Vibration damping

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2017 www.youtube.com/c/brainamplifier www.facebook.com/brainamplifier Vibration damping

Transcript of 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 < 2x105there 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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