Penalty method with Crouzeix–Raviart approximation for the Stokes equations under slip boundary condition

The Stokes equations subject to non-homogeneous slip boundary conditions are considered in a smooth domain $ \Omega \subset {\mathbb{R}}^N\enspace (N=\mathrm{2,3})$. We propose a finite element scheme based on the nonconforming P1/P0 approximation (Crouzeix–Raviart approximation) combined with a penalty formulation and with reduced-order numerical integration in order to address the essential boundary condition u · n∂Ω = g on ∂Ω. Because the original domain Ω must be approximated by a polygonal (or polyhedral) domain Ωh before applying the finite element method, we need to take into account the errors owing to the discrepancy Ω ≠ Ωh, that is, the issues of domain perturbation. In particular, the approximation of n∂Ω by $ {n}_{\mathrm{\partial }{\mathrm{\Omega }}_h}$ makes it non-trivial whether we have a discrete counterpart of a lifting theorem, i.e., continuous right inverse of the normal trace operator $ {H}^1(\mathrm{\Omega }{)}^N\to {H}^{1/2}(\mathrm{\partial \Omega })$; $ u\mapsto u\cdot {n}_{\mathrm{\partial \Omega }}$. In this paper we indeed prove such a discrete lifting theorem, taking advantage of the nonconforming approximation, and consequently we establish the error estimates $ O({h}^{\alpha }+\epsilon )$ and $ O({h}^{2\alpha }+\epsilon )$ for the velocity in the H1- and L2-norms respectively, where α = 1 if N = 2 and α = 1/2 if N = 3. This improves the previous result [T. Kashiwabara et al., Numer. Math. 134 (2016) 705–740] obtained for the conforming approximation in the sense that there appears no reciprocal of the penalty parameter ϵ in the estimates