Dragkerroin

Dragkerroin, often translated as the drag coefficient, is a dimensionless quantity used in fluid dynamics to quantify the resistance of a body moving through a fluid. It appears in the drag equation F_d = 0.5 ρ v^2 C_d A_f, where ρ is fluid density, v is velocity relative to the fluid, A_f is the reference frontal area, and C_d is the drag coefficient.

Cd depends on the object's shape, size, surface roughness, and the flow regime, described by the Reynolds number. For the same body, Cd can vary with speed and fluid properties, and is typically determined experimentally or via computational fluid dynamics.

Common reference values: a smooth sphere about 0.47, a flat plate broadside to the flow around 1.0–1.2, and streamlined cars around 0.25–0.35. Real vehicles vary widely, with Cd values influenced by trim and air flow. In sailing and cycling, aerodynamic shapes aim to minimize Cd as part of overall drag management.

Cd is used to compare aerodynamic or hydrodynamic efficiency between designs. It is only meaningful for a specified reference area and Reynolds number; changing these without adjusting the definition can lead to misleading comparisons.

See also: coefficient of drag, Reynolds number, frontal area, drag force.