CABLE
VIBRATION
AND DAMPING
Design, engineering and
production of structural
elements for high loads
MECHANICS OF VIBRATION
COMPLEXITIES
Carbon fibre composite rigging now dominates the marine industry. Carbon fibres low density, high strength and high stiffness
characteristics, combined with its environmental resistance and durability, contributes to both enhanced performance and safety
in a diverse and increasingly regulated industry. However, in some cases serviceability problems involving cable vibration have been
experienced and observed. Multiple sources of vibration exist aboard a sailing vessel - such as engine and generator operation,
halyard and runner load and forestay sag - but the common excitation for rigging is vortex shedding. Vortex shedding can be seen
at a much larger scale also, as a meteorological phenomenon.
Although a considerable amount of analytical and experimental research exists, there still lacks a standard approach to the concise
analytical understanding of vortex shedding. Combine the inherent complexities of a yachts mast, rigging and hull form with
external factors such as sea state, variable wind and rain, there is no standard analytical method available to calculate a solution
to the problems driven by vortex shedding. Subsequently, an interdisciplinary approach involving scientific fields such as structural
dynamics, fluid dynamics, meteorology, statistical and probabilistic methods are required.
DAMPING
What is vortex shedding?
Vortex shedding is the instance where alternating low pressure
zones are generated on the downwind side of a cable, causing
passing air to periodically detach. These alternating low pressure
zones (known as the von Karmen vortex street) can cause the
cable to move towards the low pressure, facilitating movement
perpendicular to the wind direction (see illustration below).
Mitigating vortex shedding
Although complex, mitigating the affects of vortex shedding is possible through damping (not dampening!). The aim of damping is
to reduce, restrict or prevent large amplitude oscillations - most effectively through ensuring resonant frequencies are not induced.
Damping is produced by a process of dissipating the energy stored in oscillations. Structural elements have a certain level of
inherent damping, whereby the cable itself is able to the dissipate the energy which makes it vibrate. However, it is often the case
that supplemental damping is required to increase the energy lost and reduce free oscillation time.
When the critical wind speed for a specific cable configuration
is reached, these forces can excite the cable at its resonant
frequency leading to large amplitude oscillations and deflections.
Vortex shedding in clouds
behind an island
Vortex shedding
fo: Resonant Frequency
When does vortex shedding matter?
Cable vibration is a well known phenomenon across a diverse
range of applications where low density, small diameter cables
are anchored at two ends under tension. A significant amount of
vibration goes unnoticed or is inherently damped.
Every system has a wind speed at which vortex shedding occurs.
When these variables align it encourages excitation of a cable
at its resonant frequency. The resulting oscillations can cause
a number of issues such as audible resonance, visible cable
deflection and uncomfortable vibration transferring into the
mast and/or deck.
These oscillations can lead to a rigging system experiencing
stress cycles which can induce fatigue into cable terminations.
Our rigging is designed with considerable safety parameters
against vibration, given the dynamic forces which act upon a
sailing vessel often leading to considerable cable acceleration in
turbulent wind and exaggerated sea states.
Understanding vortex shedding
Combating vortex shedding is a complex process. It is a prominent engineering consideration across all structural engineering from
street lamps to skyscrapers. Given the interrelation of multiple variables aboard a sailing vessel - such as the Reynolds number,
flow velocity and turbulence, surface roughness, cross-sectional shape and tension - excitation can be caused across a range of
resonant frequencies for a single configuration. Subsequently, evaluating and understanding the source of cable vibration requires
a spectrum of analytical tools from theoretical formulations to numerical algorithms.
ANALOGY - THINK OF IT LIKE A SWING!
Supplemental damping
Supplemental damping comes in many shapes and sizes, with no one-design option available. Design and engineering is driven by
each specific application and excitation source/s. Several measures can be considered:
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A tensioning or loosening of the member (easing or tensioning of stays),
Increasing the mass,
Increasing the damping,
Change in the aerodynamic characteristics, thus increasing the turbulence of air flow to disrupt vortex shedding
Of these alternatives, increasing the damping is the most desirable solution.
CARBO-LINK’s TUNED MASS DAMPER
Carbo-Link offers an effective counter-measure against vortex shedding by adding a supplemental damper to the exterior of the
cable. The Carbo-Link damper is a superior supplementary mass damper designed to dissipate the energy of cable oscillations. It
can be tuned to yield optimum damping across a range of identified and analysed frequencies.
The damping system is custom. The engineering is driven by analysis and modelling of your systems resonant frequencies,
measured accurately with high-tech vibration sensors. The system features a bespoke mass configuration designed to dampen the
frequencies causing the large scale oscillations. With advanced carbon casing and machined end cones, the system is designed for
aggressive cable acceleration and direct impact whilst being simple to install and aesthetically pleasing.
SYSTEMATIC APPROACH
Step 1 - Identify cable vibration - whether audible, visual and/or through mast/deck vibration
Step 2 - Visual inspection of cable oscillation whilst logging wind speed, direction, angle, the time and length of vibration
Step 3 - Install Carbo-Link vibration sensors to the cable
Step 4 - Transfer data to Carbo-Link for detailed FFT modelling and analysis to identify resonant frequencies
Step 5 - Carbo-Link engineer and manufacture a custom damper designed for your specific resonant frequencies
Pushing a person in time with the natural interval of the swing (its resonant frequency) makes the swing go higher and higher
(increasing amplitude), while pushing the swing at faster or slower tempos produces smaller arcs (decreasing amplitudes).
Step 6 - Install according to specific instructions delivered with your damper
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VIBRATION SENSORS
GLOSSARY
Amplitude
The displacement of a vibrating body from its equilibrium position
Critical damping
The minimum damping required to allow a vibrating system to return to equilibrium
Damping
Dissipation of vibration energy to reduce the amplitude of resonance and restore a cable state to its equilibrium
Damping ratio
Ratio of actual damping to critical damping at a resonance frequency
Dynamic mass
Ratio of applied force to resulting acceleration during resonance
Equilibrium
The conditions of a system in which all influences are balanced
Excitation
External force that causes a system to respond, such as critical wind speed
Frequency (f)
Number of cycles per second
Mode
Pattern of motion in which all parts of the system move sinusoidally
Natural frequency
The frequency at which a cable will vibrate when it is set into free oscillation - the specific preferential frequency
Node
A point along a standing wave where the wave has minimum amplitude
Overdamped
The system exponentially returns to equilibrium without oscillating
Oscillation
Movement back and forth in a regular rhythm
Resonance
Forced motion ‘in tune’ with a cables natural frequency driving a system to oscillate with a greater amplitude
Resonant frequency
Vibrating system or external force driving another system to oscillate with greater amplitude at a specific frequency
Sinusoid
Mathematical curve that describes a repetitive oscillation
Reynolds number (Re)
A dimensionless number used in fluid mechanics to characterize the transition between laminar and turbulent flow
Underdamped
The system oscillates at its natural frequency (reduced from undamped) with the amplitude gradually decreasing to zero
Installed sensors
Vibration sensors
INSTALLED DAMPER UNIT
RESEARCH AND DEVELOPMENT
Installed damper
We continue to develop a more in-depth understanding of the underlying mechanics of wind-induced vibration on our rigging
cables. Our aim is to enhance the general understanding of how best to practically mitigate the objectionable levels of vibration. We
are working on a model which has the capability to predict - for an arbitrary cable - the following characteristics:
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Damper
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Preferred mode: the full-scale measurements have indicated that wind-induced vibrations tend to occur in a preferred mode
over a fairly wide range of wind speeds; for a given stay, which damper design will be preferred?
Wind speed and direction: over what range of wind speeds and wind directions will the problematic vibrations occur?
Damping levels: how much damping is necessary to adequately suppress vibrations?
Amplitudes, forces, and power: what will be the steady-state amplitudes as a function of damping ratio? What levels of force
may be expected in the damper, and what will be the power dissipation demands?
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Undermuelistrasse 25
8320 | Fehraltorf
Switzerland
0041 (0) 58 201 25 00
[email protected]
www.carbo-link.com