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In this work, we investigate the effect of dissolved gas concentration on cavitation inception and cavitation development in a transparent sharp-edged orifice, similar to that previously analyzed by Nurick in the context of liquid injectors. The working liquid is water, and carbon dioxide is employed as a non-condensable dissolved gas. Cavitation inception points are determined for different dissolved gas concentration levels by measuring wall-static pressures just downstream of the orifice contraction and visually observing the onset of a localized (vapor) bubble cloud formation and collapse. Cavitation onset correlates with a plateau in wall-static pressure measurements as a function of a cavitation number. An increase in the amount of dissolved carbon dioxide is found to increase the cavitation number at which the onset of cavitation occurs. The transition from cloud cavitation to extended-sheet or full cavitation along the entire orifice length occurs suddenly and is shifted to higher cavitation numbers with increasing dissolved gas content. Volume flow rate measurements are performed to determine the change in the discharge coefficient with the cavitation number and dissolved gas content for the investigated cases. CFD analyses are carried out based on the cavitation model by Zwart et al. and the model by Yang et al. to account for non-condensable gases. Discharge coefficients obtained from the numerical simulations are in good agreement with experimental values, although they are slightly higher in the cavitating case. The earlier onset of fluid cavitation (i.e., cavitation inception at higher cavitation numbers) with increasing dissolved carbon dioxide content is not predicted using the employed numerical model. [ABSTRACT FROM AUTHOR]

Due to the scale effect, tip vortex cavitation (TVC) is the earliest type of cavitation that occurs on real ship propellers. As a result, experts in the ship field have been paying close attention to the accurate prediction of propeller TVC inception for a long time. The motion and growth of the microscopic nuclei in the water have a significant influence on TVC inception. However, the minimum pressure coefficient method—a common method at present—based on the traditional Eulerian framework, neglects the influence of microscopic nuclei and therefore cannot accurately predict the cavitation inception. Moreover, the numerical prediction method for cavitation inception, which is based on bubble dynamics models and considers the influence of nuclei, has not established a set of unified and specific discrimination criteria applicable to propeller cavitation inception. In order to make up for the shortcomings of traditional prediction models and the existing methods based on bubble dynamics in the prediction of TVC inception, we propose a new discrimination method for propeller TVC inception based on bubble dynamics in this paper. The comparison with experimental results demonstrates that our proposed method allows us to predict propeller TVC inception more accurately. In addition, the effect mechanism of tip vortex flow characteristics on nuclei evolution is further investigated, and it is found that when approaching the low-pressure region at a vortex core under the influence of tip vortex suction, nuclei grow explosively under the continuous action of the low pressure at the vortex core until they reach their maximum sizes and then collapse rapidly. [ABSTRACT FROM AUTHOR]

ObjectiveThis paper aims to suppress the adverse effects of tip clearance flow on the hydrodynamic performance and unsteady excitation force of a pump-jet propulsor. MethodAs for the pre-swirl stator pump-jet propulsor, an annular flexible seal structure closely matched with a rotor tip ring is used to study the validity of suppressing the clearance flow on the rotor tip. The rotor thrust and torque of the propulsor are measured by keeping the shroud approximately rigidly fixed, enabling the rotor open water efficiency to be obtained. In addition, the point of cavitation inception at each design condition is observed and recorded carefully with the help of a high-power stroboscope, and the cavitation inception curves of the propulsor with/without tip clearance are obtained through calculation. Finally, tests of shaft vibration acceleration on the pump-jet propulsor with/without tip clearance are conducted under conditions of cavitation and non-cavitation in order to evaluate the effects of diminishing tip clearance. ResultsThe results show that the thrust and torque of the rotor of the pump-jet propulsor with a flexible seal structure are significantly increased, which in turn renders open water efficiency significantly increased at low and medium advance coefficients, unchanged near the design point and slightly decreased at the high advance coefficient. Moreover, the cavitation performance of the pump-jet propulsor without clearance is better at a wide range of advance coefficient, 0.85

Copyright of China Rural Water & Hydropower is the property of China Rural Water & Hydropower Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this work, we investigate the effect of dissolved gas concentration on cavitation inception and cavitation development in a transparent sharp-edged orifice, similar to that previously analyzed by Nurick in the context of liquid injectors. The working liquid is water, and carbon dioxide is employed as a non-condensable dissolved gas. Cavitation inception points are determined for different dissolved gas concentration levels by measuring wall-static pressures just downstream of the orifice contraction and visually observing the onset of a localized (vapor) bubble cloud formation and collapse. Cavitation onset correlates with a plateau in wall-static pressure measurements as a function of a cavitation number. An increase in the amount of dissolved carbon dioxide is found to increase the cavitation number at which the onset of cavitation occurs. The transition from cloud cavitation to extended-sheet or full cavitation along the entire orifice length occurs suddenly and is shifted to higher cavitation numbers with increasing dissolved gas content. Volume flow rate measurements are performed to determine the change in the discharge coefficient with the cavitation number and dissolved gas content for the investigated cases. CFD analyses are carried out based on the cavitation model by Zwart et al. and the model by Yang et al. to account for non-condensable gases. Discharge coefficients obtained from the numerical simulations are in good agreement with experimental values, although they are slightly higher in the cavitating case. The earlier onset of fluid cavitation (i.e., cavitation inception at higher cavitation numbers) with increasing dissolved carbon dioxide content is not predicted using the employed numerical model.

Due to the scale effect, tip vortex cavitation (TVC) is the earliest type of cavitation that occurs on real ship propellers. As a result, experts in the ship field have been paying close attention to the accurate prediction of propeller TVC inception for a long time. The motion and growth of the microscopic nuclei in the water have a significant influence on TVC inception. However, the minimum pressure coefficient method—a common method at present—based on the traditional Eulerian framework, neglects the influence of microscopic nuclei and therefore cannot accurately predict the cavitation inception. Moreover, the numerical prediction method for cavitation inception, which is based on bubble dynamics models and considers the influence of nuclei, has not established a set of unified and specific discrimination criteria applicable to propeller cavitation inception. In order to make up for the shortcomings of traditional prediction models and the existing methods based on bubble dynamics in the prediction of TVC inception, we propose a new discrimination method for propeller TVC inception based on bubble dynamics in this paper. The comparison with experimental results demonstrates that our proposed method allows us to predict propeller TVC inception more accurately. In addition, the effect mechanism of tip vortex flow characteristics on nuclei evolution is further investigated, and it is found that when approaching the low-pressure region at a vortex core under the influence of tip vortex suction, nuclei grow explosively under the continuous action of the low pressure at the vortex core until they reach their maximum sizes and then collapse rapidly.

The oil film between the friction disks of the hydro-viscous clutch is taken as the research object. Considering the groove structure of the friction disks, the mathematical model of the oil film flow field is established. The finite difference algorithm is utilized to solve the velocity, the pressure, and the pressure coefficient of the oil film. The incipient position of oil film shear cavitation under different groove structure parameters is carried out. The numerical results indicate that the groove has a great influence on the oil film velocity and pressure. The oil film cavitation inception occurs first in the upstream position of the groove area. The area fraction of cavitation inception is directly proportional to the width and depth of the groove and inversely proportional to the section parameter and inclination angle of the groove. This research provides an efficient and accurate method for predicting the oil film shear cavitation inception. [ABSTRACT FROM AUTHOR]

Cavitation inception predetermines a jump of noise radiated from marine vehicles. Usually marine propellers are the main sources of such a noise. In the situation of cavitation inception near the blade's leading edge, its prediction remains a challenge. Though contemporary CFD tools for fully turbulent flows satisfactorily predict pressure distribution around cavitation-free blades and with cavities of length comparable with the blade size, analysis of blade cavitation inception is a difficult task for these tools. On the other hand, there are validated computational tools for 2D multizone flows capable of predicting cavitation inception. There is the possibility of considering the real 3D flow around the leading edges of blades as a 2D flow with the known pressure distribution along the blade section; the cavitation inception number is computed in this 2D cavitating flow, and correlations between this number and the pressure minimum in cavitation-free flow around the same section are determined. Such a correlation would be usable with any tool for cavitation-free flow. The issue of their applicability to arbitrary blades can be solved with the employment of asymptotic solutions for the pressure around contours with rounded leading edges. [ABSTRACT FROM AUTHOR]

Cavitation cloud formation by the backscattering of focused ultrasound from a bubble interface in gelatin has been studied experimentally. A laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. The maximum size of the laser-induced bubble is about 2 mm in radius. After the laser-induced bubble collapses, a small bubble whose radius is about 0.3 mm remains around the laser focus. In the present experiment, this residual bubble is also used as a reflector bubble, and the effect of reflector bubble size on cavitation cloud formation has been evaluated. Cavitation inception and cavitation cloud formation near the reflector bubble has been observed with a high-speed camera. It is shown that the cavitation cloud grows along with the propagation axis of the incident wave. The larger the reflector bubble size becomes, the earlier the first cavitation inception near the reflector occurs. On the other hand, regardless of the reflector bubble size, the distance between the top interface of the cavitation cloud and the geometrical focus of the focused ultrasound at the maximum expansion of the cloud is almost constant. The results also show that when a residual bubble is used as a reflector, the growth rate of a cavitation cloud in the direction of the propagation axis becomes higher than that when a laser-induced bubble is used. This may be due to the difference of the interface shape of the tip of developing cavitation cloud. [ABSTRACT FROM AUTHOR]

In this paper, the inception of tip vortex cavitation in weak water has been predicted using a numerical simulation, and a new scaling concept with variable exponent has also been suggested for cavitation inception index. The numerical simulations of the cavitating flows over an elliptic planform hydrofoil were performed by using the RANS approach with a Eulerian cavitation model. To ensure the accuracy of the present simulations, the effects of the turbulence model and grid resolution on the tip vortex flows were investigated. The turbulence models behaved differently in the boundary layer of the tip region where the tip vortex is developed, which resulted in different pressure and velocity fields in the vortex region. Furthermore, the Reynolds stress model for the finest grid showed a better agreement with the experimental data. The tip vortex cavitation inception numbers for the foil, predicted by using both wetted and cavitating flow simulation approaches, were compared with the measured cavitation index values, showing a good correlation. The current cavitation scaling study also suggested new empirical relations as a function of the Reynolds number substitutable for the two classic constant scaling exponents. This scaling concept showed how the scaling law changes with the Reynolds number and provided a proper scaling value for any given Reynolds numbers under turbulent flow conditions. [ABSTRACT FROM AUTHOR]

The evaluation of cavitation characteristics is one of the most significant tasks whilst properly designing a water turbine. This work investigates the steady and unsteady cavitating flows through a small Francis turbine, based on the two-phase mixture model. Detailed flow field analysis with and without cavitation is performed for three operating regimes namely the Best Efficiency Point (BEP), part-load (OP1), and over-load (OP2). The effect of reducing the cavitation number on the hydraulic performance is assessed, and the range of safe operation in respect of cavitation-free is revealed. Under specific operating conditions the structure of cavitation in the runner and the evolution of the vortex rope in the draft cone are shown. The part-load operation gives rise to undesired phenomena of pressure pulsations due to the vortex rope generated at the runner outlet. Moreover, the pressure fluctuations and the torque oscillation seem to be more pronounced at part-load and over-load operating conditions. The obtained results are encouraging and may help in designing and testing the small Francis turbines which behave differently compared with the large-scale models. • The large regions of flow recirculation and swirl flow affect the operational stability of Francis turbine. • The effects of reducing the cavitation number on the hydraulic performance are clarified. • The simulation of unsteady cavitating flows allowed monitoring the development of cavities. • The formation and evolution of the vortex rope in the draft tube are illustrated. • The cavitation has strong effects on the pressure fluctuations and torque oscillation. [ABSTRACT FROM AUTHOR]

Resistance to cavitation is an important requirement in the design of hydrokinetic turbines for marine or river applications due to cavitation has been found to contribute to the turbine blade wear, corrosion, vibration, and fatigue. The presence of cavitation in the blades can lead to a decrease in the turbine performance and the reduction of its useful life. Therefore, it is crucial to include a cavitation study in the analysis and development of hydrokinetic systems. In this work, the elements to be considered in a cavitation study of the blades of a hydrokinetic turbine are presented. The comparison between the distribution of the pressure coefficient (Cpre) on the Eppler 420 hydrofoil and the number of cavitation (σ) was presented as a criterion to determine the cavitation occurrence. Cpre was calculated by numerical simulation by using the Ansys Fluent software. The results showed that the Eppler 420 hydrofoil could be used for the design of the blades of hydrokinetic turbines. [ABSTRACT FROM AUTHOR]

Cavitation inception predetermines a jump of noise radiated from marine vehicles. Usually marine propellers are the main sources of such a noise. In the situation of cavitation inception near the blade’s leading edge, its prediction remains a challenge. Though contemporary CFD tools for fully turbulent flows satisfactorily predict pressure distribution around cavitation-free blades and with cavities of length comparable with the blade size, analysis of blade cavitation inception is a difficult task for these tools. On the other hand, there are validated computational tools for 2D multizone flows capable of predicting cavitation inception. There is the possibility of considering the real 3D flow around the leading edges of blades as a 2D flow with the known pressure distribution along the blade section; the cavitation inception number is computed in this 2D cavitating flow, and correlations between this number and the pressure minimum in cavitation-free flow around the same section are determined. Such a correlation would be usable with any tool for cavitation-free flow. The issue of their applicability to arbitrary blades can be solved with the employment of asymptotic solutions for the pressure around contours with rounded leading edges.

Pumps used in professional appliances process a solution of water, soils residues and detergents. These affect vapor tension, viscosity and rheology of the solution, mainly due to the presence of surfactants and polymers. Only a few studies have been found on how these substances can influence pump performances. Therefore, an experimental analysis has been carried out with aqueous solutions of a detergent component, the Polyox WSR 301, in the concentration range of 100–7000 ppm, to evaluate their influence on pump performances and cavitation. Some properties of the solutions have been preliminary characterized with a rheometer. Then, each solution has been tested in a dedicated test rig, to compare the performance curves of a centrifugal pump used in professional warewashing machines with those obtained with pure water. A non-intrusive method, based on the investigation of high frequency vibrations and noise signals, has been developed to detect cavitation at its early stage of inception. It was observed that polymer mitigates cavitating pump vibrations, with a reduction of the acceleration to less than one g. The analysis has provided the data necessary for the successive development of a control strategy for pump operation in professional appliances. [ABSTRACT FROM AUTHOR]

In the field of ocean engineering, cavitation is widespread, for the study of cavitation nuclei transient characteristics in cavitation inception, we applied theoretical analysis and molecular dynamics (MD) simulation to study Lennard-Jones (L-J) fluid with different initial cavitation nuclei under the NVT-constant ensemble in this manuscript. The results showed that in cavitation inception, due to the decrease of liquid local pressure, the liquid molecules would enter the cavitation nuclei, which contributed to the growth of cavitation nuclei. By using molecular potential energy, it was found that the molecular potential energy was higher in cavitation nuclei part, while the liquid molecular potential energy changes greatly at the beginning of the cavitation nuclei growth. The density of the liquid and the surface layer changes more obvious, but density of vapor in the bubble changes inconspicuously. With the growth of cavitation nuclei, the RDF peak intensity increased, the peak width narrowed and the first valley moved inner. When cavitation nuclei initial size reduced, the peak intensity reduced, the corresponding rbin increased. With the decrease of the initial cavitation nuclei, the system pressure and total energy achieved a balance longer, and correspondingly, they were smaller. In addition, at the beginning of the cavitation nuclei growth, the total energy and system pressure changed greatly.

In this paper, the inception of tip vortex cavitation in weak water has been predicted using a numerical simulation, and a new scaling concept with variable exponent has also been suggested for cavitation inception index. The numerical simulations of the cavitating flows over an elliptic planform hydrofoil were performed by using the RANS approach with a Eulerian cavitation model. To ensure the accuracy of the present simulations, the effects of the turbulence model and grid resolution on the tip vortex flows were investigated. The turbulence models behaved differently in the boundary layer of the tip region where the tip vortex is developed, which resulted in different pressure and velocity fields in the vortex region. Furthermore, the Reynolds stress model for the finest grid showed a better agreement with the experimental data. The tip vortex cavitation inception numbers for the foil, predicted by using both wetted and cavitating flow simulation approaches, were compared with the measured cavitation index values, showing a good correlation. The current cavitation scaling study also suggested new empirical relations as a function of the Reynolds number substitutable for the two classic constant scaling exponents. This scaling concept showed how the scaling law changes with the Reynolds number and provided a proper scaling value for any given Reynolds numbers under turbulent flow conditions.

Tip vortex cavitation (TVC) is an important cavitation phenomenon in marine propeller. The formation and evolution of tip vortex cavitation are hot topics consistently both in engineering application and mechanism research. In this paper some recent studies on tip vortex cavitation inception and the noise of tip vortex cavitation evolution are presented. The effects of both flow field and water qualities on tip vortex cavitation inception are considered by experiments and numerical simulations. The results show that besides the average minimum pressure in the vortex core the turbulence fluctuation and water qualities including air content and nuclei distribution have great influence on tip vortex cavitation inception. Based on the idea of first nucleus cavitating in tip vortex core new prediction formula for tip vortex cavitation inception is proposed. The synchronous technique of high speed video observation and noise measurement are adopted to study the development of tip vortex cavitation. S-type total noise characteristics are obtained when cavitation number from low to high. Vortex singing is found in the case where the tip vortex cavitation just before leaves the tip region. The excited mechanism of vortex singing is proposed by analyzing the wave propagation on the interface of vortex cavity. [ABSTRACT FROM AUTHOR]

• Cavitation inception in slots on the flat walls is analyzed in the steady flow approach using a viscous-inviscid interaction method. • The comparison shows a satisfactory agreement of computations with the measured data. Cavitation inception in slots on the flat walls is analyzed in the steady flow approach using a viscous-inviscid interaction method. It was assumed that, in the accordance with known observations, cavities appear as spherical bubbles in the cores of vortices near the slot corners. Computed cavitation inception numbers are compared with published experimental results of other authors within wide ranges of inflow parameters. The comparison shows a satisfactory agreement of computations with the measured data. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The promotion/prevention mechanism of dissolved gases on cavitation inception is essential for many high-tech industries and research. In the present study, large-scale molecular dynamics simulations are performed to investigate the effects of water cavitation caused by different gas types by using nitrogen and oxygen gases with TIP4P/2005 water. The cavitation inception behaviour is analyzed via Mean First Passage Time method. Water with dissolved gases has a higher nucleation rate and is easier to cavitate than pure water. At the same gas concentration, the cavitation of water with nitrogen is promoted to a greater extent than that with oxygen. The number and energy of hydrogen bond (HB) are further calculated by the Acceptor-Hydrogen-Donor method to explain this promotion mechanism. The number and energy of HB in water with gases decrease compared with those in pure water. The introduction of gases weakens the HB network and promotes cavitation inception because of weaker interactions between gas and water molecules. A model is developed to describe the relationship between nucleation rate and HB energy. Gas molecules assemble on the surface of bubbles during water cavitation, which may decrease the free energy of bubble surface, maintain the existing bubble, and contribute to the growth process. [ABSTRACT FROM AUTHOR]

Abstract In reversible pump-turbines at pump mode, local flow separation on the leading-edge overlaps the impeller's low-pressure side and causes a sudden pressure-drop. Hence, cavitation easily incepts on leading-edge especially at off-design conditions. To solve this cavitation problem, a NACA0006 profile was used as a simplified model of pump-turbine impeller blade to understand the cavitation inception mechanism. Numerical results indicate multiple favorable-pressure-gradients, which were caused by the quickly-varied geometry, on the hydrofoil surface. The minimum pressure occurs in one of these gradients' region and usually locates around leading-edge because of the quickly-varied geometry of leading-edge arc. Then, Diffusion-angle Integral design method was developed to design the blade leading-edge shape. It has only three design parameters by effective geometry deconstruction. Based on orthogonal test, the reasonable values of design parameters can be determined. With optimal design parameters, the NACA0006 foil and pump-turbine impeller blade were completely redesigned and improved. The inception cavitation number of the redesigned foil was obviously reduced especially at large incidence angles. The redesigned pump-turbine also has a reduced inception cavitation number and delayed cavitation inception especially at off-design conditions. The design concept, method and application in this study provide useful reference for anti-inception-cavitation design of turbomachinery blades. Highlights • The relationship among foil geometry, pressure gradient and inception cavitation is quantitatively analyzed. • The Diffusion-angle Integral design method for pump-turbine blades is established. • The inception cavitation of a pump-turbine in pump mode is improved by blade profile redesign. [ABSTRACT FROM AUTHOR]

Highlights • Effects of Venturi geometries on cavitation was studied by experiment and simulation. • Hydrodynamic cavitation increases flow resistance. • Small outlet angle is favorable for hydrodynamic cavitation to generate microbubbles. • The amount of microbubble generation increases with dissolved air in water. Abstract Venturi tubes have been extensively used to generate microbubbles through hydrodynamic cavitation. Due to their simple and flexible design, Venturi tubes have been used in a wide range of applications including mineral flotation. Although the impacts of the geometries of Venturi tubes on hydrodynamic cavitation have been studied in different specific contexts, no clear relation between cavitation and geometry parameters of Venturi tubes has been reported. In this work, we investigated numerically and experimentally the influence of several geometrical parameters on the cavitation inception detected by a hydrophone and the microbubble generation measured by a high-speed camera. Using a dimensionless number analysis, we found that the cavitation inception was determined by flow resistance, which significantly depends on the geometrical design of Venturi tube. In the cavitation regime, the flow resistance induced by cavitation increases linearly with reducing downstream cavitation number while the upstream cavitation number becomes constant regardless of the geometry of Venturi tube. A small outlet angle results in a low cavitation inception and a high microbubble production. Moreover, the degree of microbubble generation was found to increase with extra flow resistance and the dissolved gas concentration. The insights from this study provide a guideline for the design of efficient Venturi tubes for hydrodynamic cavitation system. [ABSTRACT FROM AUTHOR]

In the field of ocean engineering, cavitation is widespread, for the study of cavitation nuclei transient characteristics in cavitation inception, we applied theoretical analysis and molecular dynamics (MD) simulation to study Lennard-Jones (L-J) fluid with different initial cavitation nuclei under the NVT-constant ensemble in this manuscript. The results showed that in cavitation inception, due to the decrease of liquid local pressure, the liquid molecules would enter the cavitation nuclei, which contributed to the growth of cavitation nuclei. By using molecular potential energy, it was found that the molecular potential energy was higher in cavitation nuclei part, while the liquid molecular potential energy changes greatly at the beginning of the cavitation nuclei growth. The density of the liquid and the surface layer changes more obvious, but density of vapor in the bubble changes inconspicuously. With the growth of cavitation nuclei, the RDF peak intensity increased, the peak width narrowed and the first valley moved inner. When cavitation nuclei initial size reduced, the peak intensity reduced, the corresponding rbin increased. With the decrease of the initial cavitation nuclei, the system pressure and total energy achieved a balance longer, and correspondingly, they were smaller. In addition, at the beginning of the cavitation nuclei growth, the total energy and system pressure changed greatly. [ABSTRACT FROM AUTHOR]

Cavitation inception behind 3D surface irregularities often predetermines the vehicle acoustical performances. There are three flow scales influencing the inception phenomenon: the body size; the irregularity heights; the radius of the core of vortices drifting in separation zone behind the irregularity. Cavities appear in these vortices. Existence of three scales challenges computations. The suggested multi-step iterative numerical method is a modification of the method already used by the author for cavitation inception behind 2D irregularities. The initial unsteady problem is broken down into two steady problems. The first of them is determination of the average separated flow behind the irregularity. This problem is solved using viscous-inviscid interaction approach, with 3D pressure computation. The second of them is determination of cavity equilibrium within the vortex core. This problem is solved with the assumption on a small influence of the vortex on the surrounding flow, but the vortex characteristics are linked to the computed turbulence characteristics in the separation zone. Intermediate validations with various experimental data are shown for all steps of the computational methods. Comparisons of computed and earlier measured cavitation inception numbers for hemispheres and cylinders submerged in the flat wall boundary layer are provided. [ABSTRACT FROM AUTHOR]

The growth of a cavitation bubble cloud and the corresponding pressure near the tip of the cavitation bubble cloud formed by the backscattering of the focused ultrasound from a laser-induced bubble have been analyzed experimentally. The laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. In the present study, optical observation by a high-speed video camera with frame rate up to 1.25 Mfps and the pressure measurement by the fiber optic prove hydrophone (FOPH) are carried out simultaneously to estimate the cavitation inception pressure. It is shown that the cavitation bubble cloud grows conically along with the propagation axis of the incident wave. The distance between the top interface of the cavitation bubble cloud and the geometrical focus of the focused ultrasound becomes a limited value even though the focused ultrasound keeps on irradiating, which indicates that the cavitation bubble cloud grows within the focal region with sufficient negative pressure due to backscattering. The pressure measurement reveals that the negative pressure amplitude backscattered from a free surface is larger than that of the focused ultrasound itself. Negative peaks in the pressure waveform have a long term fluctuation whose increments correspond the sudden growth of the cavitation bubble cloud. Cavitation inception pressure was measured defining it as the negative pressure at the top interface of the cavitation bubble cloud just before the FOPH was covered with cavitation bubbles. The measured cavitation inception pressure was -24 MPa in the present study. [ABSTRACT FROM AUTHOR]

A new concept of diminishing cavitation inception number is applied to sheet cavitation over contours traditional for sections of marine propeller blades. An inverse hydrodynamic problem is solved for a segment located downstream from the point of separation of laminar boundary layer at highest angles of attack of the contour. The pressure along the forward part of this segment will be constant and it will have a prescribed increase along its rear part up to the point of merging the corrected streamline with the original contour. Description of employed numerical method is supplemented by examples of enhancement of traditional hydrofoils. The substantial improvement of their baskets of minimal pressure is obtained. • Cavitation inception baskets. • Mitigation of the pressure minima for hydrofoils by correction of their shape behind viscous separation point. • Employments of ideal fluid theory and boundary layer theory. [ABSTRACT FROM AUTHOR]

Three different models,k-ω,DES and LES,are conducted in the analysis of the tip vortex flow field. In order to reduce the discrete error induced by the grid,mesh refinement is applied to the area of the tip vortex core in numerical simulations. The axis and tangential velocities of the tip vortex flow field with no cavitation are calculated,and the calculated velocities agree well with the experimental results. On the basis of this process,the influence of vortex roll-up on the tip vortex pressure filed is discussed,and bubble static equilibrium is proposed by which the tip vortex cavitation inception number is computed.

Mixed-flow pumps compromise large flow rate and high head in fluid transferring. Long-axis mixed-flow pumps with radial–axial “spacing” guide vanes are usually installed deeply under water and suffer strong cavitation due to strong environmental pressure drops. In this case, a strategy combining the Diffusion-Angle Integral Design method, the Genetic Algorithm, and the Computational Fluid Dynamics method was used for optimizing the mixed-flow pump impeller. The Diffusion-Angle Integral Design method was used to parameterize the leading-edge geometry. The Genetic Algorithm was used to search for the optimal sample. The Computational Fluid Dynamics method was used for predicting the cavitation performance and head–efficiency performance of all the samples. The optimization designs quickly converged and got an optimal sample. This had an increased value for the minimum pressure coefficient, especially under off-design conditions. The sudden pressure drop around the leading-edge was weakened. The cavitation performance within the 0.5–1.2 Qd flow rate range, especially within the 0.62–0.78 Qd and 1.08–1.20 Qd ranges, was improved. The head and hydraulic efficiency was numerically checked without obvious change. This provided a good reference for optimizing the cavitation or other performances of bladed pumps.

The inception of tip vortex cavitation is very sensitive to water quality. In order to quantify the effect of water quality on the inception of tip vortex cavitation, we develop a motion model to describe the migration and growth of nuclei in water. An analytical solution of migration of nuclei in a vortex flow is obtained so that the capture times of various nuclei can be given out directly. A criterion is built to determine the critical nucleus in a certain nuclei spectra distribution. Tensile strength of the critical nucleus is used to quantify the effect of water quality and correct the tip vortex cavitation inception number. Finally this change of cavitation inception number is compared with experimental results to validate our model. [ABSTRACT FROM AUTHOR]

We investigate the cavitation incipience and development of water flow over a thin hydrofoil placed in the test section of highspeed cavitation tunnel. Hydrofoils with smooth and rough leading edge were tested for different upstream velocities and incidence angles. Our observations clearly revealed that cavitation incipience is enhanced by roughness at incidence angle below 2°. This is in line with the former reports, according to which roughness element decreases the wettability and traps a larger amount of gas. As a result, surface nucleation is enhanced with an increased risk of cavitation. Surprisingly, for higher incidence angles (>3°), we have found that cavitation incipience is significantly delayed by roughness while developed cavitation is almost the same for both smooth and rough hydrofoils. We argue that this unexpected incipience delay is related to the change in the boundary layer structure due to roughness. We also report a significant influence of roughness on the dynamic of developed cavitation and shedding of transient cavities. [ABSTRACT FROM AUTHOR]

Prediction of inception of sheet cavitation on solid walls has been recognized to be very difficult, since it is significantly affected by the boundary layer flow characteristics, the population of free nuclei, the nuclei held in the wall roughness, the amount of dissolved air in liquid and so on. It has not sufficiently been made clear how the inception is affected by the conditions of water qualities and background flow characteristics. In this study, high speed observation of inception of sheet cavity from free nuclei is conducted for a two-dimensional convergent- divergent nozzle flow, where the sheet cavity forms just downstream of the nozzle throat. The effects of the amount of dissolved air and the free stream velocity on the inception process of sheet cavitation is examined. In addition, the bubble nuclei density, which is well known to be important factor for cavitation inception, is passively controlled by the filter installed in the tunnel. From the observations, it is confirmed that the nuclei number density significantly affects the formation of sheet cavity rather than the other two parameters. In conditions with large nuclei number density, the sheet cavity does not form, and bubbly cavitation appears instead. In the case with small nuclei number density, the sheet cavity forms from a single flowing nucleus and develops streamwisely and spanwisely. In the conditions with medium nuclei number density, the sheet cavity also forms but is shorter/ narrower streamwisely/spanwisely, due to interaction of other nuclei flowing near the formed sheet cavity. [ABSTRACT FROM AUTHOR]

A cavitation inception theory of a vapor bubble in flowing liquid is presented by taking account of a set of fluid dynamics boundary conditions for nonequilibrium evaporation at the bubble wall. The boundary conditions for the vapor temperature, pressure and density at the wall are a set of solutions of the polyatomic type of ES-BGK Boltzmann equation. The evaporation rate and the vapor-liquid temperature discontinuity at the bubble wall are treated in a physically correct way at a molecular level. The theory also considers the translational motion of the bubble in a flowing liquid with arbitrary relative velocity between the bubble and the surrounding liquid. The translational motion largely reduces the liquid pressure at the bubble wall, and the inception process of cavitation is thereby influenced greatly. Furthermore, the numerical calculation of expansion process of the vapor bubble is performed for the case where the bubble is instantaneously exposed to the low liquid pressure, and its results are compared with those of the equilibrium evaporation. A critical liquid pressure considered as a criterion of expansion and contraction of the bubble is numerically estimated for the various relative velocities between the bubble and the liquid and the various initial bubble radii, and it is compared with the pressure of the classical theory. Numerical calculation reveals that the critical pressure for the relative velocity of zero in the case of nonequilibrium evaporation or condensation is lower than the classical one in any initial bubble radius.

In hydraulic machinery, the surface of the blade can get damaged by the cavitation of the leading-edge. In order to improve the cavitation performance, the anti-cavitation optimization design of blade leading-edge is conducted. A heuristic-parallel locally-terminated improved hill-climbing algorithm, which is named as the global dynamic-criterion (GDC) algorithm was proposed in this study. The leading-edge shape of NACA 0009-mod foil profile was optimized by combining the GDC algorithm, CFD prediction, Diffusion-angle Integral (DI) design method and orthogonal test. Three different optimal foil geometries were obtained for specific incidence angles that 0, 3, and 6 degrees. According to the flow field analyses, it was found that the geometric variation of the optimized foil fits the incoming flow better at the respective optimal incidence angles due to a slighter leading-edge flow separation. The pressure drops become gentler so that the cavitation performance get improved. Results show that the GDC algorithm quickly and successfully fits the target condition by parallel running with the ability against falling into local-best tarps. The −Cpmin of the optimal foils was improved especially by +11.4% and +14.5% at 3 and 6 degrees comparing with the original foil. This study provided a reference for the anti-cavitation design of hydraulic machinery blades.

Tip vortex cavitation is the first type of cavitation to take place in most cases. Because it always produces radiated noise, avoiding or at least delaying tip vortex cavitation is crucial for achieving low radiated noise emissions. The present study aims to predict the tip vortex cavitation inception with the low-order panel method, which is widely used at the early propeller design stage. First, a velocity smoothing parameter varying with the age of vortex is adopted to align the wake. This leads to a wake shape that coincides very well with the wake pattern from a RANS solver. A consistent formula for evaluating the smoothing parameter for different cases is also achieved with the fitting method. Second, the tip vortex circulation is calculated based on the converged wake shape and doublet distribution. This procedure is found to be independent from the panel size. Finally, based on the minimum pressure distribution along the tip vortex, cavitation inception numbers are obtained for the Potsdam Propeller Test Case (PPTC) and the propeller 4383 developed in NSRDC. The results show a good agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

Erosion through synergetic effects between cavitation erosion and particle abrasion in silt-laden flow seriously affects the safe operations of hydroturbines. In this review, recent advances of cavitation inception on particles and microscopic interactions between bubbles and particles are reviewed and discussed. For cavitation inception, influences of several paramount parameters (e.g. types, sizes, shape and surface structure of particles, pressurization and memory effects) have been revealed and discussed. The interaction mechanisms between cavitation bubbles and particles are demonstrated using experimental data obtained with a single particle. Through the microscopic interactions, the particles can be accelerated by the collapsing bubbles up to 40 m/s and also be possibly split up by the cavitation, leading to deagglomeration of particle clusters. [ABSTRACT FROM AUTHOR]

Tip vortex cavitation often occurs on ship propeller. This type of cavitation can cause significant noise compared to the wet flow. Aiming at predicting the inception of tip vortex cavitation, numerous researchers have investigated the detailed flow field around the tip. According to informed studies, the inception of tip vortex cavitation is affected by complicated factors, such as the minimum pressure in the vortex core, the turbulence fluctuation and the water quality. To understand the effect of water quality on cavitation inception, the motion of nuclei in the vortex flow is investigated. The vortex flow is given by the Ranking vortex model. The one-way coupled point-particle-tracking model (PTM) is employed to simulate the trajectory of nuclei. Meanwhile, the theoretical solution of motion of nuclei is obtained and compared to the numerical results. [ABSTRACT FROM AUTHOR]

In this paper the experimental and numerical methods were combined to investigate the critical conditions of cavitation inception in diesel injector nozzles. It was found that at the beginning of cavitation inception, the discharge coefficient presents a slight increase and then a special turning point appears which can therefore be taken as the critical point for cavitation inception. Effects of needle lift, length–diameter ratio, orifice inclination angle, back pressure, and scaled-up times on critical cavitation number were studied through the experimental method. Due to the difficulties in manufacturing, influence of inlet rounding radius was investigated by using the numerical simulation method. Based on all the experimental and numerical simulation data, a correlation of critical cavitation number with above 6 geometrical and dynamic parameters for nozzle flow was developed through statistical analysis using the Quasi-Newton (BFGS) and Universal Global Optimization (UGO) methods. The relative error between predicted results and experimental data is less than 7%. These results indicate that the derived expression has good accuracy to predict the start of cavitation. [ABSTRACT FROM AUTHOR]

• Inceptions of three-phase hydrodynamic cavitation was detected acoustically. • Hydrophilic particles stabilize nuclei and promote cavitation inception. • Hydrophobic particles entrap bubbles to further promote cavitation. • Size and roughness of fine particles at a low concentration have little effect. • Hydrophobic Venturi tube promotes cavitation and screens the particle effect. In many modern industrial applications such as mineral flotation and water treatment, microbubbles have been used as an efficient aid for increasing the collision and attachment efficiencies and enhancing the recovery of valuable fine minerals. One technique to produce such micro-bubbles is to use the hydrodynamic cavitation process when gas cavities in water nucleate under low hydrodynamic pressure. To better understand the cavitation behavior in the presence of solids with different surface properties, we present an experimental study of a multi-phase system. The characteristics of hydrodynamic cavitation with a Venturi tube were experimentally investigated using acoustic detection. The effect of both particle and tube wall surface hydrophobicity and surface structure on the inception of hydrodynamic cavitation were considered. Results demonstrated that particles with lower wettability promoted hydrodynamic cavitation by efficiently trapping gas pockets that facilitate bubble nucleation. In addition, in the condition of low particle concentration (5 g/L) and fine particle size range (D50 < 20 µm), parameters including size and roughness did not play a critical role in affecting cavitation inception. Finally, tests using Venturi tubes of different surface properties showed that the wall surface behaves similarly as particles. Hydrodynamic cavitation was promoted by a more hydrophobic Venturi tube which screened the effect of particle surface properties. [ABSTRACT FROM AUTHOR]

Simulation of cavitating flow is of practical importance for many engineering systems such as propellers, pump impellers, nozzles, injectors, and torpedoes. In this study, a two-phase flow solver based on a homogeneous mixture model was developed. Computations were carried out for an axisymmetric cylinder, and the present code was validated by comparing the calculation results with experimental results. The results showed that the system is suitable for simulating evaporation and condensation processes in water flow. What's more, the changes of cavity length with various operational conditions were calculated including the water depth, angle of attack, and free stream velocity. The conditions for cavitation inception were also studied to show the relationship between the operational conditions in multiphase flows. [ABSTRACT FROM AUTHOR]

Cavitation on spillways has proven to be an undesirable condition. The formation of vapor-filled bubbles and cavities and their eminent collapse has led to significant damage to major spillway components and appurtenant structures worldwide. Although stepped spillways are thought to be less prone to cavitation damage than smooth spillways, designers continue conservative practices regarding specifying stepped spillways at many sites. Using laboratory experiments in a specialized reduced ambient pressure chamber, cavitation was shown to form on stepped geometries that are representative of typical stepped spillways currently in service. Experiments in a nonaerated closed conduit revealed the strength and extents of the highly intense shear layer that forms above the step tips, and the friction characteristics were determined and compared with results from previous researchers. Advanced techniques for detecting cavitation characteristics along with high-speed videography have given additional insight into the flow features that drive the formation of cavitation. Finally, a correlation between the critical cavitation index and the common friction factor is shown, extending the data that also includes shear layers resulting from uniformly distributed roughnesses, jets, and wakes. [ABSTRACT FROM AUTHOR]

Pumps are designed and manufactured to operate far from the cavitation condition. Pump designers define the cavitation inception point as the point where the head drops by 3 %. This definition is incorrect, because the first appearance of vapour bubbles is observed earlier than 3 % drop in the head. Therefore, using 3 % drop in head, as indication parameter for cavitation inception has always resulted in damage in pumps during operation. This paper describes visual studies conducted to determine the variation of net positive suction head with flow rate, rotational speed and water temperature. The experiments showed that the visual incipient of cavitation in the pump was existed long before drop in pump head occurs. A special correlation between the visual inception cavitation net positive suction head and net positive suction head corresponding to 3 % drop in head was developed at various operating conditions. Graphical Abstract: [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Kuiper and Jessup (1993) developed a design method for propellers in a wake based on the Eppler foil design method. The optimized section is transformed into the three-dimensional propeller flow using the approach of the effective blade sections. Effective blade sections are two-dimensional sections in two-dimensional flow which have the same chordwise loading distribution as the three-dimensional blade sections of a propeller. However, the design procedure is laborious in two aspects: finding an optimum blade section using the Eppler program requires much skill of the designer, and transforming the two-dimensional blade section into a propeller blade section in three-dimensional flow is complex. In this work, these two problems were coped with. A blade section design procedure was presented using an optimization technique and an alternative procedure for the effective blade section is developed using a lifting surface design method. To validate the method a benchmark model of a naval ship was used. This benchmark model was extended by new appendices and a reference propeller, and designed using conventional design methods. This reference propeller was optimized using the new design procedure and model tests were carried out. Special attention was given to the data of the model and the reference propeller, to make the configuration suitable for the Reynolds-Averaged Navier-Stokes (RANS) calculations. [ABSTRACT FROM AUTHOR]

The research group including two of the present authors previously discovered a surprising phenomenon in oil flows through constricted channels; at a certain flowrate when there is no cavitation occurring, an infinitesimal cavity suddenly emerges on the wall where the flow separates. The newborn cavity grows and splits releasing many minute bubbles to downstream, as the flowrate increases. To see if this phenomenon occurs in water too, water flows through rectangular and cylindrical constrictions have been meticulously observed with very high resolution in terms of both time and space. As a result it has been confirmed that the same phenomenon does occur in separating water flows as well, although the developing process of a newborn cavity in water flows is different from that in oil flows probably depending on a large difference in Reynolds numbers between both flows. When the flowrate is substantially increased from that or the cavity inception, a large attached cavity is formed on the wall just downstream of the separation point, as it occurred in corresponding oil flows. The pressure inside the cavity has proved to be almost equal to the saturation vapour pressure of water. [ABSTRACT FROM AUTHOR]

A contact motion between two solids in a liquid can give rise to a cavity. To investigate this phenomenon, processes of cavity generation in a liquid between two walls making accelerated relative motions in their normal and tangential directions have been observed in detail, using microscopes and high-speed video cameras. The results are as follows: generation and subsequent expansion of a cavity are influenced by factors such as the initial clearance between the walls, the acceleration of motion, the initial pressure, the liquid viscosity, and the contact area. In contrast, air solubility in oil does not affect generation, but the size of a bubble remaining in the liquid. The pressure in a liquid film between two parallel walls normally moving away with a constant acceleration has also been theoretically analysed, providing the basis for the generation of tensile stresses in liquid films. Finally, the following idea of mechanism has been proposed: a tensile stress occurring in a liquid film breaks the liquid-solid interface, producing a rift, which develops into a visible cavity. Considering that liquid machines have many parts of contacting solids making relative motions, it is presumed that cavitation in them can start from this phenomenon. [ABSTRACT FROM AUTHOR]

Cavitation inception in a ducted propulsor was studied numerically using Navier-Stokes computations and bubble dynamics models. Experimental observations of the propulsor model and previous numerical computations using Reynolds-averaged Navier-Stokes (RANS) codes indicated that cavitation inception occurred in the region of interaction of the leakage and trailing tip vortices. The RANS simulations failed, however, to predict correctly both the cavitation inception index value and the inception location. To improve the numerical predictions, we complemented here the RANS computations with a direct Navier-Stokes simulation in a reduced computational domain including the region of interaction of the two vortices. Initial and boundary conditions in the reduced domain were provided by the RANS solution of the full ducted propulsor flow. Bubble nuclei were released in this flow field, and spherical and nonspherical bubble dynamics models were exercised to investigate cavitation inception. This resulted in a solution in much better agreement with the experimental measurements than the original RANS solution. Both the value of the cavitation inception index and the location of the cavitation inception were very well captured. The characteristics of the emitted acoustic signals and of the bubble shapes during a cavitation event were also computed. [ABSTRACT FROM AUTHOR]

At the very beginning stage of cavitation an infinitesimal cavity suddenly emerges and rapidly grows on the wall close to the point of flow separation. The present paper intends to make clear the mechanism of this peculiar phenomenon previously discovered by the present authors in oil flows, by experimentally examining the characteristics of a separating flow. First, pressure and temperature distributions along a cylindrical smooth wall in an oil flow were measured across the point of separation, which has proved that both tensile stress and heat are really generated near the separation point. Second, a laminar air flow passing over a sharp edge was inspected by particle image velocimetry (PIV). The inspection of the separating flow based on the PIV data has revealed that fluid particle near the separation point is not only drastically stretched but also intensely sheared along the separation streamline. With all the results integrated, it has been concluded that the singular structure of a separating flow is the real cause of both tensile stress and heat generation. When thus generated tension exceeds a threshold, it possibly breaks the adhesion between the liquid and the wall, leaving a microscopic rift on the interface that would grow to a visible cavity. [ABSTRACT FROM AUTHOR]

A new type of cavitation damage has been observed on the turbines installed at the Three Gorges Power Station despite no cavitation detected during model tests. Metallurgical and fluid dynamic analysis suggests that this cavitation is triggered by boundary-layer turbulent production; the damaged (roughened) spot in turn triggers subsequent cavitation (damage) immediately down stream. This forms a sustainable dynamic process, resulting in long and equal-width streamwise damage-strips with spanwise regularity reflecting the spanwise stochastic characteristics of turbulent production. Owing to the heat effect of cavitation, intergranular corrosion takes place through sensitization process, leaving the damaged surface with a corrosion appearance. Also, bluing presents at the damaged tails, owing to the nature of low-intensity damage. Extremely large turbines are much more susceptible to this type of cavitation (damage) owing to the similarity laws currently employed for turbine development not concerning the freestream turbulence and the boundary-layer dynamics. [ABSTRACT FROM AUTHOR]

The boundary element method (BEM) is applied to solve the unsteady behavior of a bubble placed in a vortex flow field. The steady vortex field is given in terms of the viscous core radius and the circulation, both of which may vary along the vortex axis. For this study, 2DynaFS©, an axisymmetric potential flow code which has been verified successfully for diverse type of fluid dynamic problems, is extended. The modifications to accommodate the ambient vortex flow field and to model the extreme deformations of the bubble are presented. Through the numerical simulations, the time history of the bubble geometry and the corresponding pressure signal at a fixed field point are obtained. A special effort is made to continue the numerical simulation after the bubble splits into two or more sub-bubbles. Indeed, it is found that an elongated bubble sometimes splits into smaller bubbles, which then collapse with the emission of strong pressure signals. The behavior of the axial jets after the split is also studied in more detail. [ABSTRACT FROM AUTHOR]

Cavitation occurring at a sharp projection in a hydraulic oil flow was observed in as much detail as possible, using, variously, a microscope, a high-speed video camera, laser beams, an electric charge detector and a photomultiplier. At the tip of the needle employed as a projection, a tiny cavity as small as several tens of micrometres in length suddenly emerged and would not go away. As cavitation became more vigorous, flashes occurred intermittently around the needle and positive electrical charges were generated which synchronized with the flashing. The electrode inserted downstream from the needle detected negative charge from the oil, which was also synchronous with the flashing. Observing the needle tip with the microscope made it possible to determine exactly the moment of cavitation inception, which turned out to depend on the oil temperature as well as the downstream pressure. All these findings tend to reinforce the 'rip-off' hypothesis previously proposed by the present authors for the cavitation mechanism. [ABSTRACT FROM AUTHOR]

• Hydrodynamic cavitation can be promoted with the addition of sodium chloride. • Sodium chloride decreases air solubility in water, increases gas saturation level. • Hydrodynamic cavitation is promoted as gas saturation level increases. • Cavitation inception reversibly controlled by changing pH of carbonated water. Hydrodynamic cavitation is a common natural phenomenon that has positive and negative implications in several fields. In one hand, it is a major cause of corrosion and noise radiation in the marine industry. On the other hand, it is used as an innovative process for enhancing mineral flotation and water disinfection. Therefore, understanding the promotion and prevention mechanisms of hydrodynamic cavitation inception is essential for industrial applications. In this study, a Venturi tube was used as the hydrodynamic cavitation device, and the cavitation inceptions were measured using the sound-detection method. NaCl was shown to promote cavitation inception of water. However, no obvious change was observed for air-saturated NaCl solutions. These results are explained by the increase in air saturation level when salt is added into water. Based on this idea, the cavitation inception can be controlled by modifying the CO 2 solubility through changing the pH of carbonated water, a process that was shown to be reversible. These findings are applicable to industrial operations that try to avoid cavitation or those where the generation of bubbles is required. [ABSTRACT FROM AUTHOR]