Cavitation Behavior Research Articles

Effect of ultrasonic frequency on cavitation behavior, microstructure and mechanical properties of AZ80 magnesium alloy

In this paper, the effects of single-frequency ultrasonic treatment (SUT) and dual-frequency ultrasonic treatment (DUT) on the microstructure and mechanical properties of AZ80 magnesium alloy were investigated by numerical simulation and experimental research. Numerical simulation shows that the effective cavitation area of 15 kHz SUT is about twice that of 20 kHz, while the cavitation area of the 15 + 20 kHz DUT is larger than the sum of two SUT. The experimental results find that grains are refined after ultrasonic treatment, while the refinement effects of SUT and DUT are different. Under the same power condition, the refinement effect of 15 kHz is better than 20 kHz, but the DUT has higher efficiency than both SUT. The average grain size of untreated ingot is 174 μm, which is reduced to 123 μm, 99 μm and 80 μm after 20 kHz SUT, 15 kHz SUT and 15 + 20 kHz DUT, respectively. Therefore, applying DUT is more efficient in refining the grain size of the AZ80 magnesium alloy.

The obestatin/G-protein coupled receptor 39 (GPR39) system in pancreatic cancer: proliferation studies in pancreatic cancer cell lines

The objective of this paper is to investigate the cavitation evolution and excited pressure fluctuation in unsteady sheet/cloud cavitating flows in a convergent–divergent channel using a combination of experimental and computational studies. In the experiments, a simultaneous sampling technique is used to synchronize the cavitation images with wall-pressure signals. The computations are based on the Navier–Stokes equations solved with a mass transfer cavitation model and a modified RNG k–ε turbulence model. The experimental results indicate that, for non-cavitating flow, neither significant fluctuation nor periodic fluctuation can be observed. However, in the case of sheet/cloud cavitating conditions, significant quasi-periodic pressure fluctuations are captured in the entire cavitation area. The numerically predicted unsteady sheet/cloud cavitation behaviors agree well with the experimental results; both the experimental and computational results indicate that a typical quasi-periodic sheet/cloud cavitation development process can be divided into three main stages: (1) the growth of the attached cavity, (2) shedding of the attached cavity, and (3) growth and collapse of the detached cavities. The quasi-periodic pressure fluctuations in sheet/cloud cavitation are found to be highly related with the time evolution of the cavitation behaviors. The acceleration due to the changes in the cavity volume is found to be the main source of the excited pressure fluctuations. The pressure signal decreases in the middle of the growing process of the attached cavity and the detached cavity. Next, the pressure signal increases and reaches a peak with the collapse of large detached cavity near transducer 4#.

Piston-ring performance: limitations from cavitation and friction

PurposeCavitation in piston-ring lubrication is studied as part of the performance of piston-ring assemblies. Cavitation degrades performance in engineering applications and its effect is that it alters the oil film pressure, generated at the converging-diverging wedge of the interface. Studies tried to shed light to the phenomenon of cavitation and compare it with cavities that have been identified in bearings. The paper aims to discuss this issue.Design/methodology/approachLubricant formulations were used for parametric study of oil film thickness (OFT) and friction providing the OFT throughout the stroke and LIF for OFT point measurements. Lubricant formulation affects cavitation appearance and behaviour when fully developed.FindingsCavitation affects the ring load carrying capacity. Different forms of cavitation were identified and their shape and size (length and width) is dictated from reciprocating speed and viscosity of the lubricant. A clear picture is given from both techniques and friction results give quantifiable data in terms of the effect in wear and cavitation, depending on the lubricant properties.Research limitations/implicationsEngine results are limited due to manufacturing difficulties of visualisation windows and oil starvation. Therefore, full stroke length sized windows were not an option and motoring tests were implemented due to materials limitations (adhesive and quartz windows). Lubricant manufacturer has to give data regarding the chemistry of the lubricants.Originality/valueThe contribution of cavitation in piston-ring lubrication OFT, friction measurements and lubricant parameters that try to shed light to the different forms of cavitation. A link between viscosity, cavitation, shear thinning properties, OFT and friction is given.

Wavelet analysis and frequency spectrum of cloud cavitation around a sphere

In this paper, wavelet analysis of the cavitating flow over a sphere is reported. Unsteady and dynamic behaviors of cavitation were captured using large eddy simulation (LES) turbulence approach and Sauer mass transfer models. Numerical simulation is implemented under the framework of OpenFOAM within the interPhaseChangeFoam solver. The simulation is conducted over a wide range of cavitation numbers. Two more essential variations, pressures and kinetic energies, were considered at specific points in front and behind of the sphere's body for sufficient simulation period. The oscillations global frequency modes and spectral content of the cavity cloud are computed and analyzed using Fourier and continuous wavelet transformations. The computed results show that the flow fluctuations enhance by increasing the cavitation number. The low-frequency fluctuations play a pivotal role in the cavitating flow and possess almost the same magnitude in all investigated cavitation numbers. The frequencies enhance as the simulation time increases in all cases. One of the primary frequencies that happened in all cavitation numbers in the cavity cloud separation is due to a Strouhal number within the range of 0.046 and 0.05. Therefore, this Strouhal number can be used for the purpose of cloud cavitation detection.

Study on Corrosion and Cavitation Behaviors of NiTi Alloy Coated on Steel

The aim of the present paper is to investigate the corrosion and cavitation behaviors of NiTi alloy sample. NiTi alloy coated high speed steel (HSS) with 2 mm thickness layer has a good corrosion resistance in artificial seawater used as an electrolyte solution. During the tests, a film on the surface of sample was formed, which protects from the corrosion as compared with the two other sample such as HSS and NiTi alloy coated HSS with 1.2 mm thick layer, it was concluded that NiTi alloy coating with 2 mm thick exhibits excellent corrosion resistance in the artificial solution at room temperature. From SEM analysis it is confirmed that the oxide film was formed on NiTi alloy coatings with 2 mm thickly of sample. In artificial sea water, the components of the oxide film are Ti2 and Ni (OH).

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

ABSTRACTCavitation behaviour has been investigated in an Al–Zn–Mg–Cu alloy with an average grain size of 10 µm during superplastic deformation. The superplastic tensile tests were interrupted at different true strains at 530°C and 3 × 10−4 s−1. The results showed that cavity nucleation occurred above a critical strain in the optimum loading condition. It was easy for cavities to form at the triple junction due to the stress concentration caused by cooperative grain boundary sliding. Since the tensile stress was higher in the middle of the sample, the cavities were arranged in a straight line parallel to the tensile axis in the centre of the sample. A more appropriate cavity growth equation considering the critical strain was proposed to describe the cavitation behaviour.

First observation and characterization of vortex flow in steel micronozzles for high-pressure diesel injection

Vortex flows can be observed in many industrial systems. One of the most intricate vortex flows is the one formed in high-pressure fuel injector nozzles, which dynamics and influence on initial jet formation have been veiled due to great difficulties in observation of micro-scale fluids enclosed with steel nozzles. In this study, an X-ray tracer imaging method was used to visualize the transient fluid motion in steel micronozzles for the high-pressure diesel injection. The 1 μm-size tungsten particles were added to the diesel so that well-contrasted X-ray images of in-nozzle flows can be recorded with high-energy polychromatic X-ray beams. The X-ray images revealed, for the first time, the transient behavior of vortex flow, vortex-type cavitation and flow separation inside the steel nozzle. The needle-lift dependence of in-nozzle vortex flow dynamics was then modeled and correlated to those observed in large-scale optical nozzles in previous studies. The results showed that the in-nozzle vortex and emerging jet flow characteristics of the practical nozzle were similar with those of large-scale nozzles, which suggested that the results of previous fundamental studies can be closely linked to practical cases.

Numerical Analysis on the Cavitation Performance of a Seawater Cooling Pump

In this study, a centrifugal seawater cooling pump was analyzed to investigate its cavitation behavior over different operating flow rates. 3D two-phase simulations were carried out with ANSYS-CFX commercial code. The k-e turbulence and Rayleigh-Plesset cavitation models were employed in the simulations. A head drop characteristics curves for three discharge rates was built based on numerical predictions. At higher flow rates, the impeller was more vulnerable to bubble cavitation. The 3 % head drop points of the pump working at 0.7Q, Q, and 1.3Q (Q: design flow rate) corresponded with NPSHa 1.21 m, 1.83 m, and 3.45 m, respectively. The volume of vapor bubbles was estimated and cavitation locations were anticipated to visualize the development of the cavity within the impeller. Moreover, the distribution of pressure coefficient and a blade loading chart are specifically presented, bringing out the harmful impacts of cavitation on the pump operation.

Stabilization of cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs)

Unsteady cloud cavitation phenomenon is an important subject due to its undesirable effects in various applications such as ship propeller, rudder and hydraulic machinery systems. We present an efficient passive control method to control the cavitation instabilities which may be caused by the shedding of cavity structures in the vicinity of the solid surface of an immersible body. We proposed a passive control method so called Cylindrical Cavitating-bubble Generators (CCGs) on the surface of a benchmark hydrofoil and analyzed the effects of this passive controller on the dynamics of the unsteady cloud cavitation. First we modeled the unsteady cavitating flow around the hydrofoil without CCGs using a hybrid URANS model which was implemented in an open source code. Next, we studied the effect of CCGs on the mechanism of the unsteady cloud cavitation. The results show that using this method, the unsteady cavity structure was changed to a quasi-stable cavity structure compared with the cloud cavity shedding in the case without CCGs. We observed that the instability behavior of the unsteady cloud cavitation was mitigated and only small-scale cavity may be shedded from the hydrofoil in the free stream flow away from the hydrofoil surface. A notable reduction in cavitation-induced vibration and high wall-pressure peaks on the solid surface was observed.

Effect of various winglets on the performance of marine propeller

The tip vortex cavitation (TVC) is an issue of increasing interest, because the TVC plays an important role in propeller radiated noise and cavitation erosion. The marine propeller with winglets, which is inspired by the winglets of airfoil, is numerically investigated in the present paper. The blade tip of newly designed propeller tilts toward the pressure side. The difference between six propellers is the change of the rake angle at r/R = 1.0. The pressure coefficient, TVC, axial velocity field and helicity are analyzed. The numerical results show that the winglets of newly designed propeller scarcely affect the efficiency of propeller. The thrust coefficient gradually decreases with the increase in rake angle. As for the suction side, the pressure coefficient (Cp) of winglets propellers is higher than the conventional propeller in general. In addition, the winglets are beneficial to generate less cavitation behavior when the rake angle is small. However, as the rake angle is further increased, the cavitation behavior of winglets propeller is also increased, even larger than the conventional propeller. Therefore, it can be deduced that the winglets can be used to effectively improve the TVC characteristics to some extent.

Influence of draft tube water injection system on cavitation behaviour in a full-scale Francis turbine with visual access

This paper describes the results of a comprehensive and ongoing project to control unsteady fluid-dynamic behaviour and associated effects at Svorka hydro power plant (25 MW, 260 m, 600 rpm), operated by Statkraft in Norway. This Francis unit had an original air admission system installed on the draft tube wall and turbine shaft. However, these systems did not perform satisfactorily to prevent high intensity pressure fluctuations due to vortex rope at part load. To mitigate them, a complementary draft tube water injection system, designed by Flow Design Bureau AS (FDB), was installed in 2010. Moreover, the runner suffered from cavitation pitting on the blades and a measuring campaign with several accelerometers and an acoustic emission sensor concluded that the unit was susceptible to high load cavitation erosion. Given the complexity of the turbine flow, it was decided to install four transparent acrylic glass windows on the draft tube, allowing for visual access to the runner blades and outlet flow. To evaluate the influence of the draft tube injection system on the cavitation behaviour, a series of measurements of cavitation intensity were carried out. High frequency data from the sensors were processed with demodulation techniques at various expected modulation frequencies, particularly the blade passing frequency and the frequency of the draft tube vortex rope. Furthermore, the pressure pulsations were measured and quantified with pressure sensors on the draft tube wall. The results indicate that draft tube water injection has little or no effect on the measured cavitation intensity in the runner. However, other aspects of the machine, including the seasonal and daily variation of the submergence level, must be taken into consideration. In conclusion, it appears that additional long-term observations are needed to clarify the global dynamic behaviour along the entire operating range.

Investigation of flow characteristics in the U-shaped throttle valve

Experimental and numerical analysis of cavitation flow in the U-shaped throttle valve is presented in this article. Cavitation flow has been analyzed numerically using the volume of fluid model while the pictures are captured by a high-speed camera. The results reveal that the distribution of pressure is extremely inhomogeneous, and the pressure drop zone is mainly distributed in the narrow U-shaped groove. Cavitation bubble occurs around the orifice and develops continuously since the pressure drops. The nascent, developed, and collapsed zone of the cavitation within U-shaped throttle valve are also presented. In addition, a high-speed jet flow is formed when the oil flows out the U-shaped orifice, a vortex appears and cavitation bubbles appear. Then, cavitation evolution and the effect of back pressure and the valve opening on cavitation behavior have been discussed. Increasing back pressure will weaken the cavitation intensity and suppress the cavitation effectively. The valve opening also affects the cavitation flow and performance of the U-shaped throttle valve. With the increase of valve opening, cavitation area first increases and then decreases. The results and conclusions presented in this article give the basis for better understanding the cavitation phenomenon in a throttle valve.

Multilevel design optimization of hydraulic turbines based on hierarchical metamodel-assisted evolutionary algorithms

In this paper, an efficient hydraulic optimization procedure is presented and applied to the design of hydraulic turbines. For computationally expensive industrial design optimization problems, an advanced optimization tool (EASY software) and a fast CFD evaluation tool are required. EASY optimization software is a Hierarchical Metamodel-Assisted Evolutionary Algorithm (HMAEA) that can be used in both single-(SOO) and multi-objective optimization (MOO) problems. In order to minimize the CFD solver calls during the optimization design, the MAEA rely on local metamodels, trained on the fly, that are used to identify the most promising members in each population and then only these are to be re-evaluated by the CPU costly CFD solver. For additional economy in the CPU cost, the hierarchical (two-level) optimization scheme is used in this paper, where at each level, a different evaluation tool, a low and a high fidelity specific software, can be linked. The low level utilizes a low-CPU cost and low-accuracy tool to explore the design space with a minimum impact to the wall clock time and the high level, using the high fidelity, high-CPU cost tool is used to exploit the information from the low level. For the applications presented in this paper, the high fidelity model is an incompressible Navier-Stokes equation solver and the low fidelity model is based on the solution of the incompressible Euler equations. In order to optimize the geometry of hydraulic machines, an in-house automatic geometry and mesh generation tool has been integrated in the optimization tool chain. In what follows, 2 three-objective design optimization problems of 3D Francis hydraulic turbines are presented. The optimization objective functions concern the ‘quality’ of the runner outlet velocity profile, the cavitation behavior and efficiency of the runner. The optimization results of the hydraulic turbine components along with the performance of the presented optimization procedure are shown in the paper.

Detection of erosive cavitation on hydraulic turbines through demodulation analysis

Erosive cavitation in hydraulic turbines affects severely the runner structure, increasing maintenance costs and reducing the remaining useful life of the component. Actual vibro-acoustic techniques used to detect this kind of cavitation in hydraulic turbines are based on analysing high-frequency vibrations in different parts of the unit. Particularly, the demodulation of high frequency bands has proven to give relevant results regarding the erosive cavitation behaviour. However, demodulation is not absolutely conclusive since the excitation and the transfer function from the excitation to the measuring point, which depend on every particular prototype, are partially unknown. In this paper, the demodulation method to detect erosive cavitation in hydraulic turbines is reviewed and analysed in detail. To do so, first, an experimental study in a test rig in laboratory has been carried out. This test rig is based on a rotating disk instrumented with a piezoelectric patch and with different sensors, such as accelerometers and acoustic emission sensors, in the rotating and stationary parts. Different excitation patterns simulating erosive cavitation have been applied to the rotating disk. These patterns include pseudo-random excitations of different frequency bands modulated by one low carrier frequency, which model the erosive cavitation characteristics. In this way, it is possible to understand how mechanical vibrations similar to those produced by erosive cavitation are transmitted in such complex systems involving fluid and solid mediums. The knowledge obtained in the test rig helps to interpret the results obtained with demodulation analysis in prototypes. Particularly, in this paper these conclusions have been used to analyse erosive cavitation in a real Francis turbine.

Analysis of the External Conditions Affecting on the Cavitation Resistance of a Steel

The aim of this study is to analyze the effect of cavitation erosion test conditions on the surface resistance of medium carbon steel. The ultrasonic vibratory specimen system was used to perform the experiments under the laboratory conditions. The vibratory frequency and the displacement amplitude were 20 kHz and 20 μm, respectively. In design of experiments, four various cavitation behaviors have been studied based on four cavitation test conditions corresponding to those behaviors. These conditions can be summarized as follows: (1) the usage of water as a testing liquid; (2) replacement the water with 3.5% NaCl solution; (3) applying a certain voltage to cause a combined electrochemical action working simultaneously with the water; and (4) applying the same voltage by using the 3.5% NaCl solution instead of water. The obtained results showed that the mass loss differs by test modes applied.

Dynamic characteristics of the cavitation clouds of submerged Helmholtz self-sustained oscillation jets from high-speed photography

Experiments were conducted to investigate the cavitation behavior of Helmholtz self-sustained oscillation jets. In this paper, highspeed photographic technology was used to capture the flow details of cavitation with various nozzle structures and operating pressures from 1.0 MPa to 3.0 MPa. Furthermore, an in-house code based on a matrix approach that processes gray values with various statistical methods was used to quantitatively evaluate the cavitation jet of waters generated by Helmholtz nozzles. The mean values of the normalized gray levels of the images were used as a metric to measure the lengths of the cavitation clouds. When the original data were smoothed by a 15th lowess filter to exclude the noise of the images, the periods of cavitation cloud shedding were obtained accordingly, and the average period had a magnitude of 10-4. All of the smoothed results of the original data captured a two-peak shape that comprises a large peak and a mild hump. Image processing results showed that the Helmholtz nozzle produced greater cavitation intensity than a traditional conical nozzle. The shear-layer instability waves disappeared within 10d1 downstream from the injector exit. The cavitation clouds propagated a distance of approximately 1–10d1 at various pressure ratios, and the clouds were continuous within this distance. The geometry of the nozzle strongly affected the length and shedding period of the cavitation cloud. As described in the discussion section of this paper, when the cavity length was doubled, the length of the cavitation cloud decreased approximately 26.5 %; however, the period of the cavitation cloud shedding increased approximately 10 %. The longest cavitation cloud appeared when the ratio of the cavity diameter and the upper nozzle diameter was eight. Excessively small or large diameters were not conducive to the development of cavitation, whereas the effect of the pressure ratio on the cavitation cloud length was positive. When other conditions remained unchanged, the lump shedding period decreased as the pressure ratio was increased to 25. When the pressure ratio exceeded 25, the cavitation cloud shedding period remained constant.

Simulation of throttling effect on cavitation for nozzle internal flow

The quality of spray atomization governs combustion, power output, fuel economy and emissions of internal combustion engines. Spray atomization is significantly affected by cavitation, and it is closely related to various factors, such as injection conditions, nozzle geometry and throttling effect. In this work, a 5-time up-scaled transparent nozzle was custom-made and utilized for the study of the dynamics and characteristics of the nozzle internal flow with cavitation under various injection pressures and throttling strengths (by varying needle lift). High-speed imaging technique was employed for the nozzle internal flow visualisation. To further investigate the 3D behaviour of cavitation, the simulation of nozzle internal flow was carried out with Fluent 18.0. It was found that cavitation was mainly induced by the throttling effect at the crevice between the needle and needle seat and the redirection of flow at the nozzle inlet. With the rise of the needle lift, initially the fraction of cavitation increased because of increasing hydraulic force, and then as the needle lift increased further, the fraction of cavitation decreased due to the reduced throttling effect. However, when the needle lift kept constant, the cavitation strengthened quickly with the injection pressure and then kept almost constant. Cavitation shedding was observed near the lower wall of the nozzle at the high injection pressure, but not at low injection pressure. In addition, at the low injection pressure, string cavitation was observed due to the throttling effect, originating from the nozzle and extending upstream to the needle.

Cloud Cavitation Behavior on a Hydrofoil Due to Fluid-Structure Interaction

Despite recent extensive research into fluid–structure interaction (FSI) of cavitating hydrofoils, there remain insufficient experimental data to explain many of the observed phenomena. The cloud cavitation behavior around a hydrofoil due to the effect of FSI is investigated, utilizing rigid and compliant three-dimensional (3D) hydrofoils held in a cantilevered configuration in a cavitation tunnel. The hydrofoils have identical undeformed geometry of tapered planform with a constant modified NACA0009 profile. The rigid model is made of stainless steel and the compliant model of a carbon and glass fiber-reinforced epoxy resin with the structural fibers aligned along the spanwise direction to avoid material bend-twist coupling. Tests were conducted at an incidence of 6 deg, a mean chord-based Reynolds number of 0.7 × 106 and cavitation number of 0.8. Force measurements were simultaneously acquired with high-speed imaging to enable correlation of forces with tip bending deformations and cavity physics. Hydrofoil compliance was seen to dampen the higher frequency force fluctuations while showing strong correlation between normal force and tip deflection. The 3D nature of the flow field was seen to cause complex cavitation behavior with two shedding modes observed on both models.

On the application of passive flow control for cavitation suppression in torque converter stator

Purpose The purpose of this paper is to study the transient cavitation process in torque converters with a particular focus on cavitation suppression with a passive flow control technique. Design/methodology/approach The transient fluid field in a torque converter was simulated by RANS-based computational fluid dynamics (CFD) in a full three-dimensional (3D) model. A homogeneous Rayleigh–Plesset cavitation model was used to simulate the transient cavitation process and the results were validated with test data. Various secondary flow passages (SFP) were applied to the stator blade. The cavitation behavior and hydrodynamic performance were simulated and compared to investigate the effect of SFP geometries on cavitation suppression. Findings Presented results show that cavitation in the torque converter is highly unstable at stall operating condition because of the combination of a high incidence angle and high flow velocity. The addition of an SFP to the stator blade produces a disruption of the re-entrant jet and reduces the overall degree of cavitation, consequently inhibiting the unstable cavitation and reducing performance degradation. Originality/value This paper provides unique insights into the complicated transient cavitation flow patterns found in torque converters and introduces effective passive flow control techniques useful to researchers and engineers in the areas of fluid dynamics and turbomachinery.

A Study on Cavitation Behavior through an Orifice

A Study on Cavitation Behavior through an Orifice