It is well known that cavitation collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye [sonoluminescence (SL)]. Considerable attention has been devoted to the phenomenon of "single bubble sonoluminescence" (SBSL) in which a single stable cavitation bubble radiates light flashes each and every acoustic cycle. Most of these studies involve acoustic resonators in which the ambient pressure is near 0.1 MPa (1 bar), and with acoustic driving pressures on the order of 0.1 MPa. This study describes a high-quality factor, spherical resonator capable of achieving acoustic cavitation at ambient pressures in excess of 30 MPa (300 bars). This system generates bursts of violent inertial cavitation events lasting only a few milliseconds (hundreds of acoustic cycles), in contrast with the repetitive cavitation events (lasting several minutes) observed in SBSL; accordingly, these events are described as "inertial transient cavitation." Cavitation observed in this high pressure resonator is characterized by flashes of light with intensities up to 1000 times brighter than SBSL flashes, as well as spherical shock waves with amplitudes exceeding 30 MPa at the resonator wall. Both SL and shock amplitudes increase with static pressure.
This paper describes a spherical acoustical resonator system used to study acoustic cavitation phenomena in liquids as part of an effort to scale up the energy density of collapse of transient cavitation. The resonator is formed by a stainless steel spherical shell 24.1 cm in diameter (OD) and either 1.27 cm or 1.90 cm thick designed for generating transient cavitation at high static pressures. An external transducer attached to the surface of the resonator was used to excite an acoustic standing wave in the liquid in order to generate a pressure maximum near the center of the liquid. We will present the results of our characterization of this device, including hydrophone measurements of the acoustic pressure generated in the liquid and vibration analysis on the surface of the resonator carried out using laser Doppler vibrometry. The resonance frequency spectrum and modal structure are compared to numerical predictions using theory developed by (Mehl, JASA 78(2), 782–788 (1985)) [Work supported by SMDC Contract No. W9113M-07-C-0178.]
In highly degassed, clean liquids, transient acoustic cavitation can be triggered by fast neutrons, a phenomenon that has been known since ∼1960’s. The kinetic energy acquired by the nuclei allows it to ionize a small volume (∼100 nm dia.) in the liquid, creating a vapor cavity that would normally last a couple of microseconds. If the acoustic amplitude and phase are right, this cavity expands by several orders of magnitude (∼500 to 1500 microns dia.) and then collapses, emitting a short flash of light (1 to 40 nsec). The bubble continues to expand and collapse for several hundred cycles, eventually evolving into a larger (2 to 6 mm dia.) bubble cloud which lasts several milliseconds, depending on the conditions. The time duration of the light pulses is longer and their amplitude larger than those of single bubble sonoluminescense in water (∼100 to 300 psec, 105 photons/flash). The amplitude and time evolution of the light flashes has been analyzed as a function of the driving conditions and compared with computer simulations in an effort to infer the maximum plasma temperatures and densities, and perhaps the presence of shock waves, in the cavities.
Details of the observed evolution of a high pressure transient cavitation event are described. The combination of photomultiplier tubes, hydrophones, fiber optic hydrophone, laser to photo diode light blocking methods are used to explore the evolution of a transient bubble cloud event from ns to ms time scales. [Work supported by SMDC Contract No. W9113M-07-C-0178.]
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