arXiv2025
We study the three-dimensional incompressible Navier-Stokes equations in a smooth bounded domain $Ω$ with initial velocity $u_0$ square-integrable, divergence-free and tangent to $\partial Ω$. We supplement the equations with the Navier friction boundary conditions $u \cdot n = 0$ and $[(2Su)n + αu]_{tang} = 0$, where $n$ is the unit exterior normal to $\partial Ω$, $Su = (Du + (Du)^t)/2$, $α\in C^0(\partialΩ)$ is the boundary friction coefficient and $[\cdot]_{tang}$ is the projection of its argument onto the tangent space of $\partial Ω$. We prove global existence of a weak Leray-type solution to the resulting initial-boundary value problem and exponential decay in energy norm of these solutions when friction is positive. We also prove exponential decay if friction is non-negative and the domain is not a solid of revolution. These two results are well known in the case of Dirichlet boundary condition, but, even if they have been implicitly used for the Navier boundary conditions, the comprehensive analysis is not available in the literature. After carefully studying the Stokes semigroup for such a boundary condition, we use the Galerkin method for existence, Poincaré-type inequalities, with suitable adaptations to account for the differential geometry of the boundary, and a novel integral Gronwall-type inequality. In addition, in the frictionless case $α= 0$, we prove convergence of the solution to a steady rigid rotation, if the domain is a solid of revolution.