# 3.1. Specifying Subcycling in Input

The recommended method to turn on subcycling is to use a feature to automatically generate the coarse and fine regions in the input deck. This is done by adding the following command to the presto region.

    SUBCYCLE BLOCKS = <string list>block_names

If this command is present, Sierra/SM will automatically generate and run a new input deck that can be used for the subcycling. If the original input deck is named `input.i` the automatically generated subcycling input deck will be named `input.subcycle.i`. The block_names specified are the names of the blocks targeted for inclusion in the fine region (run with the small time step).

The algorithm to split the regions is done as follows.

- Define the trial coarse region based off of everything not in the fine region.

- Compute the critical time step of the coarse region as the smallest element time step in that region.

- Compute the maximum time step each node can be integrated at as the smallest time step of any element near the node.

- For every element in the mesh, if the element is attached to only nodes with time steps greater than or equal to the coarse region time step, place the element in the coarse region.

- For every element in the mesh, if the element is attached to any node with time steps less than the coarse region time step, place the element in the fine region.

- Split all boundary conditions defined on the coarse and fine region appropriately and write to appropriate results files. Each region will generate independent output files.

The mathematical foundations of the subcycling algorithm used in Sierra/SM can be found in [[1]]. The portions of this paper specifically used in Sierra/SM are: Explicit-Explicit, Central Difference, and Linear Interpolation method.

[1]

Willem E.M. Bruijs. Subcyling in Transient Finite Element Analysi. PhD thesis, Technical University of Eindhoven, 1234.