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.)
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