Comprehensive Manual34
© 2018 Nortek AS
Usually, the aim is to classify the incident wave field and therefore it is critical that the instrument is
deployed in a location that exposes it to an incident wave environment that is undisturbed by its
surroundings. Some neighboring structures that influence the incident waves could include piers,
breakwaters, unusual changes in bathymetry, as well as rivers (or exposure to high currents). Such
structures affect the local wave field (reflections, diffraction, refraction, evanescent modes, etc.) and
are not the best representation of the incident wave field. For example an instrument deployed in
front of a breakwater would be exposed to the combined incident and reflected waves. The PUV
approach does not have the ability to separate these two fields. Therefore close proximity to any of
the above listed is discouraged, unless of course the object of the data collection exercise is to
measure the locally influenced wave environment at the particular location of deployment.
For tips on how to configure the Vector for wave measurements, check out the "Setting up for
operation" chapter.
2.1.8.4 Corrections, Measurement Errors and Uncertainties
Corrections
In case of strong background currents, the measured waves may be affected by a Doppler shift. That
is, when currents are directed against the waves, the waves are compressed. When the currents
travel in the same direction, the waves are elongated. The resulting spectra will see the peak energy
shift slightly to lower or higher frequencies. It is not just the magnitude of the currents that is
essential but also the direction. Currents flowing in a direction perpendicular to the wave direction will
have no effect on the waves.
The degree to which the Doppler shift modifies the surface waves depends on the current speed
relative to the wave propagation speed. This means that slow propagating (short period) waves are
the most affected by currents. Measurements that infer the surface waves from either orbital velocity
or pressure measurements, like PUV, require special attention regarding background currents. This
is because the transfer function used for inferring the surface waves is wavenumber dependent, and it
is the wavenumber that is modified by the background currents. The wavenumber solution must take
into account the mean current and direction relative to the wave direction.
Measurement Errors
Measurements are estimates of the the value of something real. Given a real wave direction, each
measurement is an estimate of this direction. If the instrument happens to be measure waves with
infintely-long, parallel crests, wave direction is easy to define; it is perpendicular to the wave fronts.
However, real waves are rarely so simple. At any given time, wave spreading blurs the wave direction,
making the real wave direction meaningful only as an average.
Uncertainty
There are three primary factors in the uncertainty:
1. The actual directional spread of the waves themselves. Uncertainty in the mean wave direction is
proportional to the spreading of the waves.
2. SNR or signal/noise ratio. A noisy measurement increases the apparent spreading and the
uncertainty of the measurement.
3. Averaging. Like most estimators, averaging produces more accurate estimates.
The directional estimator is unbiased, so averaging should always reduce the uncertainty. In
contrast, the spreading estimator is biased. Averaging still helps, but you will always have a residual
bias, the magnitude of which depends on the amount of spreading.
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