Vibration damping

2017
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Vibration damping

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SR.NO. CONTENT PAGE NO.
1. Wind induced oscillations 2
1.1 Aeolian vibration 2
1.2 Gallop 2
1.3 Simple swinging 2
2. Types of bodies 3
2.1 Bluff/blunt bodies 3
3. Vortex shedding 5
3.1 Reasons for vortex shedding 6
3.2 Governing equation 6
4 The Reynolds number 7
4.1 Relationship with the Reynolds number 8
5. What is Aeolian vibration? 9
5.1 Effect of Aeolian vibration 10
6 Working of vibration damper 11
6.1 Stock bridge damper 11
6.2 Spiral vibration damper 12
6.3 Tuned mass damper 13
6.3.1 Working principle 13

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WIND INDUCED OSCILLATIONS
 Wind-induced vibration of overhead conductors is common worldwide and can cause conductor
fatigue near a hardware attachment.
 As the need for transmission of communication signals increase, many Optical Ground Wires
(OPWG) are replacing traditional ground wires.
 In the last twenty years All Aluminium Alloy Conductors (AAAC) has been a popular choice for
overhead conductors due to advantages in both electrical and mechanical characteristics.
Unfortunately AAAC is known to be prone to Aeolian vibration.
 Vibration dampers are widely used to control Aeolian vibration of the conductors and earth wires
including Optical Ground Wires (OPGW).
 In recent years, AAAC conductor has been a popular choice for transmission lines due to its high
electrical carrying capacity and high mechanical tension to mass ratio. The high tension to mass
ratio allows AAAC conductors to be strung at a higher tension and longer spans than traditional
ACSR (Aluminium Conductor Steel Reinforced) conductors.
 Unfortunately the self-damping of conductor decreases as tension increases. The wind power
into the conductor increases with span length. Hence AAAC conductors are likely to experience
more severe vibration than ACSR.
Wind can generate three major modes of oscillation in suspended cables:
AEOLIAN VIBRATION
Aeolian vibration (sometimes termed flutter) has amplitude of millimetres to centimetres and a
frequency of 3 to 150 Hz.
GALLOP
Gallop has amplitude measured in metres and a frequency range of 0.08 to 3 Hz
These vibrations are low frequency and high amplitudes. These vibrations are caused by the wind
blowing over conductors which are not circular. Hence, conductors are designed to be circular to
prevent gallop-type vibrations. These vibrations can result in breaking of the conductor. They can
also result in flash over if the conductors come too close to each other during oscillations.

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SIMPLE SWINGING
This kind of vibration occurs in the horizontal direction as the conductors swing under the
influence of wind. This kind of swinging does not have any major impact. However, it needs to be
ensured that the lines to not come too close to each other or the tower to cause a flash over.
 Wake-induced vibration has an amplitude of centimetres and a frequency of 0.15 to 10 Hz
TYPES OF BODIES
Bodies subjected to fluid flow are classified as being streamlined or blunt/bluff, depending on their
overall shape.

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BLUFF/BLUNT BODY
It can be defined as a body that, as a result of its shape, has separated flow over a substantial part
of its surface. Anybody which when kept in fluid flow, the fluid does not touch the whole boundary
of the object. An important feature of a bluff body flow is that there is a very strong interaction
between the viscous and inviscid regions.
When the flow separates from the surface and the wake is formed, the pressure recovery is not
complete. The larger the wake, the smaller is the pressure recovery and the greater the pressure
drag. The art of streamlining a body lies, therefore, in shaping its contour so that separation, and
hence the wake, is eliminated, or at least in confining the separation to a small rear part of the body
and, thus, keeping the wake as small as possible. Such bodies are known as streamlined bodies.
Otherwise a body is referred to as bluff and a significant pressure drag is associated with it.
 Cylinders and spheres are considered bluff bodies because at large Reynolds numbers the drag is
dominated by the pressure losses in the wake.
Therefore, when the drag is dominated by a frictional component, the body is called a streamlined
body; whereas in the case of dominant pressure drag, the body is called a bluff body.
A body is said to be streamlined if a conscious effort is made to align its shape with the anticipated
streamlines in the flow. Streamlined bodies such as race cars and airplanes appear to be contoured
and sleek. Otherwise, a body (such as a building) tends to block the flow and is said to be bluff or
blunt. Usually it is much easier to force a streamlined body through a fluid, and thus streamlining
has been of great importance in the design of vehicles and airplanes.

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VORTEX SHEDDING
Vortex shedding is the process by which vortices formed continuously by the aerodynamic
conditions associated with a solid body in gas or air is carried downstream by the flow in the form
of a vortex street.
A vortex street is a regular stream of vortices or parallel streams of vortices carried downstream by
the flow of a fluid over a body. These are sometimes made visible by vapour condensation as in the
vortex trails from the wing tips of an aeroplane.
Vortex shedding causes wind induced vibration and it occurs close to the resonance wind velocity
of the structure. It causes the structure to resonate resulting in large forces and deflections which
can result in fatigue failure.
When the wind blows across a slender prismatic or cylindrical structure, rhythmic vortices are shed
alternatively from opposite sides causing alternating low pressure zones. Consequently the
structure is acted upon by a load perpendicular to its length and perpendicular to the wind
direction. These alternating low pressure zones can cause a structure to move towards the low
pressure zone causing movement perpendicular to the direction of the wind. When the critical wind
speed is reached, these forces can cause the structure to resonate causing large forces and
deflections to occur if the structure is not heavily damped. This occurs at wind velocities close to
the resonance wind velocity which is calculated using the dimensions of the structure and the
Strouhal number.
If the bluff structure is not mounted rigidly and the frequency of vortex shedding matches
the resonance frequency of the structure, the structure can begin to resonate, vibrating
with harmonic oscillations driven by the energy of the flow. This vibration is the cause for overhead
power line wires "singing in the wind",

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REASONS FOR VORTEX SHEDDING
It is the inherent non uniformity in the inlet flow that triggers the vortex shedding; this is the answer
to why shedding happens first from top or bottom of cylinder.
but if the inlet flow specified is uniform, without turbulence and the grid generated is also symmetric
about, say, horizontal line passing through centre of cylinder, then in that case round off errors play a
role.
A non-zero curvature perturbation of "dividing" streamline will lead to a slight centrifugal force which
in turn leads to a change in pressure thereby amplifying the ripple and stronger and stronger
amplification and shedding the vortices.
GOVERNING EQUATION
The frequency at which vortex shedding takes place for an infinite cylinder is related to the
Strouhal number by the following equation:
Where
Sr - the Strouhal number
f - the vortex shedding frequency
L - the diameter of the cylinder
V - The flow velocity.
 The Strouhal number depends on the body shape and on the Reynolds number.
 For large Strouhal numbers (order of 1), viscosity dominates fluid flow, resulting in a collective

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oscillating movement of the fluid "plug". For low Strouhal numbers (order of 10−4
and below), the
high-speed, quasi steady state portion of the movement dominates the oscillation. Oscillation at
intermediate Strouhal numbers is characterized by the build-up and rapidly subsequent shedding
of vortices.
 For spheres in uniform flow in the Reynolds number range of 8x102
< Re < 2x105
there co-exist two
values of the Strouhal number. The lower frequency is attributed to the large-scale instability of
the wake and is independent of the Reynolds number Re and is approximately equal to 0.2. The
higher frequency Strouhal number is caused by small-scale instabilities from the separation of the
shear layer.
THE REYNOLDS NUMBER
It is the ratio of inertial forces to viscous forces within a fluid which is subject to relative internal
movement due to different fluid velocities, in what is known as a boundary layer in the case of a
bounding surface such as the interior of a pipe.
A similar effect is created by the introduction of a stream of higher velocity fluid, such as the hot
gases from a flame in air. This relative movement generates fluid friction, which is a factor in
developing turbulent flow. Counteracting this effect is the viscosity of the fluid, which as it
increases, progressively inhibits turbulence, as more kinetic energy is absorbed by a more viscous
fluid. The Reynolds number quantifies the relative importance of these two types of forces for given
flow conditions, and is a guide to when turbulent flow will occur in a particular situation.

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With respect to laminar and turbulent flow regimes:
 laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is
characterized by smooth, constant fluid motion;
 Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend
to produce chaotic eddies, vortices and other flow instabilities.
RELATIONSHIP WITH THE REYNOLDS NUMBER
The type of flow occurring in a fluid in a channel is important in fluid dynamics problems and
subsequently affects heat and mass transfer in fluid systems. The dimensionless Reynolds
number is an important parameter in the equations that describe whether fully developed flow
conditions lead to laminar or turbulent flow.
The Reynolds number is the ratio of the inertial force to the shearing force of the fluid—how fast
the fluid is moving relative to how viscous the fluid is, irrespective of the scale of the fluid system.
Laminar flow generally occurs when the fluid is moving slowly or the fluid is very viscous. As the
Reynolds number increases, such as by increasing the flow rate of the fluid, the flow will transition
from laminar to turbulent flow at a specific range of Reynolds numbers, the laminar-turbulent
transition range depending on small disturbance levels in the fluid or imperfections in the flow
system. If the Reynolds number is very small, much less than 1, then the fluid will exhibit Stokes or
creeping flow, where the viscous forces of the fluid dominate the inertial forces.
The specific calculation of the Reynolds number, and the values where laminar flow occurs, will
depend on the geometry of the flow system and flow pattern.
For such systems, laminar flow occurs when the Reynolds number is below a critical value of
approximately 2,040, though the transition range is typically between 1,800 and 2,100.[4]
For fluid systems occurring on external surfaces, such as flow past objects suspended in the fluid,

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other definitions for Reynolds numbers can be used to predict the type of flow around the object.
The particle Reynolds number Rep would be used for particle suspended in flowing fluids, for
example. As with flow in pipes, laminar flow typically occurs with lower Reynolds numbers, while
turbulent flow and related phenomena, such as vortex shedding, occur with higher Reynolds
numbers.
The onset of turbulence can be predicted by the Reynolds number, This ability to predict the onset
of turbulent flow is an important design tool for equipment such as piping systems or aircraft wings,
but the Reynolds number is also used in scaling of fluid dynamics problems, and is used to
determine dynamic similitude between two different cases of fluid flow, such as between a model
aircraft, and its full size version. Such scaling is not linear and the application of Reynolds numbers
to both situations allows scaling factors to be developed. A flow situation in which the kinetic
energy is significantly absorbed due to the action of fluid molecular viscosity gives rise to a laminar
flow regime. For this the dimensionless quantity the Reynolds number (Re) is used as a guide.
With respect to laminar and turbulent flow regimes:
 laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is
characterized by smooth, constant fluid motion;
 Turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend
to produce chaotic eddies, vortices and other flow instabilities.
 While there is no theorem directly relating the non-dimensional Reynolds number to turbulence,
flows at Reynolds numbers larger than 5000 are typically (but not necessarily) turbulent, while
those at low Reynolds numbers usually remain laminar. In Poiseuille flow, for example, turbulence
can first be sustained if the Reynolds number is larger than a critical value of about 2040;
moreover, the turbulence is generally interspersed with laminar flow until a larger Reynolds
number of about 4000.
 The transition occurs if the size of the object is gradually increased, or the viscosity of the fluid is
decreased, or if the density of the fluid is increased.
WHAT IS AEOLIAN VIBRATION?
 Wind-induced vibration or Aeolian vibration of transmission line conductors is a common
phenomenon under smooth wind conditions. The cause of vibration is that the vortexes shed
alternatively from the top and bottom of the conductor at the leeward side of the conductor.
 The vortex shedding action creates an alternating pressure imbalance, inducing the conductor to
move up and down at right angles to the direction of airflow.
 The conductor vibration results in cyclic bending of the conductor near hardware attachments,
such as suspension clamps and consequently causes conductor fatigue and strand breakage.
 When a “smooth” stream of air passes across a cylindrical shape, such as a conductor or OHSW,
vortices (eddies) are formed on the back side. These vortices alternate from the top and bottom
surfaces, and create alternating pressures that tend to produce movement at right angles to the
direction of the air flow. This is the mechanism that causes Aeolian vibration.
 The term “smooth” was used in the above description because unsmooth air (i.e., air with
turbulence) will not generate the vortices and associated pressures. The degree of turbulence in
the wind is affected both by the terrain over which it passes and the wind velocity itself.
 It is for these reasons that Aeolian vibration is generally produced by wind velocities below 15
miles per hour (MPH). Winds higher than 15 MPH usually contain a considerable amount of
turbulence, except for special cases such as open bodies of water or canyons where the effect of
the terrain is minimal.
 The frequency at which the vortices alternate from the top to bottom surfaces of conductors
and shield wires can be closely approximated by the following relationship that is based on the
Strouhal Number.

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 Vortex Frequency (Hertz) = 3.26 V / d
 Where: V is the wind velocity component normal to the conductor or OHSW in miles per hour
 d is the conductor or OHSW diameter in inches
 3.26 is an empirical aerodynamic constant.
 One thing that is clear from the above equation is that the frequency at which the vortices
alternate is inversely proportional to the diameter of the conductor or OHSW.
 The self-damping characteristics of a conductor or OHSW are basically related to the freedom of
movement or “looseness” between the individual strands or layers of the overall construction.
 In standard conductors the freedom of movement (self-damping) will be reduced as the tension
is increased. It is for this reason that vibration activity is most severe in the coldest months of
the year when the tensions are the highest.
 Aeolian vibrations mostly occur at steady wind velocities from 1 to 7 m/s. With increasing wind
turbulence the wind power input to the conductor will decrease. The intensity to induce
vibrations depends on several parameters such as type of conductors and clamps, tension, span
length, topography in the surrounding, height and direction of the line as well as the frequency
of occurrence of the vibration induced wind streams.
 Hence the smaller the conductor, the higher the frequency ranges of vibration of the conductor.
The vibration damper should meet the requirement of frequency or wind velocity range and also
have mechanical impedance closely matched to that of the conductor. The vibration dampers
also need to be installed at suitable positions to ensure effectiveness across the frequency
range.
EFFECT OF AEOLIAN VIBRATION
 It should be understood that the existence of Aeolian vibration on a transmission or distribution
line doesn’t necessarily constitute a problem. However, if the magnitude of the vibration is high
enough, damage in the form of abrasion or fatigue failures will generally occur over a period of
time.
 Abrasion is the wearing away of the surface of a conductor or OHSW and is generally associated
with loose connections between the conductor or OHSW and attachment hardware or other
conductor fittings.
 Abrasion damage can occur within the span itself at spacers Fatigue failures are the direct result
of bending a material back and forth a sufficient amount over a sufficient number of cycles.
 In the case of a conductor or OHSW being subjected to Aeolian vibration, the maximum bending
stresses occur at locations where the conductor or OHSW is being restrained from movement.
Such restraint can occur in the span at the edge of clamps of spacers, spacer dampers and Stock
bridge type dampers.
 However, the level of restraint, and therefore the level of bending stresses, is generally highest at
the supporting structures.
 When the bending stresses in a conductor or OHSW due to Aeolian vibration exceed the
endurance limit, fatigue failures will occur.

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 In a circular cross-section, such as a conductor or OHSW, the bending stress is zero at the center
and increases to the maximum at the top and bottom surfaces (assuming the bending is about
the horizontal axis). This means that the strands in the outer layer will be subjected to the
highest level of bending stress and will logically be the first to fail in fatigue.
WORKING OF VIBRATION DAMPER
When a vibration wave passes the damper location, the clamp of a suspension type damper
oscillates up and down, causing flexure of the damper cable and creating relative motion between
the damper clamp and damper weights.
Stored energy from the vibration wave is dissipated to the damper in the form of heat. For a damper
to be effective, its response characteristics should be consistent with the frequencies of the
conductor on which it is installed.
 When the damper is placed on a vibrating conductor, movement of the weights will produce
bending of the steel strand. The bending of the strand causes the individual wires of the strand
to rub together, thus dissipating energy. The size and shape of the weights and the overall
geometry of the damper influence the amount of energy that will be dissipated for specific
vibration frequencies.
 Since, as presented earlier, a span of tensioned conductor will vibrate at a number of different
resonant frequencies under the influence of a range of wind velocities, an effective damper
design must have the proper response over the range of frequencies expected for a specific
conductor and span parameters.
STOCK BRIDGE DAMPER
 Some dampers, such as the VORTX Damper utilize two different weights and an asymmetric
placement on the strand to provide the broadest effective frequency range possible.
 The “Stockbridge” type vibration damper is commonly used to control vibration of overhead
conductors and OPGW. The vibration damper has a length of steel messenger cable. Two metallic
weights are attached to the ends of the messenger cable. The centre clamp, which is attached to
the messenger cable, is used to install the vibration damper onto the overhead conductor.
 Placement programs, such as those developed by PLP for the VORTX Damper, take into account
span and terrain conditions, suspension types, conductor self-damping, and other factors to
provide a specific location in the span where the damper or dampers will be most effective.

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 The asymmetrical vibration damper is multi resonance system with inherent damping. The
vibration energy is dissipated through inter-strand friction of the messenger cable around the
resonance frequencies of the vibration damper. By increasing the number of resonances of the
damper using asymmetrical design and increasing the damping capacity of the messenger cable
the vibration damper is effective in reducing vibration over a wide frequency or wind velocity
range.
Vibration dampers effectively prevent fatigue damage to conductor and static wires caused by
SPIRAL VIBRATION DAMPER
 For smaller diameter conductors (< 0.75”), overhead shield wires, and optical ground wires
(OPGW), a different type of damper is available that is generally more effective than a
Stockbridge type damper.
 The Spiral Vibration Damper has been used successfully for over 35 years to control Aeolian
vibration on these smaller sizes of conductors and wires.
 The Spiral Vibration Damper is an “impact” type damper made of a rugged non-metallic material
that has a tight helix on one end that grips the conductor or wire. The remaining helixes have an
inner diameter that is larger than the conductor or wire, such that they impact during Aeolian
vibration activity. The impact pulses from the damper disrupt and negate the motion produced
by the wind.

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TUNED MASS DAMPERS
Tuned mass dampers (also called vibration absorbers or vibration dampers) reduce low and high frequency
vibrations in machines, systems and structures. The function of a damper is based on a spring/mass
system, which counteracts and reduces extraneous vibrations. The weight of the damper depends on the
weight of the system being damped.
The dampers are set to the required frequencies, although they can still be adjusted on site to the
prevailing conditions.
WORKING PRINCIPLE
Unwanted resonance vibrations are broken down into two individual vibrations. These peaks are damped
simultaneously, so that only two vibrations with a small deflection remain.

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