The expression that a physical derived quantity is expressed by the product of several basic quantities is called the dimensional formula of the physical quantity, which is called dimension for short. It is a formula expressed in units of basic physical quantities after the unit system is selected. All physical quantities with dimensions can be dimensionless. Dimensionless appeared in the early development of fluid mechanics. At that time, the level of mathematical methods and numerical calculation was very limited. In order to theoretically analyze some fluid phenomena (such as the flow in the boundary layer near the wing and the hull), it is necessary to simplify the governing equation of viscous fluid, so the target fluid is given a characteristic length and a characteristic velocity. By using the characteristic length and characteristic velocity (usually a large number relative to the boundary layer), some variables (such as x, y and v become x/l "1 or y/l" 1 or v/u" 1) can reduce the number of variables in the governing equation. For the flow outside the boundary layer, the viscous potential flow model is used to solve it without simplification. Therefore, dimensionless plays an extremely important role in the development history of fluid mechanics, especially aerodynamics.