CASO DI UNA SEZIONE CIRCOLARE PROFILO DEI VORTICI DI KARMAN

	Il vortice a valle di un cilindro causa alte vibrazioni quando la frequenza del vortice risulta prossima a una delle frequenze modali del cilindro.
	Si generano aree alternativamente in pressione e in depressione….nella direzione trasversale alla direzione del vento.Come risultato il cilindro comincia a oscillare nella direzione trasversale alla direzione del vento. Le cylindre est sollicité alternativement dans des directions perpendiculaires à la direction du vent et se met, de ce fait, à vibrer perpendiculairement à la direction du vent.
	Esempio di un vortice nel flusso a valle di un cilindro.

Caso di una travatura da ponte

PROPOSED SOLUTIONS

A structure is vibrating when the energy resulting of from the vortex cannot be absorbed by the structural damping of the structure. Two solutions are possible to avoid resonance :

• avoid the vortex by adding special devices to break them
• to increase the structural damping by adding dampers

FIRST SOLUTION :

Adding a set of helical strakes on a stack
Avantages
The helical strakes are breaking the vortex . This solution could be adopted on cylinder structure if the Scruton number is bigger than 8.
In some rare cases they provide the most economical solution.

Disavantages
Double the wind load on the upper 13 of the stack, increasing stack weight significantly.
Nearly always more expensive than a vibration damper.
Complex geometry make fabrication difficult.
Can interfere with other attachments ( ie.ladders,platforms, etc.).
Foundation and Anchor Bolts usually increase in size.
Do not work in the case of interfering stacks.
Some codes do not allow their use for Scruton Numbers< 8 (which cover most stacks with vibration issues).
Conclusion
Only of interest for small stack with small diameters. In all other cases a solution with a damper proves to be more economical.

SECOND SOLUTION :

Installing a vibration Damper
A vibration damper absorbs the energy due to the vibration
and to make the vibration phenomena disappear

There is no universal damper to solve all vibration problems. Case by case taking into account local conditions (acceleration, amplitude, frequency, space.) will advise you of a given type of damper. dampers are used to suppress horizontal or vertical vibrations. Torsion vibration problems may be solved by the coupling of two or more vertical dampers.

Placing a damper is an elegant modern solution, which has no disadvantage. This solution is always cheaper than the solution with strakes, except on rare occasions when the stack is small and with a small diameter.

Different types of damper could be used. There is no universal damper :each one has its advantages and disadvantages. On the following pagesyou can see different types of dampers.On bridges, the phenomenon ismore complicated than on cylindrical slender structures and horizontaldampers, vertical dampers and sometimes rotation dampers could berequested.