Anemometer - Measurement of Wind Speed

What is Anemometer?

The first mechanical ‘Anemometer’ was designed by an Italian mathematician and artist Leon Battista Alberti in the year of 1450. Since then anemometer has been changed and developed in various forms and designs. The word ‘Anemometer’ is of Greek origin. The word ‘anemos’ is a Greek word which basically means wind or air. Anemometer is a scientific device which is used not only to measure the speed of the flowing wind but also to know the direction of the moving wind. In short, anemometer measures velocity of the wind. Furthermore, it has an application to measure the pressure of the wind or any stream (current) of a gas. This device has a significant place in the field of meteorology as it is beneficial to study the patterns of the weather which eventually aids to monitor and estimate the weather conditions. Anemometer is an important tool to perform experiments for physicists as well.

 

How to Make an Anemometer?

A simple anemometer can be made at home using objects that can easily be found at home. 

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First get five plastic or paper cups. Make holes in all the four cups in the middle on the side at equal distance from the rim. In the fifth cup, create four holes at the edge of the rim of the cup and the holes should be perpendicular to each other. One pair of hole should be at a little distance from the rim, otherwise straws will obstruct each other if they pass through. One hole in the middle of the cup at the bottom is also needed. The size of the hole should be big enough so that the straws can be fitted in the holes. Put a straw in the fifth cup through the bottom hole and in each four holes which were created perpendicular to each other at near the rim. Next, insert the other four cups individually at each straw coming out from the fifth cup’s rim. Therefore, the whole set up consists of a fifth cup in the middle with a straw sticking out from the bottom and four cups are stuck on the straws passing through the holes at the rims of the fifth cup. The opening of each of the four cups should face the back of the other cup. Now, the anemometer is ready. As the wind would flow, the four cups would start rotating. Cups would rotate at higher speed if a strong wind flows. Hence, one can calibrate the rotation of the cup per minute with the speed of the wind. A wind pane can be attached with the vertical straw that would denote the direction of the flow of the wind. This is the simplest kind of anemometer that comes under the category of cup anemometer.

 

Types of Anemometer

On the basis of the units (of velocity or pressure) on which they are based, anemometers can broadly be categorized in two kinds as given below:

A. Velocity Anemometers

B. Pressure Anemometers

Furthermore, depending on the function and design, each category as mentioned above comprises of different kinds of anemometers, discussed in detail. Numerous kinds of Velocity Anemometers are as following:

 

Cup Anemometer (Or Rotational Anemometer): Cup anemometer is a mechanical anemometer which is not just one of the most popular but also the oldest and the simplest anemometers. This anemometer may consist of two or three or four hollow hemispheres cups mounted separately on different horizontal rods (or arms) which altogether mounted on a vertical rod. Two, three and four cup anemometers were discovered by John Patterson and John Thomas Robinson, respectively.

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To understand the working of the cup anemometer, consider the two cups anemometer. In this case, the two cups are arranged in such a way that the hollow side of each hemisphere faces opposite to each other. That is, the hollow side of one cup and the backside of the other cup will face the wind. The function of the cup anemometer depends on the different drag coefficient of the hollow and backside of the hemisphere of the cups. Due to the difference in drag coefficient, a different magnitude of forces is exerted by the wind on the hollow and backside of the two cups and hence torque is created which results in the rotation of the anemometer. 

As the wind starts to flow, depending on the direction of the flow, cups begin to rotate either in the clockwise or anticlockwise direction. The rotational speed is directly proportional to the speed of the wind. The device is connected to a readable digital sensor. This digital reader calculates the number of rotations per minute of the cups and subsequently, determines the velocity of the flowing wind. As this tool depends on the rotation of the cups due to wind, hence, it is also known as a rotational anemometer. The hollow hemisphere cups are made of good quality and have the ability to endure the rough weather. Therefore, cup anemometer works perfectly for a long duration in bad weather. On the other hand, the only shortcoming of this device is that it is vulnerable to the icing effect.

 

Vane Anemometer (Or Windmill Or Propeller Anemometer): Vane anemometer also comes under the group of mechanical anemometer and the working principle is quite similar to the cup anemometer. However, unlike the cup anemometer, the rotational axis of the rod in the Vane anemometer is horizontal, which is parallel to the flow of the wind. Hence, the rod rotates horizontally.

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The horizontal rod consists of a propeller and tail attached at either of the ends. Therefore, as the wind flows the rod spins horizontally and changes accordingly with the variation in the direction of the flow of the wind or air. This is also commonly known as windmill or propeller anemometer. Higher the wind speed/flow, the higher will be the rotational speed of the propeller attached at the end of the horizontal rod of the anemometer. This anemometer can stand the strong wind flow.

 

Hot Wire Anemometer: The hot wire anemometer works on a unique principle. It determines the speed of the wind by measuring the difference in the temperature of the wire in the wind and in the absence of the air. This device consists of an extremely thin wire of the length of several order of micrometer. This wire is to be heated via electrical method. The wire must be hot enough so that the temperature of the wire is above the temperature of the ambient environment. The wire loses the heat energy and cools down as the wind passes the hot wire. Hence, the temperature of the wire drops down with the constant flow of the air or wind. Moreover, it is known that resistance of an electrical wire changes with the variation in temperature.Therefore, current needs to flow to maintain a particular resistance in the wire. A relation can be established between the wind flow and the current required (which is basically a change in resistance). Ultimately, this relationship basically determines the velocity of the wind with the loss of heat energy. Following Ohm’s law, in order to maintain a variable such as voltage, current or temperature constant, this instrument produces a resultant output voltage due formation of an internal circuit.

The hot wire anemometer can be divided further in the subgroups:

Constant Current Anemometer

Constant Voltage Anemometer

Constant Temperature Anemometer

Hot wire anemometers have several advantages over other anemometers. The hot wire anemometers have extremely high response. Having no rotating part in the device makes it a long-lasting device.

 

Laser Doppler Anemometer: In this device, one of the spitted laser light beams is allowed to pass through the Laser Doppler anemometer. There are air molecules flowing along with the air at the exit of the laser beam. The beam, hence, gets reflected from the particles, back to the detector. The reflected beam is measured in reference to the original beam.

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Doppler shift is observed when the laser beam interacts with the fast moving particles. This shift is used to measure the speed of the particles and henceforth to calculate the wind speed.

 

Ultrasonic (Or Sonic) Anemometer: Ultrasonic waves were employed to determine the wind speed for the first time in 1950. In this anemometer, the time is measured for an ultrasonic waves to travel back and forth from the transducers. The signals from the different pairs of transducers are combined to generate the one dimensional, two dimensional or three-dimensional pattern of the wind speed flow.

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Moreover, the ultrasonic anemometer is had a spatial and temporal resolution of 10-20 cm and 20 Hz respectively which makes the device suitable for turbulence measurement. 

This anemometer is highly stable and resistant to bad unlike cup anemometers whose performance deteriorates as the wind consist of sand or salt. And also it is independent of the icing effect. However, there is the obstruction in the wind flow due to the structures holding the transducers and hence require extra effort to process the signal. One more shortcoming is that this is highly expensive and therefore, is rarely used.

 

Acoustic Anemometer: This is one of a kind of ultrasonic anemometer. In this tool, there is a cavity that consists of numerous ultrasonic transducers. These transducers help in creating a particular standing wave in the cavity. As the wind flows through it, there is a phase shift in the standing wave. Phase shift data is collected using sensors and thereafter is processed mathematically to find out the speed and the direction of the wind.

 

Ping Pong Anemometer: This device comprises of a ping pong ball which is attached to a thin string. As the wind flows, ball-string being light-weighted gets displaced horizontally from the original position. The speed of the wind is calculated by finding the angle between the displaced ball and the vertical.

Pressure Anemometers measure the pressure of the flowing wind. Following are the types of pressure anemometer:

 

1. Plate Anemometer: As the name suggests, these anemometers consist of flat plates of either circular or square shape. These plates are suspended from the top which is kept normal to the wind by a wind pane. The plate gets deflected due to the flow of the wind and hence, exerts pressure on the face of the plate. The exerted pressure is balanced by a spring. The compression of the spring due to the force applied by the wind on the plate is read by a gauge which ultimately determines the pressure of the wind. This tool is not suitable for light wind.

 

2. Tube Anemometer: This device comprises of a U-shaped tube with a liquid manometer. One end of the U-shaped tube is bent horizontally and faces the flow of the wind while the other end is straight in the vertical direction and is parallel to the flow of the wind. When a wind is blown on one end, it increases the pressure in the monometer on the other end. Hence, the flowing wind results in a pressure gradient at the two ends of the monometer that eventually results in the change in the height of the liquid. Therefore, the difference is calibrated to calculate the pressure of the wind.

 

3. The Pilot Tube Static Anemometer: In this device, two different tubes are known as the Pilot and the static tubes are used. The Pilot tube measures the dynamic pressure of the wind while the static pressure is measured by the static tube. Later on, Bernoulli’s equation is used to determine wind speed. Pilot tube static anemometer works perfectly even in strong wind. Moreover, this device demonstrates a quick response. Though it is inefficient in case of fluctuating wind.

 

Uses of Anemometer

Anemometers are used for detecting the weather condition, wind speed and the pressure of the wind. Professionals studying the environment and weather for research can use an anemometer.

Anemometers can be used for the following cases:

In Weather Station: To report weather condition for the general public

At the Airports: Important to know the weather before the flight takes off or lands.

On Ships or Sailboats: The anemometer plays an important role in the sailboat to monitor the directions and weather.

Anemometer can be used to determine the flow of air in the wind tunnels or gas chambers.

People living in undeveloped areas can make their own anemometers and can use them to determine the weather.

Windsurfers also use anemometers to know the weather which eventually affects the waves in the ocean.