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Recently van Rijsbergen et al. (2012), by simultaneous observation of cavitation and acoustic emission measurements, and Petkovsek and Dular (2013), by simultaneous observation of both cavitation structures and cavitation damage, have pointed to the fact that the small scale structures and the topology of the cavitation clouds play a significant role in cavitation erosive potential. Despite the two, before mentioned, studies opened some new insights to the physics of cavitation damage, many new questions appeared. In the present study we attached a thin aluminum foil to the surface of a transparent Venturi section using two sided transparent adhesive tape. The surface was very soft – prone to be severely damaged by cavitation in a very short period of time. Using high speed cameras, which captured the images at 30,000 frames per second, we simultaneously recorded cavitation structures (from several perspectives) and the surface of the foil. Analysis of the images revealed that five distinctive damage mechanisms exist – spherical cavitation cloud collapse, horseshoe cavitation cloud collapse, the “twister” cavitation cloud collapse and in addition it was found that pits also appear at the moment of cavitation cloud separation and near the stagnation point at the closure of the attached cavity. [ABSTRACT FROM AUTHOR]

We are comparing results of numerical simulations against high-speed simultaneous observations of cavitation and cavitation erosion. We performed fully compressible, cavitating flow simulations to resolve the formation of the shock waves at cloud collapse--these are believed to be directly related to the formation of the damage. Good agreements were noticed between calculations and tests. Two high pressure peaks were found during one cavitation cycle. One relates to the cavitation collapse and the other one corresponds to the cavitation shed off, both contributing to a distinctive stepwise erosion damage growth pattern. Additional, more precise, simulations with much shorter time step were performed to investigate the processes of cavitation collapse and shedding off in more detail. There the importance of small cavitation structures which collapse independently of the main cloud was found. The present work shows a great potential for future development of techniques for accurate predictions of cavitation erosion by numerical means only. [ABSTRACT FROM AUTHOR]

Abstract: Despite a large ensemble of works on the relationship between the macroscopic cavitation structures and their erosive potential a study that would link individual cavitation events with specific damage has not yet been made. In the present study we attached a thin aluminum foil to the surface of a transparent Venturi section using two sided transparent adhesive tape. The surface was very soft-prone to be severely damaged by cavitation in a very short period of time. Using two high speed cameras we simultaneously recorded cavitation structures and the surface of the foil. Analysis of the results revealed that damage only occurs at cavitation cloud collapse, and that the size of the cloud and its distance from the wall at collapse do not influence the extent of the damage and that an irregular or “broken” type of cavitation cloud causes the most damage to the foil. Also and probably the most important part of the entire study shows the sequence where one can see the separation and the collapse of cavitation cloud and the corresponding appearance of cavitation erosion. [Copyright &y& Elsevier]

• The re-entrant jet induced cloud shedding is confirmed in hot water. • Whole stages of the cavity clouds developing and collapsing are presented. • Shedding dynamics of cavitation over a wide tempetature range are identified. • The thermal transition and increase-decrease trend of cavity length are analyzed. The effects of temperature on hydraulic cavitation dynamics are investigated under various operating conditions, in a close loop cavitation tunnel with a small-scale venturi type section. A systematic study is performed with temperatures varying between 24°C and 85°C, using a high-speed visualization system to observe the cavitating flow. The image processing methods provided in the paper present a quantitative comparison of the cavitation dynamics and structure development. The results show that the increase of the fluid temperature induces the growth of the cavitation volume up to about 55°C, thereafter an additional increase in temperature has the opposite effect. This evolution is interpreted as a competition between a Reynolds effect and the well-known thermal effect. Cavitation is more closely investigated within the temperature range 50°C - 65°C, to analyze the changes in the structure and the cavitation dynamics. For the prediction of thermal suppression head, the thermal effect parameter Σ which can be used empirically, is derived at the maximum cavitation length. This fluid thermodynamic parameter Σ(T trans) at the transition peak can be referred to to avoid the maximum cavitation aggressiveness induced vibration or erosion for thermos-fluids around the thermal transition temperature. Finally, the factors influencing cavitation length and shedding frequency are presented and analyzed. [ABSTRACT FROM AUTHOR]

• Cavitation dynamics in water changes drastically between 20 °C and 100 °C. • At 60 °C cavitation reaches its peak in extent and in number of structures. • Cavitation dynamics corresponds well to cavitation erosion. • Extrapolation of the results from surrogate liquids to cryogenics is questionable. Understanding and predicting thermodynamic effects is crucial when the critical point temperature is close to the operating temperature of the fluid, like in cryogenics. Due to the extreme difficulties of experimental investigation, predicting of thermodynamic effects in cavitation often bases on data in liquids other than cryogenics. Most often used surrogate liquids are hot water or certain refrigerants, which are selected by a single fluid property, most commonly by the thermodynamic parameter ∑. The paper presents a systematic study of the cavitation dynamics in water at 20 °C, 40 °C, 60 °C, 80 °C and 100 °C and in addition in liquid nitrogen (LN2). Cavitation dynamics on a 4.8 mm (tip diameter) ultrasonic horn tip, which oscillated at 20 kHz was investigated by high-speed visualization at 300,000 frames per second (fps). Simultaneously acoustic emissions were recorded by a high frequency pressure transducer. Measurements were performed under variation of the acoustic power in a closed, insulated vessel, where pressure could be optionally set. The main purpose of the presented investigation is to determine whether hot water can act as a surrogate liquid to cryogenics. The results may implicate the future investigations and development of a new generation of rocket engines, which also feature the possibility of re-ignition while in orbit – understanding and predicting of cavitation behaviour is becoming a crucial part at the (liquid oxygen – LOX and liquid hydrogen – LH2) turbo-pump design. [ABSTRACT FROM AUTHOR]

Venturi-type cavitation reactors still appear as the most promising candidates for industrial-scale production due to their cheapness and ease of construction, scaling, and replicability. The effects of temperature on hydrodynamic cavitating flows in a Venturi section are investigated to find the optimum reacting conditions enhancing cavitating treatment intensity. The flow conditions are varied with 4 different flow rates and a wide range of temperatures between 28 ℃ to 63 ℃. Results show that both the cavitation length and the transition between sheet and cloud cavitation regimes are influenced by a combination of the pressure drop (indicated by the cavitation number σ), the inertial/viscous effects (controlled by the Reynolds number Re), and the thermal effect (indicated by the thermodynamic parameter Σ). As the temperature is elevated, both the cavitation length and thickness increase first, and then decrease. The cavitation intensity peaks at a transition temperature of 58 ℃. With the increase of cavitation length and thickness, the regimes tend to switch earlier from the attached sheet cavity to periodical cloud shedding, and the shedding frequency decreases accordingly. When the temperature is progressively increased, the changing of cavitating flow structures is illustrated through Proper Order Decomposition analysis. This study allows us to understand the instability, size evolution, shedding regime transition of partial cavities considering thermodynamic effects. Recommendations are provided to beer-brewing, biodiesel production, or water treatment industries that working under a 55 ℃ to 60 ℃ temperature range will attain the highest cavitation intensity. • Temperature, Reynolds number and cavitation number affect HC performance. • The increase–decrease trend of cavity length is confirmed and analyzed. • POD method is developed to distinguish sheet cavitation and cloud cavitation. • Cavitation intensity peaks and shedding regimes transition happens at 58 °C. [ABSTRACT FROM AUTHOR]

Abstract: Previous investigations showed that a single cavitation bubble collapse can cause more than one erosion pit (Philipp & Lauterborn [1]). But our preliminary study showed just the opposite – that in some cases a single cavitation pit can result from more than one cavitation event. The present study shows deeper investigation of this phenomenon. An investigation of the erosion effects of ultrasonic cavitation on a thin aluminum foil was made. In the study we observed the formation of individual pits by means of high speed cameras (>1000fps) and quantitatively evaluated the series of images by stereoscopy and the shape from shading method. This enabled the reconstruction of the time evolution of the pit shape. Results show how the foil is deformed several times before a hole is finally punctured. It was determined that larger single pits result from several impacts of shock waves on the same area, which means that they are merely special cases of pit clusters (pit clusters where pits overlap perfectly). Finally it was shown that a thin foil, which is subjected to cavitation, behaves as a membrane. It was concluded that the physics behind erosion depends significantly on the means of generating cavitation (acoustic, hydrodynamic, laser light) and the specimen characteristics (thin foil, massive specimen), which makes comparison of results of materials resistance to cavitation from different experimental set-ups questionable. Further development of the shape from shading method in the scope of cavitation erosion testing will enable better evaluation of cavitation erosion models. [Copyright &y& Elsevier]