[TowerTalk] Thrust Bearing, etc: more answers from UST calcs

[Michael Tope]

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..............