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Daniel Ginsberg

Publications and source records attributed to Daniel Ginsberg.

16 recordsLinked to original sources
arXiv2025

On the existence of fibered three-dimensional perfect fluid equilibria without continuous Euclidean symmetry

Following Lortz, we construct a family of smooth steady states of the ideal, incompressible Euler equation in three dimensions that possess no continuous Euclidean symmetry. As in Lortz, they do possess a planar reflection symmetry and, as such, all the orbits of the velocity are closed. Different from Lortz, our construction has a discrete m-fold symmetry and is foliated by invariant cylindrical level sets of a non-degenerate Bernoulli pressure. Such examples narrow the scope of validity of Grad's conjecture that the only solutions with a continuous symmetry can be fibered by pressure surfaces.

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arXiv2025

On the fragility of laminar flow

Inviscid laminar flow is a stationary solution of the incompressible Euler equations whose streamlines foliate the fluid domain. Their structure on symmetric domains is rigid: all laminar flows occupying straight periodic channels are shear and on regular annuli they are circular. Laminarity can persist to slight deformations of these domains provided the base flow is Arnold stable and non-stagnant (non-vanishing velocity). On the other hand, flows with trivial net momentum (and thus stagnate) break laminarity by developing islands (regions of contractible streamlines) on all non-flat periodic channels with up/down reflection symmetry. Here, we show that stable steady states occupying generic channels or annuli and stagnate must have islands. Additionally, when the domain is close to symmetric, we characterize the size of the islands, showing that they scale as the square root of the boundary's deviation from flat. Taken together, these results show that dynamically stable laminar flows are structurally unstable whenever they stagnate.

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arXiv2024

The stability of irrotational shocks and the Landau law of decay

We consider the long-time behavior of irrotational solutions of the three-dimensional compressible Euler equations with shocks, hypersurfaces of discontinuity across which the Rankine-Hugoniot conditions for irrotational flow hold. Our analysis is motivated by Landau's analysis of spherically-symmetric shock waves, who predicted that at large times, not just one, but two shocks emerge. These shocks are logarithmically-separated from the Minkowskian light cone and the fluid velocity decays at the non-time-integrable rate 1/(t(\log t)^{1/2}). We show that for initial data, which need not be spherically-symmetric, with two shocks in it and which is sufficiently close, in appropriately weighted Sobolev norms, to an N-wave profile, the solution to the shock-front initial value problem can be continued for all time and does not develop any further singularities. In particular this is the first proof of global existence for solutions (which are necessarily singular) of a quasilinear wave equation in three space dimensions which does not verify the null condition. The proof requires carefully-constructed multiplier estimates and analysis of the geometry of the shock surfaces.

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arXiv2024

Long time regularity for 3d gravity waves with vorticity

We consider the Cauchy problem for the full free boundary Euler equations in $3$d with an initial small velocity of size $O(\epsilon_0)$, in a moving domain which is initially an $O(\epsilon_0)$ perturbation of a flat interface. We assume that the initial vorticity is of size $O(\epsilon_1)$ and prove a regularity result up to times of the order $\epsilon_1^{-1+}$, independent of $\epsilon_0$. A key part of our proof is a normal form type argument for the vorticity equation; this needs to be performed in the full three dimensional domain and is necessary to effectively remove the irrotational components from the quadratic stretching terms and uniformly control the vorticity. Another difficulty is to obtain sharp decay for the irrotational component of the velocity and the interface; to do this we perform a dispersive analysis on the boundary equations, which are forced by a singular contribution from the rotational component of the velocity. As a corollary of our result, when $\epsilon_1$ goes to zero we recover the celebrated global regularity results of Wu (Invent. Math. 2012) and Germain, Masmoudi and Shatah (Ann. of Math. 2013) in the irrotational case.

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arXiv2023

Islands in stable fluid equilibria

We prove that stable fluid equilibria with trivial homology on curved, reflection-symmetric periodic channels must posses "islands", or cat's eye vortices. In this way, arbitrarily small disturbances of a flat boundary cause a change of streamline topology of stable steady states.

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arXiv2022

On the distribution of heat in fibered magnetic fields

We study the equilibrium temperature distribution in a model for strongly magnetized plasmas in dimension two and higher. Provided the magnetic field is sufficiently structured (integrable in the sense that it is fibered by co-dimension one invariant tori, on most of which the field lines ergodically wander) and the effective thermal diffusivity transverse to the tori is small, it is proved that the temperature distribution is well approximated by a function that only varies across the invariant surfaces. The same result holds for "nearly integrable" magnetic fields up to a "critical" size. In this case, a volume of non-integrability is defined in terms of the temperature defect distribution and related the non-integrable structure of the magnetic field, confirming a physical conjecture of Paul-Hudson-Helander. Our proof crucially uses a certain quantitative ergodicity condition for the magnetic field lines on full measure set of invariant tori, which is automatic in two dimensions for magnetic fields without null points and, in higher dimensions, is guaranteed by a Diophantine condition on the rotational transform of the magnetic field.

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arXiv2022

Flexibility and rigidity of free boundary MHD equilibria

We study stationary free boundary configurations of an ideal incompressible magnetohydrodynamic fluid possessing nested flux surfaces. In 2D simply connected domains, we prove that if the magnetic field and velocity field are never commensurate, the only possible domain for any such equilibria is a disk, and the velocity and magnetic field are circular. We give examples of non-symmetric equilibria occupying a domain of any shape by imposing an external magnetic field generated by a singular current sheet charge distribution (external coils). Some results carry over to 3D axisymmetric solutions. These results highlight the importance of external magnetic fields for the existence of asymmetric equilibria.

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arXiv2021

Flexibility and rigidity in steady fluid motion

Flexibility and rigidity properties of steady (time-independent) solutions of the Euler, Boussinesq and Magnetohydrostatic equations are investigated. Specifically, certain Liouville-type theorems are established which show that suitable steady solutions with no stagnation points occupying a two-dimensional periodic channel, or axisymmetric solutions in (hollowed out) cylinder, must have certain structural symmetries. It is additionally shown that such solutions can be deformed to occupy domains which are themselves small perturbations of the base domain. As application of the general scheme, Arnol'd stable solutions are shown to be structurally stable.

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arXiv2020

On quasisymmetric plasma equilibria sustained by small force

We construct smooth, non-symmetric plasma equilibria which possess closed, nested flux surfaces and solve the magnetohydrostatic (steady three-dimensional incompressible Euler) equations with a small force. The solutions are also `nearly' quasisymmetric. The primary idea is, given a desired quasisymmetry direction $ξ$, to change the smooth structure on space so that the vector field $ξ$ is Killing for the new metric and construct $ξ$--symmetric solutions of the magnetohydrostatic equations on that background by solving a generalized Grad-Shafranov equation. If $ξ$ is close to a symmetry of Euclidean space, then these are solutions on flat space up to a small forcing.

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arXiv2019

On the breakdown of solutions to the incompressible Euler equations with free surface boundary

We prove a continuation critereon for incompressible liquids with free surface boundary. We combine the energy estimates of Christodoulou and Lindblad with an analog of the estimate due to Beale, Kato, and Majda for the gradient of the velocity in terms of the vorticity, and use this to show solution can be continued so long as the second fundamental form and injectivity radius of the free boundary, the vorticity, and one derivative of the velocity on the free boundary remain bounded, assuming that the Taylor sign condition holds.

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arXiv2019

Local well-posedness for the motion of a compressible, self-gravitating liquid with free surface boundary

We establish the local well-posedness for the free boundary problem for the compressible Euler equations describing the motion of liquid under the influence of Newtonian self-gravity. We do this by solving a tangentially-smoothed version of Euler's equations in Lagrangian coordinates which satisfies uniform energy estimates as the smoothing parameter goes to zero. The main technical tools are delicate energy estimates and optimal elliptic estimates in terms of boundary regularity, for the Dirichlet problem and Green's function.

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arXiv2018

On the lifespan of three-dimensional gravity water waves with vorticity

We prove a long-term regularity result for three-dimensional gravity water waves with small initial data but nonzero initial vorticity. We consider solutions whose vorticity vanishes on the free boundary and use this to derive a system for the evolution of the free boundary which reduces to the Zakharov/Craig-Sulem formulation in the irrotational case. We are able to continue the solution until a time determined by the size of the initial vorticity in such a way that if the vorticity is zero, one recovers a lifespan $T\sim ε^{-N}$ where $N$ can be taken arbitrarily large if the initial data is taken to be arbitrarily smooth.

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arXiv2018

A priori estimates for a relativistic liquid with free surface boundary

We prove energy estimates for a relativistic free liquid body with sufficiently small fluid velocity in a general Einstein spacetime. These estimates control Sobolev norms of the fluid velocity and enthalpy in the interior as well as Sobolev norms of the second fundamental form on the boundary.

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arXiv2011

Analytical and Numerical Results on the Positivity of Steady State Solutions of a Thin Film Equation

We consider an equation for a thin-film of fluid on a rotating cylinder and present several new analytical and numerical results on steady state solutions. First, we provide an elementary proof that both weak and classical steady states must be strictly positive so long as the speed of rotation is nonzero. Next, we formulate an iterative spectral algorithm for computing these steady states. Finally, we explore a non-existence inequality for steady state solutions from the recent work of Chugunova, Pugh, & Taranets.

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