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Francis turbines with medium or high specific speeds may experience a particular type of instability in the upper part load in which the precessing vortex has an elliptical shape. The occurrence of the upper part-load instability (UPLI) is accompanied by large-amplitude pressure fluctuations at a distinct frequency between 2 and 4 times the runner rotational speed. This paper experimentally investigates UPLI for a reduced-scale Francis turbine. To investigate the causal factors of this instability, draft tube pressure measurements, particle image velocimetry, and high-speed flow visualizations have been performed at several operating points under cavitation and cavitation-free conditions. It is shown for the first time that for an operating point within the UPLI range, the vortex always features a circular section in cavitation-free conditions, which is preserved even after the initial appearance of cavitation. It is only below a certain Thoma number that the vortex section turns into an ellipse and shows an abrupt increase in pressure fluctuations. Analysis of the phase-averaged velocity fields reveals that a concentrated vortex with a large precession radius is a prerequisite for UPLI, while the instantaneous velocity fields clearly illustrate the asymmetric velocity distribution around the elliptical vortex. The existence of a breathing mode and the intermittent formation of two side vortices along the elliptical vortex rope are also evidenced by high-speed flow visualizations. These results provide a much deeper insight into the flow structures that favor the development of UPLI and help delimit its thresholds to higher precision, and thus, prevent its occurrence during turbine operations.

One of the most important reasons for the wearing of the blades in the slurry pumps is cavitation. The current article investigates the inverse profile of the blade on the cavitation and erosion performances of the slurry pumps. In this article, the inverse profile of the blade is considered as straight and curved lines. The flow simulation is conducted as steady and 3-dimensional, using Realizable k—e as the turbulence model. The results of the simulation demonstrated that in the curved profile volume fraction and the explosion of bubbles at the tip of the blade is lower than that of the straight line. Furthermore, in this article, the blades were tested in both states, showing that when the inverse profile of the blade changes from straight line to curved, cavitation decreases hence the lifespan of the blade increases. In the experimental results, a blade with a straight line inverse profile eroded and completely destroyed after 12 days of functioning, while in a curved inverse profile, the blade can work well—enough for 23 days (91% increase in lifespan).

Snapping shrimps close their special shaped claw to generate a cavitating water jet with high speed, and the pressure pulse can be generated by cavitating bubble bursting. The cavitating jet is produced by its pincers, with the characteristic of high energy focusing. Therefore, for the cavitation jet, it is of great significance to explore the law of cavitation and the focusing principle, which can inspire us to investigate the cavitation effects and develop the bionic cavitation device. In this paper, a bioinspired snapping-claw apparatus with ideal cavitation effect was obtained after a series of optimization designs. And a hypothesis on the principle of cavitation generated by the bioinspired apparatus was proposed. The general process is as follows: on the basis of the existing CT scan and geometric surface reconstruction of the actual shrimp, a 3D bionic model that preserves the basic geometric features was established. By solving the dynamic equation of the bubble, the critical conditions of cavitation, temperature and maximum pressure at the moment of bubble bursting are obtained. To explore the characteristics of pincers that have influences on energy focusing mechanism and cavitation effects, on the basis of the results obtained through CFD simulation, the optimized bionic structure and the influencing principle was obtained through continuous improvement of several groups of simulation. The typical parts of bionic structures that influence submerged cavitation jets were summarized as well. Furthermore, a series of corresponding control groups were set for the sizes of these typical parts of structures respectively, and the influences of these typical parts with different sizes on the cavitation effect were explored. Finally, combined with the surface morphology of real pincers of snapping shrimps, the qualitative hypothesis of cavitation mechanism during the closure of the snapper-claw was raised based on the simulation results finished.

Cavitation plays a central role during creep-fatigue. During recent years, fundamental models for initiation and growth of creep cavities that do not involve any adjustable parameters have been developed. These models have successfully been used to predict creep rupture data for austenitic stainless steels again without using adjustable parameters. However, it appears that basic models have not yet been applied to creep-fatigue assessments. A summary of the fundamental cavitation models is given. A model for monotonous deformation is transferred to cyclic loading. The parameter values are kept except that the dynamic recovery constant is raised due to increased interactions between dislocations during cycling. This model is successfully compared with observed LCF and TMF hysteresis loops. A new model for cavity growth due to plastic deformation is presented. The model is formulated in such a way that the condition for constrained growth is automatically satisfied. In this way, it is avoided to overestimate the growth rate.

Although research on microbubbles (MBs) has intrigued a lot of researchers, very few studies have been done about the influences of MBs on the free radical formation and also on the degradation rates of organic compounds by ultrasonic irradiation. Here we report the degradation of Acid Orange 7 by the ultrasound irradiation in the solution of MBs for the first time. It was found that acceleration of the degradation rate of Acid Orange 7 was observed only for the 45 kHz ultrasonic irradiation to the microbubble solution. Dependence of gases inside the MBs on the sonochemical efficiencies with the existence of MBs was also measured using the KI method and it was found that the dependence of the sonochemical efficiencies on gases inside the MBs was in the order of Kr > Ar ~ N2 > O2 > He, supporting the conclusions that the observed phenomena were due to the cavitation by the 45 kHz ultrasonic irradiation. The dependences of ultrasonic frequency and power were investigated using various commercially available ultrasonic oscillators, and the influence of the size distribution of microbubbles on the sonochemical efficiencies was further investigated using a flow reactor system with two ultrasound generators. Finally, we showed that the frequency of the ultrasound was more sensitive than the size of microbubbles for the sonochemical enhancement by the 45 kHz ultrasonic irradiation to the solution of MBs and then discussed possible roles of nanobubbles in the presently observed phenomena.

Called by Green Ship of the Future to reduce the propeller noise pollution in the subsea environment and avoid the possibility of causing propeller–shaft–ship resonance, the low-frequency broadband noise (LFBN) of marine propellers was studied theoretically and experimentally. The spectral method is improved by considering the blade section thickness and anisotropy in the turbulence spectrum, both of which are found to be effective in improving the prediction accuracy when compared with the experimental results. A series of propellers with the same blade geometry but different blade number were tested in the large cavitation channel at the China Ship Scientific Research Centre. The peak values in all conditions were close to the first-order blade-passing frequency. The effects of blade number and the advance coefficient were investigated by testing the propellers operating under different conditions. The effects were also studied using both the spectral method and experiment, and the results were consistent. Furthermore, the quantitative dependence of the LFBN on the influencing parameters was investigated using the sensitivity analysis. The rotational speed and turbulence intensity were found to be the two main factors, with greater than 10% effects. In addition, the effects of thickness and anisotropy scaling factor were evaluated using the spectral method. The results of this study provide guidance for controlling the LFBN in propeller design and optimisation.

Laser surface melting (LSM) was applied on a cast Ni–Al bronze (NAB), which was a crucial material for marine ship propellers. A 720 μm-thick LSM layer with fine equiaxed and dendritic microstructures was obtained. After immersion for 30 days, the corrosion rate of cast NAB was reduced by 25% after LSM. Preferential corrosion occurred and deep corrosion pits appeared at α + κIII microstructure for the cast NAB. LSM NAB underwent general corrosion, and a much more protective film formed on the surface because of the homogenized microstructure. The mass loss of the cast NAB was approximately 2.1 times larger than that of LSM NAB after cavitation erosion (CE) in 3.5 wt.% NaCl solution for 5 h. For the two materials, the mechanical impact effect was dominantly responsible for CE damage. Therefore, the improved hardness and homogenized microstructure contributed to the improved CE resistance of LSM NAB. CE destructed the film, shifted the open circuit potential toward a more negative value, and raised the current density by an order of magnitude. Corrosion at the cast eutectoid microstructure and the dendrites of LSM NAB facilitates the degradation under the cavitation attack. CE-corrosion synergy was largely caused by corrosion-enhanced-CE.

Research on bulk nanobubbles with various applications has been widely reported in the literature. However, the majority of studies are still limited to small scales with non-continuous generation processes, thus, the results may be difficult to apply on large-scale applications. In this work, a nanobubble generator was developed based on hydrodynamic cavitation using a two-chamber swirling flow nozzle that efficiently produced nanobubbles and had the potential to be applied to continuous flow systems and processes. The bulk nanobubble characteristics were evaluated according to the hydrodynamic diameter, zeta potential, and dissolved oxygen (DO) concentration based on the influence of the gas flow rate ratio (Ql/Qg), generation time, gas type, pH value, and NaCl concentration on the generated nanobubbles. The results show that oxygen and air nanobubbles smaller than 200 nm were successfully generated in pure water. The oxygen and air nanobubbles were negatively charged in pure water. The effects of pH and salt addition were similar to those during nanobubble generation using ultrasonic cavitation. In alkaline medium, nanobubbles were smaller and more stable than in acidic medium. The addition of salt to the nanobubble suspension reduced the repulsive electrostatic forces between the nanobubbles by increasing their size and decreasing their negative zeta potential. The resulting oxygen and air nanobubbles in pure water were verified to be stable for up to 10 and 5 months, respectively, without any significant changes in size or zeta potential. These results corresponded to predictions by the ionic repulsion model based on microbubble shrinkage.

Pumped storage units are the main parts in China’s power construction, as a hot issue concerned by the industry. The pump turbine involves the two-way flows and a multiple condition operation, and its operation flow pattern is very complex. The particle image velocimetry (PIV) is a very effective test technique to determine the internal flow field of pump turbines. This paper discusses the key problems of the pump turbine, based on the PIV experiments under typical conditions of the pump turbine, especially for problems such as the S-shape problem, the hump problem, the pressure fluctuation problem and the cavitation problem. In the internal flow fields under typical conditions are determined. The vortices induced and their development are observed in the PIV test. The flow phenomenon is shown at each operating point. The typical problems of the pump turbine are closely related to the vortex distribution in the internal flow field. From the PIV test results under several working conditions and from the comparisons between the optimal condition and the part load condition, it is seen that the vortex distributions are very different. Vortices at the vane-less area between the guide vane and the runner are closely related to the strong pressure pulsation, the first hump and the S-shape curve. From the PIV results of the cavitation working points, it is seen that the flow angle is changed in the vane-less region and the runner leading edge because of the cavitation bubbles and that the flow angle deviates from the optimal setting angle. From the computational fluid dynamics (CFD) result of the second hump working points, it is concluded that the vortex shedding on the runner leading edge is the main cause of the second hump.

We discuss the interaction of the strongly nonlinear fluid motion induced by the collapse of a vapor microbubble over a planar surface and the elastic dynamics of the underlying solid. The fluid is described using an extension of the Navier-Stokes equations endowed with distributed capillary stresses in the context of a diffuse interface approach. The collapse of the bubble is triggered by overpressure in the liquid and leads to an intense jet that pierces the bubble, changing the bubble topology from spheroidal to toroidal, and impinges the solid wall inducing an intense and strongly localized load. Moreover, at bubble collapse, a compression wave is launched into the liquid surrounding the bubble. By propagating along the solid surface, the compression wave combined with the liquid jet excites the dynamics of the elastic solid, producing a complex system of waves, including, longitudinal, transversal, and Rayleigh waves, propagating in the solid. It is conjectured that the intense deformation of the solid induced by the strongly localized liquid jet may lead to the plastic deformation of the solid producing the surface pitting observed in many applications subject to cavitation-induced material damage.

Electrochemical micromachining (EMM) has become a popular technology for fabricating microscale structures. To obtain higher machining localization and minor stray corrosion, a new hybrid EMM process combining air assistance and ultrasonic assistance was introduced in this paper. Simulations and experimental investigations were conducted to study the influencing mechanism and the influencing characteristics of the assisting process energies. In the multi-physics model, a reactant transport model is included, and the direct coupling between the assisting processes and the electrochemical process is achieved. Results indicate that, when the two assisting processes are applied separately, the assisting effect tend to act on the entire workpiece surface, either promotes the overall dissolution or inhibits it, usually leading to unsatisfactory results; by combining the two assisting processes, synergistic effect occurs: dissolution in the central area is promoted by the ultrasonic assistance, while stray corrosion in the margin area is hindered by the air assistance. Thus, microstructures with enhanced aspect ratio and reduced stray corrosion can be fabricated. The results also show that the surrounding airflow can produce an electrical insulating effect on both the workpiece and the cathode. When the air pressure is low, an additional concentrating effect on the electric field can be observed; however, a high air pressure leads to a greatly hindered dissolution. When the applied amplitude is appropriate, ultrasonic assistance promotes the electrochemical dissolution through disturbance effect and cavitation phenomena. But if the amplitude is too large, massive cavitation bubbles will inhibit the dissolution greatly.

Laser-induced cavitation bubble behavior on the solid walls of different materials was experimentally investigated by focusing the pulsed laser on the wall. To examine the influence of wall reflectivity on laser-induced cavitation, high-speed observations of the bubble behavior and impulsive-force measurements were carried out on aluminum, steel, copper, and acrylic resin walls. It was found that the bubble behaviors and impulsive forces vary with the wall materials, and their behaviors depend on the wall reflectivity. Increased bubble diameter, bubble collapse time, and high impulsive force were observed on the walls with high reflectivity. This result can be due to the increased laser illumination on the highly reflective walls. This promotes the heating of liquid near the wall at the laser breakdown in the first impact on the wall, while the increased bubble diameter by this effect promotes the bubble collapse phenomenon and intensive shockwave formation, resulting in the high impulsive forces on the wall in the second impact.

During experiments, a micro-hydropower plant with a siphon water intake and an orthogonal turbine with five-bladed runners with an installed capacity of 30 kW was tested, and the characteristics of the power equipment in the available pressure and flow ranges were obtained. A series of theoretical and field studies on the use of free-flow orthogonal turbines in small hydropower was conducted, and possible layouts were considered. The results of tests of the orthogonal turbine of the experimental microhydropower plant were analyzed, and its efficiency and the operating conditions of the hydropower equipment in which cavitation occurs were determined.

The cavitation erosion performance of a mild steel, a high-carbon steel, and a Cr-Mn stainless steel was studied in 3.5 wt.% NaCl solution using a probe-type ultra-sonicator device. The cavitation behavior of the steels was analyzed in terms of the mean depth of erosion (MDE) and the mean depth erosion rate (MDER). The dynamic polarization tests were performed in the same chloride solution. The micro-mechanism of the cavitation behavior of the steels was investigated with the help of scanning electron microscopy and Raman spectroscopic analysis. The cavitation resistance of the high-carbon and the stainless steels was found to be ~2.5 and ~50 times better than that of the mild steel. The corrosion resistance of the high-carbon and stainless steels was found to be ~2 and ~21 times higher than that of the mild steel, respectively. Afterward, a strong influence of the corrosion resistance of the steels on the cavitation performance was investigated on the basis of corroded surface morphology. In addition, a strong correlation between cavitation resistance and corrosion products formed during cavitation was established. Cavitation erosion resistance of the steels is precisely predicted by the protective ability index (PAI) without the need for any mechanical characterization.

Present study deals with the extraction of oil from the waste custard apple seed (CAS) using acoustic cavitation. The process parameters such as optimization of irradiation time, temperature, power of ultrasound, different solvents and seed to solvent ratio have been studied. For a comparative study, obtained oil from the acoustic cavitation method was compared with Soxhlet method. When power increases, fall in the extraction yield increases, and with decrease in temperature, the rise in extraction yield was observed. The obtained oil was characterised by the Fourier transformation spectroscopic analysis. The effect of acoustic irradiation on chemical and physical properties of the extracted oil was checked using differential thermal analysis and thermogravimetric analysis and compared with extracted oil by conventional Soxhlet method. The compounds such as oleic acid, palmitic acid and stearic acid in the extracted oil were identified using gas chromatography mass spectroscopy analysis. Scanning electron microscopy had confirmed the cracking and microfractures of the seed surface which are responsible for cell wall disruption. The time required for the extraction oil from the CAS using ultrasound is only 10 min, while Soxhlet method requires 70 °C temperature and 4.5 h (270 min) of time to get the same yield. The amount of energy required for the extraction of the oil from the CAS using acoustic cavitation method is 84.51% less than the Soxhlet method. Acoustic cavitation is a novel, simple and energy efficient method for the extraction of oil from the CAS.

In this paper, the water-exit phenomenon has been studied experimentally and numerically. Based on the volume-of-fluid model and the Schnerr–Sauer cavitation model, the water-exit supercavitation around different shapes of cavitator projectiles is investigated using a dynamic mesh technology and three-dimensional six-degree-of-freedom method. The numerical results are compared with the experimental results obtained in our laboratory. Next, the effects of the cavitator shape on the characteristics of the projectile motion, breaking of the free surface, supercavitation size, and hydrodynamics of the water-exit supercavitation are analyzed. It was found that the hemispherical-nose projectile holds the best storage capacity and the longest displacement. The blunt disk-shaped nose projectile generates the maximum diameter of supercavity. The cavitation diameters of the conical-nose projectile and the hemispherical-nose projectile are relatively small.

In this research, the effects of ultrasonic cavitation bubbles on the transesterification process were investigated utilizing a modified Rayleigh–Plesset model. Furthermore, the influence of some operating parameters on ultrasound biodiesel production from rapeseed oil and waste oil were evaluated containing operating temperature, reaction time, and catalyst (LiOH) concentration. According to researches, the several cycles of collapse and rebound of cavitation bubble are the basis of intense sound effects especially acoustic micro-streaming and shock waves. The results of this study revealed that the operating conditions of reaction were effective on the behavior of micro bubbles and the yield of transesterification. The results confirmed the enhancement and intensification of the transesterification process by sonication. The high temperature and pressure inside bubble induced operating parameters similar to supercritical conditions. This condition around hotspots could offer the presence of micro-reactors that speed up the rate of reaction. Moreover, cultivating the interfacial area between viscous films containing gas-filled bubbles and the liquid phase of reactants could be one of the effective parameters to increase the yield of ultrasonic method comparing with conventional mechanical stirring transesterification. The catalyst concentration amount had a significant influence on the reaction yield. The highest conversion efficiency (approximately 99.4%) was observed at 35 °C, after a reaction time of 35 min with alcohol to oil molar ratio of 6:1 and 0.5%wt of catalyst (LiOH).

Increasing cavitation in a flowing liquid in a pipe system is associated with increasing levels of sound or noise and vibrations. It can lead to large loads if steam bubbles implode close to the pipe wall, which can be harmful to the pipe system. It can lead to fatigue due to vibrations and even worse, erosion of the pipe wall. In the worst case, it can lead to a pipe break. Therefore, cavitation is essential to be predicted and prevented in system design. The aim here is to experimentally verify a concept for the elimination of strong cavitation and harmful cavitation induced vibrations. It has been experimentally demonstrated that replacing a conventional orifice plate with three stages of multi-hole orifice plates in series with only a total streamwise distance of two pipe diameters, giving the same total pressure drop, is sufficient for the elimination of strong cavitation. The cavitation induced vibration levels are reduced by almost 500%. The streamwise distance is almost 500% shorter if multiple conventional orifice plates were to be employed. The same conclusions hold for tests performed with β = 0.30 and 0.40. This compact design was successfully applied at a nuclear power plant, and the reduction of the cavitation induced vibration levels was confirmed. The main contribution is the quantification of the vibration levels, how they depend on the type of orifice plates (conventional, multi-hole, and multi-stage-multi-hole), and how they scale with the dynamic pressure. This will be used for the validation of future CFD simulations including Fluid Structure Interaction (FSI) and advanced two-phase models.

The effect of the diameter and contact angle on the aggregation of solid inclusions in molten steel was investigated based on the cavitation theory. Larger inclusions aggregated more easily, owing to the lower stable energy and larger critical aggregation spacing. The critical aggregation spacing had an explicit relationship with inclusion diameters: $$ d_{{{\text{C}},{\text{Al}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} $$ = 0.195(RP1RP2)0.5−0.021(RP1+RP2), dC,MgO = 0.099(RP1RP2)0.5−0.013(RP1+RP2) and $$ d_{{{\text{C}},{\text{SiO}}_{{\text{2}}} }} $$ = 0.031(RP1RP2)0.5−0.0061(RP1+RP2). Furthermore, calculated results showed that the aggregation tendency of the three considered types of inclusions was Al2O3 > MgO > SiO2.

In a cavitating water jet, cavity clouds emerge and collapse with an unsteady, but periodic tendency where the frequencies depend on the working conditions. The presented work aims at examining and analyze the dynamic behavior and properties of the clouds under different circumstances. Computer vision and image processing were introduced as tools to define the cavitation clouds based on the Contour Recognition technique. A Canny operator and Otsu threshold fragmenting methods were used. The use of these methods allows for a better understanding of the cavitating jet clouds' behavior based on the pixel intensities and shows that for an arbitrary cloud the surface itself has a dynamic feature and depends on the cavity composition. The clouds' properties could be measured and correlated to the applied working conditions. Also, the oscillation frequencies of the area of the clouds could be determined. The analysis shows that the quality of the obtained results depends mainly on the input threshold values separating the foreground and background pixels. The difficulty of defining the threshold value is discussed in the paper, as well as the validity of using the Contour Recognition technique in this field.

This study presents a technique for adsorption of paclitaxel on Sylopute using ultrasonic cavitation bubbles and gas bubbles. Compared with the conventional adsorption (control), the adsorbed amount and adsorption rate constant increased, respectively, by 1.27–1.44 times and 7.44–9.71 times in ultrasonic adsorption (with mixing at 80–250 W), 1.14–1.27 times and 4.63–9.31 times in ultrasonic adsorption (without mixing at 80–250 W), and 1.06–1.19 times and 1.18–1.34 times in gas bubble-adsorption (without mixing at 1.15–9.41 L/min). As a result of investigating the adsorption mechanism in which cavitation bubbles were introduced, it was shown that microjets and shock waves produced by bubble collapse, rather than the bubble itself, drastically improve mass transport in the pores of the adsorbent, thereby completely eliminating intraparticle diffusion resistance. In the case of gas bubbles, although the intraparticle diffusion coefficient increased by 1.34–1.75 times compared with the control, there was a limitation in promoting intraparticle diffusion.

In this paper, the large eddy simulation (LES) method in conjunction with the Zwart cavitation model is adopted for the assessment of the erosion risk on a hydrofoil surface. The numerical results are in good agreement with the experiments. On this basis, three methods, namely the intensity function method (IFM), the time-averaged aggressiveness indicators (TAIs) and the gray level method (GLM), are applied for the assessment of the erosion risk. It is shown that the erosion intensity index of the IFM is extremely sensitive to the artificially selected thresholds, which greatly limits the application of the method. The erosion risk predicted by four indicators in the TAIs does not agree well with the experimental results. Further analysis demonstrates that the GLM using the instantaneous pressure field is relatively satisfactory, which can provide a reasonable assessment of the erosion and is not very sensitive to the artificially selected thresholds. To further improve the accuracy of the GLM for the erosion risk prediction, the time-average pressure field is adopted in the GLM for the erosion evaluation. It is suggested that the erosion assessment by using the time-averaged pressure field is in better agreement with the experimental results when compared with that by using the instantaneous pressure field.

In the present research, we propose a novel measuring method of ultrasonic cavitation by naked-eye based on the conversion of ferrous ion to ferric ion and chromogenic reaction between ferric and thiocyanate. The effects of different conditions affecting cavitation were investigated. Experiments show that this method can effectively measure the strength of the cavitation field of the system, and is simple to operate, and the results are intuitive, and the cavitation strength can be obtained quickly. The effects of size and concentration of solid particles on ultrasonic cavitation are also studied by using micro-SiO2 as the typical representativeness. It was found that the addition of solid particles could reduce the heterogeneous cavitation threshold. The results provide a basis for the industrialization of ultrasonic chemistry, especially the strengthening of the reaction process of heterogeneous systems under ultrasound.

The acoustic propagation characteristics of the cavitating flow are essential for the noise suppression, but were not well studied. In the current paper, a new technique concerning the propagation path of the monopole acoustic energy is presented and two typical thermodynamic cavitation modes (the inertial and thermal modes) are selected to investigate the effect of the cavity shedding dynamics on the acoustic propagation path. In the inertial mode, the temporal variation and the spatial distributions of the monopole acoustic energy as well as the divergence of the monopole acoustic pressure are both more powerful and concentrated than that in the thermal mode. The acoustic propagation path in the thermal mode strictly satisfies the feature of the convective amplification, while there exists another propagation direction close to the normal direction of the foil surface in the inertial mode. Furthermore, the occurrence of the normal direction propagation will make the path deviate from the convective direction.

A comparison of theoretical investigation and numerical simulation of a single bubble oscillation is carried out in this paper. The theoretical research is based on solving Rayleigh-Plesset (R-P) equation and Keller-Miksis (K-M) equation using Runge-Kutta method. The numerical method focuses on the two discrete methods in the volume of fluid (VOF) method, geometric reconstruction (GR) and modified high resolution interface capturing (MHRIC). The results show that the interface captured by MHRIC in the collapse stage is more stable, and the evolution of bubble radius agrees better with the theoretical solution. The R-P equation and K-M equation, which omit the effect of the energy equation, have limitations when the bubble collapses under ultra-high pressure.

Dynamics of spark-generated cavitation bubbles near a deformable rubber sheet are experimentally investigated in this study. The transient bubble–rubber interaction is captured by high-speed photography, including the expansion and collapse of the bubble, jetting behaviors, large deformation, and rupture of the rubber sheet. The dependence of bubble behaviors and rubber response on two governing parameters (the bubble–rubber stand-off distance $$\gamma $$ and the rubber thickness T) are systematically studied. Firstly, the bubble collapse patterns or jetting directions are categorized into three regimes. If the bubble is initially located at a large $$\gamma $$ from the rubber sheet, the jet is directed towards the boundary (regime I). Unexpectedly, when the generated bubble is close to the rubber sheet, it develops a jet away from the boundary (regime III). Bubble splitting and the formation of two axial liquid jets directing in opposite directions can only be observed with an intermediate $$\gamma $$ (regime II). A phase diagram of the bubble behaviors is given in the $$\gamma $$ –T space. For a large rubber sheet thickness (e.g., $$T=5$$ mm), only a downward jet is observed. From the perspective of jet direction, the effect of such a thick rubber sheet on the bubble dynamics is similar to that of a rigid wall. Note that the jet velocity and jet width may be slightly different. Secondly, the results reveal that bubble expansion can cause the boundary to deform. The strong axial jet towards the rubber sheet can damage and even rupture the rubber boundary. A parameter range within which the bubble jetting can rupture the rubber boundary is obtained. Besides, for a thin rubber sheet ( $$T \le 4$$ mm), the rupture is more likely to occur in regime II. For a thick rubber sheet (e.g., $$T=5$$ mm), the rupture can be seen when the jet directly impacts the rubber sheet (regime I).

In this work, cavitation during creep in AA7075-T76 was simulated employing a probabilistic cellular automata scheme. Two-dimensional cellular automata coupled with the governing equations for cavity growth were utilized to define the size and distribution of cavities during creep. Both diffusion and strain-controlled mechanisms were taken into account while the first and second neighboring cells were considered for determination of the cavity growth as well as to generate the initial microstructure. Moreover, uni-axial creep experiments on AA7075-T76 were carried out under different temperatures and applied stresses including 170 MPa at 150 °C and 150 MPa at 160 °C. The microstructural changes during creep were detected, and formation and growth of cavities were evaluated after different creep durations. Accordingly, microstructural evolution including optical metallography and scanning electron microscopy were performed to characterize the microstructural changes and the size of the cavities. The achieved results were then employed to determine the material constants as well as to validate the simulation results. A reasonable agreement was found between the simulation and the experimental data, which shows that the proposed model can be used under real conditions while the impact of significant aspects including the coalescence of the cavities can be properly considered by the model.

This article is particularly interested in the numerical modeling of water hammer in a hydraulic circuit, taking into account the prevailing water temperature. The study concerns the propagation velocity of the wave and the amplitude of unsteady phenomena encountered in the circuit, as well as the severity and collapse of cavitations that are also considered as major risks. To conjecture the consequences of these phenomena, we were led to simulate a single-phase and two-phase transient flows in a hydraulic copper pipe system in a temperature range of 4–95 °C. To do this, we have developed a solver for the dynamic and continuity equations’ resolution. The method of characteristics is chosen for its capacities to solve these equations. Its application shows that it is robust and adapted to the problem studied. Two cavitations’ models and column separation have been incorporated; in this case, the Discrete Vapor Cavity Model (DVCM) and the Discrete Gas Cavity Model (DGCM). Moreover, in addition to the classic models of quasi-stable friction, of which the models of unsteady friction have been included, like the one based on the instantaneous acceleration proposed by Brunone and the one proposed by Vardy & Brown based on the convolution integral. Although single-phase and two-phase water hammers do not behave in the same way, the results obtained with these models show that the temperature produces a great effect on the hammer.

One of the most widely used components in industrial rotary machines is the plain journal bearings. These components consist of a rotating shaft, which is separated from the outer sheath by a thin layer of lubricant and rotated in it. In this research, the journal bearings are investigated numerically in three dimensions using ANSYS-FLUENT software. To have a more realistic analysis, the cavitation, which is a common event in bearings, is simulated. The Eulerian two-phase model along with Zwart–Gerber–Belarmi method is used to simulate the cavitation phenomena. In addition, the changes in fluid viscosity due to the temperature and pressure have been considered in calculations. Moreover, the effect of the viscous dissipation term is considered that leads to a change in fluid temperature. Simulations are conducted for two angular velocities of 48 and 68 [rad/s] as well as two eccentricity ratios of 0.6 and 0.8. The comparison of the results with other numerical and experimental data shows the accuracy of the two-phase model and conducted settings in the software. The results show that the consideration of cavitation along with the viscosity changes have a significant impact on the pressure distribution, cavity volume, and load-bearing capacity.

For a traditional stepped spillway, low energy dissipation and cavitation damage may occur when the unit discharge is larger than a limit due to reduced air entrainment in the flow. In the present work, a new type of stepped spillway called hydraulic-jump-stepped spillway (HJSS) was developed, and the air was efficiently entrained into the flow by means of a hydraulic jump in the aeration basin, which was an aeration element of this stepped spillway. Compared with the traditional stepped spillway, various aspects of the hydraulic performance of the HJSS were experimentally investigated, including the flow pattern, energy dissipation, air entrainment characteristics and time-averaged pressure. Physical model results demonstrated that the HJSS achieved a higher energy dissipation, a more favorable air entrainment performance, and a more reasonable pressure distribution. The HJSS successfully increased the unit discharge from 50–60m2/s for a traditional stepped spillway to 101.47 m2/s.

The cavitating flow in different regimes has the intricate flow structure with multiple time and space scales. The present work develops a multiscale model by coupling the volume of fluid (VOF) method and a discrete bubble model (DBM), to simulate the cavitating flow in a convergent-divergent test section. The Schnerr-Sauer cavitation model is used to calculate the mass transfer rate to obtain the macroscale phase structure, and the simplified Rayleigh-Plesset equation is applied to simulate the growing and collapsing of discrete bubbles. An algorithm for bridging between the macroscale cavities and microscale bubbles is also developed to achieve the multiscale simulation. For the flow field, the very large eddy simulation (VLES) approach is applied. Conditions from inception to sheet/cloud cavitation regimes are taken into account and simulations are conducted. Compared with the experimental observations, it is shown that the cavitation inception, bubble clouds formation and glass cavity generation are all well represented, indicating that the proposed VOF-DBM model is a promising approach to accurately and comprehensively reveal the multiscale phase field induced by cavitation.

A numerical approach for modelling cavitating flows over moving hydrodynamic surfaces is presented. The operating fluid is modelled as an isothermal homogeneous mixture of water vapor and liquid phases. The flow field is governed by the Navier–Stokes equations along with a transport equation for the vapor volume fraction. The Arbitrary Lagrangian-Eulerian description of the continuum is adopted to handle fluid flow simulations on moving domains. The residual based variational multiscale method is used to model the turbulent flow together with wall modelling implemented by the weak imposition of the no-slip boundary condition. Merkle and Zwart cavitation models are implemented and compared. First, a cavitating flow over a 3D hemispherical fore-body is modeled to perform a detailed comparison between two models. Next, the cavitating flow over the INSEAN E779A marine propeller is modeled and results are compared to available experimental data showing good agreement.

The spatial and size distribution of cavitation bubbles in Al-18wt.%Cu during ultrasonic melt processing at a fixed acoustic intensity input was exhaustively studied by utilizing synchrotron x-ray photography. The result reveals that the larger bubbles concentrate within an intense cavitation zone close to the radiation face of sonotrode, which is out-surrounded by a weak cavitation zone with bubbles sparse-inhabitation. Most cavitation bubbles obey a truncated size-Gaussian distribution, and the average size decreases as distanced away from the radiation face. The intensive cavitation zone contributes to an effectively refined solidification structure.

The low pressure regions on hydrofoils causes cavitation which results in a change of fluid dynamics characteristics. To understand the effect of cavitation on hydrofoil performance, a numerical study has been performed in this work using Zwart–Gerber–Belamri cavitation model and Realizable $$\kappa -\epsilon $$ turbulence model. Computations were performed for two-dimensional unsymmetrical MHKF-180 and NACA4418 hydrofoils. The performance of the hydrofoils at different angles of attack and cavitation numbers was studied based on the lift coefficient, drag coefficient, pressure coefficient and cavity length. The results of MHKF-180 were compared with that of NACA4418. From the comparison, it was found that under cavitating conditions the lift coefficient of MHKF-180 at different angles of attack is higher than NACA4418. Also, the cavity length on MHKF-180 is found to be smaller than that of NACA4418.

The cavitation and slurry jet erosion resistance of surface-coated and thermochemically treated CA6NM 13-4 steel were evaluated in this work. The cavitation erosion experiments followed the ASTM G32 standard, while the slurry jet erosion tests were performed using a homemade tribometer. CA6NM steel was used as substrate and as reference material for all wear experiments. The surface treatments included plasma nitriding and salt bath nitro-carburization, while the coatings evaluated were thermal-sprayed WC-10Co4Cr and a commercial grade elastomeric coating. The microstructures were studied by scanning electron microscopy (SEM) and x-ray diffraction (XRD), and the mechanical properties were estimated by nanoindentation. From the results analysis, it was found that the plasma nitriding permitted high improvement against slurry jet erosion, similarly to the WC-10Co4Cr coating. Moreover, the thermochemical treatments of plasma nitriding and salt bath nitrocarburizing showed better increments of cavitation resistance when compared to the coatings evaluated. Based on these findings, comments on the challenges involved in the manufacture route for hydropower runners with enhanced performance against wear are included.

Hydrodynamic cavitating flows usually consist of 3-D intense vortical flows that are detached from solid boundaries. Detached vortical flows normally generate heaps of cavitating flow structures, which, in turn, govern the location of cavitation erosion before collapse. Thus, this study introduces a new numerical approach based on the improved delayed detached eddy simulation (IDDES) turbulence modeling for predicting cavitating flows. Then, the solution of compressible Eulerian-Eulerian two-phase flow and the IDDES turbulence model was linked to the microjet hypothesis and unsteady behavior of pressure and vapor volume to predict the corresponding erosion of cavitating flows. The method for cavitation erosion prediction, a modified version taken from previous studies, was applied as a post-processing tool. The validation of cavitating flow predictions was performed for the first time on the Grenoble axisymmetric nozzle by comparing them with 21 photos of cavitation from the previous experimental study. The results showed that the present numerical approach estimated various features of hydrodynamic cavitation well, including shedding processes and the length, shape, and collapse of cavitating structures. Using the numerical analysis, three main stages were detected for the present cavitating flow, and the vorticity-cavitation interactions were investigated by the vorticity transport equation. The streak-like and tube-like cavitating (STLIC and TULIC) structures were introduced in the second stage, initiated by flow instability, and entirely governed by corresponding turbulent flow structures. The collapse of these cavitating structures is one of the primary sources of cavitation erosion on lower and upper walls. The results of the numerical erosion predictions were compared with those of the previous erosion tests on the Grenoble axisymmetric nozzle. Satisfactory numerical performance was achieved in predicting the location and intensity of cavitation erosion.

When studying the flow dynamics and the characteristics of fluid motion, vortex structure with their interactions is the key issue. In the present work, a vortex control method is investigated based on the vortex identification system of Liutex. The numerical study is carried out in OpenFOAM by directly adding a source term to the Navier-Stokes equations, which is called the centripetal force model in Liutex method. A 2-D test case is examined to justify the proposed method in cavitating flow around Clark-Y hydrofoil, the simulation results show that the improved Liutex solver is feasible. Methodologies of controlling the rotation strength of vortices are able to change the flow field and suppress the cavitation. The applicability of vortex-based control method in 3-D flow field is also studied. The results show that cavitation surrounded by particular vortex can be effectively influenced.

Vibration is a major issue in turbomachines which causes damage to them and leads to reduction in life and efficiency. This paper reviews various methods to measure the amount of vibration in various turbomachines and also discusses various reasons for the same. This will significantly assist in articulating these vibrations. This paper lists various experiments done, causes, factors, comparison of different turbomachines and their results. Turbomachines which are taken into consideration are Francis turbine, radial turbines, axial turbines, compressors, pumps and fans. The outcome elucidates the changes in vibrations as a function of different input parameters. It can be concluded through various experiments on different types of turbomachines that the major causes of vibrations are cavitation and resonance while excess speed and longer operating periods too contribute to them.

A novel alternating-frequency ultrasonic melt treatment (AUMT) technology is used to refine the grain and improve the strength of AS41 magnesium alloy. The AUMT exhibits better grain refinement efficiency than the traditional 20 kHz single-frequency ultrasonic melt treatment method, showing the grain refinement efficiency up to 82.7%. The improvement of cavitation is suspected to be the reason for better grain refinement. The faster alternating time is beneficial to improve grain refinement efficiency. The AUMT also refines the Mg17Al12 and eliminates the agglomeration of Mg2Si. Such microstructure evolution helps improve the tensile strength and elongation of AS41.

The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions, to induce strong low-frequency pressure vibrations, and therefore, is very harmful to the safety of the hydropower unit. In the present work, three kinds of strategies are extensively investigated, i.e., the installations of the ventilation and the fin, as well as the hybrid strategy of the air admission through a fin, so as to effectively suppress the vortex rope oscillation and the pressure vibration in the draft tube of a Francis turbine, whose specific speed is 125 m-kW. For the unsteady flow simulation, the Reynolds averaged Navier-Stokes (RANS) method is applied coupled with the k-ω SST turbulence model and a homogeneous cavitation model. The flow analysis confirms that the low-frequency pressure vibrations are originated from the periodical oscillation of the vortex rope, and the cavitation usually enhances the vortex rope oscillation in the draft tube. Under the part-load condition, the dominant component of the pressure vibration in the draft tube has a frequency, for example, f1, lower than the runner rotating frequency fn. It is shown that all three strategies can be adopted to alleviate the vortex rope oscillation and the pressure vibrations in the draft tube, but their suppression mechanisms are quite different. The ventilation of an adequate amount from the turbine runner cone can change the vortex rope geometry from the spiral type to the cylindrical type, suppress the vortex rope oscillation, and consequently create the homogeneous distributions of the pressure and the pressure gradient in the draft tube. On the other hand, a fin installed at the draft tube wall can induce a small extra rope, and the interaction between the main vortex rope and the extra rope changes the flow field and alleviates the pressure vibration in the draft tube. It should be noted that a fin is much more effective to suppress the pressure vibration in the draft tube under the cavitation condition than under the non-cavitation condition. A better effect of suppressing the vortex rope oscillation can be achieved by the air admission through a fin, which is studied numerically in this paper. The result indicates that the air admission can further improve the effect of a fin for suppressing the pressure vibration in the inlet cone of the draft tube. This improvement is due to the stronger interaction between the main vortex rope and the extra air rope. However, the air admission through a fin should be carefully treated because the strong interaction may induce a larger pressure vibration in the elbow of the draft tube. Finally, it is clear that any strategy for suppressing the pressure vibration hardly changes the dominant component frequency f1, which is in the range of 0.22 fn-0.23 fn due to the main vortex rope oscillation in this study. The current results may be used in various engineering applications, where the active control of the vortex oscillation and the pressure vibrations with or without the cavitation is necessary.

During drilling fluid preparation, ultrasonic oscillations were introduced into water-bentonite suspension incorporating sulfonated wood coal (SMC) by a specially designed device. The influences of ultrasonic oscillations on fluid loss and rheological performances of the drilling fluid as well as mechanism of ultrasonic action were investigated. The experimental results showed that the filtrate volume decreased with the increase of ultrasonic time till a certain extent and then leveled off. In the presence of ultrasound, shorter time of 15 min and mild intensity of 250 W could lead to a satisfactory result in fluid loss properties, including the reasonable filtrate volume and thin and compact filter cakes. With increasing ultrasonic intensity, the fluid loss properties changed relatively little but various rheological data of the drilling fluids always increased. Adsorption tests through total organic carbon, infrared spectrum and thermogravimetic analyses as well as clay particle size analysis confirmed that as compared with the conventional agitation, ultrasound-assisted mud preparation could not only increase adsorbed amount of SMC on bentonite but also decrease average clay particle diameter attributed to acoustic cavitation. A plausible mechanism based on sonochemical thermodynamics is proposed to explain the improvement of the colloidal structure and performances of drilling fluid.

The present experimental study focuses on hydrodynamic cavitation in a transparent horizontal convergent-divergent nozzle. In the experimental section, flow first occurs through a nozzle with a half convergent angle of 21° and then a divergent angle of 8° to exit. The static pressure and the flow rate are varied to achieve different cavitation numbers in sheet mode. Pressure fluctuations are measured at three different locations in the test section of the nozzle and analyzed. Images of the cavities at the throat section are captured, and the average cavity length is measured for all cavitation numbers in sheet mode. The cavity dynamics variation with time is analyzed with the help of image captured using a high-speed camera. The study confirms that the cavitation number and average cavity length are exponentially inversely proportional. Pressure fluctuations and their corresponding frequency are obtained for the different stages of cavitation. The experimental findings show that the sheet cavitation stage is the most turbulent and the magnitude of pressure fluctuations gets smaller as the cavity volume increases.

Proposed grain refinement mechanisms during ultrasonic solidification have been explained in terms of refinement between cavitation enhanced nucleation and fragmentation of dendrites according to the casting conditions. Solidification studies also describe the activation of nucleation under pressure pulses after bubble implosion as an additional supporting mechanism for grain refinement. This study clarifies some overlooked concepts and proposes a plausible grain refinement mechanism explaining the role of cavitation in pure Zn and a Mg–6 wt pct Zn alloy. Equivalent grain size and grain density have been obtained in pure Zn and the Mg–6 wt pct Zn alloy (grain size distribution ranging from 40 to 200 µm) when UST was applied after the onset of solidification. These fine, non-dendritic grains originate from the cavitation zone beneath the sonotrode. Significant thermal undercooling surrounding the low superheat sonotrode in contact with the melt is responsible for the formation of a solidified layer (typically the thickness is equivalent to the average grain diameter) at the sonotrode–melt interface. High-frequency vibrations with or without cavitation at this interface assist the separation of these fine grains, which are then carried into the melt by acoustic streaming. A possible mechanism for the separation of fine grains produced from the cavitation zone is explained with the help of established concepts reported for the ultrasonic atomization process.

In this paper, the inner walls of oil pipes were cleaned using cavitation water jet technology after tertiary oil recovery. To improve cleaning efficiency and reduce damage caused by jet impingement, aluminum samples were used instead of oil pipe dirts. Scanning electron microscopy, 3D microscopy, and an electrochemical workstation were used to study the surface morphology, depth of impinging pits, and corrosion resistance of the aluminum samples after impingement with the cavitation water jet. According to the results, there were no obvious changes on the surface of the aluminum samples after impinging at inlet pressures lower than 13 MPa, but if the inlet pressure approached 15 MPa, the area of impinged pits on the surface of aluminum, the depth of impingement pits, and the mass loss of aluminum samples significantly increased, and the corrosion resistance of the oil pipe wall changed less after impingement. This proved that the impinging efficiency of the cavitation water jet was significantly improved and the degree of damage of the oil pipe wall was low near 15 MPa. If the inlet pressure was greater than 15 MPa, the impinging rate obviously increased, but the damage degree of the oil pipe wall was more serious.

Vitrimer is a polymer network with dynamic covalent bonds, which can be dynamically broken and reformed. Thanks to the dynamic covalent bonds, vitrimer behaves like an elastomer at high deformation rate or low temperature but a viscous fluid at low deformation rate or high temperature. In this article, we study the cavitation dynamics in a vitrimer, which is known to be an important damaging mechanism in it. In the formulation, a nonlinear three-dimensional rate-dependent constitutive model for the vitrimer is adopted, in which both of the strain stiffening and the kinetics of dynamic exchanging reactions are considered. The theory predicts strong dependence of the cavitation dynamics on the loading rate. The computational results may help to reveal some critical insights into the rate-dependent fracture in vitrimer.

The primary objective of this research is to study the cavitating effects of fluid flow past different axisymmetric cavitator in the upper sub-critical flow regime, which corresponds to the Reynolds number (2 × 104 to 2 × 105). Experiments are conducted in a water tunnel with a fluid flow velocity of 30 to 60 m/s at a constant rate of injection. The commercial software tool, ANSYS Fluent 18.1, is used to simplify three dimensional Reynolds averaged Navier Stokes equation with the compressible fluid flow by considering the pressure-based solver with standard K- ω turbulence model. The transport equation-based Schnerr and Sauer cavitation model is employed to study the cavitation phenomena. The finite volume discretisation method is implemented to evaluate the cavity length and diameter, respectively. A comparison of the numerical and experimental results shows that the numerical method can predict accurately the shape parameters of the natural cavitation phenomena such as cavity length, cavity diameter, and cavity shape. Results reported that with an increase in velocity, the cavity length and diameter increased to 250 and 20%, respectively. With a decrease in the cavitator angle at constant Reynolds number, the drag coefficient decreases up to 40%. Also, for the 30° cavitator, the drag coefficient rises by 28% when cavitation number is increased from 0.25 to 0.32.

The result from a multi-objective optimization on the outlet diffuser angle of the tidal turbine is presented with a focus on the turbine efficiency and cavitation. This study is the first to investigate the impact of the diffuser angle on the diffuser augmented tidal turbine in performance and cavitation inception. The 1/2 scale model was designed with three blades based on NACA 4616 airfoil. Seven diffuser angles were varied from 10.43° to 35.97° in tidal current velocity around 0.7 m/s. The standard k−e turbulence model and the multiple reference frame motion were conducted in steady-state for the computational analysis with Tip Speed Ratio (TSR) ranged from 1 to 5. It was shown that the diffuser angle strongly influences the turbine performance and cavitation inception. The maximum power coefficient was found at the largest diffuser angle as more space for the turbine to rotate leads to a higher pressure drop, which involves the current velocity change extracted to the torque of the rotor. However, the highest cavitation was also found at the largest diffuser angle since a higher-pressure drop increases the cavitation number. By multi-objective optimization using the GA-ANN method supported by TOPSIS, it was concluded that the optimum design of the tidal turbine was owned by a diffuser angle of 20.04°.

We construct forward self-similar solutions (expanders) for the compressible Navier–Stokes equations. Some of these self-similar solutions are smooth, while others exhibit a singularity due to cavitation at the origin.

The tip-leakage vortex (TLV) cavitation is a challenging issue for a variety of axial hydraulic turbines and pumps from both technical and scientific viewpoints. The flow characteristics of the TLV cavitation were widely studied in the past decades, but the knowledge about the tip-leakage cavitating flow is still limited. The present paper reviews the progresses in the researches of the TLV cavitation, including the numerical methods for the TLV cavitation, the flow characteristics of the TLV, the influences of the TLV cavitation on the local flow field and the control strategies of the TLV cavitation. It is indicated that the non-condensable gas may play an important role in the development of the TLV cavitation, and this fact should be considered during a careful simulation of the TLV cavitation. It is also suggested that the development of the TLV cavitation will significantly influence the distributions of the vorticity and the turbulence kinetic energy. Due to the complexity of the TLV cavitation, it is still an open question how to suppress the TLV cavitation in a simple but effective way. Finally, based on these understandings, some advanced topics for the future work are suggested to further promote the study of the TLV cavitation, for a deeper knowledge about the TLV cavitation.