Anemometers Assignment

D.I.T. Kevin St. Anemometry

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Department of Electrical Services Engineering

Aneomometry

Course Code / Year DT 018

Module Electrical services engineering and energy

management.

Lecturer Mr. Derek Kearney.

Student Name Wayne Doyle, Mark Stewart, Eamon Carroll

Paul Derwin

Student Number D06112499,D05110728,D06111916,

D07114349.

Submission Date 29th November 2011


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Word Count 9494

Declaration

I hereby certify that the material, which is submitted in this assignment/project,

is entirely my own work and has not been submitted for any academic

assessment other than as part fulfilment of the assessment procedures for the

program Bachelor of Science in Electrical Services and Energy Management

(BSc (Hons)) (DT 018).

Signature of student:…………….………

Date:…………………………


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Table of Contents:

Page No.

Declaration ...................................................................................................................... 2

Table of Contents: .......................................................................................................... 4

Table of Figures:............................................................................................................. 6

1.0 Introduction.......................................................................................................... 7

1.1 Wind. .................................................................................................................... 7

2.0 Anemometry ........................................................................................................ 9

3.0 Rotational anemometers ................................................................................. 10

3.1 Cup anemometer ............................................................................................. 10

3.2 Propeller anemometer ..................................................................................... 13

4.0 Phase shift anemometers ............................................................................... 14

4.1 LIDAR (Light Detection and Ranging) .......................................................... 14

4.2 Sonic anemometers ......................................................................................... 17

5.0 Pressure Type Anemometers......................................................................... 19

5.1 Pressure plate................................................................................................... 19

5.2 Pressure Tube Anemometer (Pitot tubes) ................................................... 20

6.0 International standards .................................................................................... 22

6.1 Cup & Propeller Anemometer Test Procedures.......................................... 22

6.1.1 IEC 61400-12-1..................................................................................... 22

6.1.2 ASTM D5096-02 ................................................................................... 24

6.2 Phase shift anemometers ............................................................................... 25

6.2.1 Sonic Anemometer Test Standards................................................... 26

6.2.2 ASTM D 6011-96 .................................................................................. 26


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6.2.3 ISO 16622.............................................................................................. 28

6.2.4 IEC standard 61400 ............................................................................. 30

6.3 Environmental conditions and inspections................................................... 31

6.4 Standards Conclusion ..................................................................................... 31

7.0 Comparison of the performance .................................................................... 32

7.1 Over speeding: ................................................................................................. 32

7.1.1 Dynamic filtering in Turbulent Winds ................................................. 33

7.1.2 Mechanical operation:.......................................................................... 33

7.2 Phase shift anemometers–Ultra Sonic anemometers, SODAR and

LIDAR anemometers. ............................................................................................. 34

7.2.1 2.1Ultra sonic anemometers ............................................................... 34

7.2.2 SODAR and LIDAR .............................................................................. 35

7.3 Thermoelectric anemometers ........................................................................ 36

7.3.1 Hot wire anemometers......................................................................... 36

8.0 The development and effect of turbulence in relation to wind

measurement .................................................................................................... 38

8.1 The Importance of Accurate Wind Measurement ....................................... 38

8.1.1 What is Turbulence .............................................................................. 38

8.2 Turbulence Effect on Wind Measurement Devices .................................... 40

8.2.1 Cup Anemometer.................................................................................. 40

8.2.2 Propeller Type ....................................................................................... 41

8.2.3 Sonic Anemometer ............................................................................... 42

8.2.4 SODAR................................................................................................... 43

8.2.5 LIDAR Anemometer ............................................................................. 43

8.2.6 Cup vs LIDAR........................................................................................ 44

9.0 References ........................................................................................................ 46


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Table of Figures:

Page No.

Fig 1 (Anon., 2011) ........................................................................................................ 7

Fig 2 (Gripe, 2004) .......................................................................................................10

Fig 3 (Dines, 1911 ) .....................................................................................................11

Fig 4 (Anon., 2010) ......................................................................................................13

Fig 5 (wright, 2006) ......................................................................................................14

Fig 6 (technology, 2011) .............................................................................................16

Fig 7 (Johnson, 2001)..................................................................................................18

Fig 8 IEC performance requirements ........................................................................23

Fig 9 (Brazier, 1914) ....................................................................................................23

Fig 10 ASTM D 5096-02 Calibration Test Speed Protocol, ascending and

descending speeds (Coquilla, 2009) .........................................................................25

Fig 11 wind tunnel requirements for ASTM D 6011-96 sonic sensor testing

(Adam Havner, 2008) ..................................................................................................27

Fig 12 Otech Eng. Wind Tunnel Laboratories (ASTM Inernational, 1996) .........29

Fig 13(Webb, 2007) ....................................................................................................38

Fig 14(Horst, 2007) .....................................................................................................39

Fig 15(Horst, 2007) .....................................................................................................39


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1.0 Introduction

Measurement of wind speed is very important to people such as pilots, sailors,

and farmers and engineers. Accurate information about wind speed is important

in determining the best sites for wind turbines. Wind speeds must also be

measured by those concerned about dispersion of airborne pollutants. Wind

speeds are measured in a wide variety of ways, ranging from simple go-no go

tests to the most sophisticated electronic systems. The variability of the wind

makes accurate measurements difficult, so rather expensive equipment is often

required.

1.1 Wind.

What is wind? Wind is created by

different in pressure. When pressure

differential exists the air is accelerated

from a higher to a lower pressure. So

when the sun strikes the earth it heats

the soil near the surface. In turn the soil

warms up the surrounding air .With

warm air being less dense than cool

air,just like a helium balloon it rises. Cool air then rushes in to take

its place and is then heated its self and continues the cycle. The

cycle is continues just like a crank shaft in a car and as long as the solar engine

is driving . (Gripe, 2004) Due to it rotating sphere the error will be deflected by

the Cariolis Effect except exactly on the equator. Globally, there are two major

driving factors of large-scale winds the atmospheric circulation, and the

differential heat between the equator and a pulse and the rotation of the plants.

The wind rises from the moves north and south in the higher layers of the

atmosphere. At around 30° in both hemispheres the cariolis force prevents the

air from moving much further. At this latitude there is a high pressure area, as

the air begins sinking down again. As the wind rises from the equator there will

be a low-pressure area close to ground level attracting wins from the north and

Fig 1(Anon., 2011)


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south. Then at the north and South Pole there will be a high pressure due to the

cooling of the air at the poles.


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2.0 Anemometry

An anemometer is device fundamental used for measuring wind speed and is a

common weather station instrument .The term its self is derived for the Greek

word anemo which means wind. The first known descriptions seam to show that

an inventor by the name of Leon Battista gives a description of an anemometer

in the year 1450. (W.E. Knowles Middleton, 1968) Anemometers can be broken

down mainly into two categories those that measure wind velocity an those that

measure wind pressure these are close linked (W.E. Knowles Middleton, 1968)

As stated above Anemometers can be segregated into four categories

according to their operational and physical characteristics.

1. Rotational anemometers – cup anemometers, and propeller

anemometers.

2. Phase shift anemometers – ultra sonic anemometers, and laser

Doppler anemometers.

3. Pressure type anemometers – pressure tube anemometers, pressure

plate anemometers, and sphere anemometers.

4. Thermoelectric anemometers – hot wire anemometers, and hot plate

anemometers. (Kearney, 2011)

This document will also review international standards from the International

Organization for Standardization (IOS), International Electro technical

Commission (IEC) and American Society for Testing Materials (ASTM).

whom govern anemometry calibration and wind measurement. The purpose

of these standards is to provide the methodology that will ensure accuracy

and reproducibility in the measurement and analysis of anemometers. They

are intended to aid manufacturers, installers and purchasers of

anemometers.


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3.0 Rotational anemometers

3.1 Cup anemometer

Cup anemometers are one off the most common types of anemometer used

today worldwide. It is a very simple design and operation. The cup anemometer

was invited the Irish astronomer Thomas T. R. Robinson of Armagh

Observatory in 1846. Miditon (1969) identified that the first anemometers had

four hemispherical cups each mounted on a horizontal arms which are then

mounted on equal angles to each other. The

unit was then mounted on a vertical access.”

The wind would then flow past any the cups

in any of the horizontal access inducing a

radial motion which was proportional to the

wind velocity therefore allowing for counting

the cup turns over a set time which allow him

to then produce an average wind velocity

over a rage of wind velocities (Spilhaus,

1953) (Dines, 1911 ).Around the end of the

1920s much research and experimenting was

carried out in relation the number of cups and

also the length of the arms. When the

incident was designed the anemometer

wrongly claimed that no matter how big the

cups or how long the arms were cups would

always move warned that the speed of the wind. This is apparently confirmed

by some early independent experiments but it turned out to be very further from

the truth Brazier and Paterson found that a three arm cup rotor is optimal with

respect to sensitivity and suppression of the unevenness in the rotation

(“wobbling”).A modern cup is shown below in figure 3. (Brazier, 1914) Paterson

and Braziar also demonstrated the literate is better on a shorter diameter.

(Brazier, 1914) (Kristensen, 2005) Patrson also determined that optimum

Fig 2(Gripe, 2004)


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number of cups is three and can be proven

in his equation 𝑓 =𝑢

𝑟𝑠 where r=radious of

the cup and U= angular velocity and S=

the cup rotor.

The wind would then flow past any one of

the cups in of the horizontal access

inducing a radial motion which was

proportional to the wind velocity therefore

allowing for counting the cup turns over a

set time which allow him to then produce

an average wind velocity over a rage of

wind velocities (Spilhaus, 1953).A modern

cup is shown in figure 3. The cup anemometers are probably the most common

type anemometer currently being used today; it can also be fitted with a vain to

determine the mean of the horizontal wind

velocity components. It has proven over time that such combinations sturdy,

reliable and robust Instrumentation package. They are simple to operate and

can be used in a wide range of applications in weather stations at airports wind-

farms, solar farms and can also be used on large structures such as high-rise

buildings and bridges which are under construction or the equipment being

used to construct them i.e. tower cranes.

The behaviour of a couple anemometers in turbulent wind can cause systematic

errors in the measurement of the wind speed. One of the main errors which

have been identified is a so-called over speeding. This is gives an increased

wind speed measurement or velocity than the actual wind speeds. This is due

to the anemometer responding quickly to an increased wind speed and

responding much slower to decrease in winds in the wind speed, this allows the

cup and anemometer to continue rotating at a faster velocity than the actual

wind speed it is measuring. “Based on an equation of the dynamic response of

cup anemometer and the contrast observations between cup anemometer and

sonic anemometer, it is shown that the error caused by the over speed of cup

anemometer is between 1% and 3 % and data processing error (DP error)

Fig 3(Dines, 1911 )


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associated with cup anemometer is the most important one because of distinct

calculated methods (vector mean and scalar mean). After the reduction for DP

error, the rational wind speed values could be obtained”. (Kristensen, 2005)

The following equation is used to demonstrate over speeding as a percentage

E = I². (1.8d – 1.4)

Where:

E is the percentage of over speeding error

I is the turbulence intensity

d is the distance constant for the anemometer(Energy, 2008)

In one form or another cup anemometers are often used with a wind vane for

determining the mean of the horizontal wind velocity component. Such

combinations are sturdy and reliable instrument packages. They are easy to

operate and are used at weather stations, airports, wind farms, and sites where

large structures, such as bridges, are under construction. Cup anemometers

are relatively in expensive to manufacture and are suitable for many wind

measurement applications. The three types of cup anemometer has now

become the standard for this type of anemometer. The cup anemometer is a

mechanical device or instrument with a long proven reliable track record which

is easy to find certification for the use in contractual applications. Researchers,

Educational Institutes, Meteorologists and many of the general public are

familiar with cup anemometers. Although they are one of the oldest ways of

measuring wind speed you will find that most people will have no problem

accepting easily understood mechanical designs. Cup anemometer designs

vary considerably according to the particular application. Several Cup

Anemometer Manufacturers offer different models which include low cost

portable wind speed measurement, homeowner anemometers , industrial wind

speed checking and high specification models for scientific research and

weather analysing High specification cup anemometers will normally be

designed to withstand changing conditions and more solid construction

materials and overall design will not only increase the lifetime of the device but


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also render it more valuable in terms of accuracy and repeatability in conditions

that are subject to change. (Tong, 2010)

3.2 Propeller anemometer

Propeller anemometer and sometimes also now as

wind mill anemometers or aero vanes usual

consist of four bladed propeller. The blades are

constructed of extremely light weight Martials such

as carbon fiber, aluminum or thermo setting

plastics. The device functions predominantly in on

the lift force. With airflow parallel to its axis the

propeller blades experience a lift force which turns

the propeller at a velocity or speed proportional to

the winds speed or velocity. In this case the wind

turns a propeller instead of cups. Small propeller anemometers are used to

check the velocity and direction of the turbulence. A propeller anemometer is

used where the direction of the wind changes, as it does out-of-doors. The

propeller blades indicate the correct wind velocity only when they face the flow

of the wind. The vane, which also shows the direction of the wind, keeps the

propeller properly aligned. Propeller types generally

have a driven shaft with an ac or dc generator integrated to the device. The

types of propeller used for wind applications have a fast response time and

produce a linear output for the changing speeds. These devices can be as

simple as a magnet on the rotating shaft passing a coil inducing a voltage which

can be represented as a wind speed depending how many times the magnet

passes the coil over a set time.

Figure 1

Fig 4(Anon., 2010)


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4.0 Phase shift anemometers

4.1 LIDAR (Light Detection and Ranging)

LIDAR (Light Detection and Ranging) was first demonstrated in the United

States of America in the late 1960 early 1970.The widespread use and

development of the technology has been hampered over the years due to the

complexity and the high cost associated with LIDAR systems. “However the

recent year’s developments of new LIDAR systems based on fibre optic

components which have been proven in the telecommunication’s industry seem

to show a large cost reduction and

improvement in the overall design and

also an improved compactness”.(wright,

2006)

LIDAR is a remote sensing technology

analogous radar .The range (distance)

to an object is determined by measuring

the time delay between transmission

and reflection of a signal. LIDAR is used

for remote sensing which will give

“temporal and spatial resolution”

(wright, 2006).Doppler LIDAR works by the emitting light energy and

backscattered from the microscopic particulates or aerosols being transmitted

by the wind such as dust pollen and water molecules. This difference in the

frequency being emitted and the frequency being received back is called the

Doppler shift which in turn is proportional to the velocity of the particles.

The need for atmospheric LIDAR to meet eye safety standards limits both the

intensity of the light energy and its highest frequency (shortest wavelength) that

can be focused and radiated. When these criteria are combined, LIDARs used

for atmospheric measurements use midrange infrared light generated by lasers

operating with wavelengths in the 1.5 to 2 μm range like High resolution

Fig 5(wright, 2006)


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Doppler LIDAR (HRDL).Modern equipment is highly sensitive, they are fitted

with electronic amplifiers which have allowed LIDAR technics like HRDL to

measure wind fields to a distance of several kilometres even when the air is

clean and it contains a very low number of particles that result in backscatter

(wright, 2006). Generally LIDAR is used to measure atmospheric wind in two

general forms continues wave (CW) or pulsed. CW LIDAR continuously emits

its light energy through its optics which focus the beam in a predetermined

radios ahead of the instrument. Within the focus of radios, the backscatter

energy is collected and the velocity of the wind is measured from using the

Doppler shift. (N.D. Kelley, 2007) CW wind-finding LIDARs typically perform a

conical scan sequence in which one or more 360º scans are performed at a

fixed elevation angle and focus position. (N.D. Kelley, 2007) CW LIDARS only

produce measurements of the mean horizontal wind vector at a few heights. But

as is highlighted in by the national renewable energy laboratory it has proven to

be very accurate when some optimal smoothing is applied.

Pulse LIDAR emits regular spaced emissions of highly regulated light energy

which is spaced apart at regular emissions of highly collimated light energy for a

specific and predetermined time (pulse length) similar to Doppler SODAR. The

return signals then isolated to a period of time which corresponds to a specific

segment of radial distance along a beam called “the range gate” (N.D. Kelley,

2007) The backscattered signals which are digital extracted from the gate are

then processed to “derive the line of sight (LOS) or radial velocities along the

path of the LIDAR(N.D. Kelley, 2007) Pulsed LIDAR typically operates with a

pulse repetition or sampling frequency ranging from 200 to 1000 pulses per

second. The resulting vertical wind profile derived from a pulsed LIDAR using a

conical scan incorporates a much greater vertical resolution than an equivalent

CW profile. (N.D. Kelley, 2007)Sound propagation in the atmosphere has been

studied for at least 200 years, but it has only been in the last 50 years

that acoustic scattering has been used as a means to study the structure of the

lower atmosphere. In the United States during World War II,

acoustic backscatter in the atmosphere was used to examine low-level

temperature inversions as they affected propagation in microwave

communication links. Beginning in the late 1960's and early 1970's, scientists at


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the U.S. National Oceanic and Atmospheric Administration demonstrated the

practical feasibility of using acoustic sounders to measure winds in the

atmosphere by means of the Doppler shift.(technology, 2011)

During the 1980s there was a similar development of Doppler SODAR systems

by other U.S companies. The Xondar SODAR system which was capable of

making wind profile and turbulence measurement. Another company was

developing their pulsed Doppler SODAR system, this system was an Invisible

Tower system.(technology, 2011)Like its cousin technologies, radar (radio

detection and ranging) and LIDAR (light detection and ranging), SODAR uses

waves for measurement; and, as the name suggests, it uses sound waves.

Over the past few decades SODAR has been typically used to research

atmospheric conditions which would include insect migration patterns and

pollution.

Commercial SODAR in today's wind industry are ground-based systems that

send up focused sonic beams in rapid succession, producing an audible chirp.

Wind turbulence sends a portion of the sound back towards the ground as an

echo. By precisely measuring the frequency and time delay of the echo (aka the

Doppler shift), the SODAR device using trigonometry measures the wind speed

and direction at any height up to around 200 meters. This is why the SODAR

are also called Doppler SODAR.(secondwind, 2011)Commercial SODAR

systems and other remote sensing systems were developed and

commercialized for the wind industry to help reduce to overall inaccuracy in

measuring wind velocity below the turbines hub height.

SODAR are now used as a resource assessment by

wind farm operators either in conjunction with a

microwave towers or as a standalone SODAR is

important to wind meteorologists because it is ideally

suited to measuring the lower bound below the sweep

of radar and above the ground. This is where wind

matters most to wind turbines as shown below in figure

6.SODAR have gained commercial acceptance in wind development

applications because they measure the wind at higher Fig 6(technology, 2011)


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heights than is practical using meteorological towers.Sonic Detection and

ranging SODAR systems are classified as remote sensing as they do not have

to place a sensor at the location where the measurement is required They are

used to remotely measure the vertical turbulence structure turbulence structure

and the profile of the lower layer of the atmosphere. SODAR systems operate in

a similar manner to radar (radio detection and ranging) systems except that

sound waves rather than radio waves are used for detecting. (secondwind,

2011)

Because of their ability to be easily transported, SODAR systems are also well

suited to more specialized wind development application such as wind

prospecting (also known as scouting or site finding). Wind shear measurement,

because of their ability to measure wind at many different heights, SODAR is

particularly valuable to when looking for information in relation the amount and

the type wind shear and veer. Because of the extreme difference in the velocity

of the wind and the constant changing of direction has been identified as a

substantial factor in the increase in wear and tear of wind turbines. The

information gained to understand the wind shear on a particular site helps to

identify the suitably of the site for a wind turbine.(secondwind, 2011)

4.2 Sonic anemometers

Some articles show that sonic anemometers were developed first in the 1970s

others state that it was developed by a Dr Andreas Pflitsch in 1994. And in a

report written by the department of metrology Indian the sonic anemometer was

first developed in the 1950s with some

confusion over its origins but there is no

confusion over the usefulness of this

instalment its self. But whichever year it

was developed there is no doubt in the

benefits of the device.

Wind velocity is measured based on the

time it takes for sonic pulses to travel

between a pair of transducers.Using the


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combined measurements from several transducer pairs mount on the end of

arms mounted perpendicular to each other, we can then yield a measurement

of air flow in 1, 2 or 3 dimensions. The technic used are also based on the

principles of Doppler shift. The transducers emit acoustic signals which travel

up and down through the air and because the speed of the sound moving

through still air is different from that through still air.

A typical path length between the transducers is 10 - 20cms and this gives us

the special resolution. A temporal resolution of 20 Hz or better

can be achieved which makes this technology well suited for the purposes of

turbulence measurements.(Mathew, 2009)

One of the great advantages of ultrasonic anemometers is their lack of moving

parts which makes them appropriate for usage in Automatic Weather Stations

and in harsh environments. The lack of moving parts makes them appropriate

for use in automated weather stations in the wind turbine industry and

renewable energy industry. Some limitations of sonic anemometers are when

they are exposed snowy conditions the anemometer does not work.

Fig 7(Johnson, 2001)


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5.0 Pressure Type Anemometers

5.1 Pressure plate

The pressure plate or normal plate anemometer consists of a swinging plate

which is held at the end of a horizontal arm. The arm is attached to a shaft

which it is free to rotate around. A wind vane ensures that the pressure plate is

always facing perpendicular to the wind flow. The wind blows against the plate

and the distance that the arm travels is directly calibrated to the force of the

wind. It operates on the principle that the force of moving air on a plate held

normal to the wind is

Fw = cAρ u2/2

Where

Ais the area of the plate

ρis the density of air

uis the wind speed

cis a constant depending on the size and shape of the plate but not greatly different from unity.

This force produced is then used to drive a recording device directly or as input

to a mechanical to electrical transducer. Research of gust studies is the main

application of this type of anemometer because of its very short response time.

Gusts with a duration of as little as 10ms can be examined with this type of

anemometer. A pressure plate anemometer is however not very sensitive to low

winds and is likely only to give an estimate of wind speed and is also not very

responsive to the turbulence dynamic of wind. A pressure plate anemometer is

useful for spraying agricultural chemical or for estimating wind chills. (Johnson,

2001)


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5.2 Pressure Tube Anemometer (Pitot tubes)

Pitot tubes are commonly used to measure air flow in ventilation systems

because they can be easily installed into ventilation systems through small ports

in the duct work. Accuracy can be as high as +/- 2% if great care is taken when

taking Pitot tube readings. The pressure tube anemometer measure the height

of a liquid in a u tube which has one bent into a horizontal direction to face the

wind and the other end would usually be open and facing up so the wind can

pass over, this will create a vacuum due to the Bernoulli effect. The height of

the liquid is usually measured with a float connected to a mechanical linkage to

a recording device. A disadvantage of the pressure tube anemometer is that the

moving parts can wear out. The liquid manometer can also be fragile. To get an

accurate reading the Pitot tube must be faced into the wind stream. External

weather conditions can have an effect on the performance of the pressure tube

anemometer.


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6.0 International standards

6.1 Cup & Propeller Anemometer Test Procedures

6.1.1 IEC 61400-12-1

The International Electro-technical Commission (IEC) is a global standards

organization that consists of committees that review operational practices in

electrical and electronic industry and research. The IEC also publishes

international standards in collaboration with the International Organization for

Standardization (ISO) (Coquilla, 2009).

Cup and Propeller Anemometer test procedures are carried out to determine

the transfer of the instruments voltage output to wind speed. According to the

IEC, these procedures should be conducted in a uniform, horizontal and steady

state flow of low turbulence levels these conditions are best achieved in a

purpose built wind tunnel.

The anemometer shall be mounted on a stand similar to that used in the

field in the wind tunnel.

Prior to every calibration the set up shall be verified by means of a

comparative calibration of a reference anemometer.

The anemometer shall be run for five minutes prior to calibration to avoid

the effect of large temperature variations on the mechanical friction of the

anemometer bearings.

Calibration shall be carried out in wind speeds ranging between 4 and

16m/s at 1m/s interval, the sequence of speeds are as follows, 4, 6, 8,

10, 12, 14, 16, 15, 13, 11, 9, 7, and 5m/s. This rising and falling speed

sequence is to identify any hysteresis during the test.

For data collection at the desired speed, the sampling frequency shall be

at least 1 Hz at a sampling interval of 30 seconds (Coquilla, 2009).


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below lists the IEC performance requirements for a wind tunnel calibration

facility.

Wind

characteristic

tunnel

Description Minimum requirement

Blockage ratio Anemometer ratio plus mount

frontal area to the total wind

tunnel test section area

Not to exceed 0.1 for open test

sections, 0.05 for closed

sections

Flow uniformity Percent difference in wind speed

within the test section volume

Less than 0.2% in the

longitudinal, transversal, and

vertical directions

Horizontal wind

gradient

Dynamic pressure differential at

the area covered by the rotating

cup anemometer

Must be less than 0.2%

Turbulence

intensity

Ratio between the wind speed

standard deviation to the main

speed

Must be less than 2%

Fig 8IEC performance requirements

.

It is noted by the IEC that it is important that the anemometer calibration test is

carried out with a steady uniform horizontal flow across the cup or propeller

anemometer, with no vertical or cross flow conditions. The wind tunnel section

should be turbulence free. According to the IEC, cup anemometers are to be

calibrated to a pilot-static tube

system, which is also calibrated to

an appropriate test speed (Coquilla,

2009).The IEC standard states the

reference wind speed from a Pitot-

static tube system is calculated

using the differential pressure

Fig 9(Brazier, 1914)


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measured at the inlet of the Pitot-static tube and also the measurements of the

local conditions inside the wind tunnel test section. The IEC requires equipment

used to measure pilot tubes to be traceable back to a standards authority.

6.1.2 ASTM D5096-02

ASTM D 5096-02 is a Standard Test Method for Determining the Performance

of a Cup and Propeller Anemometer Test Procedures, which were published in

1990(Smith, 2009). The ASTM (American Society for Testing Materials) is a

non-profit voluntary standards organization, which develops and publishes

standards. The purpose of ASTM D5096-02 is to provide a method for

calibration and performance characteristics for the cup and propeller

anemometers. Some of the characteristics include starting threshold and off-

axis response. Wind tunnel requirements for ASTM 5096-02 cup and propeller

anemometer testing include some of the following criteria.

Anemometer front area must be less than 5% of the test section cross-

section area of the wind tunnel testing equipment.

Turbulence must be less than 1% in the test area.

Wind speed-reading must maintain an accuracy of 0.1m/s.

Air density profile in test area must be less than 3% difference.

Flow uniformity in the test area must be constant to within 1% (Coquilla,

2009)

6.1.2.1 Steps from ASTM calibration procedure

1. Install the anemometer at an angle of attack maintained within 0.50

2. Acquire wind tunnel speed and anemometer rotation rate along with test

section environmental conditions. Each test will run between 30 and100

seconds. At each test speed collect data.

3. Calibration is carried out at two times the threshold (U0) of the

anemometer and up to 0.5 times the max application speed (Umax).

Incremental test speeds are defined in table 2 that shows ascending and

descending speeds on the next page. They are 20 different test speeds.


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4. Calculate and report wind speed residuals by determining a wind speed

value for a range of anemometer rotation rates using linear transfer

functions and then subtracting the predicted value from the measured

wind speed.

5. Construction of new data beyond the range of measurement is not

required therefore if the max test speed is 50% of Umax, then the transfer

function only applies for that test speed.

Ascending speeds Descending speeds

2 times U0

3 times U0

4 times U0

5 times U0

6 times U0

0.1times Umax

0.2 times Umax

0.3 times Umax

0.4 times Umax

0.5 times Umax

0.5 times Umax

0.4 times Umax

0.3 times Umax

0.2 times Umax

0.1 times Umax

6 times U0

5 times U0

4 times U0

3 times U0

2 times U0

Fig 10ASTM D 5096-02 Calibration Test Speed Protocol, ascending and descending speeds (Coquilla, 2009)

6.2 Phase shift anemometers

Standards that are available for the performance testing of sonic anemometers

are ASTM D 6011-96 and ISO 16622, which involves a test program that

evaluates the three-dimensional characteristics of sonic anemometers. For

sonic sensors used in research applications, where it may be necessary to map

complex flows at high resolution, such detailed test procedures for the sonic

instrument may be necessary. However, some of the most common industry


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applications for sonic anemometers generally only require a certain level of

uncertainty in two dimensional wind speed and direction measurements such as

on weather stations, airports, oceanic buoys, nuclear power plants, wind plants,

and many others. Thus, a more practical test protocol, extracted from published

test standards, may be applicable for verifying sonic anemometer measurement

6.2.1 Sonic Anemometer Test Standards

Two standards that define procedures for sonic anemometer testing are ASTM

D 6011-96 and ISO 16622. These standards describe the initial calibration for a

sonic anemometer in a zero-wind chamber, which involves the measurement of

the path length and transient times between the transmitter and receiver. ASTM

D 6011-96 is a sonic anemometer test standard, which was released in 1996

and is intended to assist wind instrument manufacturers in the development and

design of sonic anemometers. Test facilities also use it as a guide for evaluating

the performance of sonic anemometers used in specific applications.

6.2.2 ASTM D 6011-96

ASTM D 6011-96 procedures include measurements of the following

Acoustic path length

System delay

System delay mismatch

Thermal stability range

Velocity resolution

Shadow correction,

Velocity calibration range

The acceptance angle

The first five tests are done in a zero-wind chamber, which defines the initial

calibration transfer characteristics of the instrument. The following three tests,

shadow correction, velocity calibration range, and acceptance angle, define the


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necessary correction to the initial calibration transfer characteristics these are

determined in a wind tunnel

Wind tunnel

characteristic

Minimum requirement

Blockage Anemometer front area is less than 5% of the test section

C.S.A.

Wind speed

capability

Must be able to reach speeds up to 50% of the application

range and must maintain speed within +/- 0.2m/s. wind tunnel

speeds from 1.0 to 10m/s be maintained at +/- 0.1m/s.

Flow uniformity Flow profile in the test section must be constant to within 1%

Turbulence Must be less than 1% in the test section

Air density

uniformity

Density profile in test section must be less than 3% difference

Wind speed reading Maintain a relative accuracy of 0.1m/s to its traceable source

Fig 11wind tunnel requirements for ASTM D 6011-96 sonic sensor testing (Adam Havner,

2008)


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6.2.3 ISO 16622

ISO 16622, an international standard for used for anemometer performance

testing was released in September 2002 and is similar to ASTM D 6011-96, the

following steps define ISO 16622,

“Zero wind chamber tests: the offset of the measured wind speed is

determined over the operational temperature range.

Wind tunnel test: the deviation of the measured from the true velocity

(vector) is determined over the operational range of flow speed and

direction.

Pressure chamber test: the operational range of air density is

determined. Although the measuring principle does not depend on air

density, a minimum density is required to transmit detectable sound.

Field test: addresses the response to potentially adverse environmental

conditions, which are difficult to simulate in the laboratory” (ISO 16622

International Standard, 2002).

The ISO 16622 requirements for wind tunnels are similar to ASTM D 6011-96

but the wind tunnels shall be capable of producing wind speeds that cover the

full range of the anemometer to be calibrated. The wind speed must be kept

within +/- 0.2m/s. For angle testing the rotating fixture must have a 1° angular

resolution. The wind tunnel test procedures are as follows

Senor positioned at zero angle of attack, low wind speed is selected. To

test for orientation +/- 360 degrees around its vertical axis at 5-degree

increments and step has an increase in wind speed (e.g. 18%, 32%,

56% and 100% of the max test speed).

With the sensor positioned vertically at zero angle of attack and rotated

at the worst-case orientation. Tests are carried out at different

percentages of the test speed.

The steps above are repeated with the sensor tilted 15 degrees into the

wind and 15 degrees away from the wind (Coquilla, 2009).


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Fig 12 Otech Eng. Wind Tunnel Laboratories (ASTM Inernational, 1996)


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6.2.4 IEC standard 61400

Technical requirements for LIDAR and SODAR anemometers have being

revised under the IEC standard 61400-12-1. The purpose of these standards is

to ensure traceability, repeatability of SODAR/LIDAR measurements and

uncertainties of measurements. The main requests of these standards are an

accuracy test and sensitivity test/classification (Albers, 2010).

Accuracy Test & Calibration

Unlike mechanical anemometers such as cup and propeller types, LIDAR and

SODAR anemometers cannot be tested in wind tunnels due to the large size of

the wind scanning volume. The only way to calibrate the instruments is to

compare each individual LIDAR and SODAR to calibrated reference sensors in

open field tests. According to IEC 61400-12-1the comparisons of the LIDAR

and SODAR anemometers against a reference sensor must capture the wind

speed range between 4 and 16m/s similar to wind tunnel test speeds.

If the difference of the measured wind speed exceeds the reference sensor

measurement, the relation of the LIDAR/SODAR wind speed measurement and

the reference sensor measurement should be applied for a calibration of

LIDAR/SODAR. This calibration is then linked to the following; statistical

uncertainly of the comparison and uncertainty of the reference sensor.

The test and calibration cannot be more accurate than the cup anemometer

reference sensor (Albers, 2010).

6.2.4.1 Sensitivity Test/ Classification

Environmental conditions such as wind shear, turbulence intensity effects the

operation and accuracy of the LIDAR/SODAR anemometers, therefore

uncertainty of the LIDAR/SODAR wind measurements due to sensitivity to

environmental conditions needs to be assessed.

According to IEC 61400-12-1, the outline for the sensitivity test/classification is

the same as the accuracy test/calibration. The sensitivity of the LIDAR/SODAR

anemometers is based on set environmental variables. The percentage change


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of the LIDAR/SODAR and cup anemometer per 10-minute period is considered

as one environmental variable at a time.

6.3 Environmental conditions and inspections

A wind vane/ sonic anemometer should be capable of use under the following

conditions.

a) Operating temperature range:

Equipment installed outdoors − 25 °C to + 55 °C

Equipment installed indoors − 15 °C to + 55 °C.

b) Operating time: continuous.

c) Operating power range: The operating level can be set from 100V to 240V

for AC voltage, or from 12V to 24V for DC voltage. The tolerance level for

the set figures shall satisfy the requirements of IEC 60945.

6.4 Standards Conclusion

In this report, the four published standards that define procedures for calibrating

anemometers are IEC 61400-12-1, ASTM D5096-02, ASTM D6011-96 and ISO

16622. According to Coquilla the only publication related to the wind industry is

IEC 61400-12-1. Most test facilities are able to perform test procedures from the

standards listed above. The test procedures explains and defines necessary

steps manufacturers need to carry out to calibrate anemometers, this ensures

that all anemometers are traceable back to international standards organization.

LIDAR/SODAR and sonic anemometers use the same standards, ISO 16622

and IEC 61400-12-1 for calibrating.


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7.0 Comparison of the performance

The cup anemometer is the preferred wind speed measurement instrument for

site assessments or proposed wind farm performance evaluations. This

anemometer is an inexpensive and rugged sensor appropriate for turbulence

measurements for wind-energy measurement applications. Both propeller and

cup anemometers are the most suitable for a wide variety of environments

including marine applications. The propeller anemometer differs in that they

require an orienting vane to keep the propeller facing into the wind thus

provides both wind speed and direction information in a single unit. The design

of the rotational anemometer is such that the rate of rotation is linearly

proportional to the wind speed. This means the anemometer responds primarily

to that portion of the wind vector that is parallel to the rotor axis.

Aspects of rotational anemometry which may cause concern when specifying

are:

Over Speeding

In-ability to sense vertical wind components

Mechanical operation

Dynamic filtering in turbulent winds. (Energy, 2008)

7.1 Over speeding:

Over speeding occurs during fluctuating wind speeds. Instruments have time

constant characteristics, i.e. the response to change of an input parameter.

Generally the time to react to change is not linearly proportional to the

magnitude of that change in parameter however this is not the case with cup

anemometers, with fluctuating wind speed the mean indication of a cup

anemometer will be higher than the actual average wind speed, the results

showing that a cup anemometer will respond more quickly to positive changes

in wind speed and not as quick to negative changes.


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7.1.1 Dynamic filtering in Turbulent Winds

As stated above cup anemometers cannot follow the wind speed fluctuations

exactly, wind fluctuations of higher magnitude reduce the ability of the

anemometer to provide an accurate reading and indication of the changes in

magnitude of the wind vector. However by reducing the arms that support the

cups shall improve the performance and ability to operate in turbulent

conditions. Sensitivity to the wind vector depends upon the ratio of torque to

rotational inertia. As a consequence to dynamic filtering in turbulent winds the

spectral power indicated by a cup anemometer will be lower than what is the

actual power of the wind resulting in an incorrect evaluation of the true

turbulence intensity. The inability to react to the turbulent dynamic is a main

disadvantage of a rotational anemometer. This will result in directional

overshoots that can place the rotor of the main wind axis resulting in a lower

wind speed reading that the actual wind speed. This is more common in low

wind speeds with variance flow conditions and the wind direction is consistently

changing rapidly. False wind speed measurements can also occur due to off

axis flows in a vertical direction. This is particularly true for propeller – vane

anemometers installed on sloping terrain or near abrupt topographical features

which can have a constant vertical wind component or frequent large vertical

velocities. The dynamic response of a helicoid propeller to wind speed changes

decreases when the angle between the flow and axis increases, for example

when the angle with respect to the rotor axis reaches 85 degrees, its distant

constant has tripled.

7.1.2 Mechanical operation:

In the absence of mechanical friction the performance and steady state

calibration results should be perfectly linear but friction is always present to

some extent. Rotational Anemometers like their counterparts can fail totally,

however more seriously can partially fail. Due to their mechanical construction

they are more susceptible to breakdown i.e. bearing seizure. Preventative


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maintenance is necessary for the anemometer and/ or introducing forms of

redundancy to the measurement system. Such preventative maintenance and

calibration should be performed on a regular basis to maintain the low-wind

speed performance and to detect increased drag from bearing wear. (Energy,

2008)

Environmental concerns must be taken into consideration. Low temperatures

can cause particular problems for cup anemometers. The accumulation of

ice/snow in the cups will cause a change in aerodynamic behaviour, and

appropriate measures need to be taken to detect when such conditions occur.

Lightning strikes must be considered to ensure that a strike does not damage

the test equipment, a lightning finial/spike should be mounted at the top of the

mast, complete with a down conductor installed to carry the strike to Earth and

surge arrestors installed should they not be incorporated in the data logging

equipment. Another particular disadvantage of the rotational anemometer is

relative to the necessity of the erection of a mast. It may prove difficult to attain

planning permission if the mast was to impose on the environmental landscape.

7.2 Phase shift anemometers–Ultra Sonic anemometers,

SODAR and LIDAR anemometers.

7.2.1 2.1Ultra sonic anemometers

Ultra sonic anemometers main operational attribute is that they are non-

mechanical, this in turn omits many of the problems associated with cup and

propeller anemometers explained previously as the lack of moving parts

reduces continued calibration and maintenance needs, although they are similar

in that, both rotational and Ultrasonic anemometers are both linearly

proportional to the wind speed.

Accurate measurement of the three-dimensional wind vector can be obtained

using the 3-axis sonic anemometer, and provide both wind speed and direction

from a single unit as with the bi-vane propeller, where cup anemometers


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cannot. Sonic anemometers do it in a time resolution much higher than both a

cup anemometer and propeller anemometer. Although hot wire anemometers

can have sampling rates in the order of 10 kHz, their fragility in harsh

environmental conditions and the necessity to keep the wire free of dust

deposits can pose a great problem in their operation. Due to their complex

construction the cost of these anemometers can be significant. Other notable

problems associated with sonic anemometers include false readings at the

sensor can be obtained should the complex structure cause flow distortions by

impeding the wind vector. Acoustic blockages of the transducers can also be

caused by rain drops, ice, snow and other debris on the transducers, and

therefore not suited for marine applications as with the cup anemometer.

7.2.2 SODAR and LIDAR

SODAR is classed as a remote sensing system as it does not use a sensor to

directly measure the wind speed (Manwell, et al., 2003)

SODAR and LIDAR both are remote sensing device systems that are ground

level based and capable of measuring wind speeds at a range of heights .A

clear advantage is not requiring a large mast to mount the measurement

devices on thus having less visual impact on the surrounding environment.

SODAR is also primarily only able to give information on mean wind speed and

does not provide information on wind gusts. (Anon., 2011) .

SODAR and Ultra sonic anemometers both operate using acoustic principles

however differ greatly in their operating measurement range. As discussed

earlier Sonic anemometers study wind structure by emitting frequency between

closely spaced transmitters and receivers, whereas SODAR instruments look at

larger scale structures using a combined transmitter/receiver and being a

remote sensing tool, SODAR does not disturb the flow in the way that a met

mast does.

Defining wind profiles and evaluating higher elevation wind speeds using

SODAR, proves to be much more cost effective than their counterparts using

mast mounted meteorological instruments therefore SODAR is of clear

relevance to MW scale turbines. SODAR and LIDAR are ideal for short term


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data collection as they are portable, however considerations should be taken

should they be required for longer terms as the instruments could be subject to

vandalism or theft. Although portable, SODAR requires a steady surface ruling

them out of off-shore. SODAR can also measure the three dimension wind

vector should more than one antenna orientated in a different direction be

utilised. Vertical wind profiling can be derived using the Doppler effect of both

SODAR and LIDAR anemometry by measuring the propagation time.

The typical uncertainty in measurement of wind speed using SODAR is about 2

to 4%. Recent developments in SODAR have indicated improvement in

accuracy in correlation with wind speed and direction of up to 80 m/s with a 2%

difference for mean wind speed between SODAR and cup anemometry. (Lang

& McKeogh, 2011)SODAR systems are not yet in widespread use for wind

energy applications. They are not cost effective for smaller turbines and the

choice of suppliers is limited.

7.3 Thermoelectric anemometers

7.3.1 Hot wire anemometers

A hot-wire anemometer as stated earlier consists of a tungsten-wire element

heated by an electrical current. The hot wire anemometers works on the

principle of temperature coefficient of resistance. There are two types of

circuitry utilised in the construction of the heated sensing element constant-

temperature or constant-power. The constant-temperature sensor maintains a

constant temperature differential between a heated sensor and a reference

sensor; the amount of power required to maintain the differential is measured as

an indication of the mass flow rate of the wind vector. Constant-temperature

anemometers are popular because of their high-frequency response, low

electrical noise level, immunity from sensor burnout when airflow suddenly

drops, compatibility with hot-film sensors, and their applicability to liquid or gas

flows but have also serious limitations. The hot wire anemometers are

susceptible to atmospheric contamination, are expensive, require large power

inputs and do not respond to low wind speeds resulting in false readings.


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Atmospheric contamination such moisture will cause the thermometer to be

inaccurate and will ultimately lead to corrosion. They are less popular because

their zero-flow reading is not stable, temperature and velocity response is slow,

and temperature compensation is limited. The importance to correct flow

temperature changes during calibration, as well as changes which occur

between calibration and measuringsituations to standard conditions is a cause

for concern. The main cause of error in thermal anemometry is not accounting

for flow temperature changes. To reduce this affect calibration by measuring

both velocity and temperature effects of the thermal anemometer or by taking

measurements should be carried out in the same temperature controlled

apparatus that the calibration of the anemometer was completed. (Finaishand,

1994).


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8.0 The development and effect of turbulence in

relation to wind measurement

In this section will discussed the development and effect of turbulence in

relation to wind measurement and discuss the how anemometers perform in

these conditions.

8.1 The Importance of Accurate Wind Measurement

When assessing a site for a potential location for the introduction of wind energy

it is important to understand the necessity of collecting accurate information. In

turbulent environments it can be difficult to collect accurate data due to

problems such as volatile wind conditions. This type of problem can cause

errors in initial calculations of data collection for a proposed wind farm which

could lead to a project becoming a finical disaster.

8.1.1 What is Turbulence

Turbulence can be caused by surface ‘roughness’, this describes the nature of

the land in which the wind flows over (Katerina Syngellakis, 2007). It is a major

problem as it can cause wind turbine operation to perform irregularly and

ultimately cause the wind turbine to break down due to wear and tear caused by

the turbulent winds. Below table 1 gives an indication where hot spots for

turbulence may arise.

Fig 13(Webb, 2007)

In relation to wind energy/measurement the problem of turbulence can be found

in built up areas such as urban cities. In this section it is hope to explain the role

of the built environment in the creation of turbulence. For example Figure 1 and

2 show a graphic of wind flowing against a building; the arrows represent the


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current state of the wind flow at the point of interaction with the building. This is

hoped to highlight the danger of placing wind energy technology such as wind

turbines in turbulent environments without taking all the necessary

elements/data into account. The graphics in figure 1 and 2 show how this

turbulence occurs when wind flows around buildings and other obstacles found

in urban environments. These obstructions in the path of the wind flow cause

the wind flow to become distorted, this creates turbulence. These flow

disturbances can be seen in Figures 1 and 2. In Figure 1, a view of the side of

building can be seen; this shows how the wind flows over the top of the building.

The size of the arrows in the graphics demonstrates the intensity of the wind.

The shorter arrows and the arrow in the blue colour show how weak the wind

flow is while the longer arrows represent the wind at a high velocity. The section

with yellow arrows shows the area where there is the strongest wind, the

direction of this wind is mostly constant. It can be concluded from the two

graphics that if the turbine is best placed in the middle of the roof, this area is

the location with the least amount of turbulence and where the wind is also at its

strongest(Horst, 2007).

Fig 14(Horst, 2007)Fig 15(Horst, 2007)


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8.2 Turbulence Effect on Wind Measurement Devices

Traditional large scale wind turbines are designed to be located in open areas

such as rural areas or off shore where the wind characteristics can be

measured accurately. Measuring and collecting wind data for wind turbines

located in turbulent areas such as an urban environment can be a problem. In

non-turbulent areas conventional methods of collecting wind data experiences

very little fluctuations in wind speed or direction. This is not the case when it

comes to turbulent environments, in these environments conventional methods

of collecting data experiences sudden changes in wind speed and direction

frequently. This leads to the technologies which are very successful in their

traditional roles such as a cup anemometer in an open rural area becoming

inaccurate and failing to provide the correct data when applied to a turbulent

area. The following section will focus on individual types of anemometer and

explain the effects of turbulence on these technologies and discuss their

performance in turbulence environment.

8.2.1 Cup Anemometer

A cup anemometer encounters certain difficulties when expose to turbulent

areas. There are two so called difficulties that affect the devices capability to

collect accurate data.

Over speeding

Vertical component of turbulent wind

The first problem discussed will be the concept of over speeding and its

possible effects on poor data collection. Over speeding occurs in turbulent

winds where the cup anemometers mean speed result is higher than the true

speed of the wind (B Maribo Pedersen, 2003). This is created by the

fundamental nature of how the cup anemometer works. For this device to work

at all it must respond quicker to an increase in wind speed rather than response

to a decrease in wind speed. These results in a distortion of the true wind speed

as the device spend more time measuring above the mean wind speed then the

time spent below the mean wind speed. This distortion occurs with the presence


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of turbulence and will mean that the reading produced from the cup

anemometer will be larger than the true wind speed (Kristensen, 2005). The

second difficulty with the cup anemometer is the response to the vertical

element of the wind encountered in turbulent areas. It is said that cup

anemometers struggle with the three dimensional nature of the wind flow in a

complex terrain. Cup anemometers are designed to measure the horizontal

wind flow whereas an additional factor is introduced in turbulent environments,

this being the vertical component of the wind (Walker, 2004). These difficulties

have been thought to be the main factors in the problems cup anemometers

have in turbulent areas. Although this may be true, articles featuring

experiments performed have disagreed with these being the main problems and

believe a more fundamental problem is the cause for the larger readings. One

document states that over speeding only accounts for 0.2% of the errors

encountered with over speeding (B Maribo Pedersen, 2003). While another

document puts forward that the design of the cup anemometer is partly the

cause of the higher readings. It details that the shape of the cup can be the

deciding factor on how much an affect the vertical component with have on the

overall reading, stating that a design of a certain type of cup exposed to

turbulent winds can create drag and affect the overall operation of the device.

(A. Albers, 2000).

8.2.2 Propeller Type

The propeller anemometer is a rotational anemometer similar to the cup

anemometer. From research it was found that not many projects trust to

incorporate propeller anemometers solely for wind measurement in turbulent

environments. This is due to them providing limited information on turbulence or

the vertical component of the wind. In turbulent locations propeller type devices

tend to register a lower wind speed; the reason for this is the device has

difficulty maintaining its alignment with the instantaneous wind direction (D.C

Anderson, 2008). Like the cup anemometer the propeller anemometer does not

react accurately to sudden changes in wind speed or direction, this type of

environment being commonplace in turbulent/urban areas. This will have a


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crippling effect and result in essential data not being recorded by the device.

Another interesting point was found in one such article where it stated that a

propeller anemometer cannot measure the vertical component of wind (M.C.H.

Hui, 2009). In turbulent/urban locations the presence of buildings adds a strong

vertical component to the wind (D.C Anderson, 2008). This can in addition to

the reasons shown above explain why a propeller type anemometer may not be

suited to turbulent applications unless incorporated with an additional device.

This statement is backed up by a journal article where it states a propeller

device “dynamic behaviour in turbulent environments may be less than

satisfactory” (Morris et al., Not Stated). Also in this article a cup, sonic and

propeller anemometer are compared in a turbulent environment with the result

being the propeller device been labelled inferior to the two other devices. The

reason stated being the wind speed discrepancy caused by under speeding

(Morris et al., Not Stated). It may also be mentioned that the propeller type

device has problems structurally when exposed to turbulent environments.

8.2.3 Sonic Anemometer

A sonic anemometer is seen as one of the best instrument for the measurement

of turbulence (Bowen, 2008). Advantages of the sonic anemometer include no

moving parts and the fact that it can detect the vertical component of wind

successfully over an extremely quick period of time (D.C Anderson, 2008).

Another advantage is its capability to capture wind and turbulence data in very

low wind. The sonic anemometer compared to cup anemometers in low wind

speed (2ms) shows that cup devices measure higher wind speed more than

sonic 93% of the time. This once again is a problem associated with turbulent

conditions and over speeding (Bowen, 2008). This has caused a lot of

organisations to replace traditional device such as cup anemometers with sonic

anemometers. As discussed above this type of anemometer has an advantage

over cup or propeller devices in that it can measures all three components of

wind (U, V, and W), W being the vertical component which means the sonic

anemometer does not produce non-linearity’s errors which is common with the

more mechanical type devices (B Maribo Pedersen, 2003). One problem with


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this device is how it is affected by the external environment, for example the

presence of rain can affect performance.

8.2.4 SODAR

When a SODAR anemometer is utilized to perform in a turbulent area the result

usually ends up with the device underestimating data. This is due to the devices

inability to detect small scale fluctuations. A SODAR anemometer has a low

sampling rate and when used in turbulent areas it is exposed to large sampling

rates. This causes the underestimation in the devices produced wind data (R

THOMAS, 1991). Another disadvantage of SODAR is that when placed in urban

areas they tend to pick up noise disturbance causes by passing cars etc. This

distortion can lead to the device underperforming in these conditions.

8.2.5 LIDAR Anemometer

LIDAR anemometers are challenging cup anemometers in some areas of wind

measurement as hub heights increase. Remote sensors are becoming popular

in this area. This type of situation would be located at wind farms where there is

a flat terrain. The performance of LIDAR in more complex terrain where there is

unpredictable wind flow is in question. LIDAR performance in flat terrain is good

where it only shows a small percentage of error (Ferhat Bingol, 2009). Its has

been found though when LIDAR is assigned to collecting wind data in more

turbulent areas the error level can go as high as ten per cent. This error occurs

as result of the workings of the LIDAR anemometer, LIDAR use the

“assumption of horizontally homogeneous flow” (Courtney, 2011) to calculate

horizontal wind speed. This works well in non-turbulent flat areas. The error of

up to ten per cent occurs in the unpredictable wind flow which is caused by

turbulence; this turbulence interrupts the element of horizontal wind flow which

the LIDAR usually uses to create good accurate results in settled flat terrain

(Ferhat Bingol, 2009). A possible solution to this problem would be to

incorporate many LIDAR anemometers and aim each one at one point. This will


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result in the assumption of homogeneity of the flow not being required. Also a

downside of the technology is its inability to measure the horizontal component

of wind as accurately as cup or sonic devices (Courtney, 2011)

8.2.6 Cup vs LIDAR

From research it was found that the leading devices in this field are cup and

LIDAR anemometers. It is universally recognised that the cup anemometer is

leading the way but only by a short distance from the emerging LIDAR

technology. It was thought the difference between the two was random noise on

the side of the LIDAR device which caused errors in result. Though one

document argues that evidence from experiments undertaking describe the

difference more to do with the two different probing methods. The cup employs

point measurements while the LIDAR using volume measurements (Courtney,

2008). This problem stands from the fact that when LIDAR measures in volume

it takes into account the vertical and horizontal extents with non-linear

weighting, the difference between the vertical and horizontal measurements

cannot be interpreted (Michael Courtney, 2008). This problem along with the

trouble measuring the horizontal component means the cup anemometer is the

most suited device for use in turbulent environments. It should be noted that all

the devices included in this section are evolving and improving at a rapid rate.


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9.0 References

A. Albers, H. K. D. W., 2000. Outdoor Comparison of Cup Anemometers.

Messung von Wind und Leistung, 1(1), pp. 107-111.

Adam Havner, R. V. C. a. J. O., 2008. Verification Testing of Wind Speed

Measurements from 2D Sonic Anemometers.

Albers, A., 2010. How to gain acceptance for LIDAR measurements. [Online]

Available at:

http://www.windguard.de/filedmin/media/pdfs/UEber_Uns/DEWK_2010/paper_

WindGuard_LIDAR_Acceptance_DEWEK10.pdf

[Accessed 14 november 2011].

Anon.,2010.Camblescientific.[Online]

Availableat:http://www.campbellsci.com.au/05103-l

[Accessed novmber2 7 november 2011].

Anon.,2011.AboutSODAR.Online]

Available:http://www.SODAR.com/about_SODAR.htm

[Accessed 2 October 2011].

Anon.,2011:waterencyclopedia.[Online]

Available_http://www.waterencyclopedia.com/Ce-Cr/Climate-and-the-

Ocean.html

[Accessed november 2011].

ASTM Inernational, 1996. ASTM International Standards Worldwide. [Online]

Available_at:_http://www.astm.org/index.shtml

[Accessed sat 19 november 2011].

B Maribo Pedersen, T. F. P. H. K. N. v. d. B. N. K. J. Å. D., 2003. WIND SPEED

MEASUREMENT AND USE OF CUP ANEMOMETRY. Research and

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