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ADVM Theory of Operation

ADVM Theory of Operation. Streamflow Record Computation using ADVMs and Index Velocity Methods Office of Surface Water. Theory of Operation. What is an Acoustic Doppler Velocity Meter (ADVM)? How do they work? Doppler effect Velocity components What do they measure? Velocity assumption

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ADVM Theory of Operation

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  1. ADVM Theory of Operation Streamflow Record Computation using ADVMs and Index Velocity Methods Office of Surface Water

  2. Theory of Operation • What is an Acoustic Doppler Velocity Meter (ADVM)? • How do they work? • Doppler effect • Velocity components • What do they measure? • Velocity assumption • Sample section • Warnings • Sidelobes • Acoustic properties in water • Limitations • Summary

  3. Theory of Operation - What is an ADVM? • Acoustic – Sound waves • Doppler – Doppler Shift are used to determine…. • Velocity– Water velocities using an…. • Meter – Instrument

  4. Theory of Operation - What is an ADVM? An ADVM is an instrument that: • Measures water velocity using Doppler shift • Is non-intrusive, can be installed near the channel bank or anywhere within the water column • Can measure an average or instantaneous, horizontal or vertical velocity • Can profile or return an average velocity for a portion of a river RDI H-ADCP SonTek Argonaut-SL SonTek-IQ

  5. Theory of Operation Acoustic Doppler Velocity Meters use transducers to transmit and receive sound waves. • Transducers: • Are electro-mechanical elements that deform or vibrate, producing sound waves • Consist of a ceramic element protected with an epoxy or urethane coating • Are usually monostatic, meaning the transducer both transmits and receives sound waves

  6. + Crest Trough - ADCP Transducer Trumpet Sound Waves Water wave crests and troughs are points of high and low water elevations. Sound wave “crests” and “troughs” consist of bands of high and low air or water pressure.

  7. Cis the speed of sound expressed as ft/s or m/s (typical speed of sound in water is 4900 ft/s) f is the frequency of the sound. The number of waves that pass a fixed point per second. (ADCP frequencies range from 75 kHz to 3 MHz) λis the wavelength, the distance between successive crests or troughs Wavelength Observation Point Movement of Waves Speed of Sound: C = f x l + Crest Crest Crest Crest Crest -

  8. Scatterers An ADVM bounces sound waves (pulses) off particles or “scatterers” in the water

  9. Transducer Acoustic pulse Magnified view of scatterers Sound Reflects from Scatterers

  10. Theory of Operation How does an Acoustic Doppler Instrument work? • Uses Doppler shift to measure water velocity • The Doppler effect is the change in a sound's observed pitch (frequency) caused by the relative velocities of the sound source and receiver.

  11. Theory of Operation • Direction of the scatterer motion is also determined from the frequency shift: F0 < FD F0 > FD F0 = FD

  12. The Doppler Equation fD=fS *V/C fD= Doppler Shifted Frequency fS= Source Frequency (frequency of ADVM) V = Velocity of scatterers in water C = Speed of Sound (dependent on water char.)

  13. Acoustic Dopplers Hear Two Shifts

  14. Corrected Doppler Shift Equation fD=2fS*V/C

  15. Theory of Operation V=(fD / 2fS)*C V = water velocity = scatterer velocity Important There must be scatterers present for ADVMs to compute water velocities!We assume that, on average, scatterer velocity equals water velocity Violation of this assumption will lead to errors in water velocity measurement

  16. Fish: Water Water Water-velocity measurement is biased by the fish velocity Stationary object: Rock Water-velocity measurement is biased toward zero Theory of Operation Scatterer velocity = to water velocity Examples

  17. Speed of Sound V=(fD / 2fS)*C • A temperature error of 2o Celsius (5o F) or salinity error of 10-15ppt will result in a ~1% velocity error • The instrument must have an accurate temperature probe and must be configured for the correct salinity Important Speed of sound (C) must be computed accurately by the instrument.

  18. flow θ y = V*sinθ y x = V*cosθ x Beam Velocities • Velocity (V) is measured along the axis of the transducer • This is called radial or beam velocity • Since the position of the transducer is known, geometry is used to compute velocity components.

  19. vy v1 Flow vx a Radial Velocity: downstream & cross-stream velocity components v2 Velocity Components

  20. Beam Velocities

  21. Beam Velocities Beam Velocity (ft/sec) Beam Velocity (ft/sec)

  22. Velocity Components • 2-beam instruments measure in 2-D • 3- or 4-beam instruments measure in 3-D 2D 3D

  23. Theory of Operation What does an ADVM measure? • Single layer across the channel or within water column • Divides layer into one or more sample cells (bins) • Measures velocity in each cell • Not able to measure ALL water

  24. Sidelooker ADVMs

  25. Uplooker ADVMs

  26. Questions for Class • Do we need to measure as much of the channel (horizontally or vertically) as possible? Why or why not? • Which instrument do you think has the capability of profiling farther: an instrument with a lower or a higher acoustic frequency? Photo courtesy of SonTek

  27. Theory of Operation • What does an ADVM measure? • In some cases the sampled section may not extend the full width of the channel because: • The instrument may not have sufficient range • Channel geometry may cause interference from the bed or surface • Disturbed flow • The instrument does not necessarily need to sample the full width of the watercourse: • A stable relation between index velocity and mean channel velocity is all that is required

  28. Sample Cells • Sample Cells • 3- and 4-beam profilers output 3 velocity components from each cell or bin z x y

  29. Sample Cells Transmit pulse • ADVM transmit pulses have a fixed length dependent on cell size and system frequency. Because the pulse occupies a volume/length of water, scatterers outside the sample cell can affect the measurement of velocity in the cell. Cell beginning Cell End

  30. Velocity Weighting • Why is this important? • Because an obstruction may be outside of the sample cell, but still have an adverse affect on velocities measured in the cell • A large stationary object will bias velocity measurements toward zero, because the object reflections are much stronger than those from scatterers in the sample cell.

  31. Sample Cells • Knowledge of the sampling cell locations and velocity weighting will become important when selecting cell sizes for a particular site. • Also important to cell location and size is knowledge of sidelobe interference.

  32. Sidelobes • Acoustic beams have “sidelobes” which can impinge on boundaries before the main beam • Weaker energy compared to the main beam, but when reflected from a boundary, can overwhelm the reflections from the main beam

  33. Sidelobes Main beam Sidelobe zone • The main beam may clear the surface or bottom, but sidelobes may strike these boundaries and bias the measured velocity Locations affected by sidelobeinterference

  34. Signal Amplitude (Beam Checks) • The signal amplitude shows the strength of the reflected sound signal. • This depends on the number of suspended particles reflecting the sound and the distance from the transducer. ADVM Typical signal amplitude plot: The signal strength decreases as the distance from the instrument increases.

  35. Signal Amplitude • Uses of Signal amplitude plots • May indicate the location of an obstruction in the beam. • Measures stage in an upward looking transducer. • Dynamic range adjustment for Argonaut-SW.

  36. Uses of Beam Amplitude Uplooker Boundary, range 12m Object at 3.5m

  37. Limitations • Stream depth and ADVM operation • Need to consider effect of sidelobes (not just main beam) • For shallow channels the measurement volume will most likely need to be small • Obstructions in the channel or channel geometry can prevent the measurement volume from extending across the full width • Reminder: It’s not necessary to sample the full channel width. Just need a stable relation between index and mean channel velocity

  38. Cautions • Large temperature and/or salinity gradients in the measurement volume can cause: • Errors in the estimation of the speed of sound • Less uniform long term flow • Unpredictable velocity profiles • Further details included in the site selection lecture

  39. Summary • ADVMs measure water velocity and direction by measuring the Doppler shift of reflected ultrasonic waves • The sound waves are reflected off of “scatterers” (particles) in the water • It is assumed the particles have the same velocity as the water • Other “reflectors” (rocks, fish, water surface) can bias velocities low or high • Assumes flow is fairly homogenous through all beams; align perpendicular to flow • The calculated velocity depends on an accurate estimate of the speed of sound in water

  40. Summary • Velocity components measured by 2 or more transducers can be combined to calculate velocity and direction (in 2 or 3 dimensions) • Channel depth may affect the ability to accurately measure with an ADVM. Need to avoid interference from the channel bed and water surface for both main beam and side lobes • The sampled (measured) section of the channel may only be a small portion of the total cross-section • A stable relation is required between the index velocity and the mean channel velocity

  41. Any Questions? Image courtesy of SonTek/YSI

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