Characteristic length (also sometimes called characteristic dimension) is used to compute the time step that satisfies the Courant-Friedrichs-Lewy (CFL) condition for a given analysis setup.

The CFL condition is of interest mostly in explicit dynamics and computational fluid dynamics analyses. It governs the maximum time step for which a solution will be stable, and it must be met for the solution to converge. The CFL condition can be expressed as follows:

where,

such that if you know the characteristic length and material sound speed, you can determine the time step safety factor.

As h decreases, so does the time step. The definition of h varies based on element type:

Element Type	Definition of Characteristic Length (h)
Hexahedral or wedge	The volume of the element divided by the square of the longest diagonal and scaled by
Tetrahedral	The minimum distance of any element node to its opposing element face

Characteristic Length (Autodyn) calculated by the Mesh Metrics and the User Defined Function are the same.