Aerodynamic drag
In aerodynamics, aerodynamic drag is the fluid drag force that acts on any moving solid body in the direction of the fluid freestream flow. From the body's perspective (near-field approach), the drag comes from forces due to pressure distributions over the body surface, symbolized , and forces due to skin friction, which is a result of viscosity, denoted . Alternatively, calculated from the flowfield perspective (far-field approach), the drag force comes from three natural phenomena: shock waves, vortex sheet, and viscosity.
Introduction
The pressure distribution over the body surface exerts normal forces which, summed and projected into the freestream direction, represent the drag force due to pressure . The nature of these normal forces combines shock wave effects, vortex system generation effects and wake viscous mechanisms all together.
When the viscosity effect over the pressure distribution is considered separately, the remaining drag force is called pressure (or form) drag. In the absence of viscosity, the pressure forces on the vehicle cancel each other and, hence, the drag is zero. Pressure drag is the dominant component in the case of vehicles with regions of separated flow, in which the pressure recovery is fairly ineffective.