On Vortex Alignment and Boundedness of $L^q$ Norm of Vorticity

We show that the spatial $L^q$ ($q > 5/3$) norm of the vorticity of an incompressible viscous fluid in $\mathbb{R}^3$ or $\mathbb{T}^3$ remains bounded uniformly in time, provided that the direction of vorticity is H\"older continuous in the space variable, and that the space--time $L^q$ norm of the vorticity is finite. The H\"older index depends only on $q$. This serves as a variant of the classical result by P. Constantin and Ch. Fefferman (Direction of vorticity and the problem of global regularity for the Navier--Stokes equations, Indiana Univ. J. Math., 42 (1993), 775--789).