×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Natural Frequency Equation
2

Natural Frequency Equation

Natural Frequency Equation

(OP)
I'm analyzing a flag pole for wind loads and wanted to see what the critical wind speed was for vortex shedding.  I'm using Vc = fd/s where f=natural freq, d=diameter of pole, and s=Strouhal number.  I'm assuming the pole could be modeled as a cantilevered beam with fixed end?  Also, I've found a few equations for f, all of which differ slightly.  Can anyone tell me if the following sounds correct:

w (or f) = (1.875)^2 sqrt[EI/mL^4]

Since the mass is just the mass of the pole can you just use that?  Most of the examples I've seen have an object serving as the mass, being acted by springs, and dampers, etc...

Thanks in advance!

RE: Natural Frequency Equation

(OP)
Thanks for the link....that's a great site btw!

RE: Natural Frequency Equation

hi jaydigs

your welcome

desertfox

RE: Natural Frequency Equation

(OP)
I wonder how accurate it is to consider a flagpole as a cantilevered beam?  It sort of sounds reasonable because it is fixed on on end and at 90deg to the ground, but, it isn't leaning out from a structure, and in a perfect world the only load is compressive, from it's own weight, unlike the true cantilevered beam which has to support an overhung load.     

RE: Natural Frequency Equation

Hi jaydigs

Good point, however I assume your just after the natural frequency of the beam, if thats things going to vibrate then its going to do so because of wind or some other tranverse load, in that circumstance (wind loading) it would be loaded just like a beam.

desertfox

RE: Natural Frequency Equation

(OP)
Well I reviewed about 5 different equations and kept getting 1 of 2 answers.  Then it hit me, some equations were using rad/sec and others were using Hz (Duh!)  That explains why I got around 70, and around 11, I was off by a factor of 2Pi.  I ended up getting a critical speed of 4.3 m/s.  Does this mean that at 4.3 m/s my pole is going to vibrate at it's natural freq due to vortex shedding?  The wind blows on average around 5 m/s in my area, and since we already have one of these poles mounted and haven't had any issues I wonder if I'm not missing something here?

RE: Natural Frequency Equation

Reasons why you may not have a problem -

foundation is soft so your calculated frequency is wrong.

Pole is stiff enough that the aero forces are not significant.

pole material is damped.

flag damps pole motion.

flag halyards act as a vortex suppressor (bit of a long shot)

luck.





 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources