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The wonderful principle of ADCP: Do you know how to use sound waves to measure the speed of water flow?
In the fields of hydrology and ocean science, the application of acoustic wave technology has received increasing attention. Among them, the Acoustic Doppler Current Profiler (ADCP) is an important tool that measures water velocity through the Doppler effect of sound waves. This technology was first introduced by RD Instruments in the early 1980s and quickly became the leader in flow rate measurement.
The basic principle of ADCP is to use the propagation and reflection of sound waves to calculate the velocity of water flow. When sound waves encounter particles in the water, they produce echoes that return, and the frequency changes of these echoes can be used to estimate the speed of the water flow.
How it works
Acoustic Doppler flow profilers are primarily composed of piezoelectric sensors that can send and receive acoustic signals. The travel time of sound waves can be used to estimate distance, while the frequency of the echo changes in direct proportion to the water velocity. To measure velocity in three dimensions, at least three acoustic beams are needed; in rivers, two-dimensional velocities are usually sufficient, so ADCPs are generally equipped with two acoustic beams.
Today's ADCP can not only measure water flow, but also add wave and turbulence measurement capabilities. The number of equipped acoustic beams has also been increased from two to nine, increasing the accuracy and richness of data.
Data processing methods
There are three common methods when calculating Doppler shift and water velocity. The first is called "non-interferable" or "narrowband". This method is stable and provides good average velocity profiles, but its performance in spatial and temporal resolution is more limited. The repetitive sequence encoding method can improve the spatiotemporal resolution, usually by about five times. In addition, although the inter-pulse coherence processing method is only applicable to a very short profile range, its spatiotemporal resolution can be improved by up to a thousand times.
Application
According to the installation position, ADCP can be divided into side-viewing, downward-viewing and upward-viewing types. The bottom-mounted ADCP provides uniform measurement of water flow velocity and direction in the vertical direction. If it is mounted sideways on a bridge pile in a river or canal, the flow profile from shore to shore can be measured. In deep water, these instruments can be suspended from the surface by cables to take measurements. The main application of ADCP is in oceanography, and it can also be used in rivers and canals for continuous flow measurement.
ADCP can operate underwater for a long time, making it an important tool for current and wave research. Continuous data collection can last for several years, which is an extremely precious resource in scientific research.
Other measurement functions
In addition to water velocity, ADCP can also measure wave height and direction. The distance between the sea level and the instrument is estimated through a vertical acoustic beam, and the height and direction of the wave are obtained using a simple peak estimation method. In addition, ADCP can also estimate turbulence parameters. This application can help researchers gain a deeper understanding of small-scale movements in water bodies and then deduce the energy dissipation rate of turbulence.
Advantages and disadvantages
Two major advantages of ADCP are the absence of moving parts to avoid biological adhesion and its remote sensing capabilities, allowing a single stationary instrument to measure flow profiles over 1000 meters. However, the disadvantage of ADCP is the loss of data near the boundary. This phenomenon is usually called side lobe interference, which may cause up to 6%-12% of water column information to be lost. Cost is also a consideration, although the cost of ADCP is usually relatively low compared to the cost of the ships required to ensure professional deployment.
With the development of science and technology, ADCP is increasingly used and has become an indispensable tool in scientific research. This technology gives us a deeper understanding of water movement, but it also raises discussions about the ecological impact of ocean noise: When you observe an ADCP operating at sea, do you think about its interaction with the surrounding marine life? ?
