arXiv subjects

Shankar Venkataramani

Publications and source records attributed to Shankar Venkataramani.

10 recordsLinked to original sources

arXiv2026

Computation of stresses in jammed packings modeled with Tresca friction

This paper is interested in the computation of stresses within jammed packings of rigid polygonal cells. The cells are considered to follow a Tresca friction law. First, a constrained minimization problem is introduced where the friction energy is minimized while enforcing the non-interpenetration of neighboring cells as inequality constraints. The corresponding dual maximization problem is then deduced and its solutions provide normal stresses at the interface between cells. Finally, lowest order Raviart-Thomas finite elements are used to reconstruct a consistent stress field by solving local problems. Numerical results are presented to showcase the consistency and robustness of the proposed methodology.

math.NA↗

arXiv2025

Coupled KPZ equations and their decoupleability

We discuss characterizations of the decoupleability, partial and full, of trilinear or completely symmetric real $n\times n\times n$ tensors, which inform on the structure of certain coupled KPZ equations. Informally, when the tensor is partially decoupleable, one of the components in the coupled KPZ equation splits off from the others, while when the tensor is fully decoupleable, each of the $n$ components splits off from the others. Such a characterization is recast as a problem of membership of trilinear tensors in $O(n)$ orbits of subsets of fully decoupleable and partially decoupleable tensors. When $n=2$, we show these subsets are the same, and in this case give a single criterion in terms of the entries of a tensor for membership in the orbits of these subsets. When $n\geq 3$, the subsets are different. For $n\geq 3$, we characterize full decoupleability in terms of several abstract relations, which when $n=3$ are made explicit. When $n=3$, we also explicitly characterize partial decoupleability. The methods involve notions in applied invariant theory, relating $O(n)$ invariant subsets to stabilizer subgroup actions on smaller sets. When $n=3$ make use of the explicit basis of invariants found by Olive and Auffray. When $n=2$, we also supply two other more direct arguments.

math.PR↗

arXiv2020

A Modified Split Bregman Algorithm for Computing Microstructures Through Young Measures

The goal of this paper is to describe the oscillatory microstructure that can emerge from minimizing sequences for nonconvex energies. We consider integral functionals that are defined on real valued (scalar) functions $u(x)$ which are nonconvex in the gradient $\nabla u$ and possibly also in $u$. To characterize the microstructures for these nonconvex energies, we minimize the associated relaxed energy using two novel approaches: i) a semi-analytical method based on control systems theory, ii) and a numerical scheme that combines convex splitting together with a modified version of the split Bregman algorithm. These solutions are then used to gain information about minimizing sequences of the original problem and the spatial distribution of microstructure.

math.OC↗

arXiv2018

Target Patterns in a 2-D Array of Oscillators with Nonlocal Coupling

We analyze the effect of adding a weak, localized, inhomogeneity to a two dimensional array of oscillators with nonlocal coupling. We propose and also justify a model for the phase dynamics in this system. Our model is a generalization of a viscous eikonal equation that is known to describe the phase modulation of traveling waves in reaction-diffusion systems. We show the existence of a branch of target pattern solutions that bifurcates from the spatially homogeneous state when $\varepsilon$, the strength of the inhomogeneity, is nonzero and we also show that these target patterns have an asymptotic wavenumber that is small beyond all orders in $\varepsilon$. The strategy of our proof is to pose a good ansatz for an approximate form of the solution and use the implicit function theorem to prove the existence of a solution in its vicinity. The analysis presents two challenges. First, the linearization about the homogeneous state is a convolution operator of diffusive type and hence not invertible on the usual Sobolev spaces. Second, a regular perturbation expansion in $\varepsilon$ does not provide a good ansatz for applying the implicit function theorem since the nonlinearities play a major role in determining the relevant approximation, which also needs to be "correct" to all orders in $\varepsilon$. We overcome these two points by proving Fredholm properties for the linearization in appropriate Kondratiev spaces and using a refined ansatz for the approximate solution, which obtained using matched asymptotics.

math.AP↗

arXiv2015

Isometric immersions and self-similar buckling in Non-Euclidean elastic sheets

The edge of torn elastic sheets and growing leaves often form a hierarchical buckling pattern. Within non-Euclidean plate theory this complex morphology can be understood as low bending energy isometric immersions of hyperbolic Riemannian metrics. With this motivation we study the isometric immersion problem in strip and disk geometries. By finding explicit piecewise smooth solutions of hyperbolic Monge-Ampere equations on a strip, we show there exist periodic isometric immersions of hyperbolic surfaces in the small slope regime. We extend these solutions to exact isometric immersions through resummation of a formal asymptotic expansion. In the disc geometry we construct self-similar fractal-like isometric immersions for disks with constant negative curvature. The solutions in both the strip and disc geometry qualitatively resemble the patterns observed experimentally and numerically in torn elastic sheets, leaves and swelling hydrogels. For hyperbolic non-Euclidean sheets, complex wrinkling patterns are thus possible within the class of finite bending energy isometric immersions. Further, our results identify the key role of the degree of differentiability (regularity) of the isometric immersion in determining the global structure of a non-Euclidean elastic sheet in 3-space.

cond-mat.soft↗

arXiv2012

Defects and boundary layers in non-Euclidean plates

We investigate the behavior of non-Euclidean plates with constant negative Gaussian curvature using the Föppl-von Kármán reduced theory of elasticity. Motivated by recent experimental results, we focus on annuli with a periodic profile. We prove rigorous upper and lower bounds for the elastic energy that scales like the thickness squared. In particular we show that are only two types of global minimizers -- deformations that remain flat and saddle shaped deformations with isolated regions of stretching near the edge of the annulus. We also show that there exist local minimizers with a periodic profile that have additional boundary layers near their lines of inflection. These additional boundary layers are a new phenomenon in thin elastic sheets and are necessary to regularize jump discontinuities in the azimuthal curvature across lines of inflection. We rigorously derive scaling laws for the width of these boundary layers as a function of the thickness of the sheet.

math.OC↗

arXiv2012

Shape Transitions in Hyperbolic Non-Euclidean Plates

We present and summarize the results of recent studies on non-Euclidean plates with imposed constant negative Gaussian curvature in both the Föppl - von Kármán and Kirchhoff approximations. Motivated by experimental results we focus on annuli with a periodic profile. We show that in the Föppl - von Kármán approximation there are only two types of global minimizers -- flat and saddle shaped deformations with localized regions of stretching near the boundary of the annulus. We also show that there exists local minimizers with $n$-waves that have regions of stretching near their lines of inflection. In the Kirchhoff approximation we show that there exist exact isometric immersions with periodic profiles. The number of waves in these configurations is set by the condition that the bending energy remains finite and grows approximately exponentially with the radius of the annulus. For large radii, these shape are energetically favorable over saddle shapes and could explain why wavy shapes are selected by crocheted models of the hyperbolic plane.

cond-mat.soft↗

arXiv2011

Shape selection in non-Euclidean plates

We investigate isometric immersions of disks with constant negative curvature into $\mathbb{R}^3$, and the minimizers for the bending energy, i.e. the $L^2$ norm of the principal curvatures over the class of $W^{2,2}$ isometric immersions. We show the existence of smooth immersions of arbitrarily large geodesic balls in $\mathbb{H}^2$ into $\mathbb{R}^3$. In elucidating the connection between these immersions and the non-existence/singularity results of Hilbert and Amsler, we obtain a lower bound for the $L^\infty$ norm of the principal curvatures for such smooth isometric immersions. We also construct piecewise smooth isometric immersions that have a periodic profile, are globally $W^{2,2}$, and have a lower bending energy than their smooth counterparts. The number of periods in these configurations is set by the condition that the principal curvatures of the surface remain finite and grows approximately exponentially with the radius of the disc. We discuss the implications of our results on recent experiments on the mechanics of non-Euclidean plates.

math.DG↗

arXiv2003

The energy of crumpled sheets in Foppl-von Karman plate theory

We study investigate a long, thin rectangular elastic membrane that is bent through an angle $2 α$, using the Foppl--von Karman ansatz in a geometrically linear setting. We study the associated variational problem, and show the existence of a minimizer for the elastic energy. We also prove rigorous upper and lower bounds for the minimum energy of this configuration in terms of the plate thickness and the bending angle, and we also obtain results for the structure of the elastic ridge along it's length.

math.AP↗

arXiv1994

Topology of Event Horizons and Topological Censorship

We prove that, under certain conditions, the topology of the event horizon of a four dimensional asymptotically flat black hole spacetime must be a 2-sphere. No stationarity assumption is made. However, in order for the theorem to apply, the horizon topology must be unchanging for long enough to admit a certain kind of cross section. We expect this condition is generically satisfied if the topology is unchanging for much longer than the light-crossing time of the black hole. More precisely, let $M$ be a four dimensional asymptotically flat spacetime satisfying the averaged null energy condition, and suppose that the domain of outer communication $\C_K$ to the future of a cut $K$ of $\Sm$ is globally hyperbolic. Suppose further that a Cauchy surface $Σ$ for $\C_K$ is a topological 3-manifold with compact boundary $\partial§$ in $M$, and $§'$ is a compact submanifold of $\bS$ with spherical boundary in $§$ (and possibly other boundary components in $M/§$). Then we prove that the homology group $H_1(Σ',Z)$ must be finite. This implies that either $\partial§'$ consists of a disjoint union of 2-spheres, or $§'$ is nonorientable and $\partial§'$ contains a projective plane. Further, $\partial§=\partial\Ip[K]\cap\partial\Im[\Sp]$, and $\partial Σ$ will be a cross section of the horizon as long as no generator of $\partial\Ip[K]$ becomes a generator of $\partial\Im[\Sp]$. In this case, if $§$ is orientable, the horizon cross section must consist of a disjoint union of 2-spheres.}

gr-qc↗