Natural Frequency / Vortex Shedding

[TTUengr51]

I'm currently working on a 50 m tall steel tower structure that has a slender footprint of 3.8 m x 3.8 m square. The structure is made up of tube steel columns (one at each corner and one at the mid point of each face - 8 columns total) with inverted chevron bracing. A wind study has been performed with some generic assumptions by an outside consultant based upon the geometry of the structure only.

My question is in regards to determining the natural frequency, which in turn will decide whether or not vortex shedding will control the design. The wind consultant has assumed a natural frequency of 0.7 Hz. I'm hoping to find some information so that I will be able to calculate the natural frequency myself so that I am able to better verify the wind consultants numbers. Any help would be greatly appreciated!!!

 

[btrueblood]

http://naca.central.cranfield.ac.uk/reports/1954/naca-report-1191.pdf

Basically, the Strouhal number for vortex shedding past a cylinder is approx. 0.18-0.22 for Reynold's numbers of 100 thru 100,000.

Strouhal no. = nd/V, where n is freq. of shedding in Hz, d is diameter of cylinder, and V is fluid velocity.

Reynold's no. = Vd/nu, where V is velocity, d is diameter, and nu is kinematic viscosity of fluid (units of m^2/sec).

 

[btrueblood]

Hmm. I gave that reference, but it only shows data for up to Re=20,000. My old fluid dynamics textbook, Engineering Fluid Mechanics, Roberson & Crowe, Houghton-Mifflin Co., 1980, gives a plot of St vs. Re from 100 to 10^7 (from which I got my 0.18-0.22 value), and references the above report, along with the following:

Jones, G. W., Jr., "Unsteady lift forces generated by vortex shedding about a large, stationary, and oscillating cylinder at high Reynolds numbers.", Symp. Unsteady Flow, ASME (1968).