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

[Jim Lux]

On 2/9/13 11:12 AM, K8RI wrote:
> On 2/9/2013 11:43 AM, Jim Lux wrote:
>> On 2/9/13 8:21 AM, Michael Tope wrote:
>>> Jim,
>>>
>>
>> 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?


because a LOT of people start with basic texts on aerodynamics, and 
laminar flow is ALWAYS the first thing, because it lends itself to a 
theoretical analysis, with assumptions of continuity, etc. Von Karman 
and all that.

Turbulent flow was one of those "empirical data" sorts of things, and it 
is VERY hard to model effectively, so there's always caveats on the 
data. (e.g. use this as a guide, but go test in your own wind tunnel). 
Just measuring the boundary layer was a chore up until the invention of 
the laser doppler velocimeter. Smoke or bubbles (in a water tunnel) 
provided some visualization, but still, it's hard.

  Maybe today with supercomputers and good CFD codes, but even then, 
I'll bet there's not a lot of detailed modeling of things like "rough 
surfaces".


>
> 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.

Even without laminar flow, a "streamlined" strut that is inches across 
has a lot less drag than a wire or cable that is much, much smaller. 
The drag on those wire braced planes of a century ago must have been 
phenomenal.  I wonder what a pilot of one of those would think if they 
were dropped into a modern small plane (or a high performance sailplane?)
>
> 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.

well, one thing it causes is aeolian vibrations as you mention below. 
More of an issue for antenna elements than tower elements (I'll bet the 
tower element resonances are up the hundreds of Hz range.. they're 
pretty stiff)

>
> 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.

It might well be that it just doesn't make much difference in the long 
run (that is, other factors have larger effect)..

And with respect to brute force: You polish your galvanized members to a 
mirror finish to insure laminar flow?  That's a lot of brute force<grin>.


>
> 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.)
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>