traînée

Traînée is a French noun that refers to a drag force or a trailing effect produced by an object moving through a fluid medium. In the context of aerodynamics, the term evokes the resistance named « traînée parasite » (parasitic drag) or « traînée de forme » (form drag). It is calculated using the fundamental relation

F = ½ ρ v² C_D A

where F denotes the drag force, ρ the fluid density, v the relative velocity, C_D the dimensionless drag coefficient, and A the reference area. The drag coefficient itself is influenced by shape, surface roughness, and flow characteristics, and it varies with the Reynolds number.

Beyond air and water, traînée appears in other fields such as maritime engineering, where hull design aims to reduce resistance, and in astrophysics, where it describes the gravitational drag encountered by bodies moving through a stellar medium. In everyday language, traînée can also signify a literal trail or a line left behind by a moving entity, as in « la traînée d’une comète ».

The study of aerodynamic drag is crucial for efficient vehicle design, influencing the performance of aircraft, automobiles, and sporting equipment. By optimizing shape and reducing wetted surface area, engineers achieve lower C_D values and thus better fuel economy and higher speed. Traînée remains a central concept in fluid dynamics, engineering design, and environmental assessment, where minimizing drag can reduce energy consumption and emissions.