Chapter 7: Current Measurements

A: Eulerian techniques/instruments

1. Mechanical current meters

“Unidirectional” (Savonius) rotor plus vane problem:

Counting revolutions (giving average scalar SPEED) plus a single direction reading gives nonsense results in oscillating flows.This was done in early Aandeeraa RCM 4/5 units.

OK if no oscillating flows (no waves, mooring motion, turbulence, etc)

Aandeeraa RCM 4/5

Vector averaging:

Measure u,v rapidly (e.g. from frequent speed and direction readings), convert to earth coordinates (N/S, E/W), and average in THOSE components.

RCM 7/8 can do this A BIT (but whole instrument needs to turn, and read not very often because high compass current draw).

Better VACM with vanes that respond very fast, read every few seconds and 5cm of displacement.

Aandeeraa RCM 7/8

VACM

Rotor pumping

Vertical mooring motion:

U (true current)

w (mooring motion)s (total SPEED seen at rotor)

β

If rotor does NOT respond with cos β behaviour, e.g. measures an amplitde closer to s, then it will always read too high current in up/down oscillations.

Normal rotors do NOT have cos β response VMCM

VMCM

Additional problems:

- Startup friction, or stalling in low currents, usually needs 1cm/s (once turning, accuracy can be much better)

- growth/fouling: change in shape/drag calibration wrong

- measuring in own wake in oscillating flows

- compass problems (especially deviation by own housing, batteries, mounting, wire, ship, CTD rosette, etc…)

RCM 4/5

ACM

VMCM

VACM

Error/mean

Different current meters in oscillating flow

Stokes drift error

A: Eulerian techniques/instruments

2. Acoustic current meters

Acoustic current meter (3D-ACM) by FSI

Acoustic traveltime measurement within the small volume often used for small-scale and turbulence measurements.

Cost: $10,000 (?)

Sontek Argonaut Nortek Aquadopp Aanderaa RDI DVS Seaguard RCM

Modern “point” current meters (doppler measurement)All except Aanderaa can be clamped on wire and have inductive telemetry option

Special Nortek quote:--------------------

$12,000 bare$1000 clamp and fin$2500 inductive modem5% quantity discount (10)

Total cost $14,400

ADCP

The transducers see the following velocity projections along their beamsubeamE= -u sin φ ubeamW= +u sin φwbeamE= w cos φ wbeamW= w cos φ

TOTAL for each transducer:sbeamE= ubeamE+ wbeamE sbeamW= ubeamW+ wbeamW

Sum and difference:u=(sbeamW-sbeamE)/2 sin φ wE_W=(sbeamE+sbeamW)/2 cos φ

Same for N-S beams. The 2 estimates for w are averaged for <w> and differenced for “error velocity” – measure for quality of data/validity of assumptions…

Broadband ADCP uses coded pulses within a ping, and analyzes time/phase shift (via correlation techniques) to determine the distance/speed of moving scatterers.

This gives an ambiguity if particles have travelled more than a wavelength, so in high currents only short-spaced pulses can be transmitted, reducing sampling quantity and accuracy. In low currents, pulses can be spaced

further ambiguity velocity

Missing part of profile near reflector/boundary:H cos φ… 6% for 20 deg transducers !!!

Blanking period required due to ringing:

Workhorse Sentinel24-29k$(200-6000m rated)

Workhorse Quartermaser38-40k$(1500m rated)

Workhorse Longranger52k$(1500m rated)

Self-contained RDI ADCP’s, approx. to shown scale

ADCP setup example: Exercise with PlanADCP

Accuracy is affected by:-Bin size-Ambiguity speed-Ensemble size-Broadband/Narrowband mode

Resolution is affected by:-Bin size

Range is affected by:-Bin size-Ambiguity-Broadband/Narrowband mode

Proximity of first bin is affected by:- Blanking- bin size- ambiguity

Read the RDI Broadband Primer (excellent !)….

Note battery capacity:We have up to 3 packs (1 internal, 2 external) with 300 Wh at 0 °C

470 Wh at 20 °C

Vessel-mounted ADCP (vmADCP)

- typically 150 or 75kHz- now 38kHz Ocean Surveyor phased array- samples up to 1000 m depth- raw data in ship coordinates- use tilt and gyro to get earth coord.- then remove ship velocity with GPS data

Serious problem with projecting ship velocity into ocean currents:

uship

Measured:uapparent

utrueuobserved

Uerror=Uship sin φ ≈ 5m/s sin φe.g. Φ=2° 17cm/s error

uapparent

utrueuobserved

Correct for ship motion: add uship

uship with small angle errorerror

Lowered ADCP(lADCP):

use only du/dz and integrate afterwards

U= ∫ du/dz dz + Const

where Const from displacement during cast or bottom velocity

www.whoi.edu/science/ultramoor/

current meter comparison experiment:

A: Eulerian techniques/instruments

3. Electromagnetic current measurements

B

v

IElectrodes

Electric field strength

(Faraday’s law) E ~ v x B

Voltage (potential difference)

U = ΔΦ = ∫ E ds ~ v x B

More complete with return current path:E = j/σ – v x B

Marsh McBirney

S4A: Generate the magneticfield actively:

B: Use earth’s magnetic field

Cable measurement

Earth magnetic field

Fixed sensor:E = Bvert x V* where

V* conductivity-weightedvertical average

Moving sensor:E = Bvert x (V-V*)Free-falling -> measure E(z)thus V(z) except barotropic offset, i.e. only shear.Or MEASURE V with float ->get barotropic part V*

Free-falling E-M probe

RAFOS float with electrodes

• Compact Tx & Rx antennae• 360-degree view• Nominal range 50 km (other systems exist with over 100km)• 2-3 km spatial resolution• 1-hr integration time• 13 MHz carrier frequency• Measures currents in the upper metre• Measures sea state up to the saturation limit at Hs ~ 7.5 m

Coastal Radar: Technical details

Radial vectors from one station

Full vectors from 2 stations