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Yaniv Almog

Publications and source records attributed to Yaniv Almog.

18 recordsLinked to original sources
arXiv2025

On the stability of symmetric flows in a two-dimensional channel

We consider the stability of symmetric flows in a two-dimensional channel (including the Poiseuille flow). In 2015 Grenier, Guo, and Nguyen have established instability of these flows in a particular region of the parameter space, affirming formal asymptotics results from the 1940's. We prove that these flows are stable outside this region in parameter space. More precisely we show that the Orr-Sommerfeld operator $$ {\mathcal B} =\Big(-\frac{d^2}{dx^2}+iβ(U+iλ)\Big)\Big(\frac{d^2}{dx^2}-α^2\Big) -iβU^{\prime\prime}\,, $$ which is defined on $$ D({\mathcal B})=\{u\in H^4(0,1)\,,\, u^\prime(0)=u^{(3)}(0)=0 \mbox{ and }\, u(1)=u^\prime(1)=0\}. $$ is bounded on the half-plane $\Re λ\geq 0$ for $α\gg β^{-1/10}$ or $α\ll β^{-1/6}$.

math.AP↗
arXiv2025

Stability of laminar monotone shear flows in a channel for high Reynolds number

We consider the stability of a laminar flow $U\in C^4([-1,1])$ in the two-dimensional channel $\mathbb{R} \times[-1,1]$ in the large Reynolds number limit. Assuming that $U$ is strictly monotone but allowing $U^{\prime\prime}$ to vanish, we obtain that if the operator $$ {\mathcal K}_ν=-\frac{d^2}{dx^2}+\frac{U^{\prime\prime}}{U-ν} \,, $$ is strictly positive for all $ν\in\mathbb{R}$ for which $U^{\prime\prime}(U^{-1}(ν))=0$,then $U$ is stable for sufficiently large Reynolds number. This contribution generalizes previous results mostly by allowing long wave perturbations (but much shorter than the Reynolds number).

math.AP↗
arXiv2024

Minimization of the discrete interaction energy with smooth potentials

We study the pair interaction on flat tori of functions whose Fourier coefficients are positive and decay sufficiently rapidly. In dimension one we find that the minimizer, up to translation, is the equidistant point set. In dimension two, minimizing with respect to triplets we find that the minimizer is the triangular lattice.

math-ph↗
arXiv2020

On the spectrum of some Bloch-Torrey vector operators

We consider the Bloch-Torrey operator in $L^2(I,{\mathbb R}^3)$ where $I\subseteq{\mathbb R}$. In contrast with the $L^2(I,{\mathbb R}^2)$ (as well as the $L^2({\mathbb R}^k,{\mathbb R}^2)$) case considered in previous works. We obtain that ${\mathbb R}_+$ is in the continuous spectrum for $I={\mathbb R}$ as well as discrete spectrum outside the real line. For a finite interval we find the left margin of the spectrum. In addition, we prove that the Bloch-Torrey operator must have an essential spectrum for a rather general setup in ${\mathbb R}^k$, and find an effective description for its domain.

math-ph↗
arXiv2020

On the stability of laminar flows between plates

Consider a two-dimensional laminar flow between two plates, so that $(x_1,x_2)\in {\mathbb R} \times[-1,1]$, given by ${\mathbf v}(x_1,x_2)=(U(x_2),0)$, where $U\in C^4([-1,1])$ satisfies $U^\prime\neq0$ in $[-1,1]$. We prove that the flow is linearly stable in the large Reynolds number limit, in two different cases: $\bullet$ $\sup_{x\in[-1,1]} |U"(x)| + \sup_{x\in[-1,1]} |U"(x)| \ll \min_{x\in[-1,1]}|U^\prime(x)|$ (nearly Couette flows), $\bullet$ $U^{\prime\prime}\neq0$ in $[-1,1]$. We assume either no-slip or fixed traction force conditions on the plates, and an arbitrary large (but much smaller than the Reynolds number) period in the $x_1$ direction.

math-ph↗
arXiv2020

Global homogenization of a dilute suspension of spheres: Suspension rheology

A new method for rheologically homogenizing a dilute suspension composed of freely-suspended spherical particles dispersed in a Newtonian fluid is presented: The ensemble-averaged velocity and stress fields obtained for the neutrally-buoyant sphere suspension are compared with the respective velocity and stress fields obtained for a hypothetical homogeneous Newtonian fluid continuum possessing a spatially {\em non-uniform} viscosity for the same specified boundaries and ambient flow. The method is global in nature; that is, wall effects and spatial dependence of both the ambient flow and the particle number density are encountered, thereby confirming known classical results up to $O(c^{2})$ terms ($c$ = volume concentration of spheres) for the suspension viscosity which have previously been obtained by assuming {\em a priori} that the suspension is both unbounded and statistically homogeneous.

math-ph↗
arXiv2019

Existence of superconducting solutions for a reduced Ginzburg-Landau model in the presence of strong electric currents

In this work we consider a reduced Ginzburg-Landau model in which the magnetic field is neglected and the magnitude of the current density is significantly stronger than that considered in a recent work by the same authors. We prove the existence of a solution which can be obtained by solving a non-convex minimization problem away from the boundary of the domain. Near the boundary, we show that this solution is essentially one-dimensional. We also establish some linear stability results for a simplified, one-dimensional version of the original problem.

math-ph↗
arXiv2018

The spectrum of a Schrödinger operator in a wire-like domain with a purely imaginary degenerate potential in the semiclassical limit

Consider a two-dimensional domain shaped like a wire, not necessarily of uniform cross section. Let $V$ denote an electric potential driven by a voltage drop between the conducting surfaces of the wire. We consider the operator ${\mathcal A}_h=-h^2Δ+iV$ in the semi-classical limit $h\to 0$. We obtain both the asymptotic behaviour of the left margin of the spectrum, as well as resolvent estimates on the left side of this margin. We extend here previous results obtained for potentials for which the set where the current (or $\nabla V$) is normal to the boundary is discrete, in contrast with the present case where $V$ is constant along the conducting surfaces.

math-ph↗
arXiv2017

Spectral semi-classical analysis of a complex Schrödinger operator in exterior domains

Generalizing previous results obtained for the spectrum of the Dirichlet and Neumann realizations in a bounded domain of a Schrödinger operator with a purely imaginary potential $h^2Δ+iV$ in the semiclassical limit $h\to 0$ we address the same problem in exterior domains. In particular we obtain the left margin of the spectrum, and the emptiness of the essential part of the spectrum under some additional assumptions.

math-ph↗
arXiv2017

On a Schrödinger operator with a purely imaginary potential in the semiclassical limit

We consider the operator ${\mathcal A}_h=-Δ+iV$ in the semi-classical $h\rightarrow 0$, where $V$ is a smooth real potential with no critical points. We obtain both the left margin of the spectrum, as well as resolvent estimates on the left side of this margin. We extend here previous results obtained for the Dirichlet realization of ${\mathcal A}_h$ by removing significant limitations that were formerly imposed on $V$. In addition, we apply our techniques to the more general Robin boundary condition and to a transmission problem which is of significant interest in physical applications.

math-ph↗
arXiv2016

Spectral analysis of a complex Schrödinger operator in the semiclassical limit

We consider the Dirichlet realization of the operator $-h^2Δ+iV$ in the semi-classical limit $h\to0$, where $V$ is a smooth real potential with no critical points. For a one dimensional setting, we obtain the complete asymptotic expansion, in powers of $h$, of each eigenvalue. In two dimensions we obtain the left margin of the spectrum, under some additional conditions.

math-ph↗
arXiv2016

The Clausius-Mossotti formula for dilute random media of perfectly conducting inclusions

We consider a large number of randomly dispersed spherical, identical, perfectly conducting inclusions (of infinite conductivity) in a bounded domain. The host medium's conductivity is finite and can be inhomogeneous. In the dilute limit, with some boundedness assumption on a large number (proportional to the global volume fraction raised to the power of -1/2) of marginal probability densities, we prove convergence in H^1 norm of the expectation of the solution of the steady state heat equation, to the solution of an effective medium problem, where the conductivity is given by the Clausius-Mossotti formula. Error estimates are provided as well.

math.AP↗
arXiv2015

Mixed normal-superconducting states in the presence of strong electric currents

We study the Ginzburg-Landau equations in the presence of large electric currents, that are smaller than the critical current where the normal state losses its stability. For steady-state solutions in the large $κ$ limit, we prove that the superconductivity order parameter is exponentially small in a significant part of the domain, and small in the rest of it. Similar results are obtained for the time-dependent problem, in continuation of the paper by the two first authors [3]. We conclude by obtaining some weaker results, albeit similar, for steady-state solutions in the large domain limit.

math-ph↗
arXiv2014

On the spectrum of non-selfadjoint Schrödinger operators with compact resolvent

We determine the Schatten class for the compact resolvent of Dirichlet realizations, in unbounded domains, of a class of non-selfadjoint differential operators. This class consists of operators that can be obtained via analytic dilation from a Schrödinger operator with magnetic field and a complex electric potential. As an application, we prove, in a variety of examples motivated by Physics, that the system of generalized eigenfunctions associated with the operator is complete, or at least the existence of an infinite discrete spectrum.

math-ph↗
arXiv2013

Global stability of the normal state of superconductors in the presence of a strong electric current

We consider the time-dependent Ginzburg-Landau model of superconductivity in the presence of an electric current flowing through a two-dimensional wire. We show that when the current is sufficiently strong the solution converges in the long-time limit to the normal state. We provide two types of upper bounds for the critical current where such global stability is achieved: by using the principal eigenvalue of the magnetic Laplacian associated with the normal magnetic field, and through the norm of the resolvent of the linearized steady-state operator. In the latter case we estimate the resolvent norm in large domains by the norms of approximate operators defined on the plane and the half-plane. We also obtain a lower bound, in large domains, for the above critical current by obtaining the current for which the normal state looses its local stability.

math-ph↗
arXiv2010

Radially symmetric minimizers for a $p$-Ginzburg Landau type energy in $\R^2$

We consider the minimization of a p-Ginzburg-Landau energy functional over the class of radially symmetric functions of degree one. We prove the existence of a unique minimizer in this class, and show that its modulus is monotone increasing and concave. We also study the asymptotic limit of the minimizers as p \rightarrow \infty. Finally, we prove that the radially symmetric solution is locally stable for $p$ in the interval $(2,4]$.

math.AP↗