Visualization of a confined accelerated bubble

High-speed photography was used to study the collapse of a confined two-dimensional, air cavity in water, subjected to a propagating pressure disturbance. The 5–6 mm diameter cavity was confined in a rectangular duct. A sustained pressure disturbance was created by an accelerating piston in contact with the water 240 mm away from the bubble. The pressure increased from 0.1 MPa to about 0.12 MPa with a rise time of the order of 2 ms. The pressure pulse was not reflected until its arrival at the end of the duct, 320 mm from the piston. A microjet was produced at the proximal wall which penetrated the distal cavity wall, thereby producing a pair of bubbles which was thought to be regions of intense vorticity. The features of such confined bubble collapse were not found in previous investigations of unconfined bubble accelerations by weak pressure disturbances. Confinement apparently intensified the effect of the disturbance significantly. Received 18 August 1998 / Accepted 12 May 1999     

On the collapse of cavities

The collapse of a single cavity, or a cloud of bubbles has several physical consequences when in proximity to a structure or resident within a material during deformation. The earliest recognized of these was cavitation erosion of the propellers of steam ships. However, other processes include the rapid collapse of cavities leading to hot spots in explosives from which reaction ensues, or the more recent phenomenon of light generation by oscillating single bubbles or clouds. In the collapse of a cavity, the least considered but the most important mechanism is asymmetric closure. One of the consequences of this is the formation of jets leading to local high pressures and shears that result in the damage or reaction mechanisms observed. The challenge for the future remains in understanding the effects of cloud cavitation since it is likely that only one bubble in perhaps millions in a cloud catalyses an event. The review follows the author's work in the understanding of shock-induced cavity collapse and highlights several results which indicate the importance of this problem in a variety of fields. Received 27 July 2001/ Accepted 25 January 2002     

Studies on bubble dynamics

In order to clarify the behavior of bubbles which is closely related to cavitation phenomena, the research of the dynamics of bubbles has been intensively conducted and established the research field of bubble dynamics. In this review paper it is intended to describe briefly studies on bubble dynamics including the history in conjunction with the shock wave dynamics. Received 17 June 1996 / Accepted 15 August 1996     

Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber

In this paper, the influence of nozzle geometry on cavitation and near-nozzle spray behavior under liquid pressurized ambient is studied. For this purpose, eight steel drilled plates, with different diameters and degrees of conicity of their holes, are analyzed. A special near-nozzle field visualization technique, using a test rig pressurized with fuel, is used. Due to the difference in refractive index between liquid and vapor phase, bubbles present at the outlet of the orifice are visualized. The pressure conditions at which bubbles start appearing at the orifice outlet are compared with those at which choked flow appears. The results showed that pressure conditions for inception of cavitation obtained in the visualization tests differs from those seen for choked flow (5–8% in terms of cavitation number). In addition to this, the images taken are analyzed to get the angle of the jet formed by fuel bubbles, showing that it increases significantly for those conditions more prone to cavitate. Furthermore, comparison of bubbles generation when increasing or decreasing backpressure indicates the presence of hysteresis in cavitation inception phenomena.     

Cavitation phenomena in extracorporeal microexplosion lithotripsy

An experimental investigation was made of cavitation phenomena induced by underwater shock wave focusing applied to the extracorporeal microexplosion lithotripsy (microexplosion ESWL). Firstly an underwater microexplosion generated by detonation of a 10 mg silver azide pellet was studied and secondly underwater shock focusing and its induced cavitation phenomena were investgated. Underwater shock wave was focused by using a semi-ellipsoidal reflector in which a shock wave generated at the first focal point of the reflector was reflected and focused at the second focal point. It is found that an explosion product gas bubble did not produce any distinct rebound shocks. Meantime cavitation appeared after shock focusing at the second focal point where expansion waves originated at the exit of the reflector were simultaneously collected. A shock/bubble interaction is found to contribute not only to urinary tract stone disintegration but also tissue damage. The cavitation effect associated with the microexplosion ESWL was weaker in comparison with a spark discharge ESWL. The microexplosion ESWL is an effective method which can minimize the number of shock exposures hence decreasing tissue damage by conducting precise positioning of urinary tract stones.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Mechanics of submerged jet cavitating action: material properties,exposure time and temperature effects on erosion

Experimental setup with a submerged cavitating jet has been used for the study of influences of material, exposure time and working fluid temperature on the erosion process. Each of the parameters has been varied separately, and the results of erosion are analyzed in detail. Additionally, comparison of experiments with nitrated and non-nitrated material has been made in order to study the enhancement (mostly reflected as the prolonged incubation time) of erosion resistance achieved by nitrating the specimen surface.     

Shock-induced collapse of a cylindrical air cavity in water: a Free-Lagrange simulation

A Free-Lagrange CFD code is used to simulate the collapse of a cylindrical air cavity in water by a 1.9 GPa incident shock. The Lagrangian treatment allows the air/water interface to be tracked throughout the interaction. The incident shock is partially transmitted into the cavity, within which it experiences multiple reflections. The upstream cavity wall involutes to form a high-speed jet which, on impact with the far cavity wall, produces an intense blast wave. Heating of the gas within the cavity is highly non-isentropic, and is dominated by shock heating. The predicted final gas temperature is of order 12000 K, although the modelling assumptions used here lead to over-prediction of temperature during the later stages of collapse. Received 14 September 1999 / Accepted 28 June 2000     

Interfacial aeration and bubble count rate distributions in a supercritical flow past a backward-facing step

An example of high-velocity open channel flows is a supercritical flow past an abrupt drop. In such a geometry, the basic air–water flow properties were measured, including distributions of void fraction and bubble count rate, and local air and water chord size distributions, at and downstream of the backward-facing step. The bubble count rate distributions were compared with a conceptual model of streamwise distribution of air and water chords which yields a quasi-parabolic relationship between bubble count rate and void fraction. The proposed model was an attempt to explain the experimental relationship between bubble count rate and void fraction, rather a meticulous breakdown of the complex air–water structure.     

Numerical simulation of bubble breakup phenomena in a narrow flow field

Based on the boundary integral method, a 3D bubble breakup model in a narrow flow field is established, and a corresponding computation program is developed to simulate the symmetrical and asymmetrical bubble breakup. The calculated results are compared with the experimental results and agree with them very well, indicating that the numerical model is valid. Based on the basic behavior of bubbles in a narrow flow field, the symmetrical and asymmetrical bubble breakup is studied systematically using the developed program. A feasibility rule of 3D bubble breakup is presented. The dynamics of sub-bubbles after splitting is studied. The influences of characteristic parameters on bubble breakup and sub-bubble dynamics are analyzed.     

Vibration suppression in multi-tool ultrasonic machining to multi-external and parametric excitations

Ultrasonic machining （USM） is of particular interest for the machining of non-conductive, brittle materials such as engineering ceramics. In this paper, a multi-tool technique is used in USM to reduce the vibration in the tool holder and have reasonable amplitude for the tools. This can be done via dynamic absorbers. The coupling of four nonlinear oscillators of the tool holder and tools representing ultrasonic cutting process are investigated. This leads to a four-degree-of-freedom system subjected to multi-external and multi-parametric excitation forces. The aim of this work is to control the tool holder behavior at simultaneous primary, sub-harmonic and internal resonance condition. Multiple scale perturbation method is used to obtain the solution up to the second order approximations. The different resonance cases are reported and studied numerically. The stability of the system is investigated by using both phase-plane and frequency response techniques. The effects of the different parameters of the tools on the system behavior are studied numerically. Comparison with the available published work is reported.     

Characterizing texture in polycrystalline materials by ultrasonic waves

A method for characterizing texture from measurements of ultrasonic wave velocities is proposed. In polycrystalline aggregates, ultrasonic wave velocities are strongly affected by orientation distribution coefficients (ODCs), which are usually used to describe the degree of preferred grain orientation in textured materials. In this work, velocities of longitudinal and transverse waves propagating into aluminum alloy 6061 were measured under pure shear, simple shear and uniaxial tension. From the measured ultrasonic wave velocities, the ODCs W₄₀₀ and W₄₂₀ were calculated to infer the deformation-induced texture. The predicted pole figures, obtained using ultrasonic velocities, were in good qualitative agreement with the finite element polycrystal model analyzed pole figures.     

Influence of local ultrasonic forcing on a turbulent boundary layer

An experimental study was carried out to investigate the effect of local ultrasonic forcing on a turbulent boundary layer. The ultrasonic forcing system was constructed by adhering six ultrasonic transducers to a flat plate over which water was flowed. In this system, the ultrasonic waves projected into the water by the transducers caused cavitation, giving rise to an enormous number of tiny water-vapor bubbles. Stereoscopic particle image velocimetry (SPIV) was used to probe the flow characteristics. The SPIV results showed that imposition of the ultrasonic forcing caused a substantial increase in the mean wall-normal velocity but a decrease in the mean streamwise velocity. The ultrasonic forcing reduced the skin friction coefficient by up to 60% immediately downstream of the transducers; this effect gradually dissipated with moving downstream. The streamwise turbulence intensity was reduced near the wall but increased away from the wall, whereas the wall-normal turbulence intensity was not much affected near the wall but increased away from the wall. The Reynolds shear stress and the production of turbulent kinetic energy were reduced near the wall. Imposition of the ultrasonic forcing shifted the streamwise vortical structures away from the wall, leading to a reduction in skin friction.     

Influence of heat transfer on the dynamic response of a spherical gas/vapour bubble

The standard approach to analyse the bubble motion is the well known Rayleigh–Plesset equation. When applying the toolbox of nonlinear dynamical systems to this problem several aspects of physical modelling are usually sacrificed. Particularly in vapour bubbles the heat transfer in the liquid domain has a significant effect on the bubble motion; therefore the nonlinear energy equation coupled with the Rayleigh–Plesset equation must be solved. The main aim of this paper is to find an efficient numerical method to transform the energy equation into an ODE system, which, after coupling with the Rayleigh–Plesset equation can be analysed with the help of bifurcation theory. Due to the strong nonlinearity and violent bubble motions the computational effort can be high, thus it is essential to reduce the size of the problem as much as possible. In the first part of the paper finite difference, Galerkin and spectral collocation methods are examined and compared in terms of efficiency. In the second part free and forced oscillations are analysed with an emphasis on the influence of heat transfer. In the case of forced oscillations the unstable branches of the amplification diagrams are also computed.     

Visualisation and les simulation of cavitation cloud formation and collapse in an axisymmetric geometry

Visualisation and Large Eddy Simulations (LES) of cavitation inside the apparatus previously developed by Franc (2011) for surface erosion acceleration tests and material response monitoring are presented. The experimental flow configuration is a steady-state closed loop flow circuit where pressurised water, flowing through a cylindrical feed nozzle, is forced to turn 90° and then, move radially between two flat plates towards the exit of the device. High speed images show that cavitation is forming at the round exit of the feed nozzle. The cavitation cloud then grows in the radial direction until it reaches a maximum distance where it collapses. Due to the complexity of the flow field, direct observation of the flow structures was not possible, however vortex shedding is inferred from relevant simulations performed for the same conditions. Despite the axisymmetric geometry utilized, instantaneous pictures of cavitation indicate variations in the circumferential direction. Image post-processing has been used to characterize in more detail the phenomenon. In particular, the mean cavitation appearance and the cavity length have been estimated, showing good correlation with the erosion zone. This also coincides with the locations of the maximum values of the standard deviation of cavitation presence. The dominant frequency of the ‘large-scale’ cavitation clouds has been estimated through FFT. Cloud collapse frequencies vary almost linearly between 200 and 2000 Hz as function of the cavitation number and the downstream pressure. It seems that the increase of the Reynolds number leads to a reduction of the collapse frequency; it is believed that this effect is due to the agglomeration of vortex cavities, which causes a decrease of the apparent frequency. The results presented here can be utilized for validation of relevant cavitation erosion models which are currently under development.     

Simulation-based conceptual design of an acoustic metamaterial with full band gap using an air-based 1-3 piezoelectric composite for ultrasonic noise control

This paper aims at proposing a novel type of acoustic metamaterials with complete band gap composed of piezoelectric rods with square array as inclusions embedded in an air background (matrix). A modified plane wave expansion method accompanied with the principles of the Bloch–Floquet method with electromechanical coupling effect and also impedance spectra are used to get a band frequency and to investigate the passband for the selected cut of piezoelectric rods. We investigate both the electromechanical coupling coefficient and mechanical quality factor and their dependency to passband and bandwidth, which depends on both the density and the wave impedance of the matrix and the inclusions (rods). The ratio of the volume of inclusion to the matrix is used to define the fill factor or the so-called inclusion ratio, to introduce the bandwidth as a function of that. Furthermore, the fabrication method is presented in this paper. The results make a suitable foundation for design purposes and may develop an inherently passive ultrasonic noise control. In addition, the results provide the required guidance for a simulation-based design of elastic wave filters or wave guide that might be useful in high-precision mechanical systems operated in certain frequency ranges and switches made of piezoelectric materials; they also propose a novel type of elastic metamaterials, which is independent of the wave direction and has an equal sensitivity in all directions in which it reacts omnidirectionally and mitigates the occupational noise exposure.     

Distorted wave response of ultrasonic annular stator incorporating non-uniform geometry

The traveling wave ultrasonic stator is normally fabricated with teeth. The tooth geometry improves the driving speed, but it creates natural frequency splitting and mode contamination, especially a distorted traveling wave. A dynamic model of a stepped-plate periodic stator is developed to examine the distortion. The stator is treated as an annular supported by a thin mid plate, and the support stiffness is formulated by using equivalent energy principle. The effects of the tooth and mid plate on the natural frequency and vibration mode are examined by using the perturbation method. The rules governing the frequency splitting, frequency perturbation as well as mode contamination are also identified. The traveling wave response and elliptical trace on stator surface are obtained by using the mode superposition method and they are proved to be distorted due to the tooth geometry. The response at the repeated doublets becomes coupled forward and backward traveling waves, but that at the split doublets becomes coupled forward traveling, standing and backward traveling waves. The results indicate that the tooth mass instead of the stiffness decreases the vibration amplitude and driving speed of the dominant wave, but their effects are different at the repeated and split doublets. Inspection of the model implies that the distortion can be suppressed by using a suitable combination of the wavenumber, tooth count, tooth height and occupying fraction. Numerical calculations are carried out to demonstrate the tooth geometry effect on the transient waveform, driving speed and elliptical trace. The optimization of the tooth geometry that can help achieve a purer traveling wave is discussed.     

Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis

A novel technique, based on low intensity ultrasonic wave propagation, has been applied to investigate the gelation of a waxy crude oil, caused by the crystallization of paraffin fractions as the temperature reduces below a threshold value, called WAT (wax appearance temperature). Because this phenomenon significantly affects the rheological behavior of crude oils, the knowledge of the conditions under which it occurs, during oil storage and/or transportation, is a topical issue in the oil industry. In this work, an ultrasonic equipment has been set up, able to propagate longitudinal waves in the MHz range and to display in real time the behavior of ultrasonic velocity and attenuation when the crude oil sample is subjected to heating and cooling cycles. When the ultrasonic probes alternatively rotate as parallel plates of a conventional rheometer, low intensity longitudinal waves (in the megahertz range), and shear oscillations (in the hertz range) are simultaneously applied on the sample, thus widely broadening the frequency range of investigation. On cooling, the crystallization of paraffin fractions and the consequent formation of a network structure in the oil matrix are responsible of the development and growth of the crude oil elastic response, which becomes dominant over the viscous response. This process can be reliably detected by dynamic mechanical analysis and by ultrasonic analysis through the increase of the storage modulus G′ and longitudinal velocity, respectively. The growth and further association between wax crystals causes a dissipation of acoustic energy, which is indicated by the increase of the wave attenuation. The combination of rheological and ultrasonic methods has provided a better insight both on the gel transition of crude oils and the viscoelastic behavior of gelled samples. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol–gel transition in waxy crude oils. Finally, the effect of ultrasonic waves with different intensity on the gel build-up has been also evaluated. A reduction of the gel strength with increasing wave intensity has been observed and the recovery of elastic response after removing ultrasonic irradiation has been monitored. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Numerical simulation of cavitation effects behind obstacles and in an automotive fuel jet pump

By means of computational fluid dynamics (CFD) this study examines cavitation effects behind obstacles and within an automotive fuel jet pump. Especially with regard to gasoline such effects are serious issues for applications of jet pumps in automotive fuel systems. The cavitation phenomena are captured by a model based on a void region approach within the volume-of-fluid method (VOF) including the k--model of turbulence. A first-order and a second-order scheme are compared, and the potential of the numerical method is evaluated by considering benchmark cases.     

Noncontact measurement of internal temperature distribution in a solid material using ultrasonic computed tomography

Numerical simulations and experiments have been carried out for a noncontact measurement of the internal temperature distribution in a solid material using ultrasonic computed tomography (CT). The method is based on the fact that the sound propagation velocity in a material depends on its temperature as well as its density and structure. From the numerical simulations, the convolution method is found to be an effective algorithm for the reconstruction of the sound velocity distribution. To obtain an accurate temperature distribution, it is found to be necessary to measure the sound propagation time with a resolution of 1 ns. In the experiments, the temperature distributions are measured in an agar-gel cylinder of 40 mm in diameter, along the center axis of which a platinum wire with 0.1 mm in diameter is located. By comparing the experimental results with the theoretical ones, the temperature distribution inside the agar-gel can be reconstructed with an error of 0.1 K, except for the region close to the platinum heater wire where temperature gradient is high. Further, the effects of an obstacle to the sound propagation, such as an acrylic resin cylinder inside the agar-gel, are investigated. Although the obstacles causes a part of projection to be missed, by using a linear-interpolation method to compensate for the incomplete projection, the temperature distribution can be reconstructed well but with a little larger error of 0.2 K, except for the regions close to the platinum heater wire and obstacle.