[TowerTalk] Antenna element vibration damping - How

[Jim Lux]

At 11:19 AM 8/10/2003 +0200, Jiri Sanda wrote:
>To later responses.
>Jan - I would like to see some designs that DOES NOT vibrate at all. Until 
>now I have not seen such a beast !!!

Any round element will vibrate in the wind...simple mechanics and 
aerodynamics (shed vortices), and forms the basis of such things as aeolian 
harps. The vibration might not be noticable, or it might be quite 
significant, and in a high Q system (which many mechanical systems are) the 
vibration might build up to a significant amplitude.

This is a classic fluid mechanics problem...

f = S*U/D
f = frequency in Hz
S = Strouhal number (function of Reynolds number and cross section
U = velocity in m/sec
D = diameter in meters

Anything in aerodynamics is going to have something related to Reynolds 
numbers, so it's worthwhile to compute it so we know what regime we're in.
Re = Reynolds number = U*D/nu
U = velocity
D = Characteristic length (diameter of tubing in this case)
nu = fluid kinematic viscosity: 1.51E-5 m^2/sec @ 20C

Say you've got a 2" diameter element (0.05 meters) in a 20 m/sec (45 
mi/hr)  wind
Re = 20 * .05 / 1.51E-5 ... Re= 66E3 (scaling with size and wind speed..)

It looks like we're in the big medium Re regime (1000 to 1 million)
For 1E3<Re<1E6, S is typically in range 0.2 to 0.3,
so, for the same 2" tube in a 45 mi/hr wind, the vortex shedding frequency 
will be:

f = .2 * 20 / 0.05  = 80 to 120 Hz

Make the tube bigger and the frequency drops, slow the wind down, and the 
frequency drops

  All you can do is try to design to minimize the displacement: stiff 
members, make sure resonant frequency isn't at the vortex shedding 
frequency, make sure that you don't have multiple coupled resonators, etc.

Design Ideas:
1) Multiple sizes of tubing.  The driving frequency is determined to a 
certain extent by the diameter of the cylinder, so, by using 
tapered/stepped elements, you have less of the element trying to vibrate at 
the same frequency.

2) Choose stiffness/mass/damping such that the element is not mechanically 
resonant at a frequency which the aerodynamic forces will excite. (the rope 
inside the tube approach is a combination of mass (reducing resonant 
frequency) and damping (killing the Q of any resonance that is 
there)).  Most antenna design programs design for stiffness only (max 
droop, and yield point)

3) Use non circular cross section tubing (which may be better or worse, 
depending... aerodynamic analysis would tell)

Fixit Ideas

4) Put resonance killers on the element (like those triangular things you 
see hanging on power lines)... they disrupt the aerodynamics to keep the 
lines from "galloping"

5) Put something along the surface of the  tubing that breaks up the flow, 
preventing the thing from shedding one giant vortex all at the same 
time.  Perhaps something as simple as pieces of wire or plastic strip glued 
on in irregular intervals is sufficient.


>To kill the vibration with some rope or wire is fairly easy and does not 
>represent any significant cost or problem to put in.