Underwater gliders are autonomous underwater vehicles (AUVs) which
enable oceanographic studies over time and spatial scales which are
impossible to carry out by ships. Gliders move horizontally and
vertically simultaneously in a saw tooth manner as shown in
Figure 1,
while also taking important sea measurements by the use of sensors
aboard the vehicle. An underwater glider is generally about 1.5-2
metres in length, and weighs about 50 kg in air
(
Rudnick, Davis,
Eriksen, Fratantoni, & Perry, 2004).
These vehicles are designed to be very energy-efficient and therefore
propulsion is provided by what is known as a buoyancy engine,
whereby upward and downward vertical displacement is provided by
changing the volume of the glider
(
Rudnick
et al., 2004),
i.e. a decrease in volume of the vehicle results in a reduced volumetric
displacement of seawater, and hence the glider sinks; conversely, an
increase in volume results in a higher amount of displaced seawater,
and therefore the glider rises. This is achieved by means of hydraulic
pumps that use hydraulic oil to inflate and deflate a bladder, which
changes the volume and consequently the buoyancy of the glider. The
bladder is depicted in the illustration shown in
Figure 2.
Pitch control is achieved by changing the position of an internal
mass (batteries)
(
Davis,
Eriksen, & Jones, 2002)
so that there is a resulting pitch moment which changes the attitude
of the glider, i.e. the glider’s nose points towards the seabed when
the mass is moved to the front (fore), and conversely it points
towards the sea surface when the mass is moved to the rear (aft).
A glider also has two wings which provide it with lift, meaning
that both during ascent and descent, it moves forward. A symmetrical
hydrofoil does not generate any lift if the angle of attack is equal
to 0. The lift force increases as the angle of attack increases,
until it reaches a critical value known as the critical angle of
attack (
Wikipedia, 2016)
(also known as the stalling angle of attack) where no further increase
in lift is possible. On reaching the stalling angle of attack, water
is not capable of following the wing’s contour, since the fluid cannot
make such a sudden change in direction of flow. This brings the
separation point of the fluid flow forward, causing flow separation
to occur closer to the leading edge of the hydrofoil, and hence
increasing turbulence.
An underwater glider is designed, built and operated in such a way
as to maximise range per unit energy consumption. This allows these
vehicles to survey the oceans, for weeks or months on end. In order
to achieve a high range per unit energy consumption, the following
operating factors should be taken into consideration
(
Rudnick
et al., 2004):
-
Glider speed - fast is inefficient, in fact halving speed
roughly increases range by four;
-
Depth – gliders usually operate down to a depth of 1000 m;
newer versions can operate in shallow areas efficiently;
-
Ocean stratification – if there is a strong pycnocline,
such as in the area of the Maltese Islands more energy is
consumed by the glider to penetrate the buoyancy change;
conversely weak stratification is not as energy demanding,
and hence results in an increased range.
Gliders would not be able to take any measurements without having
sensors on-board. Ocean properties which are commonly measured by
gliders are temperature, salinity, density, dissolved oxygen and
chlorophyll fluorescence. Gliders are important for academic research
and, indeed, virtually indispensable in any situation where it is not
practical to have human divers take certain underwater measurements
at different locations. Data acquired from gliders can also be
incorporated into models to give more accurate forecasts of future
ocean states.
