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Study of Slocum Underwater Glider Dynamics
Master exam Colloquium
Date
: Tuesday 19 December 2006
Time
: 10.45 – 11.30 am
Location
: Faculty 3mE, Lecture Room C
Speaker
: Maaike A.M. Schilthuis
Abstract
A glider is a special type of Autonomous Underwater Vehicle. It glides through the ocean by
controlling its buoyancy and attitude using internal actuators. The combination of the attitude of
the vehicle and the buoyancy force acting on it, allow the wings and body to generate the
hydrodynamic lift and drag forces which propel the glider horizontally and vertically through
the ocean water.
The main application of underwater gliders is in oceanographic research. They are attractive for
global sampling of the oceans, because of their low cost, autonomy and capability for long
range missions.
Deep sea gliders like Seaglider and Spray turn without the use of a rudder by means of
coordinated turning. They move an internal mass sideways to roll over and change the direction
of lift, which makes them turn. The electric Slocum glider, designed to operate in relatively
shallow water, makes use of a rudder to maneuver. Use of a rudder is less energy efficient than
coordinated turning. Another drawback of a rudder is that it forms a fragile part of a glider.
The objective of this research is therefore to study the applicability of coordinated turning for a
Slocum glider in relatively shallow coastal water. For this purpose, a dynamic model of a
Slocum glider has to be determined.
In the case of underwater gliders, the hydrodynamic forces play an important role in the
dynamics of the vehicle. Since not much data is available on the hydrodynamics of a Slocum
glider, the main part of this thesis consists of the determination of a hydrodynamic model for a
Slocum underwater glider.
First, the equations of motion of a rigid, but otherwise arbitrary glider are presented. They are
written in such a way that key vehicle parameters and external forces can easily be varied and
added. The equations are verified for circular motion in the plane and compared to the
equations of motion of other underwater vehicles.
The necessary hydrodynamic and aerodynamic theory is presented in a separate chapter. The
methods to determine the hydrodynamic properties of a glider are based on those theories and
on a history of experience, especially in aerodynamics. To determine the hydrodynamic model
for a Slocum glider the presented methods are used.
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The equations of motion are then specified to the Slocum glider by including the hydrodynamic
model and the Slocum glider parameters in the equations of motion. These equations are used to
study the two and three dimensional behavior of the glider by means of Matlab simulations. The
simulated motion is validated against measurement data of sea trials and assumptions on glider
behavior. It is concluded from this validation that the presented (hydro-)dynamic model is valid
for downward motion, but invalid for upward motion.
A parameter study for downward motion shows promising results for the implementation of
coordinated turning in Slocum gliders. The glider can make a wide range of turns for small
parameter variations. The pitch of the turns is less than the maximum diving depth of a Slocum
glider, which means the glider does not have to change its buoyancy and the location of the
internal mass during a turn. From the parameter study results it is concluded that coordinated
turning can be used by a Slocum glider to maneuver in coastal water. Further research to refine
the model and to design a control is recommended.
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