Drag (sometimes called air resistance, a type of friction, or fluid resistance, another type of friction or fluid friction) refers to forces acting opposite to the relative motion of any object moving with respect to a surrounding fluid. This can exist between two fluid layers (or surfaces) or a fluid and a solid surface. Unlike other resistive forces, such as dry friction, which are nearly independent of velocity, drag forces depend on velocity. Drag force is proportional to the velocity for a laminar flow and for a squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity.
The drag coefficient (commonly denoted as: cd, cx or cw) is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment such as air or water.
A lower drag coefficient indicates the object will have less aerodynamic or hydrodynamic drag. The drag coefficient is always associated with a particular surface area.
|Cd||drag coefficient (dimensionless)|
|Fd||drag force (The force component in the direction of the flow velocity) (N)|
|ρ||mass density of the fluid (kg/m3)|
|v||speed of the object relative to the fluid (m/s)|
|A||reference area (The reference area depends on what type of drag coefficient is being measured.) (m2)|