information:filtering
At high velocity

As mentioned, the drag equation with a constant drag coefficient gives the force moving through fluid a relatively large velocity, i.e. high Reynolds number, Re > ~1000. This is also called quadratic drag.

F D = 1 2 ρ v 2 C d A, {\displaystyle F_{D}\,=\,{\tfrac {1}{2}}\,\rho \,v^{2}\,C_{d}\,A,} The derivation of this equation is presented at Drag equation § Derivation.

The reference area A is often the orthographic projection of the object, or the frontal area, on a plane perpendicular to the direction of motion. For objects with a simple shape, such as a sphere, this is the cross sectional area. Sometimes a body is a composite of different parts, each with a different reference area (drag coefficient corresponding to each of those different areas must be determined).

In the case of a wing, the reference areas are the same, and the drag force is in the same ratio as the lift force. Therefore, the reference for a wing is often the lifting area, sometimes referred to as "wing area" rather than the frontal area.

For an object with a smooth surface, and non-fixed separation points (like a sphere or circular cylinder), the drag coefficient may vary with Reynolds number R, up to extremely high values (R of the order 10).

For an object with well-defined fixed separation points, like a circular disk with its plane normal to the flow direction, the drag coefficient is constant for R > 3,500. The further the drag coefficient C is, in general, a function of the orientation of the flow with respect to the object (apart from symmetrical objects like a sphere).