----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
> That't the cool thing about that java calculator.. it does all the nice
> slow speed, low Re stuff, without you having to slog through figuring out
> what the Re is and looking up the Cd for the cylinder, etc.
>
> And, the annoying thing I've found is that the cases of real interest to
> non-airplane designers are those low speed cases: things like 2" pipes in
> 50 mi/hr winds, or whip antennas on a car, etc... and that's just where
> the
> big discontinuity in the curve is.
>
> What's great is to put in a reasonably slow speed (say, 88 ft/sec) and
> then
> step through diameters of cylinders and see how the Cd changes,
> dramatically, with pretty small changes. A diameter of 0.1 ft gives you a
> Cd of around 1, but a diameter of 0.5 ft gives you something like 0.2..
>
>
> Here's an interesting little table (calculated for 88 ft/sec = 60 mi/hr):
> dia(ft) Re Cd lb/linear ft
> 0.05 27,200 1.011 0.46
> 0.1 54,500 1.007 0.91
> 0.15 81,800 1.005 1.36
> 0.2 109,000 0.997 1.80
> 0.25 136,000 0.952 2.15
> 0.3 163,000 0.853 2.31
> 0.35 191,000 0.700 2.21
> 0.4 218,000 0.492 1.78
> 0.45 245,000 0.231 0.94
> 0.5 273,000 0.180 0.81
>
>
> What's fascinating is that a pipe 6" in diameter has about the same drag
> force as a pipe 1" in diameter, and a pipe that's 3" in diameter has more
> than twice the drag as either.
>
Jim,
Are you saying that the java calculator algorithm breaks down
in that speed range (~60 mile/hour), or are you saying that the
drag coefficient really does wander that much across the pipe
diameter range of 1" to 6" for winds speeds that are of interest
to tower designers? Per Leeson, I've been faithfully using a Cd
of ~0.67 for round members on all my tower calcs. If what you
say is true, perhaps I need to be a lot more careful. Or is the
dramatic reduction in Cd for the larger diameter pipes only valid
over a narrow range of wind speeds such that in practice you
couldn't make the projected area of a mast larger in order to
lower the overturning moment transferred to the structure
underneath it. Stated another way, what does the Cd value look
like as a function of wind speed for say the 6" diameter pipe that
has a Cd of 0.18 at 60 mi/hr? Such widely varying drag coefficients
might suggest large diameter thin-wall masts are better than small
diameter thick wall masts with the same section modulus due to
lower drag coefficients, but only if the lower drag coefficient were
valid over a fairly wide range of windspeeds.
Thanks,
Mike W4EF...............................
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