Symmetry without symmetry: Numerical simulation of axisymmetric systems using Cartesian grids

We present a new technique for the numerical simulation of axisymmetric sys tems. This technique avoids the coordinate singularities which often arise when cylindrical or polar-spherical coordinate finite difference grids are used, particularly in simulating tenser partial differential equations like those of 3 + 1 numerical relativity. For a system axisymmetric about the r axis, the basic idea is to use a three-dimensional Cartesian (x,y,z) coord inate grid which covers (say) the y = 0 plane, but is only one finite-diffe rence-molecule-width thick in the y direction. The field variables in the c entral y = 0 grid plane can be updated using normal (x, y, z)-coordinate fi nite differencing, while those in the y not equal 0 grid planes can be comp uted from those in the central plane by using the axisymmetry assumption an d interpolation. We demonstrate the effectiveness of the approach on a set of fully nonlinear test computations in 3 + 1 numerical general relativity, involving both black holes and collapsing gravitational waves.