NortekUSA Training Symposium 20 April 2006 Velocimeter Theory and applications. - Atle Lohrmann, Nortek

Transcription

1 NortekUSA Training Symposium 20 April 2006 Velocimeter Theory and applications - Atle Lohrmann, Nortek

2 What is a velocimeter? Some history Vectrino and Vector Overview How does it work? The measurement process Configuring an instrument and testing (software) Measuring turbulence In the ocean Some problem areas you should know about

3 Velocimeter Principle Vz Vz Vy Vy Focused acoustic beams for 3D measurements Signal scattering from small particles High sampling rate and small sampling volume Coherent Doppler technology for precise data Vx Vx

4 Two types of sensors

5 High resolution velocimeters Laboratory systems Self-contained ocean models Vectrino Vector

6 Why Velocimeters? Need for a new velocity sensor in physical models (1992) At the time struggling with laser systems Specifications Scale < 1cm -> 25 Hz. $10k In the field = Electro magnetic sensors Need for new instruments

7 Laboratory velocimeter hardware has stayed the same for 11 years. Started new project in 2003 A little history

8 Getting external input Developing new ideas - meeting with U.Lemmin, L. Zedel, and A.Hay in Oslo in 2003: Correlation - decorrelation mechanisms Signal statistics Signal processing. Hard limiting. Digital and analog demodulation. Doppler estimation. Transducers. Experience with parabolic or other directional schemes Dynamic range. Low signal/noise limit Conclusion: There were tricks that could improve the performance by a factor of 5-10 relative to the old 10 MHz ADV/NDV while retaining the SNR of the 10 MHz systems.

9 Vectrino Design parameter Probe Smaller probe for better geometrical response (Bill Snyder, 199..) Four receivers give two independent W-measurements Electronics Single module sensor only for low cost Non-multiplexing (all four receivers at the same time) Backward compatible with old probes Performance As good or better than MicroADV while still working at 10 MHz (SNR considerations) Faster sampling rate (max 200 Hz) Adjustable sampling volume 6 mm in diameter, 3-9 mm long Add distance measurement mode to include simple echo-sounder.

10 New Velocimeter Vectrino Old probe Snyder and Castro (1999)

11 Vectrino product structure Base electronics always the same Firmware: standard or plus Mechanical: Underwater or IP Probes: Standard 4D down cable or fixed 4D2D side-looking cable or fixed Field probe cable or fixed Shipping case: Transportation only or transport/storage ExploreV Vectrino software or Polysync

12 Vector = part of Paradopp development All plastic/titanium (2.5 kg w/battery)

13 Power management Pay for what you use

14 Vector products Cabled probe or fixed stem Orientation issues Pressure sensor range and memory options External sensors (ready to go)

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16 Calibration - tank

17 Calibration - automated

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19 Processing Techniques Pr ocessing Techniques Incoherent (+) (-) Robust Estimator Optimal range Measure close to boundaries Simple to implement Coherent Factor of improvement in spacetime resolution Hybrid Improved space-time resolution compared to incoherent systems (factor of 4-10) Limited space-time resolution Only limited applications for monostatic systems Mor e complex to design and manufacture. In wide-band mode (=large improvement in space-time r esolution): Loss of r ange Loss of measurement area close to boundaries Less robust measurements

20 Coherent Processing - Two or more pulse - Doppler shift determined from phase shift between two received signal =dφ/dtau - Works off the complex covariance method of demodulate signals... - The gist of it: -Function R(t, dtau) such that dφ =atan(r(t,tau)), Tau=pulse lag Correlation = abs(r(t,tau)) So velocity comes from dφ and the quality depends on R (<1)

21 Phase shift is measured

22 Coherent systems - limitation [Vmin Vmax]=[-pi,pi]/Tau Ambiguity velocity decreases with larger lag σv= σ(dφ(r))/tau Large lag gives small standard deviation (noise) End result: Velocity ranges (optimize noise) - see software

23 Coherent systems implication Phase change is limited [Vmin Vmax]=[-pi,pi]/τ Ambiguity velocity decreases with larger lag σv= σ(dφ(r))/ τ Large lag gives small standard deviation (noise) Low velocity noise -> low velocity range leads to the use of velocity range = different τ (software)

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25 Coherent systems - decorrelation Think of coherent systems as taking snapshots of a busy highway Mechanisms to reduce quality: The world The cars are reorienting themselves The camera In focus? Is it working?

26 Coherent systems - decorrelation Reduction in the correlation The world: Beam divergence (velocity) Turbulence Shear (??) The camera: Poor transducers Low SNR Bubbles Multiple reflections

27 Velocity of what? Acoustic return signal: I(t) = Ie i(ω t -ω D )t+iφ(t), with I = Σ I i i=1 ω t = Acoustic transmit frequency ω D = Doppler shift φ(t) = Random phase I i = Intensity of individual scatterers in sampling volume Velocity of acoustic scatterers, not water!! N

28 Turbulence Turbulence is that state of fluid motion which is characterized by random and chaotic motions. When present, it usually dominates all other flow phenomena including mixing, heat transfer, and drag. The lack of a satisfactory understanding of turbulence presents one of the great remaining fundamental challenges to scientists - and to engineers as well - since most technologically important flows are turbulent.

29 From every day life

30 Structured turbulence

31 Turbulence in data

32 Turbulent quantities Looking for the magnitude of perturbation terms: U = Umean + U, where U is the perturbation term, fluctuation, or short term variability Examples Reynolds stress: or also apparent turbulent stress Reynolds Fluxes: or apparent turbulent fluxes

33 Turbulence and noise U = Umean + U +Un, U = Fluctuations associate with turbulence Un = Instrumentation Noise In many situations, Un is a significant term: In low SNR situations Low energy situations (U 2 is small) Very turbulent flow

34 Turbulent energy U = Umean + U +Un (U-Umean) 2 = U 2 + Un 2 + < U *Un > = U 2 + Un 2 In other words, the measurements are limited by the instrument noise

35 Noise limitation (energy)

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37 Improve energy estimates with 4 beams (U.Lemmin) W = Wmean + W +Wn <W-Wmean> 2 =<(W 1 -W 1 mean)(w 2 -W 2 mean)=<w 1 W 2 > The noise two estimates of the vertical velocity is uncorrelated so the estimate of the energy is much improved Can do the same thing also for the horizontal velocity components: Optimum config is 45 degree orientation relative to the flow. (Residual noise improved a factor of 10)

38 Weak spots = when the echo from the first pulse interferes with the echo from the second pulse:

39 Ambiguity jumps

40 Fish coming by

41 Cross interference

42 Cross interference

43 Signal registered in one probe

44 Vectrino laboratory applications: 3D Physical models Flumes 3D Wave basins Scientific turbulence studies

45 Wave tank comparison - OSU Irregular waves Wave period around 3 s Output Rate 50 Hz - same for both instruments (max for MicroADV) Velocity range 100 cm/s nominal for both instruments Sampling volume 6 mm in vertical for both instruments 6 mm in horizontal for both instruments

46 Tank comparison time series m/s Time series of Vectrino and MicroADV X-components in wave tank - Oct Blue - Vectrino, Green - MicroADV Time (s) - sample rate 50 Hz both systems Sontek MicroADV Scatter plot of X-component Nortek Vectrino

47 Main axis power spectrum 10 0 Spectrum of Vectrino and MicroADV X-xomp. in wave tank - Oct Noise level Energy density (m/s)/hz First estimate show a factor of in favor of Vectrino (=theoretical estimate) Noise level corresponds to cm/s standard deviation of noise at 50 Hz 10-5 Blue - Vectrino, Green - MicroADV frequency

48 Cross axis power spectrum 10 0 Spectrum of Vectrino and MicroADV Y-xomp. in wave tank - Oct Mounting Energy density (m/s)/hz Can see more easily as sampling rate increases Vibrations can easily be induced if mounting is not fixed Can also come from the probe itself Blue - Vectrino, Green - MicroADV frequency

49 Vertical axis power spectrum Energy density (m/s)/hz 10 0 Spectrum of Vectrino and MicroADV Z-xomp. in wave tank - Oct Blue - Vectrino, Green - MicroADV Noise level Smaller than the vertical noise level by a factor of about 16 Cannot yet be seen at 50 Hz at this turbulence level Not yet experimented with processing suggested by Lemmin frequency

50 Conclusions Vectrino development First new laboratory velocimeter in a long time Performance as expected exceeding the MicroADV specifications as planned Four beams makes instrument more suitable for turbulence measurements through noise reduction

51 Vector applications: Process studies Orbital wave motion studies Surf-zone dynamics Boundary layer studies Natural low flow studies in lakes and marshes Turbulence measurements

52 Vector data from surf zone deployment 1.5 Velocity 0 (m/s) -1.5 Pressure (m) 1.5 Velocity 0 (m/s) -1.5 Pressure (m) October 1999 X Time X Time (s) Y Y Raw velocity and pressure time series observed by the Vector. The X- component is normal to the beach, positive east, and the Y- component is parallel to the beach, positive north. The lower panels show a detail of a segment from the upper panels.

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54 Vector, surf-zone data (UK)

55 Vectrino field applications: Where the Vector cannot go! Beach run-up Boundary layer Natural low flow studies in lakes and marshes