Part B | 12.3
306 Part B Autonomous Ocean Vehicles, Subsystems and Control
In contrast, a propeller is a rotating wing which
releases the vorticity into the mean flow. It produces
a more complex helical system of trailing vortices imparting swirl to the mean flow (Fig. 12.6). The trailing
vortex system from a propeller is fully three dimensional; as a result, the power needed to overcome drag
expression contains two additional terms
P e D
• " Â @w
@y
@v
@z
à 2
C
 @u
@z
@w
@x
à 2
C
 @v
@x
@u
@y
à 2
#
dx dy dz (propeller) : (12.4)
From the additional vorticity components in the vortex
trail of a propeller comes higher dissipation rates that
require greater expenditures of onboard power to maintain a given vehicle speed through the water, U (D juj).
The advantage in transport efficiency of a wing over
a propeller in water is illustrated nature through the evolution of the propulsion systems for marine mammals,
for example the blue whale (Fig. 12.7). Whales evolved
winged tails (flukes) not propellers as the basis of their
propulsion systems. Natural selection of winged tails
did not occur for lack of a joint capable of 360
ı rotation, as the shoulder joint in primates clearly proves
biomechanically viable.
12.3 Underwater Glider Attributes and Limitations
The most compelling attribute of an underwater glider
from the point of view of other types of subsurface
vehicles is its high endurance/long on-station time capability. This attribute arises because an UW glider
can readily operate over its full flight envelope, from
top speed to neutrally buoyant (determined by the
net vehicle buoyancy created by its buoyancy engine).
Therefore, it can conserve onboard propulsion energy
by moving as slowly with respect to the surrounding
medium as possible while still accomplishing its objectives. In addition, an UW glider has very low levels of
self-noise (acoustically, electrically, and magnetically),
because self-noise is created primarily by the buoyancy pump that is activated only episodically for short
intervals. Once the underwater glider changes its net
buoyancy, it can glide silently with no machinery or hydrodynamic noise until reaching the next reversal in the
sawtooth glide path. (Again, the ocean’s stable stratification minimizes disturbance from turbulence and so
minimizes the need for actuating flight controls.) The
intermittency of a glider’s self-noise is in sharp contrast
to the continuous self-noise emissions from propulsion
of prop-driven vehicles. Finally, in contrast to propdriven platforms, winged structures such as UW gliders
can fly cooperatively to improve flight efficiency in horizontal transport. This point is illustrated later in this
chapter.
The primary weakness of an UW glider compared
with other types of underwater vehicles arises from its
inability to maintain level flight in the water column – it
must change depth to propel itself forward. This weakness actually is a strength when an objective is to collect
vertical profiles of ocean properties.
As with all underwater vehicles, an UW glider is
vulnerable to damage – from surface ship collisions,
harsh weather, and entanglement – while on the ocean
surface. For missions that require covertness, the probability of platform detection is also much greater at the
surface; all of a glider’s stealthy attributes are compromised during surfacing periods. However, wideband,
inexpensive (energy-wise, size and weight-wise, and
cost-wise), two-way communications can be achieved
only after surfacing. In addition, most environmental
sources of energy in the ocean – solar, wind, and wave –
are available only at the sea surface. Once an underwater vehicle descends below the ocean surface, it must
carry onboard all of the energy supplies required to accomplish its objectives (one exception is discussed in
Sect. 12.5).
The characteristic of being autonomous for any
underwater vehicle also imparts certain benefits and
limitations. The primary benefit of autonomy is providing the platform with the ability to accomplish useful
objectives (collection of ocean measurements, cargo
transport, military-relevant missions, etc.) without direct human input. This ability almost always results
in large cost savings, and enables transits in areas too
dangerous or inaccessible for humans. The major limitation, on the other hand, is associated with this lack
of direct human input. Once it leaves the ocean surface,
an autonomous underwater platform must have onboard
all of the artificial intelligence required to accomplish
its goals. Improving the level of onboard intelligence is
the primary challenge to future applications in underwater robotics.
Given these attributes and limitations, UW gliders (and autonomous underwater gliders in particular)
are capable of performing a variety of functions and
missions. These functions can be categorized as follows [12.1].
306 Part B Autonomous Ocean Vehicles, Subsystems and Control
In contrast, a propeller is a rotating wing which
releases the vorticity into the mean flow. It produces
a more complex helical system of trailing vortices imparting swirl to the mean flow (Fig. 12.6). The trailing
vortex system from a propeller is fully three dimensional; as a result, the power needed to overcome drag
expression contains two additional terms
P e D
• " Â @w
@y
@v
@z
à 2
C
 @u
@z
@w
@x
à 2
C
 @v
@x
@u
@y
à 2
#
dx dy dz (propeller) : (12.4)
From the additional vorticity components in the vortex
trail of a propeller comes higher dissipation rates that
require greater expenditures of onboard power to maintain a given vehicle speed through the water, U (D juj).
The advantage in transport efficiency of a wing over
a propeller in water is illustrated nature through the evolution of the propulsion systems for marine mammals,
for example the blue whale (Fig. 12.7). Whales evolved
winged tails (flukes) not propellers as the basis of their
propulsion systems. Natural selection of winged tails
did not occur for lack of a joint capable of 360
ı rotation, as the shoulder joint in primates clearly proves
biomechanically viable.
12.3 Underwater Glider Attributes and Limitations
The most compelling attribute of an underwater glider
from the point of view of other types of subsurface
vehicles is its high endurance/long on-station time capability. This attribute arises because an UW glider
can readily operate over its full flight envelope, from
top speed to neutrally buoyant (determined by the
net vehicle buoyancy created by its buoyancy engine).
Therefore, it can conserve onboard propulsion energy
by moving as slowly with respect to the surrounding
medium as possible while still accomplishing its objectives. In addition, an UW glider has very low levels of
self-noise (acoustically, electrically, and magnetically),
because self-noise is created primarily by the buoyancy pump that is activated only episodically for short
intervals. Once the underwater glider changes its net
buoyancy, it can glide silently with no machinery or hydrodynamic noise until reaching the next reversal in the
sawtooth glide path. (Again, the ocean’s stable stratification minimizes disturbance from turbulence and so
minimizes the need for actuating flight controls.) The
intermittency of a glider’s self-noise is in sharp contrast
to the continuous self-noise emissions from propulsion
of prop-driven vehicles. Finally, in contrast to propdriven platforms, winged structures such as UW gliders
can fly cooperatively to improve flight efficiency in horizontal transport. This point is illustrated later in this
chapter.
The primary weakness of an UW glider compared
with other types of underwater vehicles arises from its
inability to maintain level flight in the water column – it
must change depth to propel itself forward. This weakness actually is a strength when an objective is to collect
vertical profiles of ocean properties.
As with all underwater vehicles, an UW glider is
vulnerable to damage – from surface ship collisions,
harsh weather, and entanglement – while on the ocean
surface. For missions that require covertness, the probability of platform detection is also much greater at the
surface; all of a glider’s stealthy attributes are compromised during surfacing periods. However, wideband,
inexpensive (energy-wise, size and weight-wise, and
cost-wise), two-way communications can be achieved
only after surfacing. In addition, most environmental
sources of energy in the ocean – solar, wind, and wave –
are available only at the sea surface. Once an underwater vehicle descends below the ocean surface, it must
carry onboard all of the energy supplies required to accomplish its objectives (one exception is discussed in
Sect. 12.5).
The characteristic of being autonomous for any
underwater vehicle also imparts certain benefits and
limitations. The primary benefit of autonomy is providing the platform with the ability to accomplish useful
objectives (collection of ocean measurements, cargo
transport, military-relevant missions, etc.) without direct human input. This ability almost always results
in large cost savings, and enables transits in areas too
dangerous or inaccessible for humans. The major limitation, on the other hand, is associated with this lack
of direct human input. Once it leaves the ocean surface,
an autonomous underwater platform must have onboard
all of the artificial intelligence required to accomplish
its goals. Improving the level of onboard intelligence is
the primary challenge to future applications in underwater robotics.
Given these attributes and limitations, UW gliders (and autonomous underwater gliders in particular)
are capable of performing a variety of functions and
missions. These functions can be categorized as follows [12.1].