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

• Local data from NGLI• Note tides plus ambient current

Eulerian: Moored

• Mechanical (rotors or propellers)• Electromagnetic• Acoustic– Travel time– Doppler

• Radar (“Codar”)

Mechanical

• Aanderaa– Savonious Rotor

• Endeco– Impeller

• Disadvantages:– Fouling– Dead zones

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

• Echoes from particles or just the water

• Doppler shift

• Timing for distance

• Frequencies• Tradeoff between

resolution and range– Depth and

currents• Size:– Low F = Big ducers

Applications

• Current profiling from mooring

• Current profiling from ship

• Lateral flow • Tracking– AUV/ROV

Data!

• 0-100m• Flow vs time• Lots of

engineering data too