Comprehensive Manual52
© 2018 Nortek AS
scattering and the bottom echo - if the water scattering is high the whole issue goes away.
The Vectrino Profiler can make use of adaptive ping intervals to reduce weak spot interference. This
allows the ping intervals to be adjusted dynamically during data collection to account for changing
conditions within the environment. Options range from Once (perform adaptive check only during the
configuration phase at the start of acquisition) or 1/sec (dynamic checking). Using the Max Interval
algorithm (also accessible in the Configuration dialog) allows the time interval to be adjusted
manually. After setting the velocity range as low as possible, the range can be adjusted to a slightly
higher range, to move the weak spots out of the sampling window.
2.4.6 Interference from Highly Reflective Bottoms
The bottom material used in a lab environment can have a marked effect on the quality of the data.
Surfaces which are acoustically highly reflective will cause very large return signals which pollute the
water column in such a way that obtaining reliable data is not possible. We have seen metal
bottomed flumes that have very large returns from the bottom; so large, in fact, that they caused
large areas of interference in profiles that were stepped up from the bottom.
You can tell how large the return is by looking at the signal return from the bottom check. If the
instrument is to be used in such an environment (e.g. a metal bottomed flume), some sort of
absorptive material should be placed along the bottom underneath the probe. Thick plexi-glass, a
thin layer of acrylic or rubber layer or a concrete block have both been shown to work quite well in
removing the bottom reflection interference by providing some acoustic dampening to the return.
2.4.7 Mooring Vibrations
High frequency mooring vibrations due to vortex shedding are caused by current flow past a mooring
structure, such as surface and subsurface buoys, mooring lines, risers etc. Alternating vortex
shedding produces pressure fluctuations with a dominant component in the direction across the flow
direction. The mooring structures, if not mounted rigidly, are very sensitive to excitation from vortex
shedding, especially if the frequency of vortex shedding matches the resonance frequency of the
structure. This results in resonant vibrations transverse to the flow direction of the ocean currents.
The occurrence of vortex induced motion also increases the mean drag force in the current direction,
contributing to aggravating the data collection. Increased offset implies increased mooring line
tensions.
The part covering vibrations on the mooring is especially important to users of the Vector. This
instrument has a high sampling rate, making it especially prone to data contamination due to
vibrations. Excessive mooring vibration can adversely affect the data; vibration introduces spurious
velocities and interferes with the proper operation of the tilt sensor. The impact of mooring
oscillations must be carefully considered. A general solution to improve on data quality is to design a
more stable deployment by considering the components that generate mooring vibrations. The
addition of aerodynamic devices may partly prevent and partly reduce the strength of vortex induced
vibrations.
You may be able to detect intervals of excessive vibration by looking closely at your data. For more
accurate measurements of vibration, an accelerometer is the solution. An Integrated Motion Unit
(IMU) is a system that consists of an accelerometer, a gyroscope and a magnetometer that samples
at 10 kHz, and can be integrated with the Nortek Vector. By measuring the acceleration, the tilt and
the heading, one can correlate vortex induced vibrations with the sampled current velocities, and
during post-processing the measured sensor motions can be removed from the velocity data. Note
that the use of IMU demands the use of 4GB recorder, due to the amount of data collected.
2.4.8 Acoustic Streaming (Vectrino and Vectrino Profiler)
The instruments generate acoustic streaming, or secondary flows, as the pulse is transmitted. If
ambient flow velocities are of the order of potential acoustic streaming velocities (below 8 cm/s),
acoustic streaming may represent a significant source of bias.
This effect can be observed by placing the instrument in still water and observing the flow over a
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