Anemometer

As anemometer ( from Ancient Greek anemos ( ἄνεμος ) =, wind ', and métron ( μέτρον ) = measure ') or anemometer various measuring instruments for local measurement of the velocity of a flow field are referred, in particular wind speed.

History

The oldest known anemometer is the winch plate, which is also called plates, vibration plates, pressure plates, or even Deflektionsplatten anemometer. This instrument was invented probably in 1450 by the Italian architect Leon Battista Alberti, and then described by Leonardo da Vinci around the year 1500 in a sketch by our own design.

In 1667, the Royal Society in London again mentioned a vibrating plate anemometer. The re- invention is Robert Hooke attributed. Sir Robert Hooke and Sir Christopher Wren had a common interest in meteorology, both started very early with their discoveries and they developed at the age of 15 years, the first thermograph. 1663, the apparatus was expanded with a precipitation and wind sensors. Between 1672 and 1678, Hooke has added his own specially developed combined weather recording device, a first vane anemometer. A description of it was published in 1726 in Weatherford Wiser. After the completion of his weather recording device he worked on the development of the Flügelanemometers and presented in 1683 a mobile device from which show the number of turns and was able to change the angle of the blades to the wind.

According to another embodiment of the impeller anemometer by Christian Wolff in 1743 Will man invented in 1790, the water flow meter with impeller. The instrument had two wings and a revolution counter. They say ye man after he had proposed to use this as anemometer.

1837 was built by the chief engineer of a French mine Charles Combes a derivative of Will 's water flow meter anemometer. This had a protective tire around the impeller and also a rotation counter. Between 1845 and 1862 the meter has been expanded to include a switch with which the counter was a coupled and decoupled. This apparatus was later extended by a return way, with the counter of the counter could be set to zero. This anemometer could be used for wind speeds from 0.4 to 5 m / s, as it was required in the mine ventilation.

In England, they were named after Benjamin Biram Biram 's anemometer as who first received a patent in 1842 on this design. They have been manufactured since 1845 by John Davis in Derby, from around 1862 with a peripheral ring of protection Casarelli.

By L. Casella of London then an anemometer was around 1870 designed to measure the ventilation in a hospital. This instrument has now been measured currents from 0.27 to 50 m / s.

Another measurement method using a dynamic pressure tube is named after James Lind Lind 's anemometer.

The first manometric anemometers to such is at the anemometer was Lind'schen, 1721 described by Pierre Daniel Huet, Bishop of Avranches in Normandy, France. The apparatus consisted of a mercury filled U-tube. One end of the tube was angled at 90 degrees and is aligned with the opening to the wind. The difference in the mercury levels in the U-tube is proportional to the wind pressure, and thus to the square of flow rate.

The English Pastor Stephen Hales in 1743 described a similar device. This apparatus was filled with water and was so sensitive for lower wind speeds.

1775 Lind went on to describe in his work Description and use of a portable windgauge a pressure tube anemometer, which was the first practical instrument of its kind.

1846 then the wind strength was measured by astronomer Thomas Robinson by a newly developed by him anemometer with hemispherical shells. This construction had initially only two such shells, and later another pair of shells was placed crosswise in order to more uniform torque. This type of anemometer is the cup anemometer today for meteorological measurements in use. It is probably the best known design of an anemometer.

Winch plate

Two well-known types: vibrating plate and Deflektionsplatte.

Vibration plates anemometer: Rectangular plate is rotatably mounted at its upper edge. Depending on the wind speed, the plate is more or less moved from its vertical position. On a scale behind the plate the same wind strength can be read from the angle.

Deflektionsplatten anemometer, also called pressure -plate anemometer: Here is a plate -area head-on into the wind and is displaced from this without changing the angle of inclination on a slide linearly to the wind direction. A pull rope which is held taut over a steering wheel with a weight that moves a pointer over a visible scale.

This simple measuring devices can not accurately display the wind speed, partly due to the fact that it is in the nature of the wind to flow evenly rare, partly by the fact that in the reaction of a winch plate inertia prevents an immediate response. In addition, commutes or swings the plate 's laws of gravity or spring-back - depending on the design - obeying and then shows different results.

Vane anemometer

A vane anemometer is essentially a small, almost free-running wind turbine: For fluid flowing therethrough, the rotor rotates just fast enough that the usually six to ten steep salaried leaves are flows almost parallel to the sheet surface. The potentially much larger buoyancy compensates only from the flow resistance and the friction of the bearing and possibly a display device. For a low starting resistance, the wheel is very slightly built and the axis thin; a surrounding cylindrical ring is used for mechanical protection. In modern instruments with optical or magnetic encoder, the impeller can less (down to 14 mm diameter) fail, because there is no counter or eddy current speedometer has to be driven. The measured velocity of the flow is calculated electronically from the angular velocity of the impeller with modern appliances, in older devices this was mechanically transmitted to a display that is scaled for matching the angular velocity of the impeller is acknowledged.

Vane anemometer suitable for measuring low currents, but keep because of their idling too fast flows and the operation in liquids. Handsets can be seen in use during the flight and sailing, for private use, but also in professional applications among others in the ventilation and heating technology. Stationary vane anemometer to measure the wind require a Yaw. They are better suited as a cup anemometer for operation with regenerative donors.

Cup anemometer

The cup anemometer, cup anemometer also, or Kugelschalenanemometer has a vertical rotor axis - a yaw system is redundant. The speed at which circulate the most three semi -spherical shells, is added upstream of the wind speed, while downstream of the apparent wind is less. This variation of the inflow is compensated by the Richtungsabhängkeit the drag coefficient. The fact that the wind both drives and brakes, is to fit the wind speed is established. The tip speed ratio is depending on the shape of the shells and possibly resistance of a display device 0.3 to 0.4. Compared to the impeller, the torque does not vary so strongly with the tip speed ratio, so that the speed less responsive to changes in wind speed.

Cheap handsets speak from about 1 m / s ( walking speed ), more expensive even at low speeds.

Staudruckanemometer

A Staudruckanemometer measures the pressure difference between total pressure and static pressure. The total pressure in addition to the static pressure includes the kinetic energy of the flow per unit volume and adjusts, if the flow is dammed to a standstill. A Prandtl probe, as shown in Figure, is directed against the flow opening to measure the total pressure and small on its circumference openings is parallel to the flow to measure the static pressure, the pressure differential previously detected by a differential pressure transducer, with a U- tube manometer or displayed. About the density can be inferred on speed. In particular, when used in gases is important to ensure that the density of pressure and temperature dependent, and in liquids, the hydrostatic pressure can be felt, especially in accelerated probe.

Since the kinetic energy depends on the square of the speed, Staudruckanemometer are not particularly well suited for measuring small wind speeds. Staudruckanemometer formerly belonged as part of the gusts writer of the standard equipment of meteorological stations.

Sonic anemometer

Ultrasonic waves are the medium in which they propagate, carried, so that the term of signals over a measured distance fixed-length depends on the flow through the test section. Runtimes with high frequencies (more precisely, high-bandwidth, see pulse compression ) can be determined precisely, so that high frequencies are used for short distances. Since the speed of sound depends on the temperature and the humidity, running time is always determined in both directions. From the difference of the transit times and the virtual temperature can be calculated.

A sonic anemometer usually has several measuring distances between ultrasonic transmitters and receivers, on the alternating the speed of sound is measured in different spatial directions. It calculates an electronic measuring the horizontal and vertical wind speed. Advantages of the sonic anemometer are the higher accuracy, the lack of inertia in the system and the possibility of additional acquisition of the vertical wind component. The sampling rate depends on the sound propagation time on the measured distances. In three test sections of 20 centimeters in length, which are measured sequentially in both directions, the total sound duration is approximately five milliseconds. Thus, up to 200 measurement cycles per second are possible.

Ultrasonic wind sensors based on acoustic resonance

A recent development in the field of ultrasonic wind measurement is based on acoustic resonance. While conventional ultrasonic anemometer based on the measurement of the transit time is reflected at the wind sensors with acoustic resonant ultrasonic wave in a small cavity.

In the cavity there are several vibrating membranes that generate and receive ultrasonic acoustic waves. By repeated reflection between the reflectors a quasi- standing wave is generated perpendicular to the wind direction and a transverse wave in parallel to the wind direction. Air flows along the axis between the reflectors, which affects the propagation speed of the wave, and generates a phase shift is measured. To compensate for changes in the speed of sound, for example, by temperature changes, the ultrasonic frequency is continuously adjusted so that the cavity is operated in resonance. Thereby, the measurement of the air velocity regardless of the variable speed of sound.

From successive measurements of membrane pairs of the vector components of the air flow can be determined and thus calculate wind speed and direction mathematically.

Other principles

After the design and the measurement principle, a distinction is beside the anemometers already mentioned above:

• Hot-wire anemometer ( thermal anemometer ): In this case, a heating wire is heated and flows around the wind. So the wire, heat is extracted and associated heat loss can be concluded that the wind speed.
• In the laser Doppler anemometry (LDA ) is a stationary interference pattern is established with a laser light in two directions. Flows in fluids entrained particles scatter the light as it passes through the interference pattern. The intensity of the scattered light is measured and determined in its time profile the speed of the flow perpendicular to the stripes of the interference pattern.
• Particle Image Velocimetry
• Surface pattern image velocimetry
• Thermoelectric anemometer
• The wind speed can be estimated phenomenologically even without the use of a measuring instrument based on the Beaufort scale.

Areas of application

Together with a wind direction sensor and a recording device are fixed anemometer part of weather stations. Measurement data from the temporary structures in the appropriate amount necessary for the siting and design of wind turbines. On larger aerodromes are usually transmitted from a plurality of measurement points from the data of the wind measurement for air traffic control. At the upper end of the mast of a sailing boat, a cup anemometer is often mounted which determines the magnitude of the apparent wind.