Re: Drag Coefficient of Box Lights Atop a Police Cruiser?


Pete Lynn wrote:
> "TomGee" <lvlus@xxxxxxxxxxx> wrote in message
> news:1132095763.629912.59250@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Very good information, thanks. Good suggestion,
> > too, but to do that I need to know the frontal area of
> > the car. How about if I calculate the drag of the light
> > box instead? I know the bluff frontal area, 48" x 8"
> > = 384 in.sq., and the velocity = 60 mph, and an air
> > mass can be assumed of, say, 1.30 average density
> > and stationary, through which the light box moves.
> > From here, how do we calculate the hp needed to
> > push the lightbox at the given speed?
>
> Drag = 0.5*air density*Coefficient of drag*Area* air velocity squared.
> Power = drag*vehicle velocity
>
>
Thanks Pete. I get a drag coefficient of 1.21353 by using M. S.
Cramer's "Bluff Body Drag Calculator" in his site at
http://www.fluidmech.net/jscalc/cdcal26.htm, where he also calculates
the drag to be 18.25 Newtons.

Drag = (0.65 x 1.21353) x 384 in.sq. x (60 x 60) mph
(7887945 x 384) x 3600 mph =
3028970880 x 3600
Doesn't work - different units. Any idea what I'm doing wrong?
>
>
> The drag coefficient of the light box might be difficult to estimate, it
> is in a messy part of the drag coefficient curve with respect to
> Reynolds's number - in this range it is speed sensitive. It should not
> be higher than one, though might get as low as 0.1 depending on shape
> and speed. At higher speed where the power loss is significant, it will
> likely be quite low, perhaps estimate in the 0.3 range initially, (~700
> Watts at 25m/s).
>
> The air speed at the light box will be effected by the vehicle shape -
> but compared to drag coefficient this should not be a significant error.
>
> Pete.

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