Onset Of Cavitation Research Articles

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized ‘bubble collapse’ region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb’s Free Energy method using simulated collapse conditions. OHG values as high as 1x1012OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.

Cavitation modelling using real gas state equation: A conceptual study

The prediction of the onset of cavitation to allow investigations of the flow in off-design conditions during design is unavoidable for a variety of applications. Numerical modeling of the cavitation phenomena is however computationally challenging due to the nature of the phenomena. High density ratios, steep pressure gradients, the presence of multiple phase including phase change and hence to occurrence of real gas thermophysical behavior would cause a computational effort unmanageable during design. Today’s models are therefore greatly simplified representations of the reality. Conservation equations are usually solved using a mixture approach with flow properties being a volume fraction weighted average of two incompressible fluids thus neglecting real gas effects. The source term from the phase exchange however usually poses significant stability problems. In this article we will therefore present an approach for cavitation modeling using high-order polynomial real-gas state equations to accurately predict the properties of water at any state. Computational performance is improved by the use of structured tabulation using high order interpolation methods. The numerical results are then compared against measurement data and conventional cavitation model results.

A CFD-Based Comparison of Different Positive Displacement Pumps for Application in Future Automatic Transmission Systems

The efficiency requirements for hydraulic pumps applied in automatic transmissions in future generations of automobiles will increase continuously. In addition, the pumps must be able to cope with multiphase flows to a certain extent. Given this background, a balanced vane pump (BVP), an internal gear pump (IGP) and a three-dimensional geared tumbling multi chamber (TMC) pump are analyzed and compared by a computational fluid dynamics (CFD) approach with ANSYS CFX and TwinMesh. Furthermore, test bench measurements are conducted to obtain experimental data to validate the numerical results. The obtained numerical results show a reasonable agreement with the experimental data. In the first CFD setup, the conveying characteristics of the pumps with pure oil regarding volumetric efficiencies, cavitation onset and pressure ripple are compared. Both the IGP and the BVP show high volumetric efficiencies and low pressure ripples whereas the TMC shows a weaker performance regarding these objectives. In the second CFD setup, an oil-bubbly air multiphase flow with different inlet volume fractions (IGVF) is investigated. It can be shown that free air changes the pumping characteristics significantly by increasing pressure and mass flow ripple and diminishing the volumetric efficiency as well as the required driving torque. The compression ratios of the pumps appear to be an important parameter that determines how the multiphase flow is handled regarding pressure and mass flow ripple. Overall, the BVP and the IGP show both a similar strong performance with and without free air. In the current development state, the TMC pump shows an inferior performance because of its lower compression ratio and therefore needs further optimization.

A note on the cavitation phenomenon in metallic plates perforated by sharp-nosed rigid projectiles

We explore the perforation process of metallic plates impacted by rigid sharp-nosed projectiles at high velocities. In particular, we are looking at the diameters of the penetration hole in the plates through a series of 2D numerical simulations, in order to check for the occurrence of cavitation in finite-thickness plates. This phenomenon has not been observed by previous workers and we were looking for its effect on the perforation process. Our simulations show that for every projectile/plate pair there is a certain impact velocity which marks the onset of cavitation. These threshold velocities depend on the normalized thickness of the plates, as well as on their effective strength. Our simulations are supported by the results from perforation tests on plates made of a low strength lead-antimony alloy. The main conclusion from our work is that analytical models for plate perforation should take into account the cavitation phenomenon, especially for high velocity impacts.

An Analysis of Acoustic Cavitation Thresholds of Water Based on the Incubation Time Criterion Approach

Researchers are still working on the development of models that facilitate the accurate estimation of acoustic cavitation threshold. In this paper, we have analyzed the possibility of using the incubation time criterion to calculate the threshold of the onset of acoustic cavitation depending on the ultrasound frequency, hydrostatic pressure, and temperature of a liquid. This criterion has been successfully used by earlier studies to calculate the dynamic strength of solids and has recently been proposed in an adapted version for calculating the cavitation threshold. The analysis is carried out for various experimental data for water presented in the literature. Although the criterion assumes the use of macroparameters of a liquid, we also considered the possibility of taking into account the size of cavitation nuclei and its influence on the calculation result. We compared the results of cavitation threshold calculations done using the incubation time criterion of cavitation and the classical nucleation theory. Our results showed that the incubation time criterion more qualitatively models the results of experiments using only three parameters of the liquid. We then discussed a possible relationship between the parameters of the two approaches. The results of our study showed that the criterion under consideration has a good potential and can be conveniently used for applications where there are special requirements for ultrasound parameters, maximum negative pressure, and liquid temperature.

Validation of a three-phase Eulerian CFD model to account for cavitation and spray atomization phenomena

Cavitation phase change phenomenon appears in many engineering applications, often eroding and damaging surfaces, so deteriorating the performance of devices. Therefore, it is a phenomenon of great interest for the research and industry communities. In this work, three different cavitation models, the Homogeneous Relaxation Model (HRM), the Schnerr and Sauer, and the Kunz, are implemented in a Eulerian multiphase homogeneous flow Computational Fluid Dynamics (CFD) solver previously developed for simulating fully atomized sprays. The improved solver can be used then to study not only cases with cavitation, such a hydrofoil, but also situations where cavitation occurs together with liquid atomization, such as high pressure injection systems. Validation of this solver is carried out for three different cases under diverse operating conditions: a two-dimmensional throttle, a hydrofoil and a single-hole fuel injector. The Reynolds-Averaged Navier-Stokes (RANS) approach is employed for taking into account the turbulence effects. Simulation results are compared to experimental data available in the literature. Among the tested cavitation models, the HRM is the one that provides the best accuracy in the three validation cases. Nevertheless, the onset of cavitation and the area occupied by the vapor cavities are always underpredicted, by all cavitation models in all validation cases. This can be associated to the unsteady and turbulent nature of the cavitation phenomenon. Even so, the computational prediction of several parameters, such as mass flow rate through the nozzles or spray spreading angle, has an error below 5-10%, which proves the capabilities of the solver.

Analysis of Bulb Turbine Hydrofoil Cavitation

The influence of a bulb runner blade hydrofoil shape on flow characteristics around the blade was studied. Experimental work was performed on a bulb turbine measuring station and a single hydrofoil in a cavitating tunnel. In the cavitation tunnel, flow visualization was performed on the hydrofoil’s suction side. Cavitation structures were observed for several cavitation numbers. Cavitation was less intense on the modified hydrofoil than on the original hydrofoil, delaying the cavitation onset by several tenths in cavitation number. The results of the visualization in the cavitation tunnel show that modifying the existing hydrofoil design parameters played a key role in reducing the cavitation inception and development, as well as the size of the cavitation structures. A regression model was produced for cavitation cloud length. The results of the regression model show that cavitation length is dependent on Reynolds’s number and the cavitation number. The coefficients of determination for both the existing and modified hydrofoils were reasonably high, with R2 values above 0.95. The results of the cavitation length regression model also confirm that the modified hydrofoil exhibits improved the cavitation properties.

Cavitation Nucleation and Its Ductile-to-Brittle Shape Transition in Soft Gels Under Translational Mechanical Impact

Cavitation bubbles in the human body, when subjected to impact, are being increasingly considered as a possible brain injury mechanism. However, the onset of cavitation and its complex dynamics in biological materials remain unclear. Our experimental results using soft gels as a tissue simulant shows that the critical acceleration (acr) at cavitation nucleation monotonically increases with increasing stiffness of gelatin A/B, while acr for agarose and agar initially increases but followed by a plateau or even decrease after stiffness reach to ~100 kPa. Our image analyses of cavitation bubbles and theoretical work reveal that the observed trends in acr are directly linked to how bubbles grow in each gel. Gelatin A/B, regardless of their stiffness, form a localized damaged zone (tens of nanometers) at the gel-bubble interface during bubble growth. In contrary, the damaged zone in agar/agarose becomes significantly larger (> 100 times) with increasing shear modulus, which triggers the transition from formation of a small, damaged zone to activation of crack propagation.

Combined visualisation of cavitation and vortical structures in a real-size optical diesel injector

A high-speed flow visualisation set-up comprising of combined diffuse backlight illumination (DBI) and schlieren imaging has been developed to illustrate the highly transient, two-phase flow arising in a real-size optical fuel injector. The different illumination nature of the two techniques, diffuse and parallel light respectively, allows for the capturing of refractive-index gradients due to the presence of both interfaces and density gradients within the orifice. Hence, the onset of cavitation and secondary-flow motion within the sac and injector hole can be concurrently visualised. Experiments were conducted utilising a diesel injector fitted with a single-hole transparent tip (ECN spray D) at injection pressures of 700–900 bar and ambient pressures in the range of 1–20 bar. High-speed DBI images obtained at 100,000 fps revealed that the orifice, due to its tapered layout, is mildly cavitating with relatively constant cavity sheets arising mainly in regions of manufacturing imperfections. Nevertheless, schlieren images obtained at the same frame rate demonstrated that a multitude of vortices with short lifetimes arise at different scales in the sac and nozzle regions during the entire duration of the injection cycle but the vortices do not necessarily result in phase change. The magnitude and exact location of coherent vortical structures have a measurable influence on the dynamics of the spray emerging downstream the injector outlet, leading to distinct differences in the variation of its cone angle depending on the injection and ambient pressures examined.Graphic abstract

Evaluation of Pseudorandom Sonications for Reducing Cavitation With a Clinical Neurosurgery HIFU Device.

Transcranial high-intensity focused ultrasound is used in clinics for treating essential tremor (ET) and proposed for many other brain disorders. This promising treatment modality requires high energy resulting eventually in undesired cavitation and potential side effects. The goals of the present work were: 1) to evaluate the potential increase of the cavitation threshold using pseudorandom gated sonications and 2) to assess the heating capabilities with such sonications. The experiments were performed with the transcranial magnetic resonance (MR)-compatible ExAblate Neuro system (InSightec, Haifa, Israel) operating at a frequency of 670 kHz, either in continuous wave (CW) or with pseudorandom gated sonications of 50% duty cycle. Cavitation activity with the two types of sonications was compared using chemical dosimetry of hydroxyl radical production at the focus of the transducer, after propagation in water or through a human skull. Heating trials were performed in a hydrogel tissue-mimicking material embedded in a human skull to mimic a clinical situation. The temperature was measured by MR-thermometry when focusing at the geometrical focus and steering off focus up to 15 mm. Compared with CW sonications, the use of gated sonication did not affect the efficiency (60%) nor the steering abilities of the transducer. After propagation through a human skull, gated sonication required a higher pressure level (10 MPa) to initiate cavitation as compared with CW (5.8 MPa). Moreover, at equivalent acoustic power above the cavitation threshold, the level of cavitation activity initiated with gated sonications was much lower with gated sonication than with continuous sonications, almost half after propagation through water and one-third after propagation through a skull. This lowered cavitation activity may be attributed to a breaking of the dynamic of the bubbles moving from monochromatic to more broadband sonications and to the removal of residual cavitation nuclei between pulses with gated sonications. The heating capability was not affected by the gated sonications, and similar temperature increases were reached at focus with both types of sonications when sonicating at equivalent acoustic power, both in water or after propagation through a human skull (+15 °C at 325 W for 10 s). These data, acquired with a clinical system, suggest that gated sonication could be an alternative to continuous sonications when cavitation onset is an issue.

CFD Analyses of Textured Surfaces for Tribological Improvements in Hydraulic Pumps

In any hydraulic machine there are lubricated couplings that could become critical beyond certain operating conditions. This paper presents the simulation results concerning textured surfaces with the aim of improving the performance of lubricated couplings in relative motion. The texturing design requires much care to obtain good improvements, and it is essential to analyze both the geometric features of the dimples and the characteristics of the coupled surfaces, like the sliding velocity and gap height. For this purpose, several CFD simulations have been performed to study the behavior of the fluid bounded in the coupling, considering dimples with different shapes, size, and spatial distribution. The simulations consider the onset of gaseous cavitation to evaluate the influence of this phenomenon on the pressure distribution generated by the textured surface. The analyses have pointed out that it is critical to correctly predict the behavior of the textured surface in the presence of local cavitation, in fact, when cavitation occurs, the characteristic time of the transient in which the phase of the fluid change is very rapid and it is comparable to the time taken by the fluid to move from one dimple to the next.

Numerical simulation of the performance of a centrifugal pump with a semi-open impeller under normal and cavitating conditions

An important flow mechanism that can affect the performance and efficiency, as also the maintenance cost of centrifugal pumps is cavitation. Scientific research has been focusing on the mechanisms that govern cavitation in order to develop experimental and numerical tools that are able either to detect the phenomenon or to anticipate its appearance. In this study, a computational model is used in order to study the cavitation performance of a radial flow centrifugal pump with a semi open impeller. The numerical model includes and studies the effects of the blade tip clearance and its thickness, and it is validated against corresponding laboratory measurements and visualization data obtained for this pump under normal and cavitating flow conditions. The total head drop variation curves versus cavitation parameter are extracted both numerically and experimentally, and compared for various pump loading conditions with satisfactory agreement. A non-periodic pattern of flow and cavitation bubbles between the impeller blades, which is caused by the inlet pipe bending, is also well reproduced by the model. The numerical results display in detail the complex flow field and the secondary flows developed in the tip clearance area, close to the blades leading edge. The effects of the latter on the onset and development of cavitation at the suction side of the impeller, as well as on the backflow cavitation phenomenon are presented and analyzed. Backflow cavitation is found to affect the suction ability of the pump, especially for moderate cavitation conditions, as far as the pressure differences between the two blade sides remain significant.

Prediction of cavitation in orifice plates—A novel and simple rule-of-thumb

New experiments have been carried out on the generic case of the flow through orifice plates. The aim has been to validate empirical correlations used for the prediction of cavitation. The new accurate experimental data base shows that earlier proposed empirical correlations work well for the prediction of cavitation, e.g., Tullis (1993), Miller (2009), and Nilsson (2011). Using the present data together with earlier data from Tullis (1993), it has been shown that, at the onset of cavitation, the ratio between the downstream and the upstream pressure over the orifice plate is a simple linear function of the orifice plate diameter ratio, β = d/D. This has been shown to hold independent of flowrate, downstream pressure, orifice diameter (for 0.4 < β < 0.8, which corresponds to a pressure loss coefficient 1 < ξ < 100) and boundary conditions (e.g., upstream pipe bends), at moderate temperatures and as long as the pressure drop is large compared to the saturation pressure. A novel and simple rule-of-thumb for when the cavitation becomes a problem (i.e., in between critical cavitation and incipient damage) has been demonstrated to be when the ratio between the downstream and the upstream pressure over the orifice plate equals the orifice plate diameter ratio.

Cavitation onset caused by a dynamic pressure wave in liquid pipelines

When liquids flow in the pipelines, the onset of cavitation can be characterized by a variant of the Euler number known as the cavitation number (CN), which is based on the velocity and denoted by C in this paper. Conventionally, cavitation is considered to be induced when C ~ 1. However, experimental observations and several pipe bursts indicate that the CN may incorrectly predict the onset of cavitation. For example, when leakage occurs in the pipeline or a valve in the pipeline is opened, the resultant pressure loss generates a dynamic pressure wave with a small amplitude, which may lead to bubble formation, even though C ~ 1 is not satisfied. Hence, this paper proposes another CN based on the amplitude of the generated dynamic pressure wave, rather than the velocity, for ascertaining the onset of cavitation. The validity of the proposed CN was verified through experiments and a case study. The results indicated that the proposed CN can be effectively used for cavitation prediction induced by pressure fluctuations and for investigating phenomena such as pressure fluctuation, leakage, and corrosion in liquid pipelines, tanks, and pressure vessels, as well as the safety design of liquefied natural gas tanks and tankers.

Coupled distortional-dilatational failure mode concept for silicone adhesives

Nearly incompressible rubber-like materials, such as silicone adhesives, exhibit two failure modes: dilatational and distortional failure. The former corresponds to the formation and growth of cavities owing to triaxial deformation. The latter corresponds to fracture due to excessive isochoric deformation, i.e. deformation at constant volume. Both failure modes are typically assessed individually and corresponding failure criteria are often formulated in terms of stresses. However, stress-based descriptions of failure depend explicitly on the choice of constitutive law and the experimental measurement of critical stresses is challenging or even impossible. Therefore, recent studies proposed strain and stretch-based criteria for distortional failure of nearly incompressible hyperelastic materials, that can be obtained directly from optical measurements using digital image correlation.The present work exploits stretch-based descriptions of both failure modes to provide a unified coupled dilatational-distortional failure criterion which describes the complete failure surface of nearly incompressible silicone adhesives or rubber-like materials in general. For this purpose, we propose a stretch-based cavitation criterion which depends on a single parameter. Distortional failure is described by the Podgórski-criterion formulated in principal stretch space. The coupled dilatational-distortional failure criterion is validated using uniaxial tension, bulge and pancake tests of the nearly incompressible hyperelastic transparent structural silicone sealant DOWSIL™ TSSA. It is shown that the proposed failure criterion captures both the onset of cavitation and ultimate distortional failure in very good agreement with all experiments.

Simulation of Ultrasonic Induced Cavitation and Acoustic Streaming in Liquid and Solidifying Aluminum

Ultrasonic treatment (UST), more precisely, cavitation and acoustic streaming, of liquid light metal alloys is a very promising technology for achieving grain and structure refinement, and therefore, better mechanical properties. The possibility of predicting these process phenomena is an important requirement for understanding, implementing, and scaling this technology in the foundry industry. Using an established (casting) computational fluid dynamics (CFD)-simulation tool, we studied the ability of this software to calculate the onset and expansion of cavitation and acoustic streaming for the aluminum alloy A356, partly depending on different radiator geometries. A key aspect was a holistic approach toward pressure distribution, cavitation, and acoustic streaming prediction, and the possibility of two- and (more importantly) three-dimensional result outputs. Our feasibility analysis showed that the simulation tool is able to predict the mentioned effects and that the results obtained are in good agreement with the results and descriptions of previous investigations. Finally, capabilities and limitations as well as future challenges for further developments are discussed.

Psychoacoustic approach for cavitation detection in centrifugal pumps

Cavitation in a kinetic pump causes its hydraulic performance to deteriorate and damages the pump through pitting and material erosion. It can appear within the entire range of operating conditions, producing structure vibration and noise. The sound emitted can be utilized as a useful source of information. In this paper, cavitation detection during the kinetic pump’s operation is presented based on psychoacoustic metrics. Proper manipulation and processing of a sound signal enables the extraction of multiple signal descriptors which can determine the operation point of a centrifugal pump. The presented method can detect all three operating stages: no cavitation, the onset of cavitation and fully developed cavitation. Its most evident advantage over other noise emission methods is its independence from sound pressure levels, focusing only on the shape and characteristics of the waveform. This robustness makes its application suitable in an industrial environment.

Size-dependent inertial cavitation of soft materials

High-rate loading induced cavitation of soft materials, including brain and muscle, has increasingly been studied due to its importance in a variety of biomedical applications, such as potential impact-related traumatic brain injury and high-intensity focused ultrasound therapy. The size of cavitation nuclei or defects is crucial to the onset of cavitation. However, it remains challenging to introduce gaseous defects with controllable size into soft materials and to understand the underlying mechanism of impact-bubble interaction in soft materials. Here, we set up a drop-tower system to perform impact loading tests on gelatin samples with controllable monodispersed microbubbles, allowing us to mimic the general cavitation behavior of human tissues. The microbubbles are inserted into the soft matrix as cavitation nuclei by taking advantage of microfluidic flow focusing. A high-speed camera paired with a signal acquisition system is utilized to determine the critical pressure at which cavitation initiates for different defect sizes. Meanwhile, we propose a theoretical model based on surface tension to predict the time-dependent spherically symmetric cavitation of a pre-existing gas bubble in elastomers under high-speed loading. Our experiments and theoretical modeling demonstrate the strong effect of gas-filled defects on cavitation and indicate the necessity and significance of controlling defect size in mimicking targeted organs using soft gels. Our work provides a promising route to predict cavitation-induced injury in tissues with variable materials stiffness and initial defect size.

Effect of targeted gold nanoparticles size on acoustic cavitation: An in vitro study on melanoma cells

When a liquid is irradiated with high intensities of ultrasound irradiation, acoustic cavitation occurs. Since cavitation can be fatal to cells, it is utilized to destroy cancer tumors. Considering cavitation onset and bubbles collapse, the required ultrasonic intensity threshold can be significantly decreased in the presence of nanoparticles in a liquid. The effects of gold nanoparticles size on acoustic cavitation were investigated in this in vitro study. For this purpose, ultrasonic waves were used at intensities of 0.5, 1 and 2 W/cm2 and frequency of 1 MHz in the presence of F-Cys-GNPs with 15, 23 and 79 nm sizes and different concentrations (0.2, 1 and 5 µg/ml) in order to determine their effects on the viability of melanoma cells. This was performed at different incubation times 12, 24 and 36 h. The viability of melanoma cells decreased at higher concentrations and sizes of F-Cys-GNPs. The lowest viability of melanoma cells was seen in those containing 79, 23, and 15 nm F-Cys-GNPs. This finding could be explained from the concept that the nucleation sites on the surface of GNPs increase with an increase in size of GNPs, which results in an increase in the number of cavitation bubbles. Acoustic cavitation in the presence of gold nanoparticles can be used as a way for improving therapeutic effects on the tumors.

Electrostrictive Cavitation in Water Induced by a SnO2 Nanoparticle.

Cavitation phenomenon in dielectric fluids has been a recent topic of interest in theory and experiment. We study a dielectric fluid–nanoparticle system subjected to an external electric field using molecular dynamics simulations. Electric fields ranging from 0.042 to 0.25 V/Å are applied to a water and tin dioxide system. Cavitation is observed in simulations with both SPC/E water and the hydrogen bonding polarizable model. The cavitation onset time displays a stretched exponential relaxation response with respect to the applied electric field with an exponent β = 0.423 ± 0.08. This is in accordance with the exact theoretical value for systems with long-ranged forces. Cavity growth rates are divided into two phases, a spherical growth phase and a cylindrical one. Both are reported as a function of the applied electric field. The structure of the electric field is analyzed both spatially and temporally.