As an object moves through the air, the air molecules near the object are disturbed and move around the object. Aerodynamic forces are generated between the gas and the object. To properly model these effects, aerodynamicists use similarity parameters, which are ratios of these effects to other forces present in the problem. If two experiments have the same values for the similarity parameters, then the relative importance of the forces are being correctly modeled.
Aerodynamic forces depend in a complex way on the viscosity of the air.
As an object moves through the air, the air molecules stick to the surface. The flow turning reacts to the boundary layer just as it would to the physical surface of the object. The boundary layer is very important in determining the drag of an object. To determine and predict these conditions, aerodynamicists rely on wind tunnel testing and very sophisticated computer analysis. The important similarity parameter for viscosity is the Reynolds number .