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How Ultrasonic Wind Speed Anemometers Work

Updated: Aug 14

An ultrasonic anemometer is a type of wind speed measuring device that utilizes the properties of ultrasonic waves. Unlike mechanical anemometers, which rely on physical parts to capture wind speed, ultrasonic anemometers measure wind velocity based on the time it takes for ultrasonic waves to travel between sensors.

An ultrasonic anemometer consists of at least two ultrasonic transducers (or transceivers), which can both emit and receive ultrasonic sound waves. In a typical setup, these transducers are positioned in a precise arrangement (often orthogonal or at 90 degrees to each other) to provide readings of wind speed and direction across multiple axes.

The operating principle of an ultrasonic anemometer is grounded in the physics of sound wave propagation. When the anemometer emits an ultrasonic pulse, this pulse travels through the air from one transducer to the other. The velocity of the sound wave is impacted by the movement of the air masses it travels through - in other words, the wind.

In still air, the sound pulse would take the same time to travel from Transducer A to Transducer B as it would to travel from Transducer B to Transducer A. However, when there is wind blowing, it will either hasten or slow the travel time of the sound wave. If the wind is blowing in the same direction as the sound wave, it will arrive at the receiving transducer more quickly. Conversely, if the wind is blowing against the direction of the sound wave, the arrival time will be slower.

By measuring the time it takes for the ultrasonic pulse to travel between transducers in each direction, the anemometer can calculate the speed of the wind. The difference in travel time is directly proportional to the wind speed parallel to the line between the transducers.

For measuring wind direction, more than two transducers are often used, typically in a three or four transducer setup. By comparing the differences in travel times across multiple pairs of transducers, the anemometer can also calculate the direction from which the wind is blowing.

Ultrasonic anemometers are valued for their precision and reliability. They have no moving parts, making them less prone to mechanical failure and wear. They also respond quickly to changes in wind speed and direction, making them ideal for applications that require real-time or near real-time wind data, such as weather stations, wind turbines, or atmospheric research.

However, ultrasonic anemometers also have their limitations. For instance, they can be affected by temperature fluctuations, humidity, or other environmental factors, which can impact the speed of sound. To overcome these challenges, most ultrasonic anemometers incorporate temperature sensors and use sophisticated algorithms to correct for these environmental factors, ensuring accurate and reliable wind speed and direction measurements.

Additionally the measured wind speed at the position of the anemometer might not be the wind speed in front of the turbine. The rotor already took some energy out of the wind and might have deflected the wind direction as well as increased turbulence intensity behind the rotor.

Therefore, each OEM has measured some individual wind speed and direction correction functions that are applied to the wind speed measurements. Unfortunately these correction functions are not public and sometimes the algorithm even use the power output as a correction function input. Moreover, these correction functions can change over time after a software update or parameter update of the turbine.

This has a tremendous influence on the legally binding power curve of a turbine. One can even not trust that these corrections have been made individually for not well performing turbines. With Turbit's power monitoring however you can get a chance of noticing even the slightest changes in these settings. Due to the fact, that we use all these different variables that have an effect on the turbine performance as an input we can detect event slightest changes in the power to wind relation. If Turbit detects an event this can then only have two reasons, either the wind speed correction function has been updated, or we really have a change in the power output behaviour of the turbine.

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