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Drag Coefficient

The drag coefficient expresses the drag of an object in a moving fluid

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Any object moving through a fluid experiences drag - the net force in the direction of flow due to pressure and shear stress forces on the surface of the object.

Drag force can be expressed as:

Fd = cd 1/2 ρ v2 A         (1)

where

Fd = drag force (N)

cd = drag coefficient

ρ = density of fluid

v = flow velocity

A = characteristic frontal area of the body

The drag coefficient is a function of several parameters like shape of the body, Reynolds Number for the flow, Froude number, Mach Number and Roughness of the Surface.

The characteristic frontal area - A - depends on the body.

Objects drag coefficients are mostly results of experiments. Drag coefficients for some common bodies are indicated below

Type of Object Drag Coefficient
- cd -
Frontal Area
- A -
(ft2)
Old Car like a T-ford 0.7 - 0.9 frontal area
Modern Car like Toyota Prius 0.26 frontal area
Common Car like Open Vectra (class C) 0.29 frontal area
Passenger Train 1.8 frontal area
Bike Upright Commuter 1.1 5.5
Bike racing 0.88 3.9
Tractor Trailed Truck 0.96 frontal area
Dolphin 0.0036 wetted area
Bird 0.4 frontal area
Solid Hemisphere 0.42 π / 4 d2
Solid Hemisphere flow normal to flat side 1.17 π / 4 d2
Thin Disk 1.1 π / 4 d2
Cube 0.8 s2
Streamline body 0.04 π / 4 d2
Squared flat plate at 90 deg 1.17
Long flat plate at 90 deg 1.98
Hollow semi-sphere opposite stream 1.42
Hollow semi-sphere facing stream 0.38
Hollow semi-cylinder opposite stream 1.20
Wires and cables 1.0 - 1.3
Laminar flat plate (Re=106) 0.001
Turbulent flat plate (Re=106) 0.005
Subsonic Transport Aircraft 0.012
Supersonic Fighter, M=2.5 0.016
Person (upright position) 1.0 - 1.3
Ski jumper 1.2 - 1.3

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Related Topics

  • Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.

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