The Navier-Stokes equations in primitive variables

TL;DR

This paper critically examines the primitive variable formulation of the Navier-Stokes equations, revealing fundamental issues with solution uniqueness, flow instability, and prediction reliability, while establishing long-time regularity results for incompressible flows.

Contribution

It demonstrates the non-uniqueness and potential singularities in primitive solutions and provides a priori bounds for vorticity, challenging traditional assumptions in fluid dynamics.

Findings

Primitive solutions lack essential a priori bounds for uniqueness.

Sequences of solutions can be scaled into singularities.

Long-time regularity of the Navier-Stokes and Euler equations is established.

Abstract

The Navier-Stokes equations in the primitive formulation for incompressible flow describe the evolution of velocity and pressure, without recourse to vorticity. We show that, beyond the finite Leray-Hopf regularity interval, every postulated strong solution is accompanied by infinitely many diffusion-dominated percolations of arbitrary size, while the momentum deficit caused by the non-linearity is compensated by the pressure gradient. In the upper half space, we demonstrate how sequences of these collective companions can be re-scaled into an absurd singularity. Owning to the passive nature of the pressure, there exist no essential a priori bounds for establishing the uniqueness of primitive solutions. With the illustration of well-exploited examples of closed-form basic flows, we elucidate the reason why perturbations, infinitesimal or finite, instigate indeterminate states that render the concept of flow instability inadmissible. An effort has also been made to reappraise a number of important issues in fluid dynamics. Unfortunately, the primitive theory cannot serve as a reliable tool for prediction. Nevertheless, a dedicated effort has been made to elaborate a priori bounds for vorticity dynamics for ideal as well as real fluids. As a result, we are able to establish long-time regularity of the Cauchy problem for incompressible flows. A main conclusion is that no events of finite-time blow-up can ever occur in the Euler or Navier-Stokes equations for initial data of finite energy.