Anemometer

Windmill type anenometer (known as of Robinson)

The windmill type anenometer was invented by John Thomas Romney Robinson. It is composed of 3 half-shells (of the size of a ball of Ping-pong) laid out on horizontal arms laid out with 120 degrees and rises on a vertical axis equipped with a device of counting of turns; the number of revolutions of the anemometer is proportional to the speed of the wind.

Indeed, one measures the speed of the wind by the following formula: $V = R \ times \ Omega \,$
with:

R \,

: average radius of the arms :

\ Omega \,

: number of revolutions in radian a second

\ Omega = 2. \ pi. NR \,

N \,

: mode of rotation out of turn a second

Example: R = 4 cm = 0.04 m

V = 0.04 X 2 X 3.1415 X NR = 0.25 NR

V = 0.25 NR

For the standard anemometer (diameter of the cups of 6 centimetres), a number of revolutions of a turn a second corresponds to a blowing wind at 1 m/s, that is to say 3,6 km/h. This type of anemometer is able to measure the speeds of wind ranging between 0 and nearly 200 km/h. High speeds would produce constraints which the cups could not support. But of the winds of such a violence meet only in the Tornade S or the Cyclone S.

When the wind blows, it meets alternatively a cup digs then convex. According to the laws of aerodynamics, a hollow opposes more resistance than a form convex to the passage of the surface. This difference causes the rotation of the anemometer.

The two great merits of this anemometer are its simplicity and practically the absence of a limitation in the measurable speed range; but if it is used without equipment of electronic data record, a short gust but forces is not recorded. Unfortunately, when Robinson presented its anemometer, he declared that the size of the cups and the length of the arms did not have an influence on the result of measurement. This assertion was apparently confirmed by some independent experiments. It proved later that the relationship between the speed of the wind and the dimension of the cups (the factor) is not constant and depends mainly on the dimension of cups and the length of the arms. It results from this that the values quoted in the official publications of the 19th century comprise errors going up to 60%.

Propeller anemometers

Resembling small planes, they are coupled with a Girouette and are directed in the direction of the wind. The propeller, which measures the speed of the wind, turns around a horizontal axis.

Among the types of less current anemometers, one finds anemometers made up of two propellers turning around two fixed, horizontal and perpendicular axes. One calculates the force and the direction of the wind by correlating the number of revolutions of both hélices.aussi for the wind mills.

Anemometer with ultrasound

There exist also anemometers with Ultrason S. the measurement of the wind is based to the measure of the duration of displacement of an ultrasonic wave. Two couples of ultrasonic transducers are alternatively transmitting and receivers of an ultrasonic wave train. Times of transits outward journey and return are measured and one from of deduced, by difference, the speed of the wind along the axis formed by the two transducers. The interest of this type of anemometer is not to have parts moving and to be able to measure a turbulent wind.

Pressure plate anemometer

Simplest of the anemometers of this type consists of a square or circular plate maintained opposite the wind by a Girouette. One measures the force exerted by the wind on the plate. The instruments of this type are very precise for the winds weak or during slow variations of this one.

Tube anemometer

The anemometer of Lind consists simply of a tube out of U containing a liquid with an end folded horizontally to face the wind.

The blowing wind in the opening of the tube causes a pressure which can be transmitted to any instrument. The tube can be guided by a wind vane.

Practically, the system can be sensitive to any wind which can make turn the wind vane. Its advantage is that it can be installed in places of difficult access as in top of a post with the device of recording to its foot.

It can be installed without needing lasting maintenance of the years.

Tube anemometer of Pitot

The tube of Pitot owes his name to the French physicist Henri Pitot (1695-1771) who was the first in 1732 to propose a “machine to measure the speed of running water and the wake of the vessels”. The concept is taken again and improved by Henry Darcy then by Ludwig Prandtl which thinks of using the tube in a drain to measure local speeds of flow of the fluids.

In aeronautics from the technological point of view, he is the successor of the Etévé system. The tube of Pitot is a component of the anemobarometric system. It consists of two concentric bent tubes of which the openings, in communication with the fluid of which one wants to measure speed, are laid out in a particular way. One, placed orthogonally, has a relative speed v equal at the speed of the fluid and a static pressure PS equal to the environmental pressure.

The other, placed in the direction of the flow, has a null relative speed and a total pressure Pt, sum of the dynamic pressure and static pressure. The difference between these pressures gives the speed air of which one can deduce the ground speed, these two parameters being paramount information for the pilot who needs some to calculate his displacement in space (navigation) and the consumption of his vehicle. It is then enough to apply the theorem of Bernoulli by neglecting term Z to have a direct relationship between speed and dynamic pressure Pt - PS which one measures with a pressure pick-up or a simple pressure gauge. V ² /2 + Ps/P = 0 + Pt/P from where V ² = 2 (Pt - PS) /P

PS: static pressure Pt: total pressure P: mass volume V: speed

Hot wire anemometer

One heats a wire while making there cross an electric current. Electrical resistance increases with the temperature. The wire is cooled by the wind. The more the wind blows extremely, the more the wire is cooled and the more electrical resistance decreases.

Other devices to study the wind

ultrasonic Measurement based on the Doppler effect
Measurement of the displacement of a particle with two laser beams.
the Air intake

See too

Wind vane
Air intake

External bonds

See examples of anemometer.

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The anemometer is also the apparatus which makes it possible to measure the relative wind on an aircraft. See the article Instruments of flight .

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