Abstract. It is a standard assumption in the error analysis of finite element methods that the underlying finite element mesh has to resolve the physical domain of the modeled process. In case of complicated domains appearing in many applications such as ground water flows this requirement sometimes becomes a bottleneck. The resolution condition links the computational complexity a priorily to the number (and size) of geometric details. Therefore even the coarsest available discretization can lead to a huge number of unknowns. In this paper, we will relax the resolution condition and introduce coarse (optimal order) approximation spaces for Stokes problems on complex domains. The described method will be efficient in the sense that the number of unknowns is only linked to the properties of the solution and not to the problem data. The presentation picks up the concept of composite finite elements for the Stokes problem presented in a previous paper of the authors. Here, the a priori error and stability analysis of the proposed mixed method is generalized to quite general, i.e. slip and leak boundary conditions that are of great importance in practical applications.
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Problem settingWe consider the stationary Stokes equationsdescribing the motion of a viscous incompressible fluid in a bounded Lipschitz domain Ω under the general mixed boundary conditions proposed by Navier [25] Thereby we use the following notationBoth, the equations (1.1) and the boundary conditions (1.2), can be generalized by replacing the zeros on the right hand sides by some given functions. In this paper, we are especially interested in the limit cases of the boundary conditions, i.e., Dirichlet (λ ν = λ τ = 1), Neumann (λ ν = λ τ = 0), slip (λ ν = 1, λ τ = 0) and leak (λ ν = 0, λ τ = 1) boundary conditions. In particular, we assume the boundary Γ := ∂ Ω to consist of four relatively closed disjoint partsThe leak and slip parts Γ s and Γ l are supposed to be of class C 1 . We define the coefficient functions from (1.2) in the following way:This choice leads to the following set of boundary conditionsWhile the mathematical literature on Dirichlet and Neumann boundary conditions is vast, leak and slip boundary conditions have been studied less extensively. However, they are of great practical interest. For theoretical studies of these boundary conditions we refer to [38; 13; 34] and for some applications to [19; 20].The Sobolev space that contains those velocity fields which fulfill the essential parts (conditions on the left in (1.6)) of the boundary conditions is denoted byThe (mixed) weak formulation of problem (1.1) together with the boundary conditions (1.6) reads:The bilinear forms a :In general, problem (1.8) is not uniquely solvable. The bilinear form a has a nontrivial kernel given by the set of rigid body motionsMoreover, every v ∈ R is divergence-free, i.e. the pairs (v, q), q ∈ R, is a solution of the homogeneous Stokes problem. Thus, a solution of (1.8) can only be unique up to elements of H 1 ess ∩ R.
Remark 1.To assure ...