In order to examine the growth and collapse of cavitation bubbles, to which much attention had not been paid, the authors produced cavitation bubbles in some liquids by a nickel transducer of frequency 5 kHz and observed their growth and collapse by means of the time change of the light intensity scattered by the cavitaion bubbles(Fig. 1). In this paper are reported the experimental results relating to the growth and collapse and the erosion effect of the cavitation bubbles produced near the sound-radiating surface. First of all, the growth and collapse of cavitation bubbles of one-cycleness were examined by means of flashlight photography and the following facts were confirmed:(1)The oscillogram showing the time change of the light intensity scattered by cavitation bubbles approximately indicate the growth and collapse of cavitation bubbles(Fig. 2〜7). (2)As the acoustic pressure in the liquid is increased, the aspects of the growth and collapse of the cavitation bubbles gradually become to change and show a multi-cycleness(Fig. 6), and these experimental results can be qualitatively explained by using a simple model of cavitation bubbles. Next, changing the gas content, vapor pressure and surface tension of the liquid, some erosion experiments were conducted with a test piece made of mild steel(Fig. 7). At the same time, the growth and collapse of the cavitation bubbles produced near the sound-radiating surface, i. e. near the surface of the test piece, were examined by the frequency analysis of the time change of the light intensity scattered by the cavitation bubbles(Figs. 9 and 16). Moreover, by measuring the root-mean-square height of the eroded surface of test piece(Figs. 10, 13 and 17), the distribution of the collapse intensity of the cavitaion bubbles produced near the sound-radiating surface was estimated. As a result, the following facts were confirmed:(1)The cavitaion bubbles produced near the sound-radiating surface can be classified into two groups, i. e. one is on the central part and the other is on the circumferential part of the sound-radiating surface. (2)With increase of the gas content and vapor pressure of the liquid tested, the collapse intensity of the cavitation bubbles produced near the central part of the sound-radiating surface is extremely weakened in comparison with that of the cavitation bubbles produced near the circumferential part.
Usually, we can easily design a torsional resonator whose vibration mode and velocity of torsional wave are already known, but it is very difficult to know the resonant frequency of the torsional resonator, for example, a bar with rectangular cross section, or of complex resonance with the other vibration mode such as coupled mode of longitudinal and torsional vibrations. In these cases, we must often determine its resonant frequency by actual experiments, but few devices for these purposes have been developed. The authors divised electro-magnetic type torsional vibration driver-detector for measurement of a resonant frequency and Q-factor, without any mechanical contact, and also these converters are intended not to have any acoustical influences upon a resonator under test. In this paper, we describe the mechanism and the construction of transducer(Fig. 1〜Fig. 3), method of measurement(Fig. 4)and some characteristics which was got by experiments, of the trially-made convertor. Besides, we investigated the torque factor AT(N-m/Amp)or(Volt-sec)of this reversible transducer. Inclination of AT vs. air gap between the converter and sample:0. 1mm〜2. 0mm, dimensions:10mm〜40mmφ and materials;Steel;Al and brass of sample, and the frequency;20kHz〜50kHz are shown in corresponding figures. The value of AT under the conditions prescribed, was about 10^<-5>〜10^<-4>(N-m/Amp). From these results and experiments, we got some data for reformation of the transducer and whole apparatus, and also refer to some notices for the way of measurement. Finally, according to the way described in this paper, we can easily measure a torsional resonant frequency of sample, and with very care, the electro-magnetic disturbance was comparable with input noise level;0. 2μV(rms)of the pre-amplifier.