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The collapse of the cavitation bubble near the rigid wall emits shock waves and creates micro-jet, causing cavitation damage and operation instability of the hydraulic machinery. In this paper, the millimeter-scale bubble near the rigid wall was investigated experimentally and numerically with the help of a laser photogrammetry system with nanosecond-micron space-time resolution and the open source package OpenFOAM-2212. The morphological characteristics of the bubble during its growth phase, collapse phase and rebound phase were observed by experiment and numerical simulation, and characteristics of the accompanying phenomena including the shock wave propagation and micro-jet evolution were well elucidated. The numerical results agree well with the experimental data. The bubble starts from a tiny small size with high internal pressure and expands into a sphere with a radius of 1.07 mm for γ = d / Rmax = 1.78. The bubble collapses into a heart shape and moves towards to the rigid wall during its collapse phase, resulting in a higher pressure load for the rigid wall in the second collapse. The maximum pressure of the shock wave of the first bubble collapse phase reaches 5.4 MPa, and the velocity of the micro-jet reaches approximately 100 m/s. This study enriches the existing experimental and numerical results of the dynamics of the near-wall cavitation bubble. [ABSTRACT FROM AUTHOR]

Cavitation jet drilling has been extensively employed for the exploitation of geo-energy resources. The dynamics of cavitation bubbles in close proximity to the solid boundary have been a subject of great interest during jet drilling, as they play a crucial role in determining the cavitation performance. In present work, the dynamics of a single cavitation bubble near a solid surface is numerically investigated by using the axisymmetric Navier-Stokes equations and the volume of fluid method with considering the surface tension of gas-liquid interface, liquid viscosity and compressibility of gas in bubble. The simulated profiles are qualitatively and quantitatively consistent with the experimental images, which proves the reliability of employed numerical model. The effects of stand-off distance on the bubble profiles, bubble volume and collapse time have been analysed. Moreover, the cavitation erosion patterns towards the solid wall are also revealed for different dimensionless standoff distances. The simulation results reveal two distinct collapse patterns for the bubble profiles. The solid wall significantly impedes the shrinkage rate of the bubble, resulting in the longest collapse time when the dimensionless stand-off distance is 1.0. Three erosion patterns of cavitation bubbles towards the solid wall are observed, with the shock wave and micro-jet both contributing significantly to the damage caused by cavitation erosion. The shock wave sweeps the wall resulting in circular corrosion pits with a severely eroded centre, while the micro jet penetrates the wall leading to small spot corrosion pits. [ABSTRACT FROM AUTHOR]

Non-equilibrium molecular dynamics simulations are performed to explore the evolution of one- and two-dimensional micro jets induced by square shock wave in single crystal Al with conical and sinusoidal defects. The particle source of micro jet is obtained using the forward and backward particle tracing techniques, and it is discovered that the structure of particle source in different parts of micro jet closely depends on the defect type, and the formation of micro jet is a layered aggregation process from the defect’s surficial layer to its inner layer. The jet head velocity in the conical cases initially experiences a sharp increase, then a rapid decline to be constant, and jet volume in both conical and sinusoidal cases undergoes a nonlinear increase before remaining unaltered. A further analysis on the relationship among particle count, density, velocity, and temperature is performed, where the ejecta velocity along the shock direction fundamentally shows a linear variation and the maximum velocity is in the jet head in the conical cases and sinusoidal case of shock velocity up = 6 km/s, but the jet velocity presents a fluctuant variation and the head velocity lowers than that in the post-head in the sinusoidal case of up = 2 km/s. More importantly, it is revealed that two mechanisms, the necking for the one-dimensional jet and void growth and coalescence for the two-dimensional jet, dominate the jet breakup. In fact, velocity, temperature and pressure perturbations all contribute the jet breakup, with the velocity perturbation having the most important influence. Furthermore, the thickening of void border is due to the velocity gradient, which produces an increase in density in the jet head. [ABSTRACT FROM AUTHOR]

The magnetorheological jet polishing (MJP) technique, as a revolutionary flexible contact polishing approach, is exceptionally suitable for the smooth and ultra-smooth machining of functional surfaces with tiny or microstructures due to the following essential advantages. Machine tool accuracy is not required and there is nearly no tool wear in addition to high polishing efficiency, minimal surface damage, great surface shape adaptation, superior material removal process selectivity, and so on. This work examines the machining mechanism, the development of machining devices, and the optimization of the process parameters in MJP. This review also covers the MJP technique's existing limitations and opportunities. [ABSTRACT FROM AUTHOR]

The magnetorheological jet polishing (MJP) technique, as a revolutionary flexible contact polishing approach, is exceptionally suitable for the smooth and ultra-smooth machining of functional surfaces with tiny or microstructures due to the following essential advantages. Machine tool accuracy is not required and there is nearly no tool wear in addition to high polishing efficiency, minimal surface damage, great surface shape adaptation, superior material removal process selectivity, and so on. This work examines the machining mechanism, the development of machining devices, and the optimization of the process parameters in MJP. This review also covers the MJP technique’s existing limitations and opportunities.

In recent years, the piezoelectric jet and atomization devices have exhibited tremendous advantages including their simple construction, and the fact that they are discreet and portable as well as low cost. They have been widely used in cell printing, spray cooling, drug delivery, and other industry fields. First, in this paper, two different concepts of jet and atomization are defined, respectively. Secondly, based on these two concepts, the piezoelectric jet and atomization devices can be divided into two different categories: piezoelectric micro jet device and piezoelectric atomization device. According to the organizational structure, piezoelectric micro jet devices can be classified into four different models: bend mode, push mode, squeeze mode, and shear mode. In addition, their development history and structural characteristics are summarized, respectively. According to the location of applied energy, there are two kinds of piezoelectric atomization devices, i.e., the static mesh atomization device and the vibration mesh atomization device, and both their advantages and drawbacks are discussed. The research achievements are summarized in three aspects of cell printing, spray cooling, and drug delivery. Finally, the future development trends of piezoelectric jet and atomization devices are prospected and forecasted. [ABSTRACT FROM AUTHOR]

Experiments were carried out to control an incident shock-induced separation associated with 22° shock generator in a Mach 3.5 flow using an array of steady micro-jet actuators. Four micro-jet actuator configurations based on the variation in their pitch angle (β), skew angle (α) and span-wise spacing were used. Each of these configurations were placed 14δ upstream of the interaction and operated with injection pressures (Poj) varying from 140 to 643 kPa. While no major variations in separation characteristics were observed for Poj < 140 kPa, significant modifications were observed beyond Poj of 140 kPa and until 208.5 kPa. Amongst all the four control configurations, micro-jet vortex generator 2 (α=180∘;β=45∘) showed the best control with a 2δ downstream shift in separation point location relative to no-control. The shift is also accompanied with a change in maximum zero-crossing frequency towards higher frequency (almost twice), a reduction in the intermittency length and an increase in the correlation value between the boundary layer just upstream of the interaction and the intermittent region. These results indicate that the effectiveness of micro-jet vortex generator 2 is probably due to the improved entrainment levels in the shear layer induced by the micro-vortices which are generated downstream of these devices. The increase of the skew angle (α) from 180° to 270° for the same pitch angle of β = 45° (micro-jet vortex generator 3) seems to have no major impact on the separation characteristics. The reduction in the span-wise spacing (micro-jet vortex generator 4) resulted in deterioration of the flow field due to the jet-to-jet interaction with increasing injection pressures. [ABSTRACT FROM AUTHOR]

In recent years, the piezoelectric jet and atomization devices have exhibited tremendous advantages including their simple construction, and the fact that they are discreet and portable as well as low cost. They have been widely used in cell printing, spray cooling, drug delivery, and other industry fields. First, in this paper, two different concepts of jet and atomization are defined, respectively. Secondly, based on these two concepts, the piezoelectric jet and atomization devices can be divided into two different categories: piezoelectric micro jet device and piezoelectric atomization device. According to the organizational structure, piezoelectric micro jet devices can be classified into four different models: bend mode, push mode, squeeze mode, and shear mode. In addition, their development history and structural characteristics are summarized, respectively. According to the location of applied energy, there are two kinds of piezoelectric atomization devices, i.e., the static mesh atomization device and the vibration mesh atomization device, and both their advantages and drawbacks are discussed. The research achievements are summarized in three aspects of cell printing, spray cooling, and drug delivery. Finally, the future development trends of piezoelectric jet and atomization devices are prospected and forecasted.

The aim of this paper is to demonstrate the possibly of using the cavitation phenomenon, primarily a cavitating water jet, for the controlled surface modification of metallic biomaterials. Stainless steel 316 (an austenitic face centered cubic metal) was subjected to high-speed submerged cavitating jets under certain working conditions, and different exposure times. The force generated by the collapse of cavitation bubbles is used to modify the surface topography on micro- and nano levels. Optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), and interferometric profiling were used to characterize the surface morphology before and after cavitation treatment. The results confirm that at short exposure times, the observed characteristic features in the microstructure – holes and hills without material loss and wavy configuration – can be related to plastic deformation, while longer exposure times lead to erosion accompanied by material loss. The results related to the different stages of cavitation damage demonstrate the possibility to use cavitation as a micro-nano fabrication method for the modification of biomaterial surfaces, e.g. for the controlled and convenient increase of surface roughness. [ABSTRACT FROM AUTHOR]

Hollow metal nanoparticles were produced from a low-melting point alloy, by preventing oxidation of molten metal during fabrication. Molten metal droplets were generated in the water above it by disintegrating bubbles in the molten metal, resulting in the fabrication of metal nanoparticles, accompanied by cooling of the water phase. The hollow structure of the metal nanoparticles was theorized to have occurred due to the Kirkendall effect during oxidation. With the method of applying ultrasonically generated gas-microbubbles, solid/hollow particles were selectively generated by changing the gas supply from oxidizing to non-oxidizing. [ABSTRACT FROM AUTHOR]

An in situ synchrotron radiographic study of a molten Al–10 wt% Cu alloy under the influence of an external ultrasonic field was carried out using the Diamond-Manchester Branchline pink X-ray imaging at the Diamond Light Source in UK. A bespoke test rig was used, consisting of an acoustic transducer with a titanium sonotrode coupled with a PID-controlled resistance furnace. An ultrasonic frequency of 30 kHz, with a peak to peak amplitude at 140 microns, was used, producing a pressure output of 16.9 MPa at the radiation surface of the 1-mm diameter sonotrode. This allowed quantification of not only the cavitation bubble formation and collapse, but there was also evidence of the previously hypothesised ultrasonic capillary effect (UCE), providing the first direct observations of this phenomenon in a molten metallic alloy. This was achieved by quantifying the re-filling of a pre-existing groove in the shape of a tube (which acted as a micro-capillary channel) formed by the oxide envelope of the liquid sample. Analytical solutions of the flow suggest that the filling process, which took place in very small timescales, was related to micro-jetting from the collapsing cavitation bubbles. In addition, a secondary mechanism of liquid penetration through the groove, which is related with the density distribution of the oxides inside the groove, and practically to the filtration of aluminium melt from oxides, was revealed. The observation of the almost instantaneous re-filling of a micro-capillary channel with the metallic melt supports the hypothesised sono-capillary effect in technologically important liquids other than water, like metallic alloys with substantially higher surface tension and density. [ABSTRACT FROM AUTHOR]

The aim of this paper is to establish a possible application of the cavitation phenomenon as an efficient method to modify surface properties at the nano and micro levels. Commercial-purity copper was subjected to high submerged cavitating jets under different initial conditions, for time periods between 15 and 1,800 s. The force generated by jet cavitation is employed to modify the surface roughness in the order of nano and micro scales. The target surface was analyzed with optical as well as scanning probe electron microscopy. The results showed the possibility to use cavitation bubbles to establish a nanofabrication method for the surface preparation, shoot-less surface peening (nano/micro level). Also, the cavitation is assumed to be one of the miniaturized testing methods that have to be developed to reliably measure mechanical properties in small dimensions and to identify the behavior caused by the size dependence. With optical, SEM, and atomic force microscopy observation techniques in this study, the deformation mechanism and the formation of planar or wavy slip were also studied. The results indicate that even at short exposure times, observed roughness having a characteristic 'serpentine' configuration can be related to the start of the plastic deformation of the specimen surface. Longer exposure times inevitably result in a greater number of jet-specimen interactions leading to specimen fracture. [ABSTRACT FROM AUTHOR]

This paper describes the demonstration of successful fabrication and initial characterization of micromachined pressure sensors and micromachined jets (microjets) fabricated for use in macro flow control and other applications. In this work, the microfabrication technology was investigated to create a micromachined fluidic control system with a goal of application in practical fluids problems, such as UAV (Unmanned Aerial Vehicle) -scale aerodynamic control. Approaches of this work include: (1) the development of suitable micromachined synthetic jets (microjets) as actuators, which obviate the need to physically extend micromachined structures into an external flow; and (2) a non-silicon alternative micromachining fabrication technology based on metallic substrates and lamination (in addition to traditional MEMS technologies) which will allow the realization of larger scale, more robust structures and larger array active areas for fluidic systems. As an initial study, an array of MEMS pressure sensors and an array of MEMS modulators for orifice-based control of microjets have been fabricated, and characterized. Both pressure sensors and modulators have been built using stainless steel as a substrate and a combination of lamination and traditional micromachining processes as fabrication technologies. [ABSTRACT FROM AUTHOR]

One of the most feasible ways to measure duct airflows is by tracer gas techniques, especially for complex situations when the duct lengths are short as well as their access, which makes extremely difficult or impossible other methods to be implemented. One problem associated with the implementation of tracer gas technique when the ducts lengths are short is due to the impossibility of achieving complete mixing of the tracer with airflow and its sampling. In this work, the development of a new device for the injection of tracer gas in ducts is discussed as well as a new tracer-sampling device. The developed injection device has a compact tubular shape, with magnetic fixation to be easy to apply in duct walls. An array of sonic micro jets in counter current direction, with the possibility of angular movement according to its main axle ensures a complete mixing of the tracer in very short distances. The tracer-sampling device, with a very effective integration function, feeds the sampling system for analysis. Both devices were tested in a wind tunnel of approximately 21 m total length. The tests distances between injection and integration device considered were: X/Dh = 22; X/Dh = 4; X/Dh = 2; and X/Dh = 1. For very short distances of X/Dh = 2 and X/Dh = 1, semi-empirical expressions were needed. A good reproducibility of airflow rate values was obtained. These preliminary tests showed that the practical implementation of tracer gas techniques in HVAC systems for measuring airflow rates with a very short mixing distance is possible with the devices developed. [Copyright &y& Elsevier]