[TowerTalk] Wind loading comment misdirected? W7VP

Jim Lux jimlux at earthlink.net
Sun Mar 26 20:01:02 EST 2006

At 04:13 PM 3/26/2006, Bill wrote:
>Yes Jim and I wish I could get my aerodynamics students to see it beyond
>that level of simplicity.  Indeed the coefficient of drag will increase
>rapidly when there is boundary layer separation, which occurs within a range
>of Reynolds Numbers.  After separation the flow behind the round object (or
>for that matter any object) becomes turbulent, changing the pressure
>distribution behind the object hence increasing the drag.  In teaching
>pilots about this it is often a chore to get them to appreciate the
>significance of the Reynolds Number to the flow situation since most of them
>do not have the math necessary to do the evaluations using the compressible
>flow fluid calculations.

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.


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