 Lubrication theory

Lubrication theory describes viscous flow through a gap whose thickness is much smaller than its streamwise length; pressure is nearly uniform across the gap and controls a locally parabolic velocity profile.

Lubrication theory is a branch of fluid mechanics that studies the motion of thin fluid films, typically in situations where the fluid serves to reduce friction between surfaces in relative motion. This theory simplifies the governing equations of fluid flow by assuming one dimension (thickness of the lubricant film) is small compared to the other dimensions (length and width of the surfaces in contact).