1 research outputs found
A multigrid method for the Helmholtz equation with optimized coarse grid corrections
We study the convergence of multigrid schemes for the Helmholtz equation,
focusing in particular on the choice of the coarse scale operators. Let G_c
denote the number of points per wavelength at the coarse level. If the coarse
scale solutions are to approximate the true solutions, then the oscillatory
nature of the solutions implies the requirement G_c > 2. However, in examples
the requirement is more like G_c >= 10, in a trade-off involving also the
amount of damping present and the number of multigrid iterations. We conjecture
that this is caused by the difference in phase speeds between the coarse and
fine scale operators. Standard 5-point finite differences in 2-D are our first
example. A new coarse scale 9-point operator is constructed to match the fine
scale phase speeds. We then compare phase speeds and multigrid performance of
standard schemes with a scheme using the new operator. The required G_c is
reduced from about 10 to about 3.5, with less damping present so that waves
propagate over > 100 wavelengths in the new scheme. Next we consider extensions
of the method to more general cases. In 3-D comparable results are obtained
with standard 7-point differences and optimized 27-point coarse grid operators,
leading to an order of magnitude reduction in the number of unknowns for the
coarsest scale linear system. Finally we show how to include PML boundary
layers, using a regular grid finite element method. Matching coarse scale
operators can easily be constructed for other discretizations. The method is
therefore potentially useful for a large class of discretized high-frequency
Helmholtz equations.Comment: Coarse scale operators are simplified and only standard smoothers
used in v3; 5 figures, 12 table