# Scale-invariant estimates and vorticity alignment for Navier-Stokes in the half-space with no-slip boundary conditions

## TL;DR

This paper establishes new regularity criteria for the Navier-Stokes equations in a half-space with no-slip boundary conditions, focusing on vorticity alignment and scale-invariant estimates to identify regular points.

## Contribution

It introduces improved geometric regularity criteria based on vorticity direction continuity and scaled Morrey estimates, extending previous results to boundary cases.

## Key findings

- Vorticity direction continuity implies regularity at potential blow-up points.
- New scaled Morrey estimates are developed and are of independent interest.
- The criteria apply to solutions near the flat boundary with large vorticity.

## Abstract

This paper is concerned with geometric regularity criteria for the Navier-Stokes equations in $\mathbb{R}^3_{+}\times (0,T)$ with no-slip boundary condition, with the assumption that the solution satisfies the `ODE blow-up rate' Type I condition. More precisely, we prove that if the vorticity direction is uniformly continuous on subsets of $$\bigcup_{t\in(T-1,T)} \big(B(0,R)\cap\mathbb{R}^3_{+}\big)\times {\{t\}},\,\,\,\,\,\, R=O(\sqrt{T-t})$$ where the vorticity has large magnitude, then $(0,T)$ is a regular point. This result is inspired by and improves the regularity criteria given by Giga, Hsu and Maekawa (2014). We also obtain new local versions for suitable weak solutions near the flat boundary. Our method hinges on new scaled Morrey estimates, blow-up and compactness arguments and `persistence of singularites' on the flat boundary. The scaled Morrey estimates seem to be of independent interest.