Energy generation from vortex induced vibrations report

Energy Generation from Vortex Induced Vibrations
1. Introduction
The issue of global climate change and the growing energy demand induce a need
for innovative energy harvesting devices. Geophysical flows represent a widely
available source of clean energy, useful to tackle the global energy demand using for
example wind turbines, marine turbines or wave energy converters. Yet, the energy
density in geophysical flows is small, and large systems are required in order to
harvest significant amount of energy.
The turbine generator is the most mature method for flow energy harvesting.
However, the efficiency of conventional turbines reduces with their sizes due to the
increased effect of friction losses in the bearings and the reduced surface area of the
blades. Furthermore, rotating components such as bearings suffer from fatigue and
wear, especially when miniaturised. These drawbacks of turbine generators urges
emergence of a new area in energy harvesting, i.e. energy harvesting from flow
induced vibration. The flow here includes both liquid flow and air flow. There are
three main types of energy harvester of this kind. They are energy harvesting from
vortex-induced vibration (VIV), flutter energy harvesters and energy harvesters with
Helmholtz resonators.
Flow-induced vibration, as a discipline, is very important in our daily life,
especially in mechanical engineering. Generally, scientists try to avoid flow-induced
vibration in buildings and structures to reduce possible damage. Recently, such
vibration has been investigated as an energy source that can be used to generate
electrical energy. Two types of flow-induced vibration are studied so far: vortex-induced
vibration and flutter
Dept of mechanical Engg. SJCET, Palai Page 1

2. Vortex Induced Vibrations
The non-linear resonance phenomenon known as Vortex-Induced Vibration (VIV)
has much relevance in several branches of mechanical engineering. For example, it
can be observed in civil structures, like slender chimneys stacks, tall buildings,
electric power lines or bridges, to name a few. It is also usual in offshore structures or
in the tubes of heat exchange devices. Because its practical and scientific interest,
VIV has lead to a large number of fundamental studies. Usually, VIV is considered as
an undesirable effect, as it may seriously affect the structural integrity or the
reliability of performance, but along this report we will see that if the vibration is
substantial, it can be used to extract useful energy from the surrounding flow
An original way to extract energy from these flows is to take advantage of flow-induced
vibrations, [2]. For instance, several devices based on fluid-elastic
instabilities like transverse galloping or flutter have already been introduced. [2–3].
Another kind of flow-induced oscillations that can be useful to harvest energy from a
flow is the vortex-induced vibrations (VIV) of a bluff body [1,2].The model is
presented and the generic case of energy extraction using VIV of an elastically-mounted
short rigid cylinder is analyzed.
3. Principle
When a fluid flows toward the leading edge of a bluff body, the pressure in the fluid
rises from the free steam pressure to the stagnation pressure. When the flow speed is
low, i.e. the Reynolds number is low, pressure on both sides of the bluff body remains
symmetric and no turbulence appears. When the flow speed is increased to a critical
value, pressure on both sides of the bluff body becomes unstable, which causes a
regular pattern of vortices, called vortex street or Kármán vortex street. Certain
transduction mechanisms can be employed where vortices happen and thus energy
can be extracted [3] . This method is suitable both air flow and liquid flow.

Figure 1 : Flow around a bluff body
3.1 Physics Theory
Vortex shedding is a widely occurring phenomenon applicable to nearly any
bluff (non streamlined) body submerged in a fluid flow. Since any real fluid flow is
viscous, there will be a significant boundary layer on the bodies’ surface for all but
the lowest Reynolds number flows. At some point along the bodies’ surface,
separation of the boundary layer will occur, depending on the exact surface geometry.
This separated layer, which bounds the wake and free stream, will tend to cause fluid
rotation, since its outer side, in contact with the free stream, moves faster than its
inner side, in contact with the wake. It is this rotation which then results in the
formation of individual vortices, which are then shed from the rear of the body and
travel down the wake. Typically, a pattern of periodic, alternating vortex shedding
will occur in the flow behind the body, which is referred to as a vortex street.
Depending on the characteristics of the flow, mainly the Reynolds number, different
types of vortex streets may form, which will be discussed later in more detail. When
the pattern of shed vortices is not symmetrical about the body, which is the case in
any vortex street, an irregular pressure distribution is formed on the upper and lower
sides of the body, which results in a net lift force perpendicular to the flow direction.
Since the vortices are shed in a periodic manner, the resulting lift forces on the body
also vary periodically with time, and there for can induce oscillatory motion of the
body. This occurrence alone would qualify as vortex induced vibration; however,
there is a more interesting and important phenomenon, similar to linear resonance,
which can occur when the frequency of vortex shedding (fs) is close to the natural
frequency of the body in motion, (fn). In this phenomenon, referred to as “lock in”,
the vortex shedding frequency actually shifts to match the bodies’ natural frequency,
and as a result, much larger amplitudes of vibration can occur. It is this particular
aspect of vortex induced vibration, lock in, which has traditionally been of greatest

concern to structural engineers, since it poses the greatest risk of damage or failure.
Accordingly, the range of shedding frequencies which lock in can occur over is one of
the most important research areas within vortex induced vibration.
3.1.1 Vortex Shedding
Like many fluid flow phenomenon, vortex shedding has been observed to be directly
dependent on the Reynolds number of the flow, which is defined in Eq. 2-1.
Re = (U*D)/υ ........Eq. 2-1
U is the free stream velocity, D is the cylinder diameter, and υ is the kinematic
viscosity of the fluid. As a note, most studies in literature were in fact performed
using a submerged cylinder, which is the geometry later used in the experimental
methodology, so the correlation length of cylinder diameter used in Re is appropriate
and widely applicable, as many submerge structures are typically cylindrical in shape.
Figure 2 : Formation of vortices for various Reynolds number

3.1.2 Strouhal Number
An additional non-dimensional parameter has been established to relate the
frequency of vortex shedding (fs) to the flow conditions. This is given by the
Strouhal number S, and is defined in Eq. 2-2.
S = D *(fs) /U ........Eq. 2-2
Again, U is the free stream velocity, and D is the cylinder diameter. For a wide
range of Reynolds number, the Strouhal number varies very little, and can
essentially be taken as constant, as seen in Figure 3.
Figure 3 : Reynolds number and Strouhal number relationship
3.1.3 Lock In
As introduced earlier, lock in is a particular aspect of VIV which can result in
relatively large amplitudes of forced vibration. An analytical theory of lock in
based on first principles does not presently exist, and much of the research

encountered only gives descriptive or semi empirical evidence. As a result, the
present analysis only focuses on the key findings which are relevant to achieving
large amplitude vibrations, for the purpose of energy generation. Lock in is similar
to linear resonance in that the vibration amplitudes increase as the natural
frequency of the cylinder is approached by the vortex shedding frequency.
However, the analogy stops here, as lock in is a highly non-linear phenomenon,
affected by feedback loops referred to as fluid structure interaction. Additionally,
lock in does not result in the classic large amplitude spike at exactly the natural
frequency, as in linear resonance. Instead, lock in has been described as both a
self-limiting and self-governing occurrence, as the cylinder vibrations themselves
effect the vortex shedding process, and vice versa. It is self-limiting in the sense
that as the cylinder displacement increases, the vortex shedding is weakened, and
hence tends toreduce further motion.
3.1.4 Boundary Gap
Another modeling constraint affecting the oscillation of the cylinder is the
boundary gap ratio. The gap ratio is equal to the minimum distance between the
cylinder and lower flow surface boundary divided by the diameter of the cylinder.
The coefficient of viscous drag and lift coefficient were directly related to the gap
ratio. As the gap ratio increases, viscous drag decreases and lift increases. This is
due to the effect of the gap ratio on vortex shedding. When the cylinder is in close
proximity to the flow surface boundary, flow over the cylinder is uneven. Normal
vortex shedding patterns are weakened or disrupted completely. It was found that,
for a boundary gap value of about 3.0 or greater, the effect of the boundary gap on
vortex shedding was negligible. To calculate an appropriate gap distance for a
1.25” diameter cylinder, as will be used in the test apparatus, multiply the cylinder
diameter by three: 3*1.25” = 3.75”. This yields a gap ratio of 3, rendering the
effects of the boundary on vortex shedding negligible.
4. Energy Harvesting In Liquid Flow
The energy harvester based on Kármán vortex street is shown in the
“Electromagnetic energy harvesting from vibrations induced by Karman vortex street
“ (Dung-An Wang , Chun-Yuan Chiu, Huy-Tuan Pham)[3]. One approach to harvest

energy is to convert mechanical energy of ambient vibration into electrical energy by
electromagnetic induction. Electromagnetic harvesters have been proposed and
investigated by many researchers. Electromagnetic energy-harvesting device based on
vibration induced by Karman vortex Street is illustrated in figure 4(a), a flow channel
with a flexible diaphragm is connected to a flow source. A permanent magnet is glued
to a bulge on top of the diaphragm and a coil is placed above the magnet. The
pressure fluctuation due to vortex shedding from a bluff body drives the diaphragm
into vibration. As shown in Fig 4(b). the increase of the pressure causes the
diaphragm to deflect in the upward direction. As the pressure increases to the
maximum, the diaphragm reaches its highest position. When the pressure drops, the
diaphragm moves downward shown in Fig 4(c). As the pressure decreases to the
minimum, the diaphragm reaches its lowest position .Thus, by connecting the energy
harvester to a flow source, the oscillating movement of the diaphragm with an
attached magnet under a coil makes the energy harvesting possible.

Figure 4: Flutter energy harvesting
5. Vortex Induced Vibration Aquatic Clean Energy (VIVACE)
The Vortex Induced Vibration Aquatic Clean Energy converter design was
patented in 2008 by Professor Michael Bernitsas of the University of Michigan. The
converter harnesses energy from water flow using vortex induced vibrations. The
VIVACE system is composed of a cylinder secured horizontally in a stationary frame
and allowed to oscillate transverse to the direction of water flow. The cylinder is
connected to the frame at the ends of the cylinder, where magnetic sliders move up
and down over a rail containing a coil. The motion of the magnet over the coil creates
a DC current, which can be stored or converted to AC to be sent into the grid. This
technology is superior to dam technology in several ways. It is capable of producing
energy from fluid flow without altering the local environment, posing any danger to
nearby residents, changing the landscape in any visible way, or interfering with water
traffic in any slow moving waterway (0.5-5 knots). Energy generation from VIV has
significant potential for coastal areas as well. Energy demand in coastal regions is
much larger than demand inland. Scalability and versatility are two of the greatest
strengths of this technology. Modules can range in size from single-cylinder arrays to
thousand-cylinder, mega-watt producing power plants. In their initial report, Bernitsas
et al. outline array specifications for 1kW to 1000MW cylinder arrays. Areas of

potential power production include ocean water bodies and rivers. Flow in the prime
production speeds required for this technology is significantly lower than for other
turbine based hydrokinetic technologies.
According to Bernitsas, VIVACE has superior energy density compared with
other nonturbine ocean energy technologies. As of August 2010, Bernitsas’ start-up
company, Vortex Hydro Energy, has begun open water tests in the St. Clair River in
Port Huron, MI
Figure 5: Cylinder arrangement in VIVACE

5.1. Physical Model
A simple schematic of a single module of the VIVACE Converter considered
in this paper is depicted in Figure 6. The elements of this module are: a circular rigid
cylinder of diameter D and length L, two supporting linear springs each of stiffness
k/2, system damping system, one or more generators, generator damping,
transmission damping , and the energy generating damping . The cylinder is placed
with its axis in the z direction perpendicular to the flow velocity U, which is in
direction x. The cylinder oscillates in the y direction, which is perpendicular to its axis
in z and the flow velocity in x. As discussed in Section V, the VIVACE Converter
design is modular, scalable, and flexible in the sense of geometry and configuration.
Thus, converters of various sizes can be developed by assembling modules of various
sizes and properties in a variety of configurations.
Figure 6 : Simple Schematic of a VIVACE Module with Coordinate System
Figure 7 shows artist’s rendition of a small array of VIVACE Converter for an
offshore power plant. The supporting piles, which house all the transmission and
electricity generating components, are hydrodynamically faired to prevent their own

VIV. The oscillating cylinders are attached by small pins to sliding bearings on a steel
rod with springs and damping to provide an elastic support to achieve VIV of the
cylinders. The PTO system presently used in the VIVACE Converter lab models
consists of a gear-belt transmission system and an off-the-shelf rotary generator.
Alternatives such as a hydraulic system or a linear generator are possible.
Figure 7 : VIVACE setup
There is use of a hydraulic system to connect multiple VIVACE modules to
one generator. Direct transmission to mechanical energy through hydraulics to pump
water for irrigation or raise pressure for water desalination is being studied as well. In
addition to the quantities used to define a module, for a VIVACE Converter assembly,
the following geometric variables need to be defined as shown in Figure 8: h = water
depth, d = draft of the VIVACE Converter assembly, t = vertical distance between
centers of cylinders, p = horizontal distance between centers of cylinders.

Figure 8 : Arrangement of cylinders
5.2. Benchmarking
Two benchmarking methods are used in this section. First, VIVACE is
compared to traditional and alternative energy resources based on data [4].The
comparison results are shown in Figure 9 in terms of $/kWh. The assumptions behind
these calculations are summarized in Tables 1, 2, 3. Table 5 shows the fuel cost per
BTU; Tables 2 and 3show the assumptions for conventional and alternative energy
generation, respectively. The assumptions behind the VIVACE Converter are
summarized in Table 4..
Table 1:Fuel cost assumptions

Figure 9 : Comparison of energy sources
Table 2:Assumption of cost estimate of conventional energy source

Table 3:Assumption of cost estimation of alternate energy source

Table 4 : Data regarding 100 MW VIVACE converter

6. Vortex Shedding Vertical Axis Turbine (VOSTURB)
Current hydro-turbines aim to capture the immense energy available in tidal
movements, however commonly applied technologies rely on principles more
applicable in hydroelectric dams. Tidal stream currentsin some areas are not strong
enough to make such turbines both efficient and economically viable. A new low-energy
vortex shedding vertical axis turbine (VOSTURB) to combat the inefficiencies
and challenges of hydro-turbines in low velocity free tidal streams is available. Some
of the energy in tidal streams is extracted naturally from vortex shedding; as water
streams past a bluff body, such as pier, low pressure vortices form alternatively on
each side, inducing a rhythm of pressure differentials on the bluff body and anything
in its wake. VOSTURB aims to capture this energy of the vortices by installing a
hydrofoil subsequent to the bluff body. This foil, free to oscillate, translates the vortex
energy into oscillatory motion, which can be converted into a form of potential
energy. It aims to harvest such foil motion, or the ability of VOSTURB to capture
vortex energy, and begin to use the amount of tidal energy that can be theoretically
harnessed. A small scale model of VOSTURB, a cylindrical bluff body with a
hammer shaped hydrofoils shown below. Ultimately it was found that the frequency
of the VOSTURB foil oscillations corresponded highly with the theoretical frequency
of vortex shedding for all moderate to high flow speeds [6]. Low speeds were found
to produce inconsistent and intermittent small oscillations. This signifies at moderate
to high flow speeds, VOSTURB was able to transform some vortical energy into
kinetic. The maximum average power obtained 8.4 mW corresponded to the highest
flow velocity 0.27 m/s [6]. Scaled to prototype conditions this represented 50 W at a
flow velocity of 0.95m/s, the maximum available . Although it was ascertained that
VOSTURB could consistently capture some of the vortical energy; the percentage of
which could not be calculated with certainty. Thus, the average kinetic power
assessments of the foil were compared to the available power of the mean flow for
each flow speed calculated by two methods: (1) over the foil's swept area; (2) the area
of fluid displaced by the bluff body immediately in front of the foil. The maximum
efficiency of the foil, found for the fastest flow speed was at 18% and 45%
respectively. It was found that both average foil power, available flow power, and
efficiency all decreased with a decrease in flow velocity. This study can serve as only
a preliminary study for the effectiveness of VOSTURB as a hydro-turbine for tidal

power. In the experiments, the foil was allowed to oscillate freely with little
resistance. Future testing of VOSTURB needs to observe whether the vortex energy
can overcome the resistive torque introduced by a generator to induce oscillatory
motion as well as further optimize the foil design.
Figure 10: Schematic of VOSTURB capturing kinetic energy from vortex shedding

7. Energy harvesting in air flow
One method of energy harvesting based on Kármán vortex street, called
flapping-leaf. The flapping-leaf energy harvester had the same principle as the
‘energy harvesting eel’ while it was only designed to work in airflow. The device
consisted of a PVDF cantilever with one end clamped on a bluff body and the other
end connected to a triangular plastic leaf. When the airflow passed the bluff body, the
vortices produced , fluctuated the leaf and thus the PVDF cantilever to produce
electrical energy. The energy harvester generated a maximum output power of 17μW
under the wind of 6.5m/s [5]. It consists of a flexible plate with piezoelectric
laminates which was placed behind a bluff body. It was excited by a uniform axial
flow field in a manner analogous to a flapping flag such that the system delivered
power to an electrical impedance load. Experimental results showed that a RMS
output power of 2.5 mW can be derived under a wind of 27m/s. The generator was
estimated to have an efficiency of 17%. The plate had dimensions of 310 mm × 101
mm × 0.39 mm and the bluff body has a length of 550 mm.Dimensions of the
piezoelectric laminate were 25.4 mm × 20.3 mm × 0.25 mm. Jung and Lee (2011)
recently presented a similar electromagnetic energy harvester as VIVACE. Instead of
operating under water, this device was designed to work under air flow. In addition,
this device had a fixed cylinder bluff body in front of the mobile cylinder. These two
cylinders had the same dimensions. It was found that the displacement of the mobile
cylinder largely depends on the distance between the two cylinders and the maximum
displacement can be achieved when this distance was between three and six times of
the cylinder diameter. In the experiments, a prototype device can produce an average
output power of 50-370 mW under wind of 2.5-4.5 m/s. Both cylinders had a diameter
of 5cm and a length of 0.85 m. Zhu et al(2010c) presented a novel miniature wind
generator for wireless sensing applications. The generator consisted of a wing that
was attached to a cantilever spring made of beryllium copper. The airflow over the
wing caused the cantilever to bend upwards, the degree of bending being a function of
the lift force from the wing and the spring constant. As the cantilever deflects
downwards, the flow of air is reduced by the bluff body and the lift force reduced
causing the cantilever to spring back upwards. This exposes it to the full airflow again
and the cycle is repeated. When the frequency of this movement approaches the
resonant frequency of the structure, the wing has the maximum displacement. A

permanent magnet was fixed on the wing while a coil was attached to the base of the
generator. The movement of the wing caused the magnetic flux cutting the coil to
change, which generated electrical power. The proposed device has dimensions of
12cm × 8cm × 6.5cm. It can start working at a wind speed as low as 2.5m/s when the
generator produced an output power of 470 μW. This is sufficient for periodic sensing
and wireless transmission. When the wind speed was 5 m/s , the output power reached
1.6 mW
Figure 11: Principle of energy harvesting in air flow
7.1 Piezoelectric Energy Harvesting
Piezoelectric transducers have been used in several designs for fluid flow
energy harvesting. Their goal is to generate power, on the scale of microwatts and
milliwatts, for small electronic devices such as remote sensors. There have been flag-like
devices built, one of which is a piezoelectric eel , which is an underwater sheet of
piezoelectric polymer that oscillates in the wake of a bluff body. Operating in air,
other devices are based on more conventional rotating turbine designs that implement
piezoelectrics driven by cam systems .In the category of wheat-like generators is an
oscillating blade generator, which uses a piezoelectric transducer to connect a steel
leaf spring to leaf-like ears. The device utilized a vertical rigid sail, fixed to a

vertically cantilevered piezoelectric transducer. These devices would oscillate in a
fixed direction when introduced to wind
A photograph of one of the devices is shown in Figure 5. The assembly would
be placed in a moving air stream, such that the plane of the sail and the piezo buzzer
was perpendicular to the flow. The sail would oscillate forwards and backward
relative to the flow, causing the piezo to bend back and forth. This, through the direct
piezoelectric effect, would cause the piezo to generate a current through any electric
load connected to it.
Figure 12 : Photograph of a piezoelectric device
7.2 Remote Sensing Application
Operating on the micro and milliwatt scale, devices of this type are not
necessarily designed to be alternatives to large scale energy generation. Instead, most
of these devices, including ours, are designed for applications where batteries or long
power cords can be eliminated. In the right application, this can lead to a savings in
capital, maintenance, or labour costs.

One application that is a prime candidate for using energy harvesting devices
is remote sensing. Environmental and structural sensors are often used in locations
where providing power and data connections is not cost effective. Instead, sensors
transmit data wirelessly, and power is provided at the individual sensor. To limit
power requirements needed for data transmission, sensor networks are often designed
where each sensor operates as a node, relaying data along from other sensors. Power
often comes from a battery, but there the capacity of the battery must be able to
handle the drain from the device long enough that it does not become too time and
labor intensive to periodically replace. This situation can be alleviated by generating
energy onsite, which is where energy harvesting devices become an option. Through
these devices, power is either continuously provided to the sensor electronics, or more
often, it is stored in a small battery or capacitor to provide more continuous power.
This is a viable solution as long as the average power output of the energy harvester is
more than the average consumption of the sensor, over periods of time for which the
intermediate storage can provide power.
Figure 13: Possible remote sensing application

8. Conclusion
A vibration energy harvester is an energy harvesting device that couples a
certain transduction mechanism to ambient vibration and converts mechanical energy
to electrical energy. Ambient vibration includes machinery vibration, human
movement and flow induced vibration. For energy harvesting from machinery
vibration, the most common solution is to design a linear generator that converts
kinetic energy to electrical energy using certain transduction mechanisms, such as
electromagnetic, piezoelectric and electrostatic transducers. Electromagnetic energy
harvesters have the highest power density among the three transducers. However,
performance of electromagnetic vibration energy harvesters reduces a lot in micro
scale, which makes it not suitable for Magneto-electromagnetic System (MEMS)
applications.
Energy harvesters from flow-induced vibration, as an alternative to turbine
generators, have drawn more and more attention. Useful amount of energy has been
generated by existing devices and the start flow speed has been reduced to as low as
2.5 m/s. However, most reported devices that produce useful energy are too large in
volume compared to other vibration energy harvesters. Thus, it is difficult to integrate
these devices into wireless sensor nodes or other wireless electronic systems. Future
work should focus on miniaturise these energy harvesters while maintain current
power level. In addition, researches should be done to further reduce the start flow
speed to allow this technology wider application.

9. References
1. Dung-An Wang ; Chun-Yuan Chiu and Huy-Tuan Pham ; Electromagnetic
energy harvesting from vibrations induced by Karman vortex street ; Mechatronics ;
Volume 22 ; 2012 ; pages 746–756;
2. D.A. Wang and K.H Chang ; Electromagnetic energy harvesting from flow
induced vibration ; Microelectronics Journal ; Volume 41 ; 2010 ; pages 356–364;
3. Antonio Barrero-Gil ; Santiago Pindado and Sergio Avila ; Extracting energy
from Vortex-Induced Vibrations: A parametric study ; Applied Mathematical
Modelling ; Volume36 ; 2012 ; pages 3153–3160 ;
4. Michael M. Bernitsas ; Kamaldev. Raghavan ; Y. Ben-Simon ; E. M. H.
Garcia ; VIVACE(Vortex Induced Vibration for Aquatic Clean Energy):A NEW
CONCEPT IN GENERATION OF CLEAN AND RENEWABLE ENERGY
FROM FLUID FLOW ; Journal of Offshore Mechanics and Arctic Engineering ;
2008 ;
5. Dibin Zhu ; Vibration Energy Harvesting: Machinery ,Vibration, Human
Movement and Flow Induced Vibration ; University of Southampton ,UK .
6. Bruder and Brittany Lynn ; Assessment of hydrokinetic renewable energy
devices and tidal energy potential at Rose Dhu Island, GA ; August 2011 .
7. C.H.K. Williamson ; and R. Govardhan ; A brief review of recent results in
vortex-induced vibrations ; Journal of Wind Engineering and Industrial
Aerodynamics ; Volume 96 ; 2008 ; pages 713–735 .
8. Philippe Meliga ; Jean-Marc Chomaz ; and Franc -ois Gallaire ; Extracting
energy from a flow: An asymptotic approach using vortex-induced vibrations and
feedback control ; Journal of Fluids and Structures; Volume 27 ; 2011; pages 861–
874 .
9. Ashwin Vinod ; Amshumaan ; Kashyap ; Arindam Banerjee ; and
JonathanKimball; Augmenting Energy Extraction From Vortex Induced Vibration
Using Strips Of Roughness/Thickness Combination ; Proceedings of the 1st Marine
Energy Technology Symposium , METS13 ; April 10‐11, 2013 ;

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