The splashes from the underwater explosions of 1 and 10 lb. charges of P.E. no. 2 and Nobel’s Explosive ‘808’ at various depths have been photographed with cine-cameras. The experiments were carried out in a sheltered pond which enabled the surface phenomena to be studied in greater detail than has been done hitherto, and a number of new phenomena have been observed. Measurements of the initial velocity of rise of the water were carried out with the use of a high-speed cine-camera, and it is shown that for the range of depths
0.3
<
W
1
3
/
D
<
4
(where
W
is the weight of the charge in lb. and
D
is the depth in metres) the initial velocity of rise of the water in m./sec. was given to an accuracy of about 10 % by the relation
V
= 66(
W
1/2/
D
—0.1) From this relation the peak pressure P of the shock wave sent out by the explosion was evaluated in terms of the distance from the charge, i.e. a value was obtained for
C
in the theoretical relation
P = CW
1/3/
D
. This was found to be somewhat higher than that obtained by other workers using pressure gauges. Away from the centre of the dome, the water rose with decreasing initial velocity, and it was found that in calm water the contour of the dome was a cubic of the form H = B[
D
2
/
D
2
+
R
2
-
C
] where
H
is the height of a point at a distance
R
from the centre of the splash and
D
the depth of the charge,
B
and
C
being constants depending on the nature of the charge. High-speed photography showed a new phenomenon during the first few milliseconds after detonation, which was provisionally termed the ‘crack’. It is seen as a whitening below the surface of the water, which grows at a rate closely corresponding to the arrival at the surface of the shock wave. The ‘black ring’ observed around the domes obtained from deep charges was photographed with telephoto lenses and shown to consist of a roughening of the water, resulting from the intensification of ripples already present when the water was at all rough, and from the formation of individual ‘spikes’, in water which was originally very calm. Several theories of the formation of domes and black rings are considered, and it seems clear that several mechanisms are operative in producing the phenomena observed. In particular, the enhancement of ripples already present in the water and cavitation beneath the surface appear to occur simultaneously. The role played by the bubble of gas formed by the explosion is also discussed, and phenomena resulting from its oscillations are described. It is suggested that the central column of water observed with shallow charges is produced as a result of the initial expansion of the bubble, and this theory is shown to account for the observed characteristics of the central column.
Shock Wave Motion Induced by Blade Pitching Oscillation in Transonic Cascade. Development of High-Speed Photography System.
