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Retzler. C.H.

English

This thesis examines the dynamics of a long flexible horizontal circular cylinder
when immersed in water and exposed to waves. The cylinder exhibits a
flexural resonance, dependent on its stiffness, which is stimulated by particular
combinations of wave frequency and angle. Linearity is assumed and analysis
performed in the frequency domain.

Experimental work is based on a 16 metre long 125 millimetre diameter
model consisting of 40 segments with motorised joints of controllable stiffness.
The bending moment and relative angular velocity are measured at each joint.

The model is tested in a three-dimensional wave tank in which multiple wavefronts
of specified amplitude, frequency, angle and phase can be generated. The
model response to single wavefronts is displayed as an array of plots of bending
moment against distance along the cylinder axis. The shape and size of the
plots vary strongly with wave frequency and angle, and cylinder stiffness.
Two theoretical descriptions are explored. One treats the model as a finite
continuous beam, combining beam stiffness with hydrodynamic forces in an
equation which is solved analytically. The other is a more exact nodal analysis
treating each segment as a rigid body, specifying the forces and moments on it,
and solving by a matrix operation for all segments.

Both approaches require knowledge of the body hydrodynamics as a function
of frequency. This is obtained in a set of experiments using short cylinders in a
two-dimensional wave tank. Each experiment measures the wave field, the force
on the cylinder and its velocity when the cylinder is driven in the water and
acted on by waves. A matrix calculation is performed on the data to extract the
wave force coefficient and the radiation impedance in a single operation which
eliminates the masking effect of wave reflections in the tank.

When these hydrodynamic data are used with the nodal beam theory to
predict bending moments in single wavefronts there is good agreement with
experiment. The model is then tested in multiple-wavefront sea-states representative
of the North Atlantic. The results are compared with calculations for
each sea-state made by superposing the theoretical responses of the cylinder to
the component wavefronts. The agreement is good enough to allow the use of
nodal beam theory as a predictive tool

Retzler, Chris-Heinz

Edinburgh Research Archive

The dynamics of a long flexible horizontal circular cylinder in water waves

Retzler, C.H

OpenGrey Repository

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The dynamics of a long flexible horizontal circular cylinder in water waves.

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