Table of contents

This article is a survey concerning the state-of-the-art mathematical theory of the Euler equations for an incompressible homogeneous ideal fluid. Emphasis is put on the different types of emerging instability, and how they may be related to the description of turbulence.

The theory of the motion of a free surface over a body of water is a fascinating subject, with a long history in both applied and pure mathematical research, and with a continuing relevance to the enterprises of mankind having to do with the sea. Despite the recent advances in the field (some of which we will hear about during this Workshop on Mathematical Hydrodynamics at the Steklov Institute), and the current focus of the mathematical community on the topic, many fundamental mathematical questions remain. These have to do with the evolution of surface water waves, their approximation by model equations and by computer simulations, the detailed dynamics of wave interactions, such as would produce rogue waves in an open ocean, and the theory (partially probabilistic) of approximating wave fields over large regions by averaged `macroscopic' quantities which satisfy essentially kinetic equations of motion. In this note we would like to point out open problems and some of the directions of current research in the field. We believe that the introduction of new analytical techniques and novel points of view will play an important rôle in the future development of the area.

This paper is devoted to the proof of global existence and uniqueness results for the three-dimensional incompressible Euler equations with a particular geometrical structure. The focus is on so-called axisymmetric solutions without swirl and on helicoidal solutions. The aim is to prescribe regularity conditions on the vorticity as close as possible to those formulated in the two-dimensional setting by V.I. Yudovich.

This short survey is written by a physicist. It contains neither theorems nor precise definitions. Its main content is a description of the results of numerical solution of the equations of fluid mechanics in the regime of developed turbulence. Due to limitations of computers, the results are not very precise. Despite being neither exact nor rigorous, the findings may nevertheless be of interest for mathematicians. The main result is that the isolines of some scalar fields (vorticity, temperature) in two-dimensional turbulence belong to the class of conformally invariant curves called SLE (Scramm-Löwner evolution) curves. First, this enables one to predict and find a plethora of quantitative relations going far beyond what was known previously about turbulence. Second, it suggests relations between phenomena that seemed unrelated, like the Euler equation and critical percolation. Third, it shows that one is able to get exact analytic results in statistical hydrodynamics. In short, physicists have found something unexpected and hope that mathematicians can help to explain it.

This is a survey of new results related to the study of the full Navier-Stokes-Fourier system for a general compressible, viscous, and heat conducting fluid, and its asymptotic behaviour as the Mach number approaches zero. The classical Navier-Stokes system for an incompressible fluid with lift, combined with the corresponding heat equation, is a limiting case.

More than 160 years after their invention by Hamilton, quaternions are now widely used in the aerospace and computer animation industries to track the orientation and paths of moving objects undergoing three-axis rotations. Here it is shown that they provide a natural way of selecting an appropriate orthonormal frame—designated the quaternion-frame—for a particle in a Lagrangian flow, and of obtaining the equations for its dynamics. How these ideas can be applied to the three-dimensional Euler fluid equations is then considered. This work has some bearing on the issue of whether the Euler equations develop a singularity in a finite time. Some of the literature on this topic is reviewed, which includes both the Beale-Kato-Majda theorem and associated work on the direction of vorticity by Constantin, Fefferman, and Majda and by Deng, Hou, and Yu. It is then shown how the quaternion formalism provides an alternative formulation in terms of the Hessian of the pressure.

It is proved that the first boundary-value problem for the stationary Navier-Stokes equations of viscous gas dynamics has renormalized solutions for all values of the adiabatic exponent in the interval .

A class of conditions sufficient for local regularity of suitable weak solutions of the non-stationary three-dimensional Navier-Stokes equations is discussed. The corresponding results are formulated in terms of functionals invariant with respect to the scaling of the Navier-Stokes equations. The well-known Caffarelli-Kohn-Nirenberg condition is contained in the class as a particular case.