SUMMARYConventional least-squares finite element methods (LSFEMs) for incompressible flows conserve mass only approximately. For some problems, mass loss levels are large and result in unphysical solutions. In this paper we formulate a new, locally conservative LSFEM for the Stokes equations wherein a discrete velocity field is computed that is point-wise divergence free on each element. The central idea is to allow discontinuous velocity approximations and then to define the velocity field on each element using a local stream-function. The effect of the new LSFEM approach on improved local and global mass conservation is compared with a conventional LSFEM for the Stokes equations employing standard C 0 Lagrangian elements.

SUMMARYIn this paper, we develop least‐squares finite element methods (LSFEMs) for incompressible fluid flows with improved mass conservation. Specifically, we formulate a new locally conservative LSFEM for the velocity–vorticity–pressure Stokes system, which uses a piecewise divergence‐free basis for the velocity and standard C0 elements for the vorticity and the pressure. The new method, which we term dV‐VP improves upon our previous discontinuous stream‐function formulation in several ways. The use of a velocity basis, instead of a stream function, simplifies the imposition and implementation of the velocity boundary condition, and eliminates second‐order terms from the least‐squares functional. Moreover, the size of the resulting discrete problem is reduced because the piecewise solenoidal velocity element is approximately one‐half of the dimension of a stream‐function element of equal accuracy. In two dimensions, the discontinuous stream‐function LSFEM [1] motivates modification of our functional, which further improves the conservation of mass. We briefly discuss the extension of this modification to three dimensions. Computational studies demonstrate that the new formulation achieves optimal convergence rates and yields high conservation of mass. We also propose a simple diagonal preconditioner for the dV‐VP formulation, which significantly reduces the condition number of the LSFEM problem. Published 2012. This article is a US Government work and is in the public domain in the USA.

Classic multigrid methods are often not directly applicable to nonelliptic problems such as curl-type partial differential equations (PDEs). Curl-curl PDEs require specialized smoothers that are compatible with the gradient-like (near) null space. Moreover, recent developments have focused on replicating the grad-curl-div de Rham complex in a multilevel hierarchy through smoothed aggregation based algebraic multigrid. These approaches have been successful for Nédélec finite elements (i.e., H(curl) edge elements), but do not extend naturally to high-order representations. In this paper we consider hierarchical high-order Whitney elements for the curl-curl eddy current problem and devise a scalable multilevel approach. Our method generates a hierarchy similar to p-type multigrid, which results in a coarse level that is amenable to further coarsening through the established process of a multilevel complex. The natural hierarchy of the elements induces an effective interpolation operator and motivates the construction of a compatible gradient smoothing process. We detail the multilevel solver for a hierarchical H(curl) basis and present numerical results in support of the method.