In:
Journal of Applied Physics, AIP Publishing, Vol. 17, No. 7 ( 1946-07-01), p. 541-555
Abstract:
Shadows of shock waves in water were photographed by means of a point-spark of less than one-tenth micro-second duration; the waves were formed when high velocity spheres struck the water surface. The shock wave front had the form of an elliptical arc of small eccentricity; the ellipticity was caused by the greater strength and velocity of the wave at its center. The absence-of-light band had a varying width which depended on the strength of the wave. A large number of secondary waves behind the impact wave were observed; these were produced by the vibrating spherical missile as it traveled through the water. The period of vibration was determined from the spacing of the waves for 4/32″, 6/32″, and 8/32″ spheres; these agreed to within 19 percent of the periods calculated on the basis of Lamb's theory. Interference of the vibration waves was also observed when two spheres were shot into the water simultaneously. The cavity behind the missile was nearly conical in shape with the forward tip being distorted by pressure changed immediately ahead of the missile. A sphere produced a cusp at the forward tip of the cavity; cylinders and cones each produced their own characteristic shadow pattern. When a grid was placed in front of the tank, distortions in its shadowgram were observed. These were caused by the high pressures surrounding the missile and, from the shift in the grid pattern, the direction and magnitude of the pressure gradient could be inferred. When the shock waves were reflected with the wave front nearly normal to the surface there was an indication that the stem, which is characteristic of Mach reflections, was present. All other reflections were regular. Focusing of the wave by a brass mirror was demonstrated in the spark shadowgram. Waves reflected near normal incidence from substances having a lower acoustic impedance than water were reflected with a presence-of-light band ahead of the usual absence-of-light band. At glancing incidence this reversal occurred even when the reflecting substance had a higher acoustic impedance. Striations frequently appeared in the shadowgrams when waves were reflected. These striae were from 0.02 to 0.08 cm apart and represented a quasi-stationary state. They were believed to be formed when the reflected vibration waves passed through the incident waves.
Type of Medium:
Online Resource
ISSN:
0021-8979
,
1089-7550
Language:
English
Publisher:
AIP Publishing
Publication Date:
1946
detail.hit.zdb_id:
220641-9
detail.hit.zdb_id:
3112-4
detail.hit.zdb_id:
1476463-5
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