2023 Simons Collaboration on Wave Turbulence Annual Meeting

The 2023 annual meeting of the Collaboration on Wave Turbulence took place at the Simons Foundation on November 30th and December 1st. It was preceded by a three-day workshop on the same topic held at the Courant Institute, which focused on highlighting the work of the junior collaborators. Progress on a multitude of research directions were reported on, ranging from substantial breakthroughs in the theoretical aspects to novel applications in geophysical and other systems, which formed the basis for detailed conversations and lively discussions during the workshop. This set the scene for the annual meeting, which featured overview talks on emerging and related topics by senior members of the collaboration and invited guest speakers. The broad spectrum of talks included applications of machine learning to ocean turbulence modeling, wave turbulence in the solar wind, progress in surface wave turbulence, and the ever-increasing scope of detailed solutions to the kinetic equations. The meeting closed with a talk on rotating turbulence simulations and fluid fragmentation in turbulent sprays of complex fluid systems.

Laure Zanna

 (Machine) Learning Multiscale Ocean Interaction  

Ocean mesoscale eddies (horizontal scale of 10 km-100 km) are critical in setting the global ocean circulation (scales of 1000’s km) and for mixing of momentum and tracers in the ocean. Mesoscale eddies are a key player in the ocean energy cycle, as they form the bulk of the kinetic energy and can backscatter energy to the mean flow through an inverse energy cascade. Yet, most climate models only partially resolve them, and we must therefore find approximations to represent their effects on the large-scale flow. Here, we will present results using data from numerical simulations and machine learning techniques to learn equations that represent the effects of mesoscale eddies on the large-scale flow. We will discuss the physical and mathematical properties of the equations learned, and examine their role in improving coarse-resolution simulations of the ocean.
 

Pierre Germain

The analysis of the kinetic wave equation
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The kinetic wave equation is at the heart of wave turbulence theory. As far as rigorous mathematics go, impressive progress has been made on its derivation but very little is known on its dynamics, or even more basic questions such as existence and uniqueness. A long-term aim is to build up the theory towards the application to turbulent spectra, ie Kolmogorov-Zakharov spectra and their stability. I will review the state of the art and describe open problems.
 

Sebastien Galtier

Wave turbulence in the solar wind
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The Sun can be seen as a gigantic natural wind tunnel, producing a wind at speeds of 400 to 800km/s. This wind is made up of particles (mainly electrons and protons), and fields (magnetic and electric) that can be precisely measured by ESA and NASA spacecraft. The two main properties of the solar wind are waves and turbulence, so it is natural to think of wave turbulence as a relevant regime for describing this medium.

In this presentation, I will review the main properties of the solar wind observed from in situ measurements, and then present the models used, which are based on plasma physics. I will present the main properties of weak wave turbulence theory in compressible magnetohydrodynamics (MHD) and electron MHD, which are anisotropic problems. This will be illustrated by direct numerical simulations. A comparison will eventually be made with in situ data. In particular, I will introduce the notion of collisionless wave turbulence, particularly useful for the solar wind, which is a dilute plasma. Finally, I will show the link between this topic and rapidly rotating turbulence.
 

Sergey Nazarenko

Universal scalings in evolving and stationary wave turbulence

Using the Nonlinear-Schrodinger (NLS) equation as a master model, I will present analytical and numerical results concerning several types of universal scaling regimes in wave turbulence. In stationary turbulence, these will be concerned with a revised theory of the famous Kolmogorov-Zakharov (KZ) spectra, both the direct and the inverse cascades. In evolving wave turbulence, the universal scalings manifest themselves in self-similar asymptotics (referred to as “non-thermal fixed points” in some recent papers). The latter behaviour comes in three flavours: self-similarity of the first, second and third kinds respectively.  The self-similarity of the first kind appears as a large time asymptotic of the spectrum propagating toward high frequencies. Its scaling is fully determined by energy conservation.  The self-similarity of the second kind appears as a finite time blow-up of the wave-kinetic equation (WKE) at the zero frequency: it is related to a physical phenomenon of the Bose-Einstein condensation. The scaling of this self-similarity is non-trivial: it cannot be found from conservation laws, and it is determined by solving a “nonlinear eigenvalue problem”. The self-similarity of the third kind appears in the forced-dissipated settings as a final stage of transition to the KZ spectrum and it takes the form of a frequency-space wave reflected from the low-frequency dissipative range. Its scaling is inherited from the previous (blowup) self-similar stage. I will present numerical results testing the analytical predictions arising from simulations of both the WKE and the 3D NLS equation.
 

Nataša Pavlović 

Two tales of a rigorous Derivation of the Hamiltonian Structure
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Many mathematical works have focused on understanding the manner in which the dynamics of a nonlinear  equation arises as an effective equation.

• For example, the cubic nonlinear Schrodinger equation (NLS) is an effective equation for a system of N bosons interacting pairwise via a delta or approximate delta potential. In this talk, we will advance a new perspective on deriving an effective equation, which focuses on structure. In particular, we will show how the Hamiltonian structure for the cubic NLS in any dimension arises from corresponding structure at the N-particle level.
• On the other hand, the Vlasov equation in any spatial dimension has long been known to be an infinite-dimensional Hamiltonian system whose bracket structure is of Lie-Poisson type. In parallel, it is classical that the Vlasov equation is a mean-field limit for a pairwise interacting Newtonian system. Motivated by this knowledge, we provide a rigorous derivation of the Hamiltonian structure of the Vlasov equation, both the Hamiltonian functional and Poisson bracket, directly from the many-body problem. This work settles a question of Marsden, Morrison, and Weinstein on providing a “statistical basis”’ for the bracket structure of the Vlasov equation.

Jacob  Bedrosian

Nonlinear dynamics in stochastic systems
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In this overview talk we discuss several results regarding the long-time dynamics of damped-driven Hamiltonian stochastic systems in the limit of weak damping. Examples of interest include damped-driven cubic NLS or the 2D or 3D Navier-Stokes equations. In infinite dimensional systems one generally observes turbulent behavior and relatively little is known aside from some basic results characterizing dissipation anomalies. However, for high, but finite, dimensional approximations to such systems, such as Galerkin truncations of the Navier-Stokes equations, one can now say a lot more about the dynamics through a combination of ideas in random dynamical systems and PDE methods applied to stochastic dynamics. We discuss some of these results regarding quantitative estimates on positive Lyapunov exponents and estimates of the attractor dimension (i.e. almost-sure exponential sensitivity to initial conditions), sharp estimates on the spectral gaps of the Markov semigroups (i.e. of the time required for time-averaged statistics to be close to the ensemble averaged statistics), and some results regarding nonlinear energy transfer (i.e. if one leaves a subspace undamped, under what conditions does the nonlinearity transfer energy to the damped modes and give on-average bounded trajectories vs unbounded growth due to energy piling up in the undamped subspace).  This talk will mention joint works with Alex Blumenthal, Michele Coti Zelati, Sam Punshon-Smith, Kyle Liss, and Franziska Weber.
 

Yulin Pan

Topics in surface gravity wave and MMT turbulence
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In this presentation, I will discuss several selected topics in surface gravity wave and MMT turbulence.

1. Nonlocal surface gravity wave turbulence in the presence of a condensate: We develop new theory to explain the inverse-cascade spectral slope observed in experiments and simulations that differs from the KZ solution. The theory considers the condensate at large scales and describes the spectral evolution by a diffusion equation, with a stationary solution consistent with the observations. (joint work with Alexander Korotkevich, Sergey Nazarenko and Jalal Shatah)
2. Role of three-wave quasi-resonances in surface gravity wave turbulence: In the standard derivation of the Hasselmann’s kinetic equation, the quadratic terms (three-wave interactions) are removed from a Lee transformation which is supposed to be valid only at small nonlinearity level. We numerically study the role of these three-wave quasi-resonances at finite nonlinearity level and discuss the implication to Zakharov equation and kinetic equation. (joint work with Zhou Zhang)
3. MMT turbulence at kinetic limit: We numerically study wave turbulence at the kinetic limit (small nonlinearity and large domain size simultaneously) in the context of 1D MMT equation, and examine the relation of the numerically obtained spectrum with the KZ solution focusing on the Kolmogorov constant (joint work with Alex Hrabski).

Lydia  Bourouiba
Unsteadiness: instabilities, turbulence, and fragmentation

Unsteady fragmentation of fluids into droplets is ubiquitous in industrial, energy,  environmental, and health-related processes. More broadly, in a range of geophysical and health flow systems, unsteadiness contributes in shaping, often in unexpected ways, instabilities, emerging patterns, mixing, or final fragment sizes from multi-scale fluid fragmentation processes, for example. We will discuss  illustrative examples  for which the theoretical or experimental frameworks are adapted to gain insights on the rich effects emerging from unsteadiness of complex fluid systems.