arXiv subjects

Ishan Srivastava

Publications and source records attributed to Ishan Srivastava.

31 records · Page 2Linked to original sources

arXiv2022

Flow and Arrest in Stressed Granular Materials

Flowing granular materials often abruptly arrest if not driven by sufficient applied stresses. Such abrupt cessation of motion can be economically expensive in industrial materials handling and processing, and is significantly consequential in intermittent geophysical phenomena such as landslides and earthquakes. Using discrete element simulations, we calculate states of steady flow and arrest for granular materials under the conditions of constant applied pressure and shear stress, which are also most relevant in practice. Here the material can dilate or compact, and flow or arrest, in response to the applied stress. Our simulations highlight that under external stress, the intrinsic response of granular materials is characterized by uniquely-defined steady states of flow or arrest, which are highly sensitive to interparticle friction. While the flowing states can be equivalently characterized by volume fraction, coordination number or internal stress ratio, to characterize the states of shear arrest, one needs to also consider the structural anisotropy in the contact network. We highlight the role of dilation in the flow-arrest transition, and discuss our findings in the context of rheological transitions in granular materials.

cond-mat.soft↗

arXiv2022

Fluctuations and power-law scaling of dry, frictionless granular rheology near the hard-particle limit

The flow of frictionless granular particles is studied with stress-controlled discrete element modeling simulations for systems varying in size from 300 to 100,000 particles. The volume fraction and shear stress ratio $μ$ are relatively insensitive to system size fo a wide range of inertial numbers $I$. Second-order effects in strain rate, such as second normal stress differences, require large system sizes to accurately extract meaningful results, notably a non-monotonic dependence in the first normal stress difference with strain rate. The first-order rheological response represented by the $μ(I)$ relationship works well at describing the lower-order aspects of the rheology, except near the quasi-static limit of these stress-controlled flows. The pressure is varied over five decades, and a pressure dependence of the coordination number is observed, which is not captured by the inertial number. Large fluctuations observed for small systems $N\le$ 1,000 near the quasi-static limit can lead to arrest of flow resulting in challenges to fitting the data to rheological relationships. The inertial number is also insufficient for capturing the pressure-dependent behavior of property fluctuations. Fluctuations in the flow and microstructural properties are measured in both the quasi-static and inertial regimes, including shear stress, pressure, strain rate, normal stress differences, volume fraction, coordination number and contact fabric anisotropy. The fluctuations in flow properties scale self-similarly with pressure and system size. A transition in the scaling of fluctuations of stress properties and contact fabric anisotropy are measured and proposed as a quantitative identification of the transition from inertial to quasi-static flow.

cond-mat.soft↗

arXiv2022

Shear Is Not Always Simple: Rate-Dependent Effects of Flow Type on Granular Rheology

Despite there being an infinite variety of types of flow, most rheological studies focus on a single type such as simple shear. Using discrete element simulations, we explore bulk granular systems in a wide range of flow types at large strains and characterize invariants of the stress tensor for different inertial numbers and interparticle friction coefficients. We identify a strong dependence on the type of flow which grows with increasing inertial number or friction. Standard models of yielding, repurposed to describe the dependence of the stress on flow type in steady-state flow and at finite rates, are compared with data.

cond-mat.soft↗

arXiv2021

Jamming of Bidisperse Frictional Spheres

By generalizing a geometric argument for frictionless spheres, a model is proposed for the jamming density $ϕ_J$ of mechanically stable packings of bidisperse, frictional spheres. The monodisperse, $μ_s$-dependent jamming density $ϕ_J^{\mathrm{mono}}(μ_s)$ is the only input required in the model, where $μ_s$ is the coefficient of friction. The predictions of the model are validated by robust estimates of $ϕ_J$ obtained from computer simulations of up to $10^7$ particles for a wide range of $μ_s$, and size ratios up to 40:1. Although $ϕ_J$ varies nonmonotonically with the volume fraction of small spheres $f^s$ for all $μ_s$, its maximum value $ϕ_{J,\mathrm{max}}$ at an optimal $f^{s}_{\mathrm{max}}$ are both $μ_s$-dependent. The optimal $f^{s}_{\mathrm{max}}$ is characterized by a sharp transition in the fraction of small rattler particles.

cond-mat.soft↗

arXiv2020

Viscometric flow of dense granular materials under controlled pressure and shear stress

This study examines the flow of dense granular materials under external shear stress and pressure using discrete element method simulations. In this method, the material is allowed to strain along all periodic directions and adapt its solid volume fraction in response to an imbalance between the internal state of stress and the external applied stress. By systematically varying the external shear stress and pressure, the steady rheological response is simulated for: (1) rate-independent quasi-static flow; and (2) rate-dependent inertial flow. The simulated flow is viscometric with non-negligible first and second normal stress differences. While both normal stress differences are negative in inertial flows, the first normal stress difference switches from negative to slightly positive, and second normal stress difference tends to zero in quasi-static flows. The first normal stress difference emerges from a lack of coaxiality between a second-rank contact fabric tensor and strain rate tensor in the flow plane, while the second normal stress difference is linked to an excess of contacts in the shear plane compared with the vorticity direction. A general rheological model of second order (in terms of strain rate tensor) is proposed to describe the two types of flow, and the model is calibrated for various values of interparticle friction from simulations on nearly monodisperse spheres. The model incorporates normal stress differences in both regimes of flow and provides a complete viscometric description of steady dense granular flows.

cond-mat.soft↗

arXiv2020

Granular packings with sliding, rolling and twisting friction

Intuition tells us that a rolling or spinning sphere will eventually stop due to the presence of friction and other dissipative interactions. The resistance to rolling and spinning/twisting torque that stops a sphere also changes the microstructure of a granular packing of frictional spheres by increasing the number of constraints on the degrees of freedom of motion. We perform discrete element modeling simulations to construct sphere packings implementing a range of frictional constraints under a pressure-controlled protocol. Mechanically stable packings are achievable at volume fractions and average coordination numbers as low as 0.53 and 2.5, respectively, when the particles experience high resistance to sliding, rolling and twisting. Only when the particle model includes rolling and twisting friction, were experimental volume fractions reproduced.

cond-mat.soft↗

arXiv2020

Constraining the weights of Stokes Polytopes using BCFW recursions for Phi^4

The relationship between certain geometric objects called polytopes and scattering amplitudes has revealed deep structures in QFTs. It has been developed in great depth at the tree- and loop-level amplitudes in $\mathcal{N}=4~\text{SYM}$ theory and has been extended to the scalar $ϕ^3$ and $ϕ^4$ theories at tree-level. In this paper, we use the generalized BCFW recursion relations for massless planar $ϕ^4$ theory to constrain the weights of a class of geometric objects called Stokes polytopes, which manifest in the geometric formulation of $ϕ^4$ amplitudes. We see that the weights of the Stokes polytopes are intricately tied to the boundary terms in $ϕ^4$ theories. We compute the weights of $N=1,2$, and $3$ dimensional Stokes polytopes corresponding to six-, eight- and ten-point amplitudes respectively. We generalize our results to higher-point amplitudes and show that the generalized BCFW recursions uniquely fix the weights for an $n$-point amplitude.

hep-th↗

arXiv2020

Evolution of Internal Granular Structure at the Flow-Arrest Transition

The evolution of the internal granular structure in shear-arrested and shear-flowing states of granular materials is characterized using fabric tensors as descriptors of the internal contact and force networks. When a dilute system of frictional grains is subjected to a constant pressure and shear stress, the bulk stress ratio is well-predicted from the anisotropy of its contact and force networks during transient flow prior to steady shear flow or shear arrest. Although the onset of shear arrest is a stochastic process, the fabric tensors upon arrest are distributed around nearly equal contributions of force and contact network anisotropy to the bulk stress ratio. The distribution becomes seemingly narrower with increasing system size. The anisotropy of the contact network in shear-arrested states is reminiscent of the fabric anisotropy observed in shear-jammed packings.

cond-mat.soft↗

arXiv2019

A different take on the best-first game tree pruning algorithms

The alpha-beta pruning algorithms have been popular in game tree searching ever since they were discovered. Numerous enhancements are proposed in literature and it is often overwhelming as to which would be the best for implementation. A certain enhancement can take far too long to fine tune its hyper parameters or to decide whether it is going to not make much of a difference due to the memory limitations. On the other hand are the best first pruning techniques, mostly the counterparts of the infamous SSS* algorithm, the algorithm which proved out to be disruptive at the time of its discovery but gradually became outcast as being too memory intensive and having a higher time complexity. Later research doesn't see the best first approaches to be completely different from the depth first based enhancements but both seem to be transitionary in the sense that a best first approach could be looked as a depth first approach with a certain set of enhancements and with the growing power of the computers, SSS* didn't seem to be as taxing on the memory either. Even so, there seems to be quite difficulty in understanding the nature of the SSS* algorithm, why it does what it does and it being termed as being too complex to fathom, visualize and understand on an intellectual level. This article tries to bridge this gap and provide some experimental results comparing the two with the most promising advances.

cs.AI↗

arXiv2019

Flow-Arrest Transitions in Frictional Granular Matter

The transition between shear-flowing and shear-arrested states of frictional granular matter is studied using constant-stress discrete element simulations. By subjecting a dilute system of frictional grains to a constant external shear stress and pressure, friction-dependent critical shear stress and density are clearly identified with both exhibiting a crossover between low and high friction. The critical shear stress bifurcates two nonequilibrium steady states: (i) steady state shear flow characterized by a constant deformation rate, and (ii) shear arrest characterized by temporally decaying creep to a statically stable state. The onset of arrest below critical shear stress occurs at a time $t_{c}$ that exhibits a heavy-tailed distribution, whose mean and variance diverge as a power law at the critical shear stress with a friction-dependent exponent that also exhibits a crossover between low and high friction. These observations indicate that granular arrest near critical shear stress is highly unpredictable and is strongly influenced by interparticle friction.

cond-mat.soft↗

arXiv2018

Effect of Shape and Friction on the Packing and Flow of Granular Materials

The packing and flow of aspherical frictional particles are studied using discrete element simulations. Particles are superballs with shape $|x|^{s}+|y|^{s}+|z|^{s} = 1$ that varies from sphere ($s=2$) to cube ($s=\infty$), constructed with an overlapping-sphere model. Both packing fraction, $ϕ$, and coordination number, $z$, decrease monotonically with microscopic friction $μ$, for all shapes. However, this decrease is more dramatic for larger $s$ due to a reduction in the fraction of face-face contacts with increasing friction. For flowing grains, the dynamic friction $\tildeμ$ - the ratio of shear to normal stresses - depends on shape, microscopic friction and inertial number $I.$ For all shapes, $\tildeμ$ grows from its quasi-static value $\tildeμ_0$ as $(\tildeμ-\tildeμ_0) = dI^α,$ with different universal behavior for frictional and frictionless shapes. For frictionless shapes the exponent $α\approx 0.5$ and prefactor $d \approx 5\tildeμ_0$ while for frictional shapes $α\approx 1$ and $d$ varies only slightly. The results highlight that the flow exponents are universal and are consistent for all the shapes simulated here.

cond-mat.soft↗

arXiv2016

Local Plasticity as the Source of Creep and Slow Dynamics in Granular Materials

Creep mechanisms in uniaxially compressed 3D granular solids comprised of faceted frictionless grains are studied numerically using a constant pressure and constant stress simulation method. Rapid uniaxial compression followed by slow dilation is predicted on the basis of a logarithmic creep phenomenon. Micromechanical analysis indicates the existence of a correlation between granular creep and grain-scale deformations. Localized regions of large strain appear during creep and grow in magnitude and size with time. Furthermore, the accumulation of non-affine granular displacements increases linearly with local strain, thereby providing insights into the origins of plastic dissipation during stress-driven creep evolution. The prediction of slow logarithmic dynamics in the absence of friction indicates a universality in the role of plastic dissipation during the creep of granular solids.

cond-mat.soft↗

arXiv2014

Variable-cell method for stress-controlled jamming of athermal, frictionless grains

A new method is introduced to simulate jamming of polyhedral grains under controlled stress that incorporates global degrees of freedom through the metric tensor of a periodic cell containing grains. Jamming under hydrostatic/isotropic stress and athermal conditions leads to a precise definition of the ideal jamming point at zero shear stress. The structures of tetrahedra jammed hydrostatically exhibit less translational order and lower jamming-point density than previously described `maximally random jammed' hard tetrahedra. Under the same conditions, cubes jam with negligible nematic order. Grains with octahedral symmetry jam in the large-system limit with an abundance of face-face contacts in the absence of nematic order. For sufficiently large face-face contact number, percolating clusters form that span the entire simulation box. The response of hydrostatically jammed tetrahedra and cubes to shear-stress perturbation is also demonstrated with the variable-cell method.

cond-mat.soft↗