fluid dynamics, drag coefficient

in fluid dynamics, 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. It is used in the drag equation, where 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.

The drag coefficient of any object comprises the effects of the two basic contributors to fluid dynamic drag: skin friction and form drag. The drag coefficient of a lifting airfoil or hydrofoil also includes the effects of lift induced drag. The drag coefficient of a complete structure such as an aircraft also includes the effects of interference drag.

The drag coefficient Cd is defined as:

C_d= \dfrac{F_d}{\dfrac{1}{2} \rho v^2 A},

where

Fd is the drag force, which is by definition the force component in the direction of the flow velocity,[6]
ρ is the mass density of the fluid, [7]
v is the speed of the object relative to the fluid, and
A is the reference area