Drag Coefficient
The drag coefficient is used to express the drag of an object in moving fluid
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Any object moving through a fluid will experience a drag - the net force in direction of flow due to the pressure and shear stress forces on the surface of the object.
The 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 as 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.
Most of the information about the drag on objects is a result of experiments. Drag coefficients for some common bodies:
| 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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