On the stability of residual-free bubbles for convection-diffusion problems and their approximation by a two-level finite element method

We consider the Galerkin finite element method for partial differential equations in two dimensions, where the finite-dimensional space used consists of piecewise (isoparametric) polynomials enriched with bubble functions. Writing L for the differential operator, we show that for elliptic convection-diffusion problems, the component of the bubble enrichment that stabilizes the method is equivalent to a Petrov-Galerkin method with an L-spline (exponentially fitted) trial space and piecewise polynomial test space; the remaining component of the bubble influences the accuracy of the method. A stability inequality recently obtained by Brezzi, Franca and Russo for a limiting case of bubbles applied to convection-diffusion problems is shown to be slightly weaker than the standard stability inequality that is obtained for the SDFEM/SUPG method, thereby demonstrating that the bubble approach is in general slightly less stable than the streamline diffusion method. When the trial functions are piecewise linear, we show that residual-free bubbles are as stable as SDFEM/SUPG, and we extend this stability inequality to include positive mesh-Peclet numbers in the convection-dominated regime. Approximate computations of the residual-free bubbles are performed using a two-level finite element method. (C) 1998 Elsevier Science S.A. All rights reserved.