Hydrodynamics is a branch of fluid mechanics that involves mathematical analysis of the forces occurring at a fluid-object interface, e.g., between a submarine and the surrounding water or between a pipe wall and the flowing water. In general, it does not matter mathematically whether the object or the fluid is moving, rather it is the relative motion between the two that is important. The fluid is considered to be composed of particles that flow in layers. These layers become distorted and slide over one another (like playing cards) when the fluid intercepts an object.

Historically, hydrodynamics focused on theoretical equations for the flow of a fictitious (ideal) fluid that was assumed to be inviscid (i.e., totally without viscosity). In other words, this fluid would experience no frictional effects between its own moving layers or between fluid layers and object surfaces. These equations, though mathematically correct, indicated...