A CHARACTERISTICS-MIXED FINITE-ELEMENT METHOD FOR ADVECTION-DOMINATED TRANSPORT PROBLEMS

We define a new finite element method, called the characteristics-mixed method, for approximating the solution to an advection-dominated transport problem. The method is based on a space-time variational form of the advection-diffusion equation. Our test functions are piecewise constant in space, and in time they approximately follow the characteristics of the advective (i.e., hyperbolic) part of the equation. Thus the scheme uses a characteristic approximation to handle advection in time. This is combined with a low-order mixed finite element spatial approximation of the equation. Boundary conditions are incorporated in a natural and mass conservative fashion. The scheme is completely locally conservative; in fact, on the discrete level, fluid is transported along the approximate characteristics. A postprocessing step is included in the scheme in which the approximation to the scalar unknown is improved by utilizing the approximate vector flux. This has the effect of improving the rate of convergence of the method. We show that it is optimally convergent to order one in time and at least suboptimally convergent to order 3/2 in space.