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Measuring the flow of water
• LaGrangian– Follow a parcel of water over time– Use satellite tagged drifters
• Eulerian– Meausure the flow of water at a single point over time– Usually from a mooring
• Combined?– Measure flow from a moving ship– Calculate the flow based on glider drift
Drifters
• Usually at the surface• High-drag so that
they attach to the flow
• Usually include both GPS and Argos positioning but only Argos is required
• How it works• Polar Orbiting
Satellites• Receiving stations• Positions calculated
by meauring Doppler shift from consecutive transmissions
Eulerian: Moored
• Mechanical (rotors or propellers)• Electromagnetic• Acoustic– Travel time– Doppler
• Radar (“Codar”)
Electromagnetic
• Interocean S-4• Marsh McBirney
The S4 Electromagnetic Current Meter measures the voltage resulting from the motion of a conductor (water flow velocity) through a magnetic field according to Faraday's law of electromagnetic induction. Simply stated, Faraday's law defines the voltage produced in a conductor as the product of the speed of the conductor (water flow velocity) times the magnitude of the magnetic field times the length of the conductor. In the case of the S4, the conductor length is the effective path between the sensing electrodes. The magnetic field intensity is generated by a circular coil, internal to the S4, driven by a precisely regulated alternating current. The use of an alternating magnetic field and synchronous detection techniques to measure the voltage at the sensing electrodes provides an extremely stable, low noise current measurement. Two orthogonal pairs of electrodes and an internal flux gate compass provide the current vector.- Interoceansystems.com
Acoustic: travel time
• BASS: Benthic Acoustic Stress Sensor
• MAVS: Modular Acoustic Velocity Sensor
MAVS Specifications• Toughness– Carried by submarine
landslide 550 m down Monterey Canyon. Still recording pressure although sensor rings sheared off and tube bent 90°.
• Fouling sensitivity– Stops measuring only when
flow is totally blocked as in illustration
– Insensitive to particles, bubbles, or perfectly clear water. Only hard obstruction or swim bladder stops acoustic signal
Acoustic Doppler
• Measure Doppler shift in 4 directions
• Single depth• Low vulnerability to
fouling• Somewhat
dependent on particles
• Can be done with lasers too (LDV)
• ADCP = Acoustic Doppler Current Profiler– Acoustic: typically 38-1200 kHz– Doppler: uses Doppler shift to measure speed– Current: measures the flow of water– Profiler: measures current vector simultaneously
in as many as 100 distance bins• Distance can be depth below ship or height above
bottom• More bins isn’t always better; statistics
Brands
• RD instruments (RDI)– Oldest, first, biggest
• Sontek– Smaller instruments
• LinkQuest– New on the market; quality?
• AAnderAA? Simrad? Others
Applications
• Current profiling from mooring
• Current profiling from ship
• Lateral flow • Tracking– AUV/ROV
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