Nortek Vector Comprehensive Manual

Also see for Vector: Quick start guide

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Nortek Vector Comprehensive Manual Manual pdf 34 page image

Comprehensive Manual34© 2018 Nortek ASUsually, the aim is to classify the incident wave field and therefore it is critical that the instrument isdeployed in a location that exposes it to an incident wave environment that is undisturbed by itssurroundings. 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). Suchstructures affect the local wave field (reflections, diffraction, refraction, evanescent modes, etc.) andare not the best representation of the incident wave field. For example an instrument deployed infront of a breakwater would be exposed to the combined incident and reflected waves. The PUVapproach does not have the ability to separate these two fields. Therefore close proximity to any ofthe above listed is discouraged, unless of course the object of the data collection exercise is tomeasure 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 foroperation" chapter.2.1.8.4 Corrections, Measurement Errors and UncertaintiesCorrectionsIn case of strong background currents, the measured waves may be affected by a Doppler shift. Thatis, when currents are directed against the waves, the waves are compressed. When the currentstravel in the same direction, the waves are elongated. The resulting spectra will see the peak energyshift slightly to lower or higher frequencies. It is not just the magnitude of the currents that isessential but also the direction. Currents flowing in a direction perpendicular to the wave direction willhave no effect on the waves.The degree to which the Doppler shift modifies the surface waves depends on the current speedrelative to the wave propagation speed. This means that slow propagating (short period) waves arethe most affected by currents. Measurements that infer the surface waves from either orbital velocityor pressure measurements, like PUV, require special attention regarding background currents. Thisis because the transfer function used for inferring the surface waves is wavenumber dependent, and itis the wavenumber that is modified by the background currents. The wavenumber solution must takeinto account the mean current and direction relative to the wave direction.Measurement ErrorsMeasurements are estimates of the the value of something real. Given a real wave direction, eachmeasurement is an estimate of this direction. If the instrument happens to be measure waves withinfintely-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.UncertaintyThere are three primary factors in the uncertainty:1. The actual directional spread of the waves themselves. Uncertainty in the mean wave direction isproportional to the spreading of the waves.2. SNR or signal/noise ratio. A noisy measurement increases the apparent spreading and theuncertainty 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. Incontrast, the spreading estimator is biased. Averaging still helps, but you will always have a residualbias, the magnitude of which depends on the amount of spreading.