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Michele Coti Zelati

Publications and source records attributed to Michele Coti Zelati.

At least 19 recordsLinked to original sources
arXiv2026

Smooth autonomous fast dynamo action on the three-torus

We construct a nonempty $C^k$-open family of smooth, autonomous, divergence-free velocity fields on $\mathbb{T}^3$ that generate fast dynamos. The proof proceeds by first establishing fast dynamo action for a large class of smooth, time-periodic velocity fields exhibiting the classical stretch--fold--shear mechanism. We then realize the associated period map within a smooth autonomous flow using carefully tuned return dynamics to a global Poincaré section. In both settings, the proof first isolates unstable distributional eigenmodes for the ideal dynamo operator on an anisotropic Banach space of distributions, and then shows that this spectral instability persists under the singular perturbation $\varepsilon Δ$. The resulting estimates are uniform as $\varepsilon\to0$ and stable under $C^k$ perturbations of the velocity field, yielding an open set of smooth fast dynamo vector fields. This result resolves the Fast Dynamo Conjecture of Zeldovich and Sakharov, as recorded in Arnold's book of problems.

math.AP↗
arXiv2026

A fast dynamo on the three-torus

We study the kinematic dynamo equation on the three-torus and provide a rigorous proof of fast dynamo action for a time-periodic, divergence-free, Lipschitz velocity field. Our construction is based on a stretch-fold-shear mechanism generating a uniformly hyperbolic flow. To analyze the associated dynamics, we develop anisotropic Banach spaces adapted to the underlying hyperbolic structure, allowing us to recover a discrete spectral picture for the ideal dynamo operator. In the strong-chaos regime, we show that this operator admits an eigenvalue with modulus strictly larger than one. We then prove that this instability persists under the singular perturbation induced by diffusion, yielding exponential growth of the magnetic field uniformly in the vanishing resistivity limit.

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arXiv2025

Stability analysis for active Brownian particle models

We carry out a comprehensive linear stability analysis of active Brownian particle systems around a constant homogeneous state. These scalar models, being important prototypes for the continuous description of active matter, are Fokker-Planck type equations in position-orientation and are known to exhibit motility-induced phase separation. We fully characterize the linear stability and instability regimes, with an explicit threshold depending on the effective speed of the particles. In this way, we rigorously confirm a conjecture on phase separation originating in the physics and applied literature. Our sharp and quantitative (in)stability results are valid both in the non-diffusive case and in the case of small angular diffusion. In the stable non-diffusive regime, we uncover a mixing mechanism reminiscent of Landau damping for the Vlasov equation, albeit with significantly weaker decay. This decay is non-integrable in time and gives rise to substantial mathematical difficulties; in particular, it prevents the use of classical perturbative arguments to treat the case of small angular diffusion.

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arXiv2025

Nonlinear stability for active suspensions

This paper is devoted to the nonlinear analysis of a kinetic model introduced by Saintillan and Shelley to describe suspensions of active rodlike particles in viscous flows. We investigate the stability of the constant state $Ψ(t,x,p) = \frac{1}{4π}$ corresponding to a distribution of particles that is homogeneous in space (variable $x \in \mathbb{T}^3$) and uniform in orientation (variable $p \in \mathbb{S}^2$). We prove its nonlinear stability under the optimal condition of linearized spectral stability. The main achievement in this work is that the smallness condition on the initial perturbation is independent of the translational diffusion and only depends on the rotational diffusion, which is particularly relevant for dilute suspensions. Upgrading our previous linear study to such nonlinear stability result requires new mathematical ideas, due to the presence of a quasilinear term in $x$ associated with nonlinear convection. This term cannot be treated as a source, because it is not controllable by the rotational diffusion in $p$. Also, it prevents the decoupling of $x$-Fourier modes crucially used in our previous paper. A key feature of our work is an analysis of enhanced dissipation and mixing properties of the advection diffusion operator $\partial_t + (p + u(t,x)) \cdot \nabla_x - νΔ_p$ on $\mathbb{T}^3 \times \mathbb{S}^2$ for a given appropriately small vector field $u$. We hope this linear analysis to be of independent interest, and useful in other contexts with partial or anisotropic diffusions.

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arXiv2025

A Stochastic RAGE Theorem and Enhanced Dissipation for Transport Noise

We prove a stochastic version of the classical RAGE theorem that applies to the two-point motion generated by noisy transport equations. As a consequence, we identify a necessary and sufficient condition for the corresponding diffusive equation to be dissipation enhancing. This involves the identification of a non-trivial, finite dimensional subspace that is invariant for the family of self-adjoint operator characterizing the structure of the transport noise. We discuss several examples and prove a sharp enhanced dissipation rate for stochastic shear flows.

math.AP↗
arXiv2025

A noise-induced transition in the Lorenz system

We consider a stochastic perturbation of the classical Lorenz system in the range of parameters for which the origin is the global attractor. We show that adding noise in the last component causes a transition from a unique to exactly two ergodic invariant measures. The bifurcation threshold depends on the strength of the noise: if the noise is weak, the only invariant measure is Gaussian, while strong enough noise causes the appearance of a second ergodic invariant measure.

math.PR↗
arXiv2025

Alpha-unstable flows and the fast dynamo problem

We construct a time-independent, incompressible, and Lipschitz-continuous velocity field in $\mathbb{R}^3$ that generates a fast kinematic dynamo - an instability characterized by exponential growth of magnetic energy, independent of diffusivity. Specifically, we show that the associated vector transport-diffusion equation admits solutions that grow exponentially fast, uniformly in the vanishing diffusivity limit $\varepsilon\to 0$. Our construction is based on a periodic velocity field $U$ on $\mathbb{T}^3$, such as an Arnold-Beltrami-Childress flow, which satisfies a generic spectral instability property called alpha-instability, established via perturbation theory. This provides a rigorous mathematical framework for the alpha-effect, a mechanism conjectured in the late 1960s to drive large-scale magnetic field generation. By rescaling with respect to $\varepsilon$ and employing a Bloch-type theorem, we extend the solution to the whole space. Finally, through a gluing procedure that spatially localizes the instability, we construct a globally defined velocity field $u$ in $\mathbb{R}^3$ that drives the dynamo instability.

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arXiv2025

On the stability of viscous three-dimensional rotating Couette flow

We study the stability of Couette flow in the 3d Navier-Stokes equations with rotation, as given by the Coriolis force. Hereby, the nature of linearized dynamics near Couette flow depends crucially on the force balance between background shearing and rotation, and includes lift-up or exponential instabilities, as well as a stable regime. In the latter, shearing resp. rotational inertial waves give rise to mixing and dispersive effects, which are relevant for distinct dynamical realms. Our main result quantifies these effects through enhanced dissipation and dispersive amplitude decay in both linear and nonlinear settings: in particular, we establish a nonlinear transition threshold which quantitatively improves over the setting without rotation (and increases further with rotation speed), showcasing its stabilizing effect.

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arXiv2024

Three-dimensional exponential mixing and ideal kinematic dynamo with randomized ABC flows

In this work we consider the Lagrangian properties of a random version of the Arnold-Beltrami-Childress (ABC) in a three-dimensional periodic box. We prove that the associated flow map possesses a positive top Lyapunov exponent and its associated one-point, two-point and projective Markov chains are geometrically ergodic. For a passive scalar, it follows that such a velocity is a space-time smooth exponentially mixing field, uniformly in the diffusivity coefficient. For a passive vector, it provides an example of a universal ideal (i.e. non-diffusive) kinematic dynamo.

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arXiv2024

Entropy maximization in the two-dimensional Euler equations

We consider variational problem related to entropy maximization in the two-dimensional Euler equations, in order to investigate the long-time dynamics of solutions with bounded vorticity. Using variations on the classical min-max principle and borrowing ideas from optimal transportation and quantitative rearrangement inequalities, we prove results on the structure of entropy maximizers arising in the investigation of the long-time behavior of vortex patches. We further show that the same techniques apply in the study of stability of the canonical Gibbs measure associated to a system of point vortices.

math.AP↗
arXiv2024

Ill-posedness of the hydrostatic Euler-Boussinesq equations and failure of hydrostatic limit

We investigate the hydrostatic approximation for inviscid stratified fluids, described by the two-dimensional Euler-Boussinesq equations in a periodic channel. Through a perturbative analysis of the hydrostatic homogeneous setting, we exhibit a stratified steady state violating the Miles-Howard criterion and generating a growing mode, both for the linearized hydrostatic and non-hydrostatic equations. By leveraging long-wave nonlinear instability for the original Euler-Boussinesq system, we demonstrate the breakdown of the hydrostatic limit around such unstable profiles. Finally, we establish the generic nonlinear ill-posedness of the limiting hydrostatic system in Sobolev spaces.

math.AP↗
arXiv2024

Stability of viscous three-dimensional stratified Couette flow via dispersion and mixing

This article explores the stability of stratified Couette flow in the viscous $3d$ Boussinesq equations. In this system, mixing effects arise from the shearing background, and gravity acts as a restoring force leading to dispersive internal gravity waves. These mechanisms are of fundamentally different nature and relevant in complementary dynamical regimes. Our study combines them to establish a bound for the nonlinear transition threshold, which is quantitatively larger than the inverse Reynolds number $ν$, and increases with stronger stratification resp. gravity.

math.AP↗
arXiv2023

Vanishing Mach Number Limit of Stochastic Compressible Flows

We study the vanishing Mach number limit for the stochastic Navier-Stokes equations with $γ$-type pressure laws, with focus on the one-dimensional case. We prove that, if the stochastic term vanishes with respect to the Mach number sufficiently fast, the deviation from the incompressible state of the solutions (for $γ\geq 1$) and the invariant measures (for $γ= 1$) is governed by a linear stochastic acoustic system in the limit. In particular, the critically sufficient decay rate for the stochastic term is slower than the corresponding results with deterministic external forcing due to the martingale structure of the noise term, and the blow-up of the noise term for the fluctuation system can be allowed.

math.PR↗
arXiv2023

Orientation mixing in active suspensions

We study a popular kinetic model introduced by Saintillan and Shelley for the dynamics of suspensions of active elongated particles where the particles are described by a distribution in space and orientation. The uniform distribution of particles is the stationary state of incoherence which is known to exhibit a phase transition. We perform an extensive study of the linearised evolution around the incoherent state. We show (i) in the non-diffusive regime corresponding to spectral (neutral) stability that the suspensions experiences a mixing phenomenon similar to Landau damping and we provide optimal pointwise in time decay rates in weak topology. Further, we show (ii) in the case of small rotational diffusion \(ν\) that the mixing estimates persist up to time scale \(ν^{-1/2}\) until the exponential decay at enhanced dissipation rate \(ν^{1/2}\) takes over.The interesting feature is that the usual velocity variable of kinetic models is replaced by an orientation variable on the sphere. The associated \emph{orientation mixing} leads to limited algebraic decay for macroscopic quantities. For the proof, we start with a general pointwise decay results for Volterra equations that may be of independent interest. While, in the non-diffusive case, explicit formulas on the sphere allow to conclude the desired decay, much more work is required in the diffusive case: here we prove mixing estimates for the advection-diffusion equation on the sphere by combining an optimized hypocoercive approach with the vector field method. One main point in this context is to identify good commuting vector fields for the advection-diffusion operator on the sphere. Our results in this direction may be useful to other models in collective dynamics, where an orientation variable is involved.

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arXiv2023

Statistically self-similar mixing by Gaussian random fields

We study the passive transport of a scalar field by a spatially smooth but white-in-time incompressible Gaussian random velocity field on $\mathbb{R}^d$. If the velocity field $u$ is homogeneous, isotropic, and statistically self-similar, we derive an exact formula which captures non-diffusive mixing. For zero diffusivity, the formula takes the shape of $\mathbb{E}\ \| θ_t \|_{\dot{H}^{-s}}^2 = \mathrm{e}^{-λ_{d,s} t} \| θ_0 \|_{\dot{H}^{-s}}^2$ with any $s\in (0,d/2)$ and $\frac{λ_{d,s}}{D_1}:= s(\frac{λ_{1}}{D_1}-2s)$ where $λ_1/D_1 = d$ is the top Lyapunov exponent associated to the random Lagrangian flow generated by $u$ and $ D_1$ is small-scale shear rate of the velocity. Moreover, the mixing is shown to hold $\textit{uniformly}$ in diffusivity.

math.PR↗
arXiv2023

Explicit solutions and linear inviscid damping in the Euler-Boussinesq equation near a stratified Couette flow in the periodic strip

This short note provides explicit solutions to the linearized Boussinesq equations around the stably stratified Couette flow posed on $\mathbb{T}\times\mathbb{R}$. We consider the long-time behavior of such solutions and prove inviscid damping of the perturbed density and velocity field for any positive Richardson number, with optimal rates. The explicit solution is obtained through the limiting absorption principle whereas the inviscid damping is proved using oscillatory integral methods.

math.AP↗
arXiv2023

Limiting absorption principles and linear inviscid damping in the Euler-Boussinesq system in the periodic channel

We consider the long-time behavior of solutions to the two dimensional non-homogeneous Euler equations under the Boussinesq approximation posed on a periodic channel. We study the linearized system near a linearly stratified Couette flow and prove inviscid damping of the perturbed density and velocity field for any positive Richardson number, with optimal rates. Our methods are based on time-decay properties of oscillatory integrals obtained using a limiting absorption principle, and require a careful understanding of the asymptotic expansion of the generalized eigenfunction near the critical layer. As a by-product of our analysis, we provide a precise description of the spectrum of the linearized operator, which, for sufficiently large Richardson number, consists of an essential spectrum (as expected according to classical hydrodynamic problems) as well as discrete neutral eigenvalues (giving rise to oscillatory modes) accumulating towards the endpoints of the essential spectrum.

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