Civil Engineering Reference
In-Depth Information
i
Vf D t
/
=
(5.32)
Ri
Thus, there is a resonance velocity for every eigen-frequency associated with vibrations
in the across wind direction or in torsion. Experiments show that when resonance occurs
the flow and the oscillating structure will interact, and for a certain range of ensuing
wind velocity settings
f
will deviate from Eq. 5.31 and stay equal or close to
f
, as
shown on the upper right hand side of Fig. 5.6. This is what is usually called lock-in.
Such vortex shedding induced interaction is accompanied by two important load effects.
At lock-in the fluctuating load becomes better correlated in the span-wise direction, but
what is more important is that a significant motion induced part is added. However, these
effects are self-destructive in the sense that they diminish when fluctuating structural
displacements become large. Thus, vortex shedding induced vibrations are self-limiting,
as illustrated on the diagram on the lower right hand side in Fig. 5.6.
Fig. 5.6
Response characteristics associated with vortex shedding
