On the Cauchy problem to the axially-symmetric solutions to the Navier-Stokes equations

Abstract

We consider the Cauchy problem to the axisymmetric Navier-Stokes equations. To prove an existence of global regular solutions we examine the Navier-Stokes equations near the axis of symmetry and far from it separately. We derive only a global a priori estimate. To show it near the axis of symmetry we need the energy estimate, $L_\infty$-estimate for swirl, $H^2$ and $H^3$ estimates for the modified stream function (stream function divided by radius) and also expansions of velocity and modified stream function found by Liu-Wang. The estimate for solutions far from the axis of symmetry follows easily. Hence, having so regular solutions that Liu-Wang expansions hold we have the global a priori estimate $(\Omega=\mathbb{R}^3)$ $$ \|\omega_{r/r} \|_{V(\Omega^t)} + \|\omega_{\varphi/r}\|_{V(\Omega^t)}\le\phi({\rm data}),\ \ t<\infty, \qquad(*) $$ where $\omega_r$ is the radiar component of vorticity, $\omega_\varphi$ the angular, $V(\Omega^t)$ is the energy norm. Estimate $(*)$ can be treated as an a priori estimate derived on sufficiently regular solutions. Increasing regularity of solutions $(*)$ we derive the estimate $$\eqalign{ &\|v\|_{W_3^{3,3/2}(\Omega^t)}+\|\nabla p\|_{W_3^{1,1/2}(\Omega^t)}\cr &\le\phi(\phi({\rm data}),\|f\|_{W_3^{1,1/2}(\Omega^t)},\|v(0)\|_{W_3^{3-2/3}(\Omega)}),\cr} \qquad(**) $$ where $\phi$ is an increasing positive function. The estimate is proved on the local solution. Estimate $(**)$ plus existence of local solutions imply the existence of global regular solutions to the Cauchy problem.