On 2/10/13 8:30 AM, SPWoo wrote:
I'm not an engineer by profession; so I'm not saying that this is correct. In
various software programs and Excel spreadsheets I obtained from ham-related
sources use a Cd of 1.2 for round members and 2.0 for flat members. A specific
example of this is the AntennaMast Calculator that came with the ARRL Antenna
Handbook.
And Kurt's page gives similar numbers..
What's odd is that Cd of 1.2 for transverse flow over a cylinder is for
a VERY low Reynolds number (like 400 => 0.5" cylinder at 1 mi/hr) and
laminar flow.
Of course, Cd is lower than 1.2 for anything with higher Re, so maybe
it's a safe assumption. But it also doesn't mean that the drag force on
a 2" round pipe is 0.6 of the drag force on a 2" flat strap.
The curves of Cd vs Re are very different for a cylinder and a flat
plate and a square member. So what the "standard" does, probably, is
take a worst case.. Cd is never bigger than 1.2 for a cylinder. Cd is
never bigger than 2.0 for a flat plate. Both might be less, but then,
the standard overestimates the loads, and the resulting structure will
be safe.
I think one thing the Kurt's page makes clear is that the tower
standards and the "wind ratings" that derive from them are not
necessarily based on actual aerodynamic analysis. They're a standard
where you plug in the parameters to see if your proposed design meets
the standard.
For instance, it doesn't really matter what the wind speed actually is:
you use the "wind zone" (older versions) or the "reference wind speed by
county" (newer versions). What matters is that when you calculate the
"wind load" by multiply the frontal area of your tower by the
pounds/square foot number for your area.
That is, the standard gives a common and uniform way to determine what
the design loads for your structure should be (whether or not those
happen to be the real loads.. hopefully, the design load is always
GREATER than the actual loads you expect to see)
After that it's straightforward structural analysis.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|