Convergence of nonconforming finite element approximations to first-order linear hyperbolic equations

Finite element approximations of the first-order hyperbolic equation ${\mathbf {U}} \bullet \nabla u + \alpha u = f$ are considered on curved domains $\Omega \subset {\mathbb {R}^2}$. When part of the boundary of $\Omega$ is characteristic, the boundary of numerical domain, ${\Omega _h}$, may become either an inflow or outflow boundary, so it is necessary to select an algorithm that will accommodate this ambiguity. This problem was motivated by a problem in acoustics, where an equation similar to the one above is coupled to three elliptic equations. In the last section, the acoustics problem is briefly recalled and our results for the first-order equation are used to demonstrate convergence of finite element approximations of the acoustics problem.