arXiv subjects

Franck Sueur

Publications and source records attributed to Franck Sueur.

At least 19 recordsLinked to original sources

arXiv2026

A Runge-type theorem by remote forcing for the linearized resistive MHD system

In this paper, we study a quantitative Runge-type global approximation theorem for the linearized magnetohydrodynamic (MHD) system in bounded domains with arbitrary topology. In the context of magnetic relaxation, the interplay between the domain topology and magnetic field structure plays a crucial role. Recent studies illustrate a sharp contrast in the dynamics: while Enciso--Peralta-Salas (2025) highlights that the geometric complexity of magnetic fields acts as an obstruction to relaxation in non-resistive regimes, Kozono-Shimizu-Yanagisawa (2025) proves that in resistive regimes, the flow stably relaxes towards a harmonic equilibrium. Focusing on this resistive scenario, we adopt a control-theoretic viewpoint to quantitatively approximate the relaxation trajectory generated by the linearized initial-boundary value problem. Specifically, after decomposing the bounded-domain solution into the time-evolving part and the stationary part, we approximate it by a global solution on $\mathbb{R}^3$ under a remote forcing. An explicit dependence of the forcing cost on the approximation error is provided.

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arXiv2026

A Lions' type formula for some reproducing kernel Hilbert spaces of fractional harmonic functions

In \cite{Lions}, J. L. Lions considered a reproducing kernel Hilbert space (RKHS) of harmonic functions on a regular domain with Sobolev traces and obtained a formula that expresses the kernel of this space as an integral on the boundary of some derivatives of the Green function associated with the Laplace operator and the homogeneous Dirichlet boundary condition. This result was simplified and extended later by Englis, Lukkassen, Peetre, and Persson in \cite{ELPL} to more general elliptic systems of even orders. In particular, they emphasized that the resemblance between Lions' type formula and the Hadamard variational formula only appears when the operator is of order $2$. In this paper, we investigate some RKHS of $a$-harmonic functions, where $a$ in $(0,1)$ refers to a fractional exponent of the Laplace operator. For such fractional order pseudo-differential operators, the local nonhomogeneous Dirichlet problem can be addressed by means of some $a$-transmission Sobolev spaces, which were introduced by H\"ormander in the sixties and recently developed by Grubb in a series of papers. We deduce from these works a fractional Poisson formula, which is applied to obtain a Lions' type formula. We observe, in particular, that despite the order of the operator not being $2$, this formula resembles the Hadamard variational formula that we prove in the companion paper \cite{sidy-franck_1}. As a complementary remark, we observe that for a family of RKHS associated with the steady Stokes system, a second order system, there is also a Lions' type formula for their two-point kernels, which turn out not to be similar to the corresponding Hadamard variation formula.

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arXiv2026

Pointwise Hadamard variational formula for the fractional Laplacian

We establish pointwise formulas for the shape derivative of solutions to the Dirichlet problem associated with the fractional Laplacian. Specifically, we consider the equation $(-\Delta)^s u = h$ in $\Omega$ and $u=0$ in $\Omega^c$, where the right-hand side $h$ is either a Dirac delta distribution or a Lipschitz function. In both cases, we prove that the corresponding solution is shape differentiable in every direction and we derive a formula for the pointwise value of its shape derivative. These formulas involve integral on the domain's boundary and fractional Neumann's traces. This extends to the case of the fractional Laplacian the well-known Hadamard variational formula for the standard Laplacian. Our argument is in the spirit of \cite{Ushikoshi, Kozono-Ushikoshi} and is based on PDEs techniques.

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arXiv2026

A Brezis and Peletier type result for the fractional Robin function

This paper is devoted to the Laplacian operator of fractional order $s\in (0,1)$ in several dimensions. We consider the equation $(-\Delta)^su=f(x,u)$ in $\Omega$, $u=0$ in $\Omega^c$ and establish a representation formula for partial derivatives of solutions in terms of the normal derivative $u/\delta^s$. As a consequence, we prove that solutions to the overdetermined problem $(-\Delta)^su=f(x,u)$ in $\Omega$, $u=0$ in $\Omega^c$, and $u/\delta^s=0$ on $\partial\Omega$ are globally Lipschitz continuous provided that $2s>1$. We also prove a Pohozaev-type identity for the Green function and, in particular, obtain a formula for the gradient of the Robin function, which extends to the fractional setting some results obtained by Br\'ezis and Peletier in \cite{Bresiz} in the classical case of the Laplacian. Finally, an application to the nondegeneracy of critical points of the fractional Robin function in symmetric domains is discussed.

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arXiv2025

Lagrangian controllability in perforated domains

The question at stake in Lagrangian controllability is whether one can move a patch of fluid particles to a target location by means of remote action in a given time interval. In the last two decades, positive results have been obtained both for the incompressible Euler and Navier-Stokes equations. However, for the latter, the case where the fluid is contained within domains bounded by solid boundaries with the no-slip condition has not been addressed, with respect to the difficulty caused by viscous boundary layers. In this paper, we investigate the Lagrangian controllability of viscous incompressible fluid in perforated domains for which the fraction of volume occupied by the holes is sufficiently small. Moreover, we quantitatively distinguish situations depending on the parameters for holes (diameter and distance) and for fluid (size of the initial data). Our approach relies on recent results on homogenization for evolutionary problems and on weak-strong stability estimates in measure of flows, alongside classical results on Runge-type approximations for elliptic equations and on Cauchy-Kowalevsky-type theorems for equations with analytic coefficients. Here, homogenization refers to the vanishing viscosity limit outside a porous medium, where (after scaling in time) the Navier-Stokes equations are homogenized to the Euler or Darcy equations. Indeed, in the proof, we act on the Navier-Stokes equations by strong and fast forcing to leverage inviscid approximations, which is a standard technique in the theory of controllability.

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arXiv2025

Motion of a massive rigid loop in a 3D perfect incompressible flow

We consider the motion of a rigid body immersed in an inviscid incompressible fluid. In 2D, an important physical effect associated with this system is the famous Kutta-Joukowski effect. In the present paper, we identify a similar effect in the 3D case. For this, we first recast the Newtonian dynamics of the rigid body as a first-order nonlinear ODE for the $6$-component body velocity, in the body frame. Then, we focus on the particular case where the rigid body occupies a slender tubular domain with a smooth closed curve as the centerline and a circular cross-section, in the limit where the radius goes to zero, with fixed inertia and circulation around the curve. We establish that the dynamics of the limit massive rigid loop are given by a first-order nonlinear ODE with coefficients that depend only on the inertia, on the fluid vorticity, and on the limit curve through two $3$D vectors, which are involved in a skewsymmetric $6 \times 6$ matrix that appears in the limit force and torque, a structure which is reminiscent of the 2D Kutta-Joukowski effect. We also identify the limit fluid dynamics as, where, as in the case of the Euler equation alone, the vorticity evolves according to the usual transport equation with stretching, but with a velocity field that is due not only to the fluid vorticity but also to a vorticity filament associated with the circulation around the limit rigid loop. This result is in stark contrast with the case where the filament is made of fluid, with non-zero circulation, since in the latter, the filament velocity becomes infinite in the zero-radius limit. However, considering the inertia scaling that corresponds to a fixed density, we prove that there are solutions for which the solid velocity and its displacement tend to infinity over a time interval of size $\mathcal{O}(1)$.

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arXiv2025

Global small-time approximate null and Lagrangian controllability of the viscous non-resistive MHD system in a $3D$ domain with Navier type boundary conditions

We consider the incompressible viscous MHD system without magnetic diffusion in a $3D$ bounded domain with Navier type boundary condition. We establish the global small-time approximate null controllability and the Lagrangian controllability of the system, in the class of smooth solutions, by following the approach initiated in \cite{CMS} to establish the global small-time null controllability of the incompressible Navier-Stokes equations in the class of weak solutions and extended in \cite{LSZ1} to establish the global small-time null and Lagrangian controllability of the incompressible Navier-Stokes equations in the class of strong solutions. This approach makes use of controls with an extra fast scale in time and some corresponding multi-scale asymptotic expansions of the controlled solution. This expansion is constructed by an iterative process which requires some regularity. The extra-difficulty here is that the MHD system at stake is mixed hyperbolic-parabolic, without any regularizing effect on the magnetic field. Despite our strategy makes use of a quite precise asymptotic expansion, we succeed to cover the case where the initial velocity belongs the Sobolev space $H^{24}$ and the initial magnetic field belongs to the Sobolev space $H^8$.

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arXiv2025

Local exact Lagrangian controllability for 1D barotropic compressible Navier--Stokes equations

We consider a viscous compressible barotropic flow in the interval $[0,π]$ with homogeneous Dirichlet boundary conditions for the flow velocity and a constant rest state as initial data. Given two sufficiently close subintervals $I=[α_1,α_2]$ and $J=[β_1,β_2]$ of $(0,1)$, a nonempty open set $ω\subset (1,π)$, and $T>0$, we construct an external force $f$ supported in $ω$ acting on the momentum equation such that the corresponding flow map moves the fluid particles initially occupying $I$ exactly onto $J$ in time $T$.

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arXiv2024

On the analyticity of the flow map for the AHT equations

The AHT equation is a non linear and non local vectorial transport equation which was introduced in 2003 by Angenent, Haker and Tannenbaum in optimal transport theory. For this equation, classical solutions are known to exist at least locally in time, and a flow map can thus be uniquely associated with these solutions. In this paper we consider the case where the equation is set in a bounded domain with an analytic boundary and we prove that the flow map is analytic with respect to time.

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arXiv2024

A Runge-type approximation theorem for the 3D unsteady Stokes system

We investigate Runge-type approximation theorems for solutions to the 3D unsteady Stokes system. More precisely, we establish that on any compact set with connected complement, local smooth solutions to the 3D unsteady Stokes system can be approximated with an arbitrary small positive error in $L^\infty$ norm by a global solution of the 3D unsteady Stokes system, where the velocity grows at most exponentially at spatial infinity and the pressure grows polynomially. Additionally, by considering a parasitic solution to the Stokes system, we establish that some growths at infinity are indeed necessary.

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arXiv2024

A new look at the controllability cost of linear evolution systems with a long gaze at localized data

We revisit the classical issue of the controllability/observability cost of linear first order evolution systems, starting with ODEs, before turning to some linear first order evolution PDEs in several space dimensions, including hyperbolic systems and pseudo-differential systems obtained by linearization in fluid mechanics. In particular we investigate the cost of localized initial data, and in the dispersive case, of initial data which are semi-classically microlocalized.

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arXiv2024

Differential transmutations

Inspired by Gromov's partial differential relations, we introduce a notion of differential transmutation, which allows to transfer some local properties of solutions of a PDE to solutions of another PDE, in particular local solvability, hypoellipticity, weak and strong unique continuation properties and the Runge property. The latest refers to the possibility to approximate some given local solutions by a global solution, with point force controls in preassigned positions in the holes of the space domain. As examples we prove that 2D Lamé-Navier system and the $3$D steady Stokes system, can be obtained as differential transmutations of appropriate tensorizations of the Laplace operator.

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arXiv2023

A few representation formulas for solutions of fractional Laplace equations

This paper is devoted to the Laplacian operator of fractional order $s\in (0,1)$ in several dimensions. We first establish a representation formula for the partial derivatives of the solutions of the homogeneous Dirichlet problem. Along the way, we obtain a Pohozaev-type identity for the fractional Green function and of the fractional Robin function. The latter extends to the fractional setting a formula obtained by Brézis and Peletier, see \cite{Bresiz}, in the classical case of the Laplacian. As an application we consider the particle system extending the classical point vortex system to the case of a fractional Laplacian. We observe that, for a single particle in a bounded domain, the properties of the fractional Robin function are crucial for the study of the steady states. We also extend the classical Hadamard variational formula to the fractional Green function as well as to the shape derivative of weak solution to the homogeneous Dirichlet problem. Finally we turn to the in homogeneous Dirichlet problem and extend a formula by J.L. Lions, see \cite{Lions}, regarding the kernel of the reproducing kernel Hilbert space of harmonic functions to the case of $s$-harmonic functions. We observe that, despite the order of the operator is not $2$, this formula looks like the Hadamard variational formula, answering in a negative way to an open question raised in \cite{ELPL}.

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arXiv2022

A few remarks on the transport-Stokes system

We consider the so-called transport-Stokes system which describes sedimentation of inertialess suspensions in a viscous flow and couples a transport equation and the steady Stokes equations in the full three-dimensional space. First we present a global existence and uniqueness result for $L^1 \cap L^p$ initial densities where $p \geq 3$. Secondly, we prove that, in the case where $p>3$, the flow map which describes the trajectories of these solutions is analytic with respect to time. Finally we establish the small-time global exact controllability of the transport-Stokes system. These results extend to the transport-Stokes system some results obtained for the incompressible Euler system respectively by Yudovich in [40], by Chemin in [2,3] and by Coron, and Glass, in [6,14].

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arXiv2021

Uniqueness of Yudovich's solutions to the 2D incompressible Euler equation despite the presence of sources and sinks

In $1962$, Yudovich proved the existence and uniqueness of classical solutions to the 2D incompressible Euler equations in the case where the fluid occupies a bounded domain with entering and exiting flows on some parts of the boundary. The normal velocity is prescribed on the whole boundary, as well as the entering vorticity. The uniqueness part of Yudovich's result holds for Hölder vorticity, by contrast with his 1961 result on the case of an impermeable boundary, for which the normal velocity is prescribed as zero on the boundary, and for which the assumption that the initial vorticity is bounded was shown to be sufficient to guarantee uniqueness. Whether or not uniqueness holds as well for bounded vorticities in the case of entering and exiting flows has been left open until $2014$, when Weigant and Papin succeeded to tackle the case where the domain is a rectangle. In this paper we adapt Weigant and Papin's result to the case of a smooth domain with several internal sources and sinks.

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arXiv2021

Motion of several slender rigid filaments in a Stokes flow

We investigate the dynamics of several slender rigid bodies moving in a flow driven by the three-dimensional steady Stokes system in presence of a smooth background flow. More precisely we consider the limit where the thickness of these slender rigid bodies tends to zero with a common rate $\varepsilon$, while their volumetric mass density is held fixed, so that the bodies shrink into separated massless curves. While for each positive $\varepsilon$, the bodies' dynamics are given by the Newton equations and correspond to some coupled second-order ODEs for the positions of the bodies, we prove that the limit equations are decoupled first-order ODEs whose coefficients only depend on the limit curves and on the background flow. These coefficients appear through appropriate renormalized Stokes resistance tensors associated with each limit curve, and through renormalized Faxén-type force and torque associated with the limit curves and the background flow. We establish a rate of convergence of the curves of order $O( \vert \log \varepsilon \vert^{-1/2})$. We also determine the limit effect due to the limit curves on the fluid, in the spirit of the immersed boundary method. Both for the convergence of the filament velocities and the fluid velocities we identify an initial exponential relaxation within a $O(\varepsilon^2 \vert \log \varepsilon \vert)$ time.

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arXiv2021

Existence of weak solutions to the two-dimensional incompressible Euler equations in the presence of sources and sinks

A classical model for sources and sinks in a two-dimensional perfect incompressible fluid occupying a bounded domain dates back to Yudovich in 1966. In this model, on the one hand, the normal component of the fluid velocity is prescribed on the boundary and is nonzero on an open subset of the boundary, corresponding either to sources (where the flow is incoming) or to sinks (where the flow is outgoing). On the other hand the vorticity of the fluid which is entering into the domain from the sources is prescribed. In this paper we investigate the existence of weak solutions to this system by relying on \textit{a priori} bounds of the vorticity, which satisfies a transport equation associated with the fluid velocity vector field. Our results cover the case where the vorticity has a $L^p$ integrability in space, with $p $ in $[1,+\infty]$, and prove the existence of solutions obtained by compactness methods from viscous approximations. More precisely we prove the existence of solutions which satisfy the vorticity equation in the distributional sense in the case where $p >\frac43$, in the renormalized sense in the case where $p >1$, and in a symmetrized sense in the case where $p =1$.

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arXiv2020

Remote trajectory tracking of rigid bodies immersed in a 2D perfect incompressible fluid

We consider the motion of several rigid bodies immersed in a two-dimensional incompressible perfect fluid. The motion of the rigid bodies is given by the Newton laws with forces due to the fluid pressure and the fluid motion is described by the incompressible Euler equations. Our analysis covers the case where the circulations of the fluid velocity around the bodies are nonzero and where the fluid vorticity is bounded. The whole system occupies a bounded simply connected domain with an external fixed boundary which is impermeable except on an open non-empty part where one allows some fluid to go in and out the domain by controlling the normal velocity and the entering vorticity. We prove that it is possible to exactly achieve any non-colliding smooth motion of the rigid bodies by the remote action of a controlled normal velocity on the outer boundary which takes the form of state-feedback, with zero entering vorticity. This extends the result of (Glass, O., Kolumb{á}n, J. J., Sueur, F. (2017). External boundary control of the motion of a rigid body immersed in a perfect two-dimensional fluid. Analysis \& PDE) where the exact controllability of a single rigid body immersed in a 2D irrotational perfect incompressible fluid from an initial position and velocity to a final position and velocity was investigated. The proof relies on a nonlinear method to solve linear perturbations of nonlinear equations associated with a quadratic operator having a regular non-trivial zero. Here this method is applied to a quadratic equation satisfied by a class of boundary controls, which is obtained by extending the reformulation of the Newton equations performed in the uncontrolled case in (Glass, O., Lacave, C., Munnier, A., Sueur, F. (2019). Dynamics of rigid bodies in a two dimensional incompressible perfect fluid. Journal of Differential Equations, 267(6), 3561-3577) to the case where a control acts on the external boundary.

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