On 6/12/17 8:58 AM, Grant Saviers wrote:
Some more for the discussion:
2. The complexity of the wind field is largely ignored in the standards
except for the exposure classes. Then again, Computational Fluid
Dynamics has progressed to doing complex flow modeling. Again free on
the web and it works fine on a big/fast PC.
The software may be free, but the "building a model" is not, and
requires a fairly steep learning curve. Typically one starts with a
suitable 3D mechanical model (e.g. from something like Solidworks). The
sophistication comes in when deciding what is "too small to worry about".
I don't think there are any 3D models for towers that hams have. If one
searches, one might find solidworks type models (STEP files, for
instance) for 25 and 45 sections, but probably not for cables, and all
the other installation stuff.
So a specific tower site
could be modeled. Consider that NASA infrequently uses their numerous
wind tunnels (the big one at Ames needs 104 megawatts to run!).
Computers to it faster and cheaper. I have to wonder if the link re wind
tunnel testing of cell antennas refers to the difference from "rules of
thumb" vs tunnel results or CFD modeling vs tunnel?
I got the impression they were comparing the TIA/EIA-222 numbers against
actuals. The 222 numbers will be conservative - I doubt they are
derived from CFD, more from simple approximations.
For something like a Yagi, the approximations will get you pretty close,
as long as you pay attention to Cd vs Reynolds numbers and things like
that. The structure is simple, and there's not much "member to member"
interaction.
For lattice towers, it's a lot harder. There's an enormous amount of
interaction between the members of the tower aerodynamically, so simple
approximations (like projected area) might not be accurate.
OTOH, I'll bet there's actual measured data out there. I don't have it
here, but there's a big green "Applied Fluid Dynamics Handbook" by
Blevins which is full of such empirical data - you want to know the drag
of 4" reinforcing mesh for water and air? It's in there.
I don't have it to hand, so I can't go look it up, but something like
that is a good starting point.
http://adsabs.harvard.edu/abs/1984vnr..book.....B
The article showed significant differences with respect to the coax
cables, which is not surprising. Blevin's book does have things like
the drag of two cylinders next to each other, where one cylinder is a
lot larger than the other.
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