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Thermodynamic effects in cavitation become significant only when the critical-point temperature is close to the operating temperature of the fluid, as in the case of cryogenic fluids. Therefore, the understanding and the prediction of the cavitation effects in such cases is crucial in many applications - for example the turbopumps for liquid hydrogen (LH2) and oxygen (LOX) in space launcher engines. The new generation of rocket engines will also feature the possibility of re-ignition while in orbit and prolonged period of operation; hence cavitation erosion is becoming an issue at the design stage of the turbo-pumps. In the study, we show measurements of cavitation erosion in liquid nitrogen (LN2), where cavitation was generated by an ultrasonic transducer. The damage was evaluated on aluminium samples. Special care was given to accurate setting of the operation point – especially the operating pressure, which defines the size of cavitation. We show that it is less aggressive than cavitation in water and that its aggressiveness cannot be described by a single fluid property (for example the most commonly used Brennen’s thermodynamic parameter Σ), but by a combination of several (viscosity, density, vapor pressure, surface tension, thermodynamic parameter) – in the present paper we addressed this point by a simple bubble dynamics model with consideration of the thermodynamic effect to qualitatively predict the results of the measurements. Finally, we also compared performance of several other engineering materials. [ABSTRACT FROM AUTHOR]

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]

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]