ssc-367 - Ship Structure Committee

frequency (i.e. fc = fn = fv), member stiffness and mass may be
modified. The maximum amplitude of oscil1ation for the critical
velocity is an important variable, directly affecting the stress
amplitudes. The maximum amplitude of oscillation of a member
depends on member support conditions and the Ks value, reaching a
value approximately equal to member diameter for simply supported
boundary conditiens. To prevent the lock-in effect, it is desirable
to keep the member natural frequenciesto less than 70%or more than
130% of the vortex shedding frequency, whenever practical.
8.2
ANALYSES AND DESIGN FOR VORTEX SHEDDING
The interactivenature of the vortices shed and the vibration of the
cylinder makes analytical prediction of response to vortex induced
vibration (VIV) extremely difficult. Empirical formulations
(References 8.5, 8.6 and 8.7) have been developed to reflect the
state-of-the-artwith respect to VIV technology. These empirical
approaches incorporate various parameters and are based on the
comparison of specificparametricvalues with experimental results.
Empirical formulations can be effective y used to avoid VIV, but
they are less reliable at predicting the occurrence of VIV and
determining the response amplitudes.
8.2.1
Suscelltibilityto Vortex Sheddinq
Cylindrical members may experience either in-line or cross flow
oscillations for a range of flow velocity and member response
characteristicratios. To define susceptibilityof a member to VIV,
a reduced velocity (Vr) term is introduced:
v
vr=—
fnd
where: