It is a branch of continuum mechanics, a subject which models matter without using the information that it is made out of atoms; that is, it models matter from a "macroscopic" viewpoint rather than from "microscopic". Fluid mechanics, especially fluid dynamics, is an active field of research, typically mathematically complex. Many problems are partly or wholly unsolved and are best addressed by numerical methods, typically using computers. A modern discipline, called computational fluid dynamics (CFD), is devoted to this approach. Particle image velocimetry, an experimental method for visualizing and analyzing fluid flow, also takes advantage of the highly visual nature of fluid flow.