3. ROV HYDRODYNAMICS
In general, the damping of an underwater vehicle
moving in 6 degrees of freedom at high speed
is highly nonlinear and strongly coupled. Usually,
rough approximations are made based on
the assumption that the vehicle moves slowly.
The violation of this assumption, specially when
high-gain controllers are applied, creates one of
the major uncertainty in the dynamics. In this
work we investigate the hydrodynamic damping
in dependence on the geometric form and usual
range of velocities of a ROV in the operation.
A body moving through a fluid experiences a drag
force, which is of frictional and of pressure nature.
Particularly, the last force is associated with the
development of a wake behind a passing flow.
For usual ROV geometries, the pressure drag
dominates over frictional drag since their shapes
act as bluff bodies offering higher resistance to
motion than streamlined bodies. The drag force
experimented by body in relative motion with
respect to the fluid depends on the Reynolds
number defined as