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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 |
| Sports Car, sloping rear | 0.2 - 0.3 | frontal area |
| Saloon Car, stepped rear | 0.4 - 0.5 | frontal area |
| Convertible, open top | 0.6 - 0.7 | frontal area |
| Common Car like Opel Vectra (class C) | 0.29 | frontal area |
| Bus | 0.6 - 0.8 | frontal area |
| Truck | 0.8 - 1.0 | frontal area |
| Passenger Train | 1.8 | frontal area |
| Motorcyvcle and rider | 1.8 | frontal area |
| Bicycle 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 | |
| Long stream-lined body | 0.1 |
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