Cloud cavitation

Weninger KR, Camara CG, Putterman SJ (2001) Observation of bubble dynamics within luminescent cavitation clouds sonoluminescence at the nano-scale. Phys Rev E 63 016310... 

Further investigations of chemical kinetics and transformation products will be carried out during the final phase of the project. In order to truly understand sonochemical effects, the behavior of the individual bubbles and the bubble clouds must be more finely resolved. Physical characterization of cavitation bubble clouds will also be performed. Thus, a more fundamental link will be established between bulk, observable parameters and sonochemistry, via the physics and hydrodynamics of the cavitating cloud. 

The underlying premise of the cloud collapse theory is that the first bubbles to collapse are those at the periphery of a cavitation cloud, and they in turn transfer collapse energy to the core of the bubble cluster, thus intensifying water hammer pressures. 

Parlitz U, Mettin R, Luther S, Akhatov I, Voss M, Lauterbom W (1999) Spatio-temporal dynamics of acoustic cavitation bubble cloud. Philos Trans R Soc London A 357 313-334... 

The transient nature of the cavitation event precludes conventional measurement of the conditions generated during bubble collapse. Chemical reactions themselves, however, can be used to probe reaction conditions. The effective temperature realized by the collapse of clouds of cavitating bubbles can be determined by the use of competing unimolecular reactions whose rate dependencies on temperature have already been measured. The sonochemical ligand substitutions of volatile metal carbonyls were used as... 

Many authors report a fairly linear enhancement of reaction rates with increasing power density (power applied/irradiated volume ratio, W/L) [14,18-20]. A saturation power was reached in some cases [21], probably related to the formation of clouds of cavitation bubbles near the transducer, which block the energy transmitted from the probe to the fluid, at high intensities. 

Brennen CE. Cloud cavitation observations, calculations and shock waves. Multiph Sci Technol 1998 10 303-321. 

Reisman GE, Brennen CE, Wang YC. Observations of shock waves in cloud cavitation. J Fluid Mech 1998 355 255-283. 

An ultrasonic horn transducer consists of a transducer unit attached to a horn (rod) usually made from titanium alloy and which has a length a multiple of half-wavelengths of the sound wave. For the commonly encountered 20-kHz horn this corresponds to 12.5 cm. The horn is then partially inserted into the fluid medium of interest and intense ultrasound is generated at its tip so that, for adequately large intensities, a cloud of cavitation bubbles is visible. This arrangement permits significantly higher ultrasonic intensities (10-1000 W cm ) to be applied than are achievable with a bath. 

A detailed study of the nucleation of ice by power ultrasoimd has been performed using a variety of high-speed photography systems with a particular focus on the influence of cavitation. The nucleation of ice has been shown to occur predominantly within the bubble cloud produced by a commercial ultrasonic horn. An investigation of a single oscillating bubble has confirmed that ice crystals are nucleated in the immediate vicinity of the bubble. 

We observed more than 50 vapour nucleation events in the IF, which enabled us to identify the main stages of cavitation. The two-phase stable situation was recovered within about l/3s (5 to 6 images with our camera). In general, nucleation started in the broadest part of the inclusion by a foam, a milky cloud a little more contrasted than the liquid. Then, a burst of tiny bubbles, taking birth in the inclusion appendix, invaded the whole cavity (Microphotograph 7, Fig. 4). 

The process of cavitation has since been the subject of several intensive studies, for example. Cum et al., 1990, 1992 Atchley et al., 1988 Alippi et al., 1992 Leighton, 1995. One of the theories is that cavitation is the result of events that occur within a cloud of bubbles and not just within a single bubble (as assumed in the above equations), but reaction engineering approaches taken so far have been based on the classical single-bubble theories. 

The possibility of generating a cloud of droplets by means of ultrasonic waves was first reported by Wood and Lomis [29]. Two different mechanisms have been reported to explain the ultrasonic atomization capillary waves and cavitations. However, the interaction between these two approaches and hmits in which one could predominate over the other depending on the different atomizing situation are challenging for immediate understanding. 

Experiments on cavitation prove that the noise level is highly dependent on the type of cavitation. Usually it is expected that bubble cavitation is much less severe than cloud cavitation. To verify this expectation, noise measurements on a bubble stream over a hydrofoil were done at Marin (Wageningen, The Netherlands). These experiments show that there is not much difference irrespective of the fact whether the bubbly flow is clustering or not. 

It is expected from experimental research that sheet-cloud cavitation causes most of the sound production by a cavitating ship propellor. Therefore Omta [ 2] analysed the sound production of a spherical bubble cloud analitycally. At MARIN the results of Omta where verified in a water tunnel [3]. In these experiments the sound radiation of a bubble stream, injected into the fluid near a two dimensional hydrofoil, was measured. These bubbles where injected both intermitted and continouosly, in order to distinguish a cloudy bubble stream and a continuous bubble stream. The remarkable result of this research was that no difference between the two, above mentioned, situations was observed. 

The cavitation development process is the formation of a cloud of vapour-gas bubbles (bubbly cluster) under the action of a rarefaction phase in the liquid containing microinhomogeneities, such as microbubbles of free gas, solid particles or combinations of them. Under an intense development of cavitation the liquid physically transforms into a two-phase state changing substantially the structure and parameters of the applied field. In order to construct a mathematical model of this process, one should know, first of all, the state of real liquid and the mechanism of the bubbly cluster formation. 

Cavitation is defined as the formation and behavior of gas or vapor bubbles and bubble clouds in a liquid. It can be induced thermally (by boiling), acoustically, optically (with a laser), by high energy charged particles, and hydrodynamically, i.e., through a pressure fall in a flow. The first questions that arise are (1) what are the processes inducing cavitation, (2) how does a bubble evolve from its formation up to its demise, (3) what occurs inside collapsing bubbles so that they are able to produce radicals and excited species, (4) what is the nature of the interactions between bubbles and solid boundaries involved in the dramatic erosion of these solids ... 

BUBBLE CLOUD (THE PASSAGE TOWARDS TRANSIENT CAVITATION)... 

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