Antenna Dia 19.0mm x 720mm Long Wind Vibration 1

[paulmech01]

I have a Car Mounted Antenna Dia 19.0mm x 1mm Wall x 720mm Long which creates a vibration at approx. 80km/h. The Antenna has a fibreglass thin walled structure. I believe the vibration is due to the natural frequency of the material.

Can the velocity be calculated to determine critical speed where natural frequency affected?

Can this be designed out ie. longer/shorter length, increase in stiffness, etc...?

 

[GregLocock]

A more likely excitation is vortex shedding.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[paulmech01]

GregLocock,

OK, so it is vertex shedding behind the antenna. This only occurs at 80km/h and up. How can this be designed out.

 

[GregLocock]

Add a spiral rail up the outside like factory chimneys have.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[paulmech01]

That was my first thoought, but I don;t think that marketing want this look, and this will increase drag.

How can the frequency and vortex shedding be calculated?

 

[GregLocock]

http://www.pcnet.com/~jdutka/thesis/thesis.html

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[paulmech01]

I had a quick look at this thesis, it pretty full on.

Hopefully I can relate the frequency back to Velocity and might nearly be there.

Also relation of phsical properties and there effect.

 

[paulmech01]

Is there somewhere I can get a worked example of calulating a parts natural frequency, and then its vortex induced vibration frequency.

 

[fredt]

Vortex shedding is the excitation, the speed at which the vibration occurs is when the excitation frequency matches the resonant frequency of the antenna. Changing either characteristic will alter the speed when the problem occurs EG using a flexible rubber mounting that will also have some damping.

I like the idea of the helical vortex breaker. A wire wound around the OD in a helical form could do the trick and would look OK.