At 06:17 PM 3/26/2006, Michael Tope wrote:
>----- Original Message -----
>From: "Jim Lux" <jimlux@earthlink.net>
>
>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.
here's the data for a 3" pipe
v (ft/sec) re Cd lb/ft mi/hr
40 61,900 1.01 0.47 27
50 77,400 1.01 0.73 34
60 92,900 1.00 1.05 41
70 108,400 1.00 1.42 48
80 124,000 0.98 1.82 55
90 139,000 0.94 2.22 61
100 155,000 0.89 2.59 68
110 170,000 0.82 2.89 75
120 186,000 0.73 3.07 82
130 201,000 0.63 3.08 89
140 217,000 0.50 2.87 95
150 232,000 0.36 2.37 102
160 248,000 0.20 1.52 109
170 263,000 0.18 1.51 116
180 278,000 0.18 1.70 123
190 294,000 0.18 1.89 130
200 309,000 0.18 2.10 136
the peak drag is around 80-90 mi/hr, where the Cd is about .6-.7, so
Leeson's ballpark number of 0.67 is in the right ballpark, at least for 3"
diameter cylinders. If you use that, it's conservative for higher
speeds. You have to get up to around 160 mi/hr before the load (now at a
Cd of 0.18) gets to be the same.
There's another factor that's not included here, and that's the effect of
roughness.. You might get low "form drag" from a large diameter tube, but
now there's more "surface drag" because the length of the surface is bigger.
Some more fiddling is needed, clearly.
Jim
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