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pascal28 · 02-02-2013, 06:30 AM

In the equation of drag the drag coefficient (CD) is assumed not to vary and hence indicates the aerodynamic efficiency of an objects shape.

The drag is then a product of this efficiency multiplied by the frontal area, 0.5 density of fluid traveling through and the velocity squared i.e.

Drag = CD * A* 0.5 * rho * V^2

Manufacturers quote a CD, which as far as I am aware can be derived from from testing in a wind tunnel (& possibly from CFD).

Questions:
1) Is drag coefficient really a constant with speed?
2) If it isn't what would be the typical variation with normal road speeds (say 30 to 70 mph)?
3) Is there an agreed speed that manufacturers quote CD at?

I would like to know this to correctly calibrate a CFD model of the Honda Insight that has a quoted CD of 0.25.

02-02-2013, 06:30 AM	#1 (permalink)
pascal28 EcoModding Lurker Join Date: Nov 2011 Location: UK Posts: 10 Thanks: 0 Thanked 3 Times in 2 Posts	Drag Coefficient - Is it really constant In the equation of drag the drag coefficient (CD) is assumed not to vary and hence indicates the aerodynamic efficiency of an objects shape. The drag is then a product of this efficiency multiplied by the frontal area, 0.5 density of fluid traveling through and the velocity squared i.e. Drag = CD * A* 0.5 * rho * V^2 Manufacturers quote a CD, which as far as I am aware can be derived from from testing in a wind tunnel (& possibly from CFD). Questions: 1) Is drag coefficient really a constant with speed? 2) If it isn't what would be the typical variation with normal road speeds (say 30 to 70 mph)? 3) Is there an agreed speed that manufacturers quote CD at? I would like to know this to correctly calibrate a CFD model of the Honda Insight that has a quoted CD of 0.25.