10-MHz ADV Principles of Operation
The SonTek/YSI 10-MHz ADV (Figure 1) is a single-point,
high-resolution, 3D Doppler current meter. ADV Doppler processing techniques provide
several important advantages: 3D velocity measurements in a remote sampling volume;
invariant factory calibration (no periodic re-calibration required); simple operation;
direct calculation of turbulent parameters such as Reynolds stress; and excellent low-flow
performance. This document presents the basic operating principles of the ADV. To
learn more about specific ADV configurations and applications,
contact SonTek/YSI.
The ADV measures the velocity of water using a physical principle called the Doppler
effect. If a source of sound is moving relative to the receiver, the frequency of the
sound at the receiver is shifted from the transmit frequency.
Fdoppler = -2Fsource (V / C)
In this equation, V is the relative velocity between source and
receiver, C is the speed of sound, Fdoppler
is the change in frequency at the receiver, and Fsource
is the transmitted frequency.
Figure 2. Bistatic Doppler Current Meter
Figure 2 illustrates the operation of a bistatic Doppler
current meter such as the ADV (bistatic systems use separate acoustic transducers for
transmitter and receiver). Both transmitter and receiver are constructed to generate very
narrow beam patterns. The transmitter generates sound with the majority of the energy
concentrated in a narrow cone, and the receiver is sensitive to sound coming from a narrow
angular range. The transducers are mounted such that their beams intersect at a volume of
water located some distance away. The beam intersection determines the location of the
sampling volume (the volume of water in which measurements are made).
The transmitter generates a short pulse of sound at a known frequency, which propagates
through the water along the axis of its beam. As the pulse passes through the sampling
volume, the acoustic energy is reflected in all directions by particulate matter
(e.g., sediment, small organisms, bubbles). Some portion of the reflected energy travels
back along the receiver axis, where it is sampled by the ADV and processed by the
electronics to measure the change in frequency. The Doppler shift measured by one receiver
is proportional to the velocity of the particles along the bistatic axis of the receiver
and transmitter. The bistatic axis is located halfway between the center axes of the
transmit and receive beams.
Each transmitter/receiver pair measures the projection of the water velocity onto its
bistatic axis. The ADV uses one transmitter and two or three acoustic receivers (for 2D or
3D probes). The receivers are aligned to intersect with the transmit beam pattern at a
common sampling volume. The velocity measured by each receiver is referred to as the
bistatic velocity, and is the projection of the 3D velocity vector onto the bistatic axis
of the acoustic receiver. Bistatic velocities are converted by the ADV to XYZ (Cartesian)
velocities using the probe geometry. XYZ velocities give the 3D velocity field relative to
the orientation of the ADV probe. As it is not always possible to control instrument
orientation, the ADV can be equipped with an internal compass and tilt sensor. The
compass/tilt sensor allows the ADV to report velocity data in an Earth (East-North-Up or
ENU) coordinate system, independent of probe orientation.
The location of the sampling volume is 10 cm from the tip of the probe. The size of the ADV
sampling volume is determined by the sampling configuration used. The standard sampling
volume is a cylinder of water with a diameter of 6 mm and a height of 9 mm. For
specialized high-resolution applications, the height of the sampling volume can be reduced
to as little as 1.2 mm with only software modifications. For more information, see Setting the Record Straight: ADV Data
Acquisition Rates & Sampling Volume Size.
The ADV records nine values with each sample: three velocity values (one for each
component), three signal strength values (one for each receiver), and three correlation
values (one for each receiver). Naturally, the velocity data are of foremost interest;
signal strength and correlation are used primarily to determine the quality and accuracy
of ADV velocity data.
5.1. Velocity
ADV velocity data can be reported in XYZ (Cartesian) coordinates relative to probe
orientation or Earth (East-North-Up or ENU) coordinates for systems using the optional
compass/tilt sensor. The ADV's velocity output data can be used directly without post
processing. The calibration will not change unless the probe has been physically damaged.
One of the most important ADV operating parameters is the velocity range setting. This
determines the maximum velocity that can be measured by the instrument; standard settings
are ±3, ±10, ±30, ±100, and ±250 cm/s. The user should select the lowest velocity
range setting that will cover the maximum velocity expected in a given experiment.
Instrument noise in velocity data is proportional to the velocity range setting; higher
velocity ranges have higher noise levels. The typical noise level in good operating
conditions is 1% of the velocity range when transmitting data at 25 Hz (i.e., each sample
is ±1 cm/s when using the ±100 cm/s velocity range and a sampling rate of 25 Hz).
The ADV is designed to measure velocity as rapidly as possible. A single estimate of
the 3D velocity field is referred to as a ping. The ADV pings 200-250 times per second.
The noise in a single ping is too high for practical use, so the ADV averages a number of
pings before outputting a velocity sample. The number of pings averaged is set to meet the
user specified sampling rate within the range of 0.1 to 25 Hz. For example, when sampling
at 25 Hz the ADV will collect as many pings as possible over a 40-ms period and output the
average as one sample. An important result of the ADV sampling scheme is that reducing the
sampling rate decreases the noise in each sample (by increasing the number of pings
averaged per sample).
5.2. Signal Strength
Signal strength, recorded for each ADV receiver, is a measure of the intensity of the
reflected acoustic signal. The primary function of signal strength data is to verify that
there is sufficient particulate matter in the water. If the water is too clear, the return
signal may not be stronger than the ambient noise level of the electronics. Without
sufficient signal strength, the ADV is unable to make accurate velocity measurements. In
general, the ADV requires a minimal amount of scattering material (typically 10 mg/L) for
excellent operation.
Since the return signal is a function of the amount and type of particulate matter in
the water, signal strength values can be used as an indicator of sediment concentration.
While ADV signal strength data cannot be directly converted to sediment concentration, it
does provide an excellent qualitative picture of sediment fluctuations. With proper
calibration, signal strength data can be used for reasonably accurate estimates of
sediment concentration.
5.3. Correlation Coefficient
The ADV correlation coefficient is a data quality parameter that is a direct output of
the Doppler velocity calculations. The ADV computes three correlation values (one for each
acoustic receiver). Correlation is expressed as a percentage: perfect correlation of 100%
indicates reliable, low-noise velocity measurements; 0% correlation indicates that the
output velocity value is dominated by noise (no coherent signal).
Ideally, correlation should be between 70 and 100%. Values below 70% indicate that the
ADV is operating in a difficult measurement regime, the probe is out of the water, the
signal-to-noise ratio (SNR) is too low, or that something may be wrong with the ADV. In
some environments (highly turbulent flow, highly aerated water), it may not be possible to
achieve high correlation values. Low correlation values will affect the short-term
variability in velocity data (i.e., increase the noise), but will not bias the mean
velocity measurements. For mean velocity measurements, correlation values as low as 40%
can be used.
The ADV probe is available in several configurations for different measurement needs.
These variations are divided into four areas: location of the sampling volume, acoustic
sensor mounting, sensor orientation, and coordinate resolution. Probes can be constructed
with almost any combination of these configurations.
- Sampling volume location - The sampling volume is 10 cm
from the tip of the acoustic sensor.
- Sensor mounting - The sensor can be mounted on a 25-cm long stainless
steel stem or a 100-cm flexible cable. The 25-cm stem is used in field
applications where a more rugged stem is required. The 100-cm cable allows increased
flexibility in the sensor orientation, but requires more complicated mounting
arrangements.
- Sensor orientation - The ADV acoustic sensor can be oriented looking down, to
the side, or up. Down is most common as it allows for easy mounting and measurements close
to the bottom boundary. Side looking is typically used in wave tanks with the sensor
mounted looking across the direction of propagation, reducing the chance of flow
interference. Up-looking is typically used for measurements near the surface, under a
layer of ice, or near the bottom of a vessel or structure.
- Coordinate resolution - The ADV can be built to measure either 2D or 3D fluid
flow, using 2 or 3 acoustic receivers respectively. 3D is the most common configuration.
2D is most commonly used for very shallow water (minimum 3 cm) with a side-looking sensor,
or very narrow channels using a down-looking sensor.
Because of the remote 3D velocity measurements, the ADV is extremely well suited to
flow studies in boundary layers. The ADV can be used for detailed boundary layer studies
and direct measurement of turbulent parameters such as Reynolds stress. Additionally, the
ADV automatically measures and records the distance to the boundary at the start of each
data collection cycle (the boundary measurement can be made when the sampling volume is
between 2 and 25 cm from the boundary).
Under good operating conditions, the leading edge of the sampling volume can be placed
within about 0.5 mm of a boundary. The vertical extent of the sampling volume is precisely
defined; thus, this leading edge can be placed very close to a boundary without
interference. Using the reduced sampling volume size mentioned earlier, with a minimum
height of 1.2 mm, allows the user to make detailed flow measurements within a few
millimeters of a boundary.
One significant advantage of the ADV is that there is no minimum measurable velocity,
with no potential for a zero offset or zero drift. The lowest ADV velocity range, ±3 cm/s, will yield good results for flows down to about 0.1 cm/s. If working in an
environment with extremely low flows, the ADV software can be modified to use lower
velocity ranges to further improve performance. The ADV has been used to measure
calibrated flows as low as 0.04 cm/s and has shown excellent performance at even lower
levels where no reference is available.
More details about the ADV can be found at:
