The authors have published the study on the reflection loss of 10kc to 50kc ultrasound on the fish-body already. Since the dimension of the fish-body is generally larger than the wave-length of 100kc to 400kc ultrasound, an experimental formula suitable for this case has been derived: Reflection loss on the fish-body: L_P=20log_<10>√<λ/(2r)>・x/b・2/(Δρ)・1/K where λ is the wave-length of sound in the fish-body, r the radius of curvature of the back of the fish-body, b the length of the fish-body, Δρ the difference of densities between the fish-body and water, K the formcoefficient, and x the distance from the fish-body. We have confirmed this formula by measuring the reflection losses of 28kc to 400kc ultrasound for mackerel, frigate mackerel, horse mackerel, beryx and flat fish for the cases when the sound impinges on the backs, sides and heads of these materials. K is about 1 for the back, 2 for the side and 0. 4 for the head. Though the reflection loss may be different according to the toughness of the skin or the formation of the fish-body, these effects are considered to be included in the form coefficient K in this paper. More precise formula may be obtained, by taking into account the two radii of curvature of the fish-body in the directions of both length and width. But as the form of the fish-body is complicated, it is more simple for practical treatment to substitute the fish-body by a simple cylinder and to include the effect of the curvature in the length-direction into the form coefficient. And the cylinder is easy to be constructed as a standard substitute body.
(1) In order to facilitate the mathematical treatment, the formal cavitation determinable by the magnitude of the amplitude of vibrating-bubble was considered besides the practical cavitation detectable by its marked, chemical and mechanical effects. (2) According to the experimental results on the ultrasonic degradation of polymer in solution, cavitation intensity is almost independent of the ultrasonic intensity. One of causes for this fact may be ascribed to the fact that the cavitation intensity is weakened by the damping effect of gas in a vibrating bubble. (3) In order to examine the influence of gas-damping on cavitation intensity, the authors considered the behavior of gas as follows: During the expansion stage of cavitation, the dissolved gas in liquid comes into the cavity, while the free gas in the bubble goes out of the bubble during its contraction. Based on such consideration, fundamental simultaneous equations were derived, as shown in this paper. (4) As the usage of suspensions of powder materials is considered as the conventional method for detecting the directions of the stream-lines, it is stressed in this paper that in like manner polymer solution could become an useful means for determining the dynamical force of fluid flow.
(1) Conditions for the occurrence of the cavitation were investigated for various values of the external pressure, ultrasonic intensity, frequency, and the size of the bubbles, and the cavitation range was plotted (Fig. 1) (2) By taking into account the variation of the air quantity in bubble accompanying ultrasonic vibration, two kinds of stationary states, namely, stable and unstable ones, can be found theoretically. If the size of the bubble which existed before ultrasonic irradiation is less than the critical value, i. e. the size in unstable stationary state, the vibration of the bubble reaches a stable stationary state. On the other hand, if the size is above the criticla value, it becomes larger and larger. This conclusion may be correlated to ultrasonic degassing from liquid and to the ultrasonic burst of bubble as observed by Willard (1953). (3) In case of stable stationary cavitation, following three graphes were made in nondimensional expressions: (i) the maximum and minimum radii of bubble plotted against ultrasonic intensity (Fig. 4), (ii) the pressure at any distance from bubbble (Fig. 5), (iii) the bubble-radii at any time(Fig. 6).