Two-Dimensional Fourier Continuation and Applications

This paper presents a fast “two-dimensional Fourier continuation” (2D-FC) method for construction of biperiodic extensions of smooth nonperiodic functions defined over general two-dimensional smooth domains. The approach, which runs at a cost of (\mathcalO(Nłog N)\) operations for an $N$-point discretization grid, can be directly generalized to domains of any given dimensionality, but such generalizations are not considered in this contribution. The 2D-FC extensions are produced in a two-step procedure. In the first step the one-dimensional Fourier continuation method is applied along a discrete set of outward boundary-normal directions to produce, along such directions, continuations that vanish outside a narrow interval beyond the boundary. Thus, the first step of the algorithm produces “blending-to-zero along normals” for the given function values. In the second step, the extended function values are evaluated on an underlying Cartesian grid by means of an efficient, high-order boundary-normal interpolation scheme. A Fourier continuation expansion of the given function can then be obtained by a direct application of the two-dimensional fast Fourier transform (FFT). Algorithms of arbitrarily high order of accuracy can be obtained by this method. The usefulness and performance of the proposed 2D-FC method are illustrated with applications to the Poisson equation and the time-domain wave equation within a bounded domain. As part of these examples the novel “Fourier forwarding” solver is introduced which, propagating plane waves as they would in free space and relying on certain boundary corrections, can solve the time-domain wave equation and other hyperbolic partial differential equations within general domains at computing costs that grow sublinearly with the size of the spatial discretization.