On 2/9/2013 8:43 AM, Jim Lux wrote:
He then
states "conservative design, however, dictates a less aggressive
choice", referring to the choice between assuming turbulent flow or
laminar flow when doing these sorts of design calculations (for laminar
flow this transition from ~constant drag coefficient to rapidly changing
drag coefficient occurs at much higher wind speeds). UBC and EIA-222 (at
least the versions that were current when his book was published) both
appear to assume laminar flow.
yes.. I agree with Leeson. Interesting that UBC and 222 assume
laminar flow. I find the idea of laminar flow over a typical
galvanized strut somewhat unrealistic, but I admit I haven't looked at
that particular situation.
Yes, Leeson speculates that it wouldn't require much in the way of
surface roughness or other disturbance causing elements to achieve
turbulent flow across antenna and tower members. It sounds like the main
reason smooth flow assumptions are used has more to do with the simple
and conservative calculations that result than it does fidelity to the
actual the physics.
What's interesting is that a flat plate (or rectangular box) has a Cd
of about 2 at low Re.. So it's drag is twice the "flat plate area"...
This is reflected in some versions of EIA-222 (not sure about the
current version) whereby Cd = 1.2 for round members and 2.0 for flat
members (Leeson indicates EIA-222-D was written this way) whereas
RS-222-C apparently used Cd = 0.67 for round members. Some of this
difference is compensated for by offsetting differences in other
constants of the wind force formulas given in each of the respective
specifications.
73, Mike W4EF..............
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