Drag Coefficient

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

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 (1.2 kg/m3 for air at NTP)

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)
Laminar flat plate (Re=106) 0.001
Dolphin 0.0036 wetted area
Turbulent flat plate (Re=106) 0.005
Subsonic Transport Aircraft 0.012
Supersonic Fighter,M=2.5 0.016
Streamline body 0.04 π / 4 d2
Airplane wing, normal position 0.05
Long stream-lined body 0.1
Airplane wing, stalled 0.15
Modern Car like Toyota Prius 0.26 frontal area
Sports Car, sloping rear 0.2 - 0.3 frontal area
Common Car like Opel Vectra (class C) 0.29 frontal area
Hollow semi-sphere facing stream 0.38
Bird 0.4 frontal area
Solid Hemisphere 0.42 π / 4 d2
Sphere 0.5
Saloon Car, stepped rear 0.4 - 0.5 frontal area
Convertible, open top 0.6 - 0.7 frontal area
Bus 0.6 - 0.8 frontal area
Old Car like a T-ford 0.7 - 0.9 frontal area
Cube 0.8 s2
Bike racing 0.88 3.9
Bicycle 0.9
Tractor Trailed Truck 0.96 frontal area
Truck 0.8 - 1.0 frontal area
Person standing 1.0 – 1.3
Bicycle Upright Commuter 1.1 5.5
Thin Disk 1.1 π / 4 d2
Solid Hemisphere flow normal to flat side 1.17 π / 4 d2
Squared flat plate at 90 deg 1.17
Wires and cables 1.0 - 1.3
Person (upright position) 1.0 - 1.3
Hollow semi-cylinder opposite stream 1.2
Ski jumper 1.2 - 1.3
Hollow semi-sphere opposite stream 1.42
Passenger Train 1.8 frontal area
Motorcyvcle and rider 1.8 frontal area
Long flat plate at 90 deg 1.98
Rectangular box 2.1

Example - Air Resistance on a Normal Car

The force required to overcome air resistance for a normal family car with drag coefficient 0.29 and frontal area 2 m2 in 90 km/h can be calculated as:

Fd = 0.29 1/2 (1.2 kg/m3) ((90 km/h) (1000 m/km) / (3600 s/h))2 (2 m2) 

   = 217.5 N

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.

Related Documents

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