ssc-367 - Ship Structure Committee

0.4.1 In-LineVortex Sheddinq Amplitudes
The reduced velocity and the amplitude of vibrations shown on Figures
D-1 and D-2, respectively, as functions of stability parameter are
based on experimental data. The experimental data obtained are for
the cantilever mode of deflection for in-line and cross-flow
vibrations.
Sarpkaya (Reference D.1O) carried out tests on both oscillatory flow
and uniform flow and observed smaller amplitudes of vibration for the
oscillatory flow than for the uniform flow. It is also suggested by
King (Reference 0.1) that the maximum amplitude for an oscillatory
flow is likely to occur at a Vr value in excess of 1.5 (as opposed to
1.0 assumed by DnV) and that an oscillation build-up of about 15
cycles is required before “lock-in” maximum-amplitude vibration
occurs. In light of this evidence, the amplitude of vibrations shown
in Figure D-2 is based on Hallam et al (Reference D.2) rather than
the DnV (ReferenceD.8).
Since typical marine structure members have stability parameters (Ks)
in excess of 1.8, in-line vibrations of these members in air are
unlikely.
D.4.2
Cross-flow Vortex Shedding Amplitudes
The amplitude of the induced vibrations that accompanies cross-flow
vibration are generally large and creates very high stresses.
Therefore, it is desirable to preclude cross-flow induced
vibrations. Figure D-4 illustrates a curve defining the amplitude of
response for cross-flow vibrations due to current flow and based on a
cantilevermode of deflection.
Cross-flowoscillations in air may not be always avoidable, requiring
the members to have sufficient resistance. The DnV procedure
(Reference 0.8) to determine the oscillation amplitudes is derived
from a simplified approach applicable to vortex shedding due to
D-lo