Nested grids ILU-decomposition (NGILU)

A preconditioning technique is described which shows, in many cases, grid-independent convergence. This technique only requires an ordering of the unknowns based on the different levels of multigrid, and an incomplete LU-decomposition based on a drop tolerance. The method is demonstrated on a variety of well-known elliptic test problems including strongly varying coefficients, advective terms and grid refinement.