Drag coefficient

) 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.

where: The reference area depends on what type of drag coefficient is being measured.

Since this tends to be large compared to the frontal area, the resulting drag coefficients tend to be low, much lower than for a car with the same drag, frontal area, and speed.

Airships and some bodies of revolution use the volumetric drag coefficient, in which the reference area is the square of the cube root of the airship volume (volume to the two-thirds power).

Submerged streamlined bodies use the wetted surface area.

The reason for this is that the conventional definition makes the most sense when one is in the Newton regime, such as what happens at high Reynolds number, where it makes sense to scale the drag to the momentum flux into the frontal area of the object.

In particular at very low Reynolds number, it is more natural to write the drag force as being proportional to a drag coefficient multiplied by the speed of the object (rather than the square of the speed of the object).

An example of such a regime is the study of the mobility of aerosol particulates, such as smoke particles.

where: The drag equation is essentially a statement that the drag force on any object is proportional to the density of the fluid and proportional to the square of the relative flow speed between the object and the fluid.

comes from the dynamic pressure of the fluid, which is equal to the kinetic energy density.

is not a constant but varies as a function of flow speed, flow direction, object position, object size, fluid density and fluid viscosity.

Speed, kinematic viscosity and a characteristic length scale of the object are incorporated into a dimensionless quantity called the Reynolds number

A high form drag results in a broad wake.

The boundary layer will transition from laminar to turbulent if Reynolds number of the flow around the body is sufficiently great.

[14] For other objects, such as small particles, one can no longer consider that the drag coefficient

At very low Reynolds numbers, without flow separation, the drag force

equal to 1 would be obtained in a case where all of the fluid approaching the object is brought to rest, building up stagnation pressure over the whole front surface.

The graph to the left of it shows equal pressure across the surface.

In a real flat plate, the fluid must turn around the sides, and full stagnation pressure is found only at the center, dropping off toward the edges as in the lower figure and graph.

of a real flat plate would be less than 1; except that there will be suction on the backside: a negative pressure (relative to ambient).

of a real square flat plate perpendicular to the flow is often given as 1.17.

It varies with the speed of airflow (or more generally with Reynolds number

, while automobiles (and many other objects) use projected frontal area; thus, coefficients are not directly comparable between these classes of vehicles.

[21] The force between a fluid and a body, when there is relative motion, can only be transmitted by normal pressure and tangential friction stresses.

Thus, the shape of the body and the angle of attack determine the type of drag.

For example, an airfoil is considered as a body with a small angle of attack by the fluid flowing across it.

This means that it has attached boundary layers, which produce much less pressure drag.

The wake produced is very small and drag is dominated by the friction component.

This mainly occurs due to adverse pressure gradients at the top and rear parts of an airfoil.

Cylinders and spheres are taken as blunt bodies because the drag is dominated by the pressure component in the wake region at high Reynolds number.