Solving Problems on a Grid
Central to any direct computational method is the manner in which it discretizes the continuous domain of interest into a grid of many individual elements. This grid may be static, established once and for all at the beginning of the computation, or it may be dynamic, tracking the features of the result as the computation progresses. If the computation has features which one wants to track which are much smaller than the overall scale of the problem, and which move in time, then one must either include many more static grids to cover the region of interest, or adopt a dynamic scheme. The advantages of a dynamic gridding scheme are:

1) Increased computational savings over a static grid approach.

2) Increased storage savings over a static grid approach.

3) Complete control of grid resolution, compared to the fixed resolution of a static grid approach.

Introduction to Adaptive Mesh Refinement
In a series of papers, Berger, Oliger, and Colella developed an algorithm for dynamic gridding called local adaptive mesh refinement. The algorithm begins with the entire computational domain covered with a coarsely resolved base-level regular Cartesian grid. As the calculation progresses, individual grid cells are tagged for refinement, using a criterion that can either be user-supplied (i.e. mass per cell remains constant, hence higher density regions are more highly resolved) or based on Richardson extrapolation.

All tagged cells are then refined, meaning that a finer grid is overlayed on the coarse one. After refinement, individual grid patches on a single fixed level of refinement are passed off to an integrator which advances those cells in time. Finally, a correction procedure is implemented to correct the transfer along coarse-fine grid interfaces, to insure that the amount of any conserved quantity leaving one cells exactly balances the amount enter the bordering cell. If at some point the level of refinement in a cell is greater than required, the high resolution grid may be removed and replaced with a coarser one.

The image to the left shows the grid structure of an AMR calculation of a shock impacting an inclined slope. Each of the boxes is a grid; the more boxes it is nested within, the higher the level of refinments. It is adapted From Colella and Crutchfield (1994).

As the image shows, the algorithm uses high resolution grids only at the physical locations and times where they are required. This lets us solve problems that are completely intractable on a uniform grid; for example,we have used the code to model the filamentary nature of isothermal collapse down to an effective resolution of 131,072 cells per initial cloud radius, corresponding to a resolution of 10^15 cells on a uniform grid.

For more examples of our use of AMR in astrophysical fluid dynamics, see the research page.