Mechanical Engineering

# What is coefficient of drag? How can it be known? What is its significance and what is its unit?

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What is coefficient of drag? How can it be known? What is its significance and what is its unit?

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A Car has a ‘profile’ – endeavoured to be practically as ‘aero-dynamic’ as an Arrow but at the same time it also has a square or a rectangular cross-section’ related to its W x H.
If such a ‘cross-section/flat board’ is ‘inflicted’ with a blast of air in a Wind Tunnel corresponding to the road speed of the Car, it would experience a ‘push’ of say, 100 kgs.
On the other hand, if the actual Car is put in the place of the ‘flat board’, it would experience a push much lower than as above, coz of its ‘aero-dynamics’ – say 30 kgs.
Therefore, the ‘Coefficient of Drag’ is the ‘ratio’ of the above two i.e. 0.30.
Since it’s a ‘ratio’, it has no Unit and obviously, the lower it is - the better. Present day ‘Stock Cars’ have managed to achieve a “cee-dee-alpha” (coefficient of drag) of less than 0.30 – typically 0.25.

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Coefficient of drag can be explained in another way, it is the amount of kinetic energy of the flowing air which is lost as resistance work done when flown over a body. For flat rectangular surface as soon as the fluid strikes it it loses its entire kinetic energy resisting the body so the value of Cd = 1.0 but for a streamlined cross section with a Cd value of say 0.25, only 25% of the energy is resisting the body rest 75% is retained by air, so the body achieves a higher terminal velocity due to low resistance.

Mathematically Cd = (Drag force)/(0.5*(Density of air)*(Projected area of body)*(flow velocity)2)

Where the term in the denominator relates to the total kinetic energy possessed by the flowing air.

Practically it can be computed by placing the test body inside a wind tunnel. The body is attached to a mechanism which calculates the drag force, flow velocity can be measured by using a pitot tube and the density of air can be computed from thermodynamic models.

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Hi Ashwin, for the projected area of the body, how we calculate it? Can you explain it in details.

Is it a cross-section parallel to wind direction or what?

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Projected area is the cross section of the test object normal to the flow velocity, while the component parallel to the flow velocity is used to calculate the lift.

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