the distal (mu m) site had no detectable effect on the usage of the mu s site in either plasmacytomas or pre-B cells. These findings are inconsistent with the poly(A) site model. In addition, we noted that in a wide variety of organisms, the sequence at the 5' splice junction of the C mu 4-to-M1 intron is significantly different from the consensus 5' splice junction sequence and is therefore suboptimal with respect to its complementary base pairing with U1 small nuclear RNA. When we mutated this suboptimal sequence into the consensus sequence, the mu mRNA production in plasmacytoma cells was shifted from predominantly mu s to exclusively mu m. This result unequivocally demonstrated that splicing of the C mu 4-to-M1 exon is in competition with usage of the mu s poly(A) site. A key feature of this regulatory phenomenon appears to be the appropriately balanced efficiencies of these two processing reactions. Consistent with predictions of the mu s site-splice model, B cells were found to contain mu m precursor RNA that had undergone the C mu 4-to-M1 splice but had not yet been polyadenylated at the mu m site.

A New Simple Non Linear Modelling of the Quasi Statics of Granular Media: predictions, comparisons with experiments

2000 ◽  
Vol 627 ◽  
Author(s):  
Pierre Evesque
Keyword(s):  
Granular Material ◽  
Granular Media ◽  
Stress Ratio ◽  
Rheological Behaviour ◽  
Stress Strain ◽  
Linear Modelling ◽  
Non Linear ◽  
Strain Paths ◽  
Contact Distribution

ABSTRACTFirst, a non linear incremental modelling is proposed to describe rheological behaviour of granular material under different simple (i.e. triaxial-, oedometric-, undrained-) stress-strain paths. Validity of isotropic-response assumption is demonstrated whatever the stress ratio as far as deformation range remains small (ε1<5%). This contradicts some recent hypothesis made on the evolution of contact distribution during anisotropic loading.

scholarly journals Aerated granular flow over a horizontal rigid surface

2000 ◽  
Vol 424 ◽  
pp. 169-195 ◽  
Author(s):  
I. EAMES ◽  
M. A. GILBERTSON
Keyword(s):  
Granular Material ◽  
Point Source ◽  
Drag Force ◽  
Granular Flow ◽  
Gas Flow ◽  
Granular Flows ◽  
Angle Of Repose ◽  
Minimum Fluidization Velocity ◽  
Granular Pile ◽  
Particle Pressure

The effect of a vertical gas flow on the dynamics of a coulombic granular material moving over a horizontal rigid porous surface has been studied experimentally and theoretically. The presence of a fluidizing gas significantly alters the granular flow dynamics. When the gas velocity, ug, is below the minimum fluidization velocity, umf, the effect of the gas is to reduce the angle of repose θ from the value measured in the absence of a gas flow. When material is poured from a point source onto a horizontal surface it forms a pile, which adjusts through episodic avalanching to a self-similar conical shape. Under these conditions, the development of the pile is determined by the local force balance on individual particles and its extent may be expressed in terms of the volume of particles added and the angle of repose. A volume of material is poured continuously from a point source onto a surface according to Qtα. Below the minimum uidization velocity, a quasi-static description gives the encroachment distance of the granular pile as l = (2Q/(2π/3)n−1 tan θ)1/n+1tα/n+1 where n = 1 for a planar release and n = 2 for an axisymmetric release.A continuum description of fluidized granular flow has been developed by vertically averaging the mass and momentum conservation equations and including the momentum exchange between the gas and granular flow. The bulk movement is driven along the ground by horizontal gradients of particle pressure and is resisted by a viscous drag force due to the particles moving horizontally through a vertical gas flow. Above the minimum fluidization velocity the character of the granular flow is significantly altered and takes on fluid-like properties. The model predicts the shape of the fluidized granular pile and that the encroachment distance grows as l = λnα (Q(ug + umf) / ε)1/n+2tα+1/n+2, where ε is the void fraction in the bed and λnα is a constant. Below the conditions for fluidization (ug < umf), the pile of granular material grows quasi-statically when t > t∗, where t∗ ∼ (εn+1Qug + umf) / μ2+n (umf − ug)2+n) 1/1+n−α corresponds to the critical time when frictional forces are comparable to gradients of particle pressure and the drag force. Numerical solutions describing the granular flow are presented.Experimental observations of the shape and extent of planar and axisymmetric granular flows when α = 1 compare well with theoretical predictions for various values of particle volume flux Q, time t, and gas flow rate ug. The mathematical description of fluidized granular flows along rigid surfaces indicates a strong analogy with buoyancy-driven flows in a porous medium. This analogy permits extension of our description to include flows down slopes and the effect of internal stratification.

A-site model RNAs

Biochimie ◽  
2006 ◽  
Vol 88 (8) ◽  
pp. 1021-1026 ◽  
Author(s):  
T HERMANN
Keyword(s):  
Site Model ◽  
A Site

Particula: A simulator tool for computational rock physics of granular media

Geophysics ◽  
2019 ◽  
Vol 84 (3) ◽  
pp. F85-F95 ◽  
Author(s):  
Mustafa A. Al Ibrahim ◽  
Abdulla Kerimov ◽  
Tapan Mukerji ◽  
Gary Mavko
Keyword(s):  
Granular Media ◽  
Rock Physics ◽  
Coherent Noise ◽  
Irregular Shapes ◽  
Granular Dynamics ◽  
Loose Sediments ◽  
Dynamics Simulations ◽  
Grain Surface ◽  
Spherical Grains

Granular dynamics simulations provide insights to the contact-scale physics of loose sediments. However, simulations using identical spherical grains do not reflect characteristics observed in natural sediments, such as pack sorting, grading, grain sphericity, and grain roundness. We have developed software to create 3D grain packs of a range of regular and irregular shapes with geologically realistic variations in sorting and grading. An efficient approach is used to create multiple realizations of nonspherical irregularly shaped grains using coherent noise modification of the spherical grain surface. The discrete-element method is used to assemble the grain pack with different depositional styles by letting grains fall under the influence of gravity. Characterization of various parameters of random loose and dense grain packs, and comparison with previous studies, helps to establish the validity, flexibility, and consistency of the simulator. The output of this software is a digital grain pack, including metadata such as contacts and coordinates, that can be studied further using other analysis tools, e.g., by conducting fluid flow, mechanical, or electrical simulations.

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