Parabolic fractal dimension of forward-singularities for Navier-Stokes and liquid crystals inequalities

TL;DR

This paper extends partial regularity results for Navier-Stokes and liquid crystal inequalities, establishing bounds on the fractal dimension of singularities without maximum principle assumptions, and compares these to previous results.

Contribution

It proves a new upper bound on the parabolic fractal dimension of forward-singularities for solutions to inequalities related to Navier-Stokes and liquid crystals, without relying on maximum principles.

Findings

Bound of 55/13 on fractal dimension for solutions to inequalities.

Range of criteria, including boundedness of d, implying previous bounds.

Comparison with Liu's 2018 results for solutions satisfying maximum principles.

Abstract

In 1985, V. Scheffer discussed partial regularity results for what he called solutions to the "Navier-Stokes inequality". These maps essentially satisfy the incompressibility condition as well as the local and global energy inequalities and the pressure equation which may be derived formally from the Navier-Stokes system of equations, but they are not required to satisfy the Navier-Stokes system itself. One may extend this notion to a system considered by F.-H. Lin and C. Liu in the mid 1990s related to models of the flow of nematic liquid crystals, which include the Navier-Stokes system when the 'director field' $d$ is taken to be zero. In addition to an extended Navier-Stokes system, the Lin-Liu model includes a further parabolic system which implies an a priori maximum principle for $d$, which is lost when one considers the analogous 'inequality'. In 2018, Q. Liu proved a partial regularity result for certain solutions to the Lin-Liu model in terms of the parabolic fractal dimension $\textrm{dim}_{\textrm{pf}}$, relying on the boundedness of $d$ coming from the maximum principle. Specifically, Q. Liu proves $\textrm{dim}_{\textrm{pf}}(\Sigma_{-} \cap \mathcal{K}) \leq \tfrac{95}{63}$ for any compact $\mathcal{K}$, where $\Sigma_{-}$ is the set of 'forward-singular' space-time points, near which the solution blows up forwards in time. For solutions to the corresponding 'inequality', we prove here that, without any compensation for the lack of maximum principle, one has $\textrm{dim}_{\textrm{pf}}(\Sigma_{-} \cap \mathcal{K}) \leq \tfrac {55}{13}$. We also provide a range of criteria, including as just one example the boundedness of $d$, any one of which would furthermore imply that $\textrm{dim}_{\textrm{pf}}(\Sigma_{-} \cap \mathcal{K}) \leq \tfrac{95}{63}$ for solutions to the inequality, just as Q. Liu proved for solutions to the Lin-Liu system.