How Ultrasonic Measuring Wind Speed
Let us first acquaint ourselves with ultrasonics. The prefix "ultra" in ultrasonics signifies that its frequency range exceeds the auditory perception of humans, yet when analyzed from a wavelength perspective, ultrasonic waves actually possess shorter wavelengths. Scientists define the distance between two adjacent peaks or troughs of a wave as its wavelength, and the wavelengths of mechanical waves audible to the human ear range from 2 cm to 20 m (2 centimeters to 20 meters). Hence, we refer to mechanical waves with wavelengths shorter than 2 cm as "ultrasonics."
Fundamentally, ultrasonics bear no difference from the sounds we hear, save for their inaudibility to us.Upon discovering ultrasonics, people have identified several fascinating characteristics they exhibit.
Firstly, they possess high frequencies, typically beyond the range of human auditory perception, yet some animals, such as dolphins and bats, rely on ultrasonics to substitute for vision.Secondly, ultrasonics have short wavelengths. As frequency is inversely proportional to wavelength, high frequencies correspond to short wavelengths. This property enables ultrasonics to be employed for precise detection and imaging of relatively small objects or structures. Ultrasound scans (sonograms) utilize this principle.Additionally, ultrasonics exhibit strong directionality and energy concentration, allowing them to propagate in a linear fashion within a certain range. Ultrasonics can also carry immense energy. Ultrasonic cleaning, for instance, employs the substantial energy of ultrasonics to induce cavitation in liquids for cleaning purposes.
Ultrasonics are also susceptible to the influence of mediums. Like sound waves, their propagation speeds vary in different mediums due to the density, elasticity, and temperature of the medium. This variation facilitates ultrasonic flaw detection.
Measuring Wind Speed with Ultrasonics
Now that we understand the characteristics of ultrasonics, how exactly are they utilized to measure wind speed and direction?
I discovered an excellent project on GitHub, which conveniently demonstrates how to measure wind speed using ultrasonic technology.You can find the link here: https://github.com/majianjia/QingStation. If you wish to delve deeper into the subject, feel free to visit the provided link.
In essence, the principle is as follows: When ultrasonic waves (pulses) propagate within a flowing medium (air), the time taken for the waves to reach their destination differs.
The time discrepancy between forward and backward propagation reflects the flow velocity of the medium, i.e., the wind speed.The speed of sound is influenced by airflow relative to the air. Assuming a fixed speed of sound, the wind speed can be calculated accordingly.
The principle is remarkably straightforward: sound waves propagating within a medium (air) are affected by the movement of the medium itself.By utilizing the known propagation path and time of propagation, we can determine the velocity of the medium.
As illustrated in the diagram above, the travel of wind BC, combined with sound propagation AB, results in the travel path AC.
The formula can be expressed as: α = atan(2*H/D) v = H/(sin(α) * cos(α)) * (1/t1 - 1/t2) c = H/(sin(α) * (1/t1 + 1/t2))
The variables are as follows:
- α is an angle, calculated using the arctangent function atan, with 2*H/D as its argument;
- H and D are distances, representing the distances between the reflector and the sensor and between the transmitter and receiver, respectively;
- t1 and t2 are time parameters;
- v denotes wind speed;
- c signifies sound speed.
To measure wind direction, arctan2 can be employed: β = atan2(NS, EW)
The variables are as follows:
- β is an angle, calculated using the two-argument arctangent function atan2, with NS and EW as its arguments;
- NS represents the direction from north to south;
- EW indicates the direction from east to west.
atan2 is a commonly used function in computer science that returns the angle of point (EW, NS) relative to the origin, ranging from -π to +π. This function considers all four quadrants, making it more versatile than the atan function.
In designing an anemometer, it is important to note that, as depicted in the schematic, we require a reflective surface. This surface serves to extend the trajectory of the ultrasonic waves, thereby facilitating the development of algorithms for wind speed calculation. Of course, some designs opt for a vertical transmission approach, which imposes higher frequency requirements on the processor.
Let us now elucidate what transpires in this process.
Initially, our ultrasonic generator produces a pulse signal under the impetus of the driver, generating a pulsatory acoustic wave. Upon emission, this wave directly collides with the reflective surface before rebounding towards the ultrasonic receiver. During this interval, should a gust of wind traverse, its relative motion will extend the propagation path, thereby causing the signal, originally anticipated to reach the receiver at time 't,' to arrive at time 't1.'
By positioning ultrasonic sensors vertically, one can obtain the vertical component of wind speed. Given that wind speed comprises both velocity and direction—meaning it is a vector—we can calculate wind direction through vector composition. More simply, trigonometric functions can be employed to ascertain the wind direction.
Equipped with this foundation, we delve into the signal processing phase, where sampled data undergoes storage and processing, effectively segregating noise from valid data. Subsequently, by computing the difference between transmission and reception times, wind speeds in various directions are ascertained. These values are then synthesized with data from three other directions, ultimately yielding both wind speed and direction.
In essence, ultrasonic measurement of wind speed and direction leverages the influence of wind speed on the velocity of sound. This method of measurement provides insight into the quantification of numerous physical variables. It is my hope that understanding how ultrasonics measure wind speed will elucidate other approaches to measuring physical quantities.
Presently, ultrasonic anemometers find widespread application in diverse fields, including meteorological monitoring, environmental protection, and modern agriculture—all of which benefit from this anemometer devoid of moving components. Its precision and sensitivity furnish data that enhances our acuity towards the environment that surrounds us.
Perhaps one day, as we discuss the weather, the climate information we acquire will originate from ultrasonic anemometers."The wind is rather formidable today!"
Obscure Ultrasonic Facts
Measuring Temperature with Ultrasonics
At times, in the absence of a thermometer or under particular circumstances, we can employ ultrasonics to measure temperature. This is feasible because the speed of sound in air at atmospheric pressure, at room temperature, is approximately 340 m/s. In reality, for each 1°C increase in air temperature, the speed of sound varies by approximately 0.6 m/s. Adhering to this principle, one can measure air temperature given a known distance.
Manipulating Delicate Objects with Ultrasonic Tweezers
Acoustic tweezers operate by utilizing the pressure nodes created by ultrasonic standing waves.
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