Inviscid (ideal)flow around a stationary wing section.

Viscous flow around a stationary wing section.

Viscosity

Viscosity is the degree of "stickiness" or internal friction found between molecules in a fluid-in a liquid like water or in a gas like air. The degree of viscosity is related to the amount of force that has to be applied to it from the side to get it to move sideways-called "shearing." In equations where viscosity of fluid is a factor, it is indicated by the symbol µ (the Greek letter "mu.") All real fluids exhibit some amount of viscosity. Ideal fluids are said to be inviscid-there is no internal friction between the molecules. Ideal fluids are considered to determine how air behaves in theory and to serve as a starting point for examining how airflow behaves in real conditions.