1000 
t, found from the shock wave position. The radius of the cavitation region R, 
Was also calculated and the values are shown in Table II together with the — 
measured values. In the calculation it was assumed that cavitation takes place 
only in the region where the pressure drops to zero Ly ae the compressional 
diffraction wave from the edge of the plate arrives. 
It is assumed that once cavitation appears, it persists for some time after the 
diffraction wave from the edge of the plate has reached it. (See Section III, 
1, d.) The rather good agreement of the calculated and observed results for both 
sets of values argues that the assumptions made in the calculations, including 
the low value for the tension necessary for cavitation, are roughly correct. 
The target used to obtain Figure 33 was an essentially completely free plate which 
is illustrative of several experiments. A small can filled with air was hung with 
its open end down and the disk was supported at the air-water interface on small 
clips which offered no resistance to its motion. The fact that no essential 
difference is shown between these experiments and the type illustrated by Figure 
29 indicates that platés mounted as in Figure 29 were essentially "free" plates, 
(c) Cavitation from objects other than plane surfaces. -- 4A single photograph of an 
early UERL cylinder taken 200 sec after impact of a shock wave from a 65 gm 
tetryl charge 30 in. distant shdéwed a considerable region of cavitation. This is 
seen in Figure 34; the cylinder is not shown because of the restricted field of 
view. The cylinder axis is parallel to the plane of the photograph and the top 
edge of the cylinder is in view at the lower edge of the picture. Figure 35 shows 
cavitation off the side of a 4.5 x 5 x 0.011 in. paint can 129 Je see after being 
damaged by a 25 gm charge 48 in. away. } 
Figures 36, 37, and 38 demonstrate that no cavitation occurs when a 1/4 in. 
piezoelectric gage, or a Hartman type momentum gage or a 5 x 5 in. cylinder of 
steel is hit by a shock wave from 250 gm of tetryl at 30 in. 
(d) Disappearance of cavitation. -- If the cavitation bubbles consist simply of 
water vapor, it seems hard to understand why these bubbles persist for long times 
after the pressure has returned to the hydrostatic level. In an attempt to cast 
some light on this problem, a series of experiments was performed in which cavita- 
tion was allowed to disappear spontaneously, while, in another series, auxiliary 
shock waves were passed into the cavitation region. 
Figures 39, 40, 41, 42, and 43 show a series of photographs in each of which a 
0.002 in. brass diaphragm supported by a 6 in. pipe was ruptured by a 25 gm charge 
12 in. away. The picutres are taken at different times after the impact as shown. 
The finest bubbles begin to disappear by 125 Ves and there is only a trace of 
cavitation left at 400 J eco 
Figures 44, 45, 46, 47, 48, and 49 show the effect of an auxiliary shock wave. 
The auxiliary charge, data for which are given in Table III, is detonated at the 
same time as the charge causing cavitation either by means of primacord or the 
simultaneous cap method. It may be seen that increase in the pressure of the 
auxiliary shock wave results in more effective destruction of the cavitation, 
that increase in time constant has but little effect, and that the large bubbles 
are much more resistant than the fine bubbles. 
(e) Cavitation caused by oblique reflection of shock waves from air-water interfaces. - 
Experiments were performed to discover whether there was a critical angle of 
reflection from a water-air interface beyond which no cavitation would occur. The 
1/ In computing the time at which the diffraction wave relieves the tension in front 
of the plate, it must be remembered that the front of the diffraction wave may actually 
reduce the pressure in front of the plate and only in later stages raise it, since 
initially the pressure is higher in front of the plate than in the surrounding water. 
It is only when the plate is surrounded by an infinite baffle that the diffraction 
wave is always positive relative to the existing pressure. 
2% 15518 
