Cavitatton (Influence of Free Gas Content) 
presented to Association Technique Maritime et Aeronautique (x), 
using a singular perturbation method, the solution of the problem 
cannot be reduced at not order to Stokes solution. Foissey showed in 
particular that some terms, which are currently put in the equations 
are not in fact correct, even far from the walls ; a fortiori close to 
the body, in particular inside the boundary layer, since it would be 
necessary to take into account the rotation of the bubbles and their 
deformation. But the analytical calculation may have a qualitative 
interest, namely to show the screening effect. Concerning this effect 
it seems that there exists some discrepancy between the present re- 
sults and those obtained by Johnson and Hsieh from the same hypo- 
theses. For Johnssonand Hsieh, if my memory is good, the bubbles 
are kept away from the solid body and only the smallest ones starting 
from the axis of the body can reach it. On the contrary, Dr. Peterson 
tells us that bubbles starting away from the axis can reach the body 
and cavitate on it. I would like this point to be cleared. 
My second series of remarks are relative to the respective 
influence of the stream free bubbles and of the hydrophobic particles 
as cavitation nuclei. When the air content in the water of the tunnel 
is decreased, it is noted that the free bubbles are decreased in num- 
ber and diameter but that the noise remains constant, and from this 
it is concluded that hydrophobic particles are responsible for cavi- 
tation, at least for cavitation noise. I believe that this conclusion may 
be true but that it is perhaps premature. It is indeed possible that the 
bubbles which can be observed when the air content is high are not 
cavitation bubbles but gaseous bubbles or pseudo-cavitation bubbles, 
that is bubbles filled with a great quantity of air, and leading to visual 
but not noisy phenomena. The noise could be perhaps produced by 
smaller free bubbles i.e. by true cavitation bubbles and not necessa- 
rily by hydrophobic particles. 
I believe that these considerations bring us back, as ever, to 
that difficulty of defining incipient cavitation. This can be only defined 
by its effects, either bubble growth, that is a visible phenomenon, with 
the difficulty of making the distinction between cavitation and gaseous 
bubbles, or an acoustical phenomenon that presents difficulty for an 
analytical treatment. In the ITTC Cavitation Committee, we have had 
serious discussion before reaching an agreement on the definition of 
(x) FOISSEY, C., "Application d'une méthode de perturbation singu- 
liére a 1'étude de la cavitation naissante.'' Association Technique 
Maritime et Aeronautique, session 1972. 
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