The aerodynamic forces on a vehicle come primarily from differences in pressure and viscous shearing stresses . Pressure and viscosity are basic properties of fluids . There is an important connection between velocity and pressure, expressed in Bernoulli's equation , and between velocity and cross-sectional area, expressed in the continuity equation . The viscosity of air is very small, but it is not negligible. Frictional drag comes from friction between the air flow and the surfaces exposed to the air flow.
Viscosity is associated with the ability of a fluid to flow freely.
For example, honey has about 100 times the viscosity of water, and it is obvious that the two fluids flow differently, and that it takes much greater force to stir honey than water. The frictional drag also depends on the Reynolds number , because the flow is laminar at low Reynolds numbers, and turbulent at high Reynolds numbers. Pressure drag comes from the the eddying motions that are set up in air by the passage of the rider. This part of the flow is called the wake , and it is similar to the flow left behind a passing boat. It is generally not a strong function of Reynolds number.