Cavitation Growth Research Articles

Nonlinear dynamics of a bubble cluster in liquid cavity wrapped by elastic medium

Abstract According to the modified nucleation theory, gas nuclei can generate and grow into cavitation bubbles when’s the liquid pressure exceeds a threshold in a cavity. Based on the modified nucleation theory, the growth process of the gas nuclei in a liquid cavity can be simplified to two “events”, and the full confinement effects of the surrounding elastic medium of the cavity was considered by including the bulk modulus in the equation of state. The dynamical equations were derived to model the nonlinear oscillation of the multi-bubble system in the cavity. The effects of acoustic parameters, the initial radius and the total number of bubbles on bubble dynamics were investigated numerically. It is found that bubble behaviors are sensitive to the acoustic field. In the low frequency region (f <1MHz), bubble oscillation exhibits a modulated waveform, and fundamental, subharmonic and harmonic modes can be found in the acoustic response curve. The oscillations of confined bubbles diminish with increasing frequency. In the high frequency region (f >1MHz), the confinement state affects the acoustic response of the gas nuclei, and the growth rate differ by a factor of 5 difference in the presence of shelled or unbounded elastic confinement of the cavity. Therefore, the suppression or enhancement of cavitation growth and collapse in confined cavity can be modulated by a variety of factors, such as the total number of nuclei, cavity size, acoustic frequency and amplitude, negative pressure, and elastic medium, which have a synergistic influence on the confined cavitation process.

Active flow control on unsteady cloud cavitation: Insights into jet dynamics

Unsteady cloud cavitation significantly impairs the performance of marine systems. Active flow control method has shown promise in effectively reducing cavitation, yet its specific mechanisms of action require elucidation. This study investigates continuous water injection on a NACA66 (MOD) hydrofoil as an active control against unsteady cloud cavitation. Results show notable reductions in cavitation volume: 52.57 % at a cavitation number of 0.83 and 86.24 % at 1.29. Employing the Lagrangian approach, the study tracks ideal particles to explore the jet's behavior and its interaction with cavitation. The interaction between the jet and cavitation is reciprocal: cavitation draws the jet, which in turn alters the cavitation's structure. The jet's movement is synchronized with the evolution process of cavitating flow, effectively limiting the spread of attached cavitation, disrupting cloud integrity, and encircling scattered clouds to prevent their growth. The analysis identifies the jet's streamwise direction as critical in suppressing cavitation, with the normal direction providing supplementary support, and the spanwise direction having minimal impact. Furthermore, the jet promotes a condensation-dominant state in local flow regions, hindering cavitation growth by altering the water-vapor mass transport process.

Stabilize cloud cavitation with an obstacle near hydrofoil's trailing edge and conduct local entropy production analysis

Cloud cavitation always causes severe damage to the efficiency and stability of the hydraulic machinery, resulting in extra energy losses in the system. We have observed an effective and simple way to prevent cloud cavitation formation by placing an obstacle near the hydrofoil's trailing edge. Cavitating flows around four different types of hydrofoils were simulated using the stress-blended eddy simulation turbulence model: the National Advisory Committee for Aeronautics (NACA) 66 hydrofoil and the NACA 66 hydrofoil with a 1 ×1 mm2 obstacle at 0.3c, 0.5c, or 0.7c. Sheet cavitation is the predominant mode of cavity flow when the obstruction is positioned at 0.7c. To find out why the cloud cavitation growth can be stopped when the obstruction is positioned at 0.7c, the velocity field, vorticity in the Z direction, and vortex structure of the Q-criterion were computed. To study the energy loss of the cavity flow and comprehend how obstacles affect it, the local entropy production rate was computed. It was discovered that the vorticity downstream of the obstacle, positioned at 0.7c, is restructured, which helps manage the flow separation upstream of the obstacle. Consequently, the hydrofoil's suction surface vorticity nearly rotates in the same direction as the obstacle at 0.7c, and the direction of Vx upstream of the obstacle is in the positive direction of the X axis, indicating that the reentrant flow has been controlled upstream of the 0.7c obstacle. Furthermore, cavitation shedding and the entropy production rate are strongly correlated, and regulating cloud cavitation growth is advantageous for energy conservation.

Numerical investigation on the evolution of cavitating dynamics and energy loss around a hydrofoil in fluoroketone under different temperatures

To investigate the effects of variable free-stream temperatures on cavitation evolution and entropy production in fluoroketone cavitation flows. The Zwart cavitation model with the thermodynamic effect and the modified RNG k-ε turbulent model with the compressibility of the gas-liquid mixed phase are used to run the simulation. Numerical results demonstrate that the intensity of cavitation decreases as free-stream temperatures rises, resulting in the cavitation thermal effect. Because cavitation shedding caused a negative pressure gradient, the main flows on the hydrofoil pressure surface at the trailing edge went back along the suction surface. The re-entrant at the trailing edge is caused by the interaction between returned flows and the process of cavitation growth. The process of cavitation interphase mass transfer alters the internal temperature field. In addition to the local temperature decrease induced by evaporation, condensation causes a local temperature increase. And, evaporation is more intense than condensation. The entropy production caused directly by temperatures is very small, and primarily caused indirectly by the velocity pulsation in the flow field. This study could help to comprehend the unsteady cavitation flow in fluoroketone while accounting for the thermodynamic effect, as well as the rational design of fluid machinery for transporting thermo-sensitive fluids.

Numerical investigation about the unsteady behavior of a free submerged cavitation jet using the SBES approach

Submerged cavitation jet has emerged as an efficient method for the removal of ship fouling. Due to the complicated revolution of cavitation cloud, it has attracted great research interest. Ascertaining the internal flow dynamics of cavitating jets is always a challenge. Thus, this paper investigates the unsteady behavior of a free submerged cavitating jet numerically in three dimensions. The turbulent cavitation flow is simulated utilizing the Stress-Blended Eddy Simulation (SBES) turbulent model combined with the Schnerr-Sauer cavitation model. The results are compared with the data from a famous experiment, and they exhibit good agreement in both time-averaged and unsteady characteristics. The cavitation life stages, i.e., the cavitation growth, shedding, contraction, and collapse are explored. In results, cavitation cloud forms around the jet core. Cavitation cloud shedding involves complex factors, including shear layer instability, complex pressure gradients, and re-entrant motion, and it may also be related to the potential core and jet core instability. Additionally, cavitation cloud development is closely related to the jet core and exhibits synchronous behavior. After the cavitation cloud collapse, a local high pressure is observed and then cavity rebound is generated, where the collapse can be either mild or intensive.

Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil

The large eddy simulation model coupled with the modified Schnerr–Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 (National Advisory Committee for Aeronautics) hydrofoil under different operating conditions. The results show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number of 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams–Hawkings acoustic analogy approach accompanied by the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation–vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons, it can be inferred that the hydrofoil suction surface under the cavitation effect will generate quasiperiodic waves starting from upstream and moving downstream.

A Lagrangian analysis of partial cavitation growth and cavitation control mechanism

Partial cavitation has a strong unsteadiness, which will cause serious damage to the hydraulic machinery. The spanwise obstacle is nearly the most efficient method for controlling unsteady cavitation. In this study, numerical simulations of partial cavitating flows around NACA (National Advisory Committee for Aeronautics) 66 hydrofoils in two dimensions (2D) were carried out both with and without obstruction. The obstruction is placed at 0.37c, and its height is 0.1c. Utilizing the finite-time Lyapunov exponent, the Lagrangian coherent structures (LCSs) were developed to investigate the dynamic characteristics of the unsteady flow. By showing the dynamic evolution of the Lagrangian behaviors, the time-dependent LCSs over the two different flows demonstrate the effectiveness of LCSs in explaining the evolution of the vortex during the partial cavitation process. With the use of LCSs, the vortex boundary and reentrant jet can be easily located, and the link between the vortexes can be readily seen. In the meantime, the vortex's origin and destination are shown by the stable and unstable manifolds, respectively. LCSs were then utilized to examine how the obstruction had an impact, and the following conclusions were reached. First, the obstruction can stop a portion of reentrant jets from passing through it. Second, the obstruction can curve the pathway of the reentrant jet, which has passed through it. Third, the obstruction prevents the cavity from flowing downstream. Finally, the obstruction continuously obliterates the expanding cavity across it. Simply said, the Lagrangian analysis based on LCSs provides a better understanding of the vortex dynamics than traditional visualization techniques, which is essential to understanding the great performance of the cavitation-induced unsteady flow.

Cavitation flow and broadband noise source characteristics of NACA66 hydrofoil with a V groove on the suction surface

In the present work, a V groove on the suction surface is proposed to investigate the cavitation flow and noise sources of a NACA66 hydrofoil. The SST k-ω turbulence model and broadband noise source model is employed to analyze the flow pattern and noise distribution. Proper orthogonal decomposition (POD) is then used to investigate the coherent structure of cavitation flow around the hydrofoil. The results show that, under cloud cavitation conditions, the groove improves the energy characteristics of the hydrofoil and increases the lift-to-drag ratio by 2.95%. During the sheet cavitation growth phase, the groove can suppress the development of low-pressure, low-velocity regions on the hydrofoil surface, thus suppressing cavitation near the leading edge of the hydrofoil. During the cavitation collapse stage, the groove reduces the pressure impact of cavitation collapse on the hydrofoil. The groove extends a cavitation cycle and reduces the average cavitation volume during a cavitation cycle by 17.46%. Results indicate that during a cavitation cycle, the groove reduces the average dipole noise on the hydrofoil surface by 5.07% and the average quadrupole noise on the middle longitudinal section by 6.86%. POD results show that the first five orders of the modes capture more than 90% of the energy of the flow field. The primary frequency of the modal coefficients of Mode 1 and Mode 2 is the cavitation frequency, and the other frequencies are multiples of the cavitation frequency. The groove significantly affects Mode 2, which represents a change in velocity distribution in the flow field characteristics.

Unsteady cavitation around submerged and water-exit projectiles under the effect of the free surface: A numerical study

The growth, shedding, and collapse regime of cavitation under the effect of a free surface is complicated by projectiles moving near and across the free surface and three phases (air, liquid, and vapor due to evaporation) of a fluid. In this work, the flow behaviors of cavitation around projectiles moving beneath a free surface and exiting the free surface were numerically investigated in detail using a dual-time preconditioned Navier–Stokes (NS) model for three-phase flows. First, the cavitation flow structure around a submerged projectile moving parallel to the free surface at a constant velocity was computed to validate the numerical method. Comparisons between numerical and experimental data on cavity growth are presented. Second, the vertical and oblique water exits of the cavitating projectiles under different flow conditions were investigated. Special features of the flows, such as cavitation growth, shedding, and collapse under the effect of the free surface and the detailed pressure peaks on the surface of the projectiles from the collapse of the cavitation, were obtained, through which the physical mechanism of this flow was clearly observed. The findings are significant for the design and control of submerged vehicles and projectiles operating in such flow fields.

Intermittent cavitating flow field in tandem cascades of Kaplan turbines

Within the framework of the classical turbine design setup, we present a numerical investigation of unsteady cavitating flow in tandem cascades of guide vanes and rotor blades. The practical relevance of the present investigation is ensured by choosing real blade profiles and domain geometry for the large Kaplan turbines (9.5 m rotor diameter) from the Iron Gates I hydropower plant on the Danube River. The 2D tandem cascade setup is obtained with a conformal mapping of the circular guide vane cascade into a straight cascade, while the rotor blade cascade is by default a straight one for axial turbines. Both single-phase and two-phase cavitating flows results are presented and analysed. The single-phase liquid flow displays rather modest pressure and tangential force fluctuations, with the characteristic blade passing frequency, due to the interaction of the rotor blades with the wakes of guide vanes. The two-phase cavitating flow displays a completely different dynamics in comparison to the liquid flow, with periodic cavitation growth and collapse at a specific frequency lower than the blade-passing one. Moreover, the tangential force on the rotor blade displays severe intermittency and the force fluctuations are almost one order of magnitude larger than in the single-phase flow.

Investigation on axial thrust behavior of balance piston system for a rocket pump

Abstract High stability in axial direction is required for rocket pumps operated under extremely low-temperature and high-pressure conditions, turbopumps therefore uses balance piston (BP) system for balancing their axial thrust. The BP system is stable under quasi-static conditions. However, BP system might become dynamically unstable under some condition. Thus it is fundamental for stability evaluation of turbopumps to predict static/dynamic characteristics in axial direction of BP system. Furthermore, we focus on characteristic change by cavitation which often occurs in the pump inlet. In this paper, an experimental study of a model turbopump which had an unshrouded impeller equipped with BP system was carried out. We experimented it with an active magnetic bearing (AMB) test facility in order forcibly to oscillate it with an optional amplitude and frequency. In addition, we examined characteristics of BP system by three-dimensional computational fluid dynamics (3D-CFD) simulations. The results of 3D-CFD simulations were in good agreement with these tendency of BP system, and were effective in predicting its static/dynamic characteristics. Some cases showed that dynamic characteristic of BP system became unstable by growth of cavitation, therefore we suggest that the influence of cavitation must also be considered in the design of turbopump.

Experimental and Numerical Investigation on the Transient Cavitating Flows in a Mixed Flow Pump With Different Number of Blades at Startup

Abstract The objective of this paper is to experimentally and numerically investigate the transient cavitation flow during the startup process of mixed flow pump with emphasis on studying the influence of blade numbers. The transient cavitation simulation was studied based on the improved SST k–ω turbulence model and the Zwart cavitation model. Firstly, in order to obtain the relationship between transient flow rate and the variation of rotational speed at startup, a theoretical analysis based on the fast transients of centrifugal pump was first applied to mixed flow pump and was verified by the current experiment study. Subsequently, the influence of blade number on the cavitation flow in the startup was studied. It is found that the transient cavitation could be classified into four stages regardless of the number of blades: no cavitation stage, the cavitation growth stage, the cavitation reduction stage and the cavitation stabilization stage. However, the blade number does have an impact on the spatial-temporal evolution of cavitation. More specifically, when the blade number increases, the initial cavitation appeared lately, the coverage area of the triangular cavitation cloud and sheet cavitation both decreased, and the increase in blade number has a better inhibitory effect on the sheet cavitation at the cavitation growth stage, and can make sheet cavitation disappear more quickly at the cavitation reduction stage.

Experimental Research on Cavitation Evolution and Movement Characteristics of the Projectile during Vertical Launching

Aiming at the problem of unsteady cavitation during a projectile’s vertical water-exit process, scaled model experiments were carried out based on the self-designed underwater launch platform and high-speed cameras, which focus on changes in cavitation shape and projectile posture. In this paper, the general process of the cavitation evolution and projectile’s movement is described; the relationship between the re-entry jet, local cavitation number and cavitation stability is discussed. Meanwhile, the effect of head forms and launch speeds on the cavitation evolution and movement characteristics is analyzed, including 60° cone, 90° cone and hemispherical head with velocity of 16.8 m/s, 18.5 m/s and 20.0 m/s, whose launch cavitation number is 0.714, 0.589 and 0.504. The results show that the attached cavities fall off from the bottom up under the influence of the end-re-entry jet and the shedding frequency declines when the launch cavitation number decreases. The cavitation growth of 60° cone is easily disturbed by the air mass near the launcher, the cavitation development of 90° cone is characterized by small-scale and high-frequency growth and shedding, while the hemispherical head is not prone to cavitation. Moreover, increasing the speed can improve the stability of cavitation development and the projectile’s movement.

Empirical mode decomposition of ship hull pressure fluctuation induced by cavitating propeller

We report for the first time a cavitation-induced pressure fluctuation decomposition developed from empirical mode decomposition (EMD) [Huang et al., Proc. R. Soc. London, Ser. A 454, 903–995 (1998)]. The idea is to decompose the nonlinear and non-stationary time series data into a finite and usually small number of “intrinsic mode functions” based on the local properties of the signal, which admit a well-behaved Fourier transform. With this transform, we can obtain frequency characteristics that give sharp identifications of imbedded structures. The cavitation evolution and excited pressure fluctuation around a cavitating propeller in the nonuniform wake are investigated using high-speed imaging and pressure sensors. By the EMD method, we separate the pressure fluctuations induced by different types of cavitation. The high frequency components of the pressure fluctuations are mainly caused by the collapse of sheet cavitation, followed by the shrinking and growth of sheet cavitation. Furthermore, the tip vortex cavitation leads to higher frequency but contributes less to pressure fluctuations. The periodical motion of the propeller contributes to the first blade frequency, and the pressure fluctuations induced by cavitation are superimposed on it.

Experimental study of nanoelectroplating steel spot welding structure under impact tensile-shear loading

The tensile shear fracture behavior of solder joints under impact load influences the whole vehicle’s safety substantially. This paper takes the QP980 steel resistance spot welding (RSW) structure as the research object to study the tensile-shear fracture behavior of the RSW structure under tensile-shear loading. The microstructure observation of the welded spot shows that the metallographic structure is the martensite. The porosity defects in the melting zone are the primary defect reflecting the obvious plate-cracks on both sides of the nugget. The paper demonstrates the Vickers hardness test result of the spot-welded zone. According to the test, the micro-hardness distribution result shows that the higher the martensite, the greater the hardness. A softened zone emerges adjacent to the heat-affected zone on the welded base material interface. The quasi-static and dynamic tensile-shear tests on the QP980 steel RSW lap-joint specimens show that the fracture on the BM is adjacent to the welded spot under quasi-static loading but close to the heat-affected zone under dynamic loading. Under dynamic loading, the weld seam and softened zone of the welded spot have a direct influence on the fracture. On the recovered specimen’s fractured section, there are a large number of apparent dimples on the section of the BM under quasi-static loading and the section of the HAZ under dynamic loading with nucleation, growth, and aggregation of cavitation, resulting in ductile fracture.

LES INVESTIGATION ON THE INFLUENCE OF CAVITATION ON THE EVOLUTION AND CHARACTERISTICS OF TIP LEAKAGE VORTEX

In the present paper, the tip-leakage vortex (TLV) cavitation around a NACA0009 hydrofoil is numerically investigated with large eddy simulation method combined with a new Euler-Lagrangian cavitation model, which takes into account the effect of nuclei on tip-leakage cavitating flow. A satisfying agreement between the numerical and experimental results is obtained. Based on the numerical data, the evolution behavior of TLV cavitation with different gap sizes and its influence on the strength and radius of TLV, the nuclei distribution in the vortex core and the distribution of tangential velocity are discussed in detail. The results show that once cavitation occurs, the intensity of TLV is mainly affected by the evolution behavior of the sheet cavitation, while the tip-leakage cavitating flow has little effect on the strength of TLV. In addition, the results suggest that a smaller gap size will result in a more unstable sheet cavity, and the strength of TLV in turn presents corresponding quasi-periodic fluctuation as influenced by the evolution of the sheet cavitation. As the tip clearance progressively increases, the strength of the sheet cavitation gradually decreases with the reduction of instability, and the intensity of TLV gradually returns to the level without cavitation, together with its fluctuation level. Cavitation has a more significant influence on the nuclei distribution in the vortex core, and its degree of influence depends on the spatial stability of TLV and the intensity of TLV cavitation after the occurrence of cavitation. Moreover, the numerical results obtained by our simulations also indicate that cavitation has a significant influence on the radius of TLV. After cavitation occurs, the radius of TLV increases to a certain extent, and a ``rigid body rotation'' tangential velocity distribution is formed on the periphery of cavitation region, which is mainly caused by the expansion process induced by cavitation growth and the viscous effect of flow.

Study on Cavitating Flow Inside Orifice and Spray Angle Near Nozzle Tip According to the Position of Needle Using Enlarged Transparent Acrylic Nozzle

This study aims to analyze effect of needle position inside nozzle on the internal and external flow characteristics. To visualize cavitating flow inside nozzle, the transparent acrylic nozzle was used. We tested five needle positions that simulated the needle movement of injector. The cross-section of test nozzle is rectangular with a very narrow width instead of circular shaped for a better visualization of cavitating flow. The inside of the nozzle was visualized by the shadowgraphic visualization using a high-speed camera and a metal-halide lamp. From the experiments, development of the cavitating flow according to the needle positions depends on the flow velocity and the cavitation number. The cavitation length, cavitation width, and spray angle are relatively symmetrically shown when the needle was at the center. However, they were asymmetrical when the needle position was biased. When the needle positions were in the same on the vertical line, the minimum cavitation length, minimum cavitation width, and minimum spray angle were larger when the upper level. The needle position affects the development timing and cavitation growth. When the needle position was located at the upper level, the hydraulic flip occurred at a higher injection pressure than the lower level.

Preferential cavitation and friction-induced heating of multi-component Diesel fuel surrogates up to 450MPa

The present work investigates the formation and development of cavitation of a multicomponent Diesel fuel surrogate discharging from a high-pressure fuel injector operating in the range of injection pressures from 60MPa to 450MPa. The compressible form of the Navier-Stokes equations is numerically solved with a density-based solver employing the homogeneous mixture model for accounting the presence of liquid and vapour phases, while turbulence is resolved using a Large Eddy Simulation approximation. Simulations are performed on a tapered heavy-duty Diesel engine injector at a nominal fully-open needle valve lift of 350μm. To account for the effect of extreme fuel pressurisation, two approaches have been followed: (i) a barotropic evolution of density as function of pressure, where thermal effects are not considered and (ii) the inclusion of wall friction-induced and pressurisation thermal effects by solving the energy conservation equation. The PC-SAFT equation of state is utilised to derive thermodynamic property tables for an eight-component surrogate based on a grade no.2 Diesel emissions-certification fuel as function of pressure, temperature, and fuel vapour volume fraction. Moreover, the preferential cavitation of the fuel components within the injector's hole is predicted by Vapour-Liquid Equilibrium calculations; lighter fuel components are found to cavitate to a greater extent than heavier ones. Results indicate a significant increase of temperature with increasing pressures due to friction-induced heating, leading to a significant increase in the mean vapour pressure of the fuel and an increase of the mass of fuel cavitating, but at the same time to an unprecedented decrease of cavitation volume inside the fuel injector with increasing injection pressure. This has been attributed to the shift of the pressure drop from the feed to the back pressure inside the injection hole orifice as fuel discharges; as injection pressure increases, so does the pressure inside the orifice, confining the location of cavitation formation to a smaller volume attached to the upper part of orifice, thus restricting cavitation growth.

Numerical and Experimental Analysis of a Singing Propeller Having Blunt Trailing Edges

This study considers a blunt trailing-edged propeller operating both in a uniform and a nominal wake fields. Experiments are performed in the cavitation tunnel of Hyundai Maritime Research Institute. The effects of propeller rotation speed, tunnel flow speed, and blade sheet cavitation growth on the generation mechanism of the singing propeller are investigated. The cavitation, sound, and vibration characteristics related to the singing phenomena are measured by a hydrophone, a microphone, an accelerometer, and a highspeed digital camera. The natural frequencies of propeller blades are predicted using a finite element method and verified by both contact- and noncontact-type impact hammer tests in air and underwater conditions. The inflow speed and angle of attack for each section of the propeller blades are calculated using the Reynolds-averaged Navier-Stokes equation-based flow analysis. Using a detached eddy simulation, the vortex shedding patterns and their frequencies are calculated. The predicted vortex shedding frequencies are compared with the measured singing frequency and blade natural frequency for determination of consistency. Under cavitation-free regime, the vortex shedding frequencies are predicted for normalized blade radial positions of .8R and .9R. The computed values are close to the two blade natural frequencies and also consistent with the double singing phenomena in the cavitation tunnel test. For fully developed blade sheet cavitation condition, the vortex formation in the wake region is observed to be strongly influenced by the cavitation growth on the pressure side surface. Propeller singing is diminished with the continuous growth of cavitation and is finally locked-off. The significant variation of the flow-induced sound and vibration levels are also observed for the locked-in and the locked-off conditions. The singing occurrence location and frequency under uniform inflow condition are analyzed to investigate the generation mechanism of propeller singing. This study can be applied to the analysis of singing location and its frequency of a propeller operating in the hull wake, which changes the angle of attack according to the propeller rotation angle.

Initiation and growth of cavitation during the fatigue process of pre-oriented polypropylene film: In-situ SAXS study

Initiation and growth of cavitation during the fatigue process of pre-oriented polypropylene film have been studied via in-situ small angle X ray scattering (SAXS). The variation of the scattering invariant has been probed and the size of a single cavity has been fitted via a cylinder model. Two steps including initiation and growth of cavitation can be observed. In the first step, cavities initiate with a fast-growing rate at the beginning of the fatigue. The fitted length of a single cavity varies with the same frequency to the macroscopic stress, whereas the radius shows a higher frequency than the loading stress at the beginning of the fatigue process. In the second step, cavities stably grow after ca. 5 cycles. L and R deform in the same frequency with the macroscopic stress. Interestingly, the single cavity grows in the stretched direction and L enlarged with the loading stress, while in the lateral direction R decreases due to the lateral contraction. In this step, the volume of the cavity becomes stable cycle-by-cycle and the deformation of a single cavity is close to affine deformation due to the stable of the cavity. These findings could be of great importance for understanding the initiation and growth of pore during the fabrication of porous polypropylene membrane.