Underwater Robots - Gianluca Antonelli.pdf

C A ( ν )=− C T A ( ν ) ∀ ν ∈ IR 6 .
2.4 Hydrodynamic Effects 27
If the body is completely submerged in the water, the velocity islow
and ithas three planes of symmetry as common for underwater vehicles, the
following structure ofmatrices M A and C A can therefore be considered:
M A = − diag { X ˙u ,Y˙v ,Z˙w ,K˙p ,M˙q ,N˙r } ,
⎡
⎢
C A = ⎢
⎣
0 0 0 0 − Z ˙w w Y˙vv 0 0 0 Z ˙w w 0 − X ˙u u
0 0 0 − Y ˙v v X˙uu 0
0 − Z ˙w w Y˙vv 0 − N ˙r r M˙qq Z ˙w w 0 − X ˙u u N˙rr 0 − K ˙p p
− Y ˙v v X˙uu 0 − M ˙q q K˙pp 0
The added mass coefficients can be theoretically derived exploiting the
geometry of the rigid body and, eventually, its symmetry [127], by applying
the strip theory. Foracylindrical rigid body of mass m ,length L ,with circular
section of radius r ,the following added mass coefficients can be derived [127]:
X ˙u = − 0 . 1 m
Y ˙v = − πρr 2 L
Z ˙w = − πρr 2 L
K ˙p =0
M ˙q = − 1
12 πρr 2 L 3
N ˙r = − 1
12 πρr 2 L 3 .
Notice that, despite (2.40), in this case it is M A ≥ O .This result is due
to the geometrical approach tothe derivation of M A .Asamatter of fact,
if asphere submerged in afluid is considered, it can beobserved that a
pure rotational motion of the sphere does not involve any fluid movement,
i.e., it is not necessary to add an inertia term due tothe fluid. This small
discrepancy is just an example of the difficulty inrepresenting with aclosed
set of equations adistributed phenomenon as fluid movement.
In [220] the added mass coefficients for an ellipsoid are derived.
In [145], and in the Appendix, the coefficients for the experimental vehicle
NPS AUV Phoenix are reported. These coefficients have been experimentally
derived and, since the vehicle can work atamaximum depth of few
meters, i.e., it is not submerged in an unbounded fluid, the structure of M A
is not diagonal. To give anorder of magnitude of the added mass terms, the
vehicle has amass of about 5000 kg, the term X ˙u ≈−500 kg.
Adetailed theoretical and experimental discussion on the added mass
effect of acylinder moving in afluid can befound in [203] where itisshown
⎤
⎥ .
⎥
⎦