6.8.10 Reordering the Domain and Zones

Reordering the domain can improve the computational performance of the solver by rearranging the nodes, faces, and cells in memory. The Mesh/Reorder submenu contains commands for reordering the domain and zones, and also for printing the bandwidth of the present mesh partitions. The domain can be reordered to increase memory access efficiency, and the zones can be reordered for user-interface convenience. The bandwidth provides insight into the distribution of the cells in the zones and in memory.

To reorder the domain, select the Domain menu item.

Mesh $\rightarrow$ Reorder $\rightarrow$ Domain

To reorder the zones, select the Zones menu item.

Mesh $\rightarrow$ Reorder $\rightarrow$ Zones

Finally, you can print the bandwidth of the present mesh partitions by selecting the Print Bandwidth menu item. This command prints the semi-bandwidth and maximum cell distance for each mesh partition.

Mesh $\rightarrow$ Reorder $\rightarrow$ Print Bandwidth

Remember to save a new case file (and a data file, if data exist) after reordering.

About Reordering

The Reverse Cuthill-McKee algorithm [ 17] is used in the reordering process to construct a "level tree'' initiated from a "seed cell'' in the domain. First a cell (called the seed cell) is selected using the algorithm of Gibbs, Poole, and Stockmeyer [ 25]. Each cell is then assigned to a level based on its distance from the seed cell. These level assignments form the level tree. In general, the faces and cells are reordered so that neighboring cells are near each other in the zone and in memory. Since most of the computational loops are over faces, you would like the two cells in memory cache at the same time to reduce cache and/or disk swapping--i.e., you want the cells near each other in memory to reduce the cost of memory access. The present scheme reorders the faces and cells in the zone, and the nodes, faces, and cells in memory.

You may also choose to reorder the zones. The zones are reordered by first sorting on zone type and then on zone ID. Zone reordering is performed simply for user-interface convenience.

A typical output produced using the domain reordering is shown below:

>> Reordering domain using Reverse Cuthill-McKee method:
      zones, cells, faces, done.
   Bandwidth reduction = 809/21 = 38.52
   Done.

If you print the bandwidth, you will see a report similar to the following:

Maximum cell distance = 21

The bandwidth is the maximum difference between neighboring cells in the zone--i.e., if you numbered each cell in the zone list sequentially and compared the differences between these indices.