YSI SonTek ADVField, Техническая документация инструкция

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SonTek/YSI
Acoustic Doppler Velocimeter Principles of Operation (September 1, 2001) 10
A more precise method to correct ADV velocity involves calculating the exact geometry change
and re-computing the Cartesian velocity values. This is done in four steps.
1. Transform Cartesian (XYZ) velocities back to bistatic velocities.
2. Scale bistatic velocities for correct sound speed.
3. Compute corrected transformation matrix for geometry change.
4. Transform bistatic velocities to Cartesian (XYZ) velocities using corrected matrix.
This algorithm uses the transformation matrix and bistatic angles unique to each ADV probe. A
documented program for this correction is available upon request.
7.3. Near-Boundary Measurements
Because of the remote, 3D velocity measurements made by the ADV, the instrument is extremely
well suited to flow studies in boundary layers. The ADV can be used for detailed boundary layer
studies and make direct measurements of turbulent parameters such as Reynolds stress. However,
making accurate measurements near a boundary does require some additional care. This section
describes the potential sources for error and the overall system performance.
The most obvious source of interference with ADV measurements is when the sampling volume
includes the boundary. In this case, the ADV will be measuring the Doppler shift of the boundary
reflection rather than the reflection from particles in the water. This will typically bias velocities
towards zero (since the boundary is normally stationary), although this is not completely predict-
able (particularly if dealing with non-stationary or non-uniform boundaries).
Under good operating conditions, the leading edge of the sampling volume can be placed within
about 1 mm of a boundary for 16-MHz /10-MHz ADVs, and about 2 mm for 5-MHz ADV-
Oceans. As discussed in Section 4, the vertical extent of the sampling volume is precisely de-
fined, so this leading edge can be placed very close to a boundary without interference. When
making near boundary measurements, it is important that the distance from the probe to the
boundary is constant. If making measurements near the surface or near a moving bed, where the
height of the boundary can change with time, this will typically be the limiting factor in how
close measurements can be made.
Figure 7 shows the profile of signal strength for one ADV receiver when operating near a bound-
ary. This is the same as shown in Figure 2, although we now see a sharp reflection corresponding
to the reflection from the boundary. The exact size and shape of the boundary reflection will vary
with range, the type of boundary, and the relative orientation of the probe. Although the boundary
reflection comes from a direction outside the peak receiver sensitivity, the strength of the reflec-
tion is far stronger than the return signal from the water so its return can be as strong (or poten-
tially stronger) than the return from the sampling volume. The distance between the sampling
volume peak and the bottom reflection peak is used by the ADV to measure the distance from the
sampling volume to the boundary.
When working close to a boundary, measuring the distance from the sampling volume to the
boundary becomes very important. The ADV measures and reports the distance from the center
of the sampling volume to the boundary. This measurement is accurate to about
±
1 mm for
16-MHz/10-MHz ADVs, and about
±
2 mm for the 5-MHz ADVOcean. However, this accuracy
only holds if the center of the sampling volume is more than about 2 cm from the boundary. At
closer ranges, the ADV cannot distinguish the boundary from the sampling volume. To position
the ADV sampling volume very close to a boundary, use the ADV measured distance at a greater
range and move the probe a controlled distance to achieve the desired measurement location.