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9 Experimental Methods in Fluid Mechanics
currents) or by tracking tracers such as a surface float or dye patch (Lagrangian
current). Since the launch of oceanographic satellites, tracking of surface buoys
has provided reliable, long-term current trajectories for many different parts
of the world's oceans. Various types of drifters and their use for particular
applications are described by Emery and Thomson (1997) and will not be
repeated here. In this section we briefly describe current measurements at
fixed-locations. The methods available can be divided into two groups: single
point current meters and current profilers.
Single Point Current Meter. A conventional single point current meter
provides information on water flow velocity and direction at the point at which
it is located. These devices must be moored at depths exceeding 10 m, in order
to avoid 'contamination' induced by the oscillating flow due to surface waves.
Data are either recorded aboard the instrument or telemetered to shore or ship.
There are a few types of current meters used in oceanographic practice, namely
rotor type current meters, acoustic current meters, and electromagnetic current
meters. As was mentioned by Emery and Thomson (1997), the first modern
current meter was the Aanderaa Recording Current Meter (RCM) developed
by Ivar Aanderaa in Norway in the early 1960s under sponsorship of the North
Atlantic Treaty Organization (NATO). Various versions of this current meter,
such as the RCM4 or RCM5, are still in use in the oceanographic practice.
Aanderaa type current meters use a Savonious rotor to measure current speed.
The Savonious rotor consists of six axisymmetric, flat blades enclosed in a
vertical housing oriented normally to the direction of flow. Allowable sampling
periods are 3.75, 7.5, 10, 30 and 60 min., while speed is obtained from the
number of rotor revolutions for the sample interval.
The major shortcoming of rotor-type current meters is their inability to
record currents in regions affected by surface motion. In the upper ocean wave
regime, non-mechanical current meters (acoustic and electromagnetic) are better suited. Acoustic current meters measure the difference between an acoustic
source and receiver separated by a fixed distance, L. A three-axis current
meter determines the three-dimensional velocity by simultaneously measuring
time differences along three orthogonal axes.
Examples of electromagnetic current meters are the InterOcean 84 current
meter or Marsch-McBirney 512 current meter. Both instruments are commonly
used for coastal and offshore oceanographic measurements. Current meters
measure the voltage induced from the motion of a water through a magnetic
field generated by the instrument. An electromagnetic force is induced which is
directly proportional to the speed of the ocean current according to Faraday's
law of electromagnetic induction. A two-axis electromagnetic current meter is
equipped with an internal compass to produce horizontal components referred
to the Earth coordinates.
The 84 current meter is spherical in shape with a diameter of 25 cm. Electrodes are located on the surface of the sphere; the compass and all electronics,
including data storage and the power supply, are located within the sphere. At
9 Experimental Methods in Fluid Mechanics
currents) or by tracking tracers such as a surface float or dye patch (Lagrangian
current). Since the launch of oceanographic satellites, tracking of surface buoys
has provided reliable, long-term current trajectories for many different parts
of the world's oceans. Various types of drifters and their use for particular
applications are described by Emery and Thomson (1997) and will not be
repeated here. In this section we briefly describe current measurements at
fixed-locations. The methods available can be divided into two groups: single
point current meters and current profilers.
Single Point Current Meter. A conventional single point current meter
provides information on water flow velocity and direction at the point at which
it is located. These devices must be moored at depths exceeding 10 m, in order
to avoid 'contamination' induced by the oscillating flow due to surface waves.
Data are either recorded aboard the instrument or telemetered to shore or ship.
There are a few types of current meters used in oceanographic practice, namely
rotor type current meters, acoustic current meters, and electromagnetic current
meters. As was mentioned by Emery and Thomson (1997), the first modern
current meter was the Aanderaa Recording Current Meter (RCM) developed
by Ivar Aanderaa in Norway in the early 1960s under sponsorship of the North
Atlantic Treaty Organization (NATO). Various versions of this current meter,
such as the RCM4 or RCM5, are still in use in the oceanographic practice.
Aanderaa type current meters use a Savonious rotor to measure current speed.
The Savonious rotor consists of six axisymmetric, flat blades enclosed in a
vertical housing oriented normally to the direction of flow. Allowable sampling
periods are 3.75, 7.5, 10, 30 and 60 min., while speed is obtained from the
number of rotor revolutions for the sample interval.
The major shortcoming of rotor-type current meters is their inability to
record currents in regions affected by surface motion. In the upper ocean wave
regime, non-mechanical current meters (acoustic and electromagnetic) are better suited. Acoustic current meters measure the difference between an acoustic
source and receiver separated by a fixed distance, L. A three-axis current
meter determines the three-dimensional velocity by simultaneously measuring
time differences along three orthogonal axes.
Examples of electromagnetic current meters are the InterOcean 84 current
meter or Marsch-McBirney 512 current meter. Both instruments are commonly
used for coastal and offshore oceanographic measurements. Current meters
measure the voltage induced from the motion of a water through a magnetic
field generated by the instrument. An electromagnetic force is induced which is
directly proportional to the speed of the ocean current according to Faraday's
law of electromagnetic induction. A two-axis electromagnetic current meter is
equipped with an internal compass to produce horizontal components referred
to the Earth coordinates.
The 84 current meter is spherical in shape with a diameter of 25 cm. Electrodes are located on the surface of the sphere; the compass and all electronics,
including data storage and the power supply, are located within the sphere. At