Some open questions in hydrodynamics

TL;DR

This paper explores fundamental open questions in fluid mechanics, focusing on the long-term behavior of solutions to Navier-Stokes equations and the validity of relativistic hydrodynamics in high-energy physics, aiming to guide future research and experiments.

Contribution

It discusses unresolved issues in both non-relativistic and relativistic hydrodynamics, proposing ideas to address the existence of solutions and the applicability of the theory in experimental contexts.

Findings

Potential for finite-time singularities in Navier-Stokes solutions

Uncertainty about the validity of relativistic hydrodynamics in high-energy collisions

Insights into initial condition determination for fluid models

Abstract

When speaking of unsolved problems in physics, this is surprising at first glance to discuss the case of fluid mechanics. However, there are many deep open questions that come with the theory of fluid mechanics. In this paper, we discuss some of them that we classify in two categories, the long term behavior of solutions of equations of hydrodynamics and the definition of initial (boundary) conditions. The first set of questions come with the non-relativistic theory based on the Navier-Stokes equations. Starting from smooth initial conditions, the purpose is to understand if solutions of Navier-Stokes equations remain smooth with the time evolution. Existence for just a finite time would imply the evolution of finite time singularities, which would have a major influence on the development of turbulent phenomena. The second set of questions come with the relativistic theory of hydrodynamics. There is an accumulating evidence that this theory may be relevant for the description of the medium created in high energy heavy-ion collisions. However, this is not clear that the fundamental hypotheses of hydrodynamics are valid in this context. Also, the determination of initial conditions remains questionable. The purpose of this paper is to explore some ideas related to these questions, both in the non-relativistic and relativistic limits of fluid mechanics. We believe that these ideas do not concern only the theory side but can also be useful for interpreting results from experimental measurements.