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

[K8RI]

On 2/9/2013 11:43 AM, Jim Lux wrote:
> On 2/9/13 8:21 AM, Michael Tope wrote:
>> Jim,
>>
>> With regard to your comments below, are you assuming laminar or
>> turbulent flow? I just grabbed my copy of Leeson's "Physical Design of
>> Yagi Antennas" and he discusses this same issue of a rapid change in
>> drag coefficient for wind speeds and tubing diameters of practical
>> interest to antenna builders for the case of turbulent flow.
>
> Turbulent flow.. When I built the spreadsheet that does the
> calculations, I assumed turbulent flow, and used those tables: the
> airflow over the elements is likely not smooth, having been disrupted by
> other elements and environmental effects, and also the elements
> themselves are likely not smooth, so the laminar/turbulent transition
> would be tripped by hose clamps, bird droppings, surface corrosion,
> galvanized surfaces, etc. (relatively few antennas have a "mirror
> finish" on them)
>
> Laminar flow is when you have undisturbed air (e.g. clean airplane
> wings) and my own experience with trying to get laminar flow (soap box
> derby car when I was a kid, airplane wings as an adult, artificial
> tornado machines, etc. ) is that it's *really hard* to generate and keep
> laminar flow.   It just wants to go turbulent.
>

Having spent a lot of time with aircraft construction and flight with 
several thousand hours in high performance aircraft, my question, is why 
even use laminar flow calculations in this case?

BTW this is where I defer to the mechanical engineers, but I do have 
substantial experience with aircraft..

As you mentioned,  it is very difficult to intentionally achieve laminar 
flow.  With aircraft wings it requires a carefully contoured wing, or 
body to achieve that laminar flow.  It's almost impossible for round or 
flat plate members, or at junctions between members let alone a group of 
said members.

The abrupt separation on the leading edge of a round member usually 
causes rapidly changing pressures and associated drag.  How important 
this rapidly varying drag becomes...I don't know.

When the wind encounters the leading edge of a tower it creates 
turbulence that then encounters the trailing side.

Round members also have relatively strong turbulence on the trailing 
side, creating rapidly changing pressures.  Even on a relatively calm 
day it isn't unusual to hear those antenna elements just singing from 
the vibration caused by a gentle airflow.

>
>
>   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.

Having spent many thousands of hour with aircraft I find it totally 
unrealistic, but his knowledge far exceeds mine and I resort to the 
brute force method.

73

Roger  (K8RI)


>
>>
>> Leeson presents calculations from both UBC and EIA-222 formulas both of
>> which show an ~0.6 ratio between cylindrical member and flat-plate
>> member drag coefficients.
>
> Aha.. that's where the 0.6 comes from.
>
> But if you look at the classic "drag of a cylinder" graph, it starts out
> with Cd very high (10 for Re=1) and smoothly comes down to about 1 for
> Re <1E5.. then there's the big dip to 0.5-0.6 around 500,000, then it
> comes back up to around 0.8 for Re>1E7...
> That dip is from the transition to turbulent flow nearer to the front of
> the cylinder, so the boundary layer "sticks" to the cylinder longer on
> the back side.
>
>
> 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"...
>
>
>
> The overall summary is that I think just assuming a Cd of 1 works
> (conservative for cylinders), and hoping that the other design margin
> takes care of the variation on things like flat bars, angle iron, etc.
>
> If you're designing your tower such that the Cd changing by 20-30% makes
> a difference, I think you're kind of on the ragged edge anyway.  I doubt
> that you know the wind profile from ground to top that accurately, and
> that's a square law effect.  (actually, assuming the wind at the ground
> level is the same as at the top is probably a conservative assumption..
> the wind at the ground is almost always less, because of the surface
> drag of the ground, not to mention there's bushes, grass, trees, houses,
> etc.)
>
>
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