Aerodynamic drag

Aerodynamic drag is the force a fluid exerts opposite an object’s motion relative to it. Its magnitude depends on flow conditions, object shape, and speed.

Pressure drag: Drag caused by pressure differences between the front and rear surfaces of an object. Flow separation often leaves a low-pressure wake that creates this component.
Drag equation: An equation estimating drag as one-half fluid density times speed squared, reference area, and drag coefficient. It makes the main dependencies of aerodynamic drag explicit.
Automotive aerodynamics: The study and design of airflow around road vehicles. Reducing vehicle drag can lower fuel or energy use, especially at highway speeds.
Lift: The force perpendicular to a body’s motion through a fluid, commonly associated with flight. Lift and drag are perpendicular components of the same aerodynamic force.
Skin friction drag: Drag caused by viscous shear stress along a surface in contact with moving fluid. The air’s viscosity resists motion within the boundary layer against the body.
Dynamic pressure: The kinetic energy per unit volume of a moving fluid, equal to one-half density times speed squared. Dynamic pressure supplies the speed-and-density factor in the drag equation.
Aircraft design: The engineering of aircraft structures and systems to meet flight requirements. Aircraft designers balance drag reduction against lift, stability, and structural constraints.
Buoyancy: The upward force a fluid exerts on an immersed or floating body. Buoyancy depends on displaced fluid, unlike drag’s dependence on relative motion.
Boundary layer: A thin region near a surface where viscosity strongly affects fluid velocity. Its thickness and state shape surface friction and influence flow separation.
Reference area: A chosen area used to normalize aerodynamic forces and coefficients. The area convention changes reported drag coefficients, even when the force is unchanged.

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