Author:
HOVER F. S.,TECHET A. H.,TRIANTAFYLLOU M. S.
Abstract
Forces are measured at both ends of rigid cylinders with span 60
cm, performing
transverse oscillations within an oncoming stream of water, at Reynolds
number
Re≈3800. Forced harmonic motions and free vibrations of uniform
and tapered
cylinders are studied. To study free motions, a novel force-feedback control
system
has been developed, consisting of: (a) a force transducer, which
measures forces on
a section of a cylinder moving forward at constant speed; (b)
a computer using the
measured force signal to drive in real time a numerical simulation of an
equivalent
mass-dashpot-spring system; (c) a servomotor and linear table
which impose, also in
real time, the numerically calculated motion on the cylinder section. The
apparatus
allows very low equivalent system damping and strict control of the parametric
values
and structure of the equivalent system.Calculation of the cross-correlation coefficient between forces at the
two ends of the
uniform cylinder reveals five distinct regimes as a function of the nominal
reduced
velocity Vrn: two regimes, for low and high
values of
Vrn, and far away from the
value of VrS corresponding to the Strouhal
frequency,
show small correlation; two regimes immediately adjacent to, but excluding,
VrS show strong correlation, close
to 1; surprisingly, there is a regime containing the Strouhal frequency,
within which
correlation is low. Free vibrations with a 40[ratio ]1 tapered cylinder
show that the regime
of low correlation, containing the Strouhal frequency, stretches to higher
reduced
velocities, while lock-in starts at lower reduced velocities.When comparing the amplitude and phase of the lift coefficient measured
for free
and then for forced vibrations, we obtain close agreement, both for tapered
and
uniform cylinders. When comparing the cross-correlation coefficient, however,
we find
that it is much higher in the forced oscillations, especially for the uniform
cylinder.
Hence, although the force magnitude and phase may be replicated well in
forced
vibrations, the correlation data suggest that differences exist between
free and forced
vibration cases.
Publisher
Cambridge University Press (CUP)
Subject
Mechanical Engineering,Mechanics of Materials,Condensed Matter Physics
Cited by
153 articles.
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