Decomposing velocity measurements: Difference between revisions

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==Overview==
This step involves separating the [[quality-controlled Data processing of raw measurements| raw velocities]] into:
* low-frequency component comprising of the background large-scale flow
* high-frequency component comprising of the turbulence and surface wave signals. This component may also include unwanted contributions from the measurement platforms, such as wakes and vibrations associated with the frame.


This decomposition enables determining [[Velocity past the sensor|mean flow past the sensor]] and [[Surface wave statistics|surface wave statistics]], which are necessary for later choosing the appropriate [[Velocity inertial subrange model| inertial subrange model]] for [[Spectral fitting|spectral fitting]].
The quality-controlled velocities can be [[Detrending time series|detrended]] before being further analysed to determine [[Velocity past the sensor|mean flow past the sensor]] and [[Surface wave statistics|surface wave statistics]]. These quantities are necessary for later choosing the appropriate [[Velocity inertial subrange model| inertial subrange model]] for [[Spectral fitting|spectral fitting]].
Quality-controlled velocities may also need to be detrended before applying various [[Velocity despiking|despiking]]. Because estimating <math>\varepsilon</math> is done from the spectral observations, the analysis is insensitive to this step provided that we don’t filter out the scales shorter than the [[Segmenting datasets| segment length]].
 
<div><ul>
<li style="display: inline-block; vertical-align: top;"> [[File:Short timeseries.png|thumb|none|350px|512 s segment of the measured velocities after applying different [[Detrending time series|detrending methods]]]]
</li>
<li style="display: inline-block; vertical-align: top;"> [[File:Short_spectra.png|thumb|none|350px|Example velocity spectra of the short 512 s of records before and after different detrending techniques applied to the original 6h  time series. The impact of the detrending method can be seen at the lowest frequencies only]] </li>
</ul></div>
 


==Methods for decomposing==


===Long continuous sampling==
Different techniques dependent on whether measurements were collected continuously or in long bursts (define here). The high-frequency content can be obtained by:
* High-pass filtering (linear and stationary signals)
* Empirical mode decomposition  (nonlinear and/or non-stationary signal)


== Short burst sampling==
A short burst is typically at most 2-3x the expected largest turbulence length scales. As a rule of thumb, turbulence estimates from the inertial subrange of velocity rely on 5 to 15 min long-segments.
* Remove the arithmetic mean of the burst to obtain the high-frequency content
* Linear trend removal
* Empirical mode decomposition (nonlinear and/or non-stationary signal)




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Return to [[Velocity_point-measurements|Velocity point-measurements' welcome page]]
Return to [[Preparing quality-controlled velocities]]

Latest revision as of 19:08, 5 July 2022


The quality-controlled velocities can be detrended before being further analysed to determine mean flow past the sensor and surface wave statistics. These quantities are necessary for later choosing the appropriate inertial subrange model for spectral fitting. Quality-controlled velocities may also need to be detrended before applying various despiking. Because estimating [math]\displaystyle{ \varepsilon }[/math] is done from the spectral observations, the analysis is insensitive to this step provided that we don’t filter out the scales shorter than the segment length.

  • 512 s segment of the measured velocities after applying different detrending methods
  • Example velocity spectra of the short 512 s of records before and after different detrending techniques applied to the original 6h time series. The impact of the detrending method can be seen at the lowest frequencies only





Return to Preparing quality-controlled velocities