Measuring the flow of water

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

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.comElectromagnetic

Interocean S-4

Marsh McBirney

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