Measuring instruments for fluid velocity

Basically, the ADV measures the velocity of particles (sediments) at a point in the water column from the Doppler shift in frequency of the emitted and received acoustic signals (without calibration) in 2 or 3-directions, depending on the sensor arrangement. The system includes three modules: sensor, signal conditioning module and signal processing module. The measurement probe consists of four ultrasonic transducers: a transmit transducer located at the bottom end of the stem and three receive transducers, slanted about 30o from the axis of the transmit transducer and pointed at the sampling volume, which is located about 0.1 m below the probed tip. Hence, the flow velocity in the sampling volume is not disturbed by the presence of the probe. The acoustic frequency is of the order of 10 MHz. The accuracy is of the order of approx.1% of the reading.

An acoustic signal from a bottom-mounted sonar is projected in a wide horizontal fan, radiating outward in the water from the instrument package and filling the water column in shallow water. The sound scatters off particles in the water (especially bubbles) and off the bottom. Some backscattered sound returns to the sonar, where the signal is received on an array, beamformed into returns from various directions, and analyzed for frequency shift versus direction and elapsed time since transmission. For direct-path transmission and return, the time-delay since transmission translates to distance from the sonar. The frequency shift of the backscattered signal (Doppler shift) is proportional to the radial component of the velocity of scatters at the sample volume.

The backscattered signal from suspended particles in the flow can be utilised to determine the velocities of the particles. The two techniques are: coherent Doppler method, and cross-correlation method.

The coherent Doppler method is based upon pulse-to-pulse phase coherence between consecutive transmissions to measure the radial component of the velocity along the beam axis. This instrument uses Doppler shift to obtain the Doppler flow velocity. The Doppler frequency is obtained from the pulse-to pulse coherence (phase coherence). Averages over pulse pairs are taken.

The correlation method employs a pair of horizontal separated transducers directed vertically downward and cross-correlation of the backscattered signals from the transducer pairs is used to obtain the velocity.

Unlike the coherent Doppler system, the correlation method is incoherent, as it is the signal intensity that is used. The basic requirement is that there are fluctuations in the suspension field, which have spatial scales greater than the distance between the transducers and that these fluctuations can be cross-correlated. Small-scale turbulent fluctuations cannot be measured.

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