Cavitation Region Research Articles

PIV measurement of cavitating flow around a pitching hydrofoil

Particle Image Velocimetry (PIV) and hydrodynamic measurements were applied to investigate the cavitating flow of a pitching hydrofoil in the form of triangular wave motion (mean incidence α 0=10°, amplitude Δα=5° and pitching frequency f*=2Hz). The Reynolds number is Re=4.5×105. The cavitation number is set to σ=2.3, 1.36. Different typical cavity patterns are observed by PIV images in the whole pitching period, including sub cavitation, inception cavitation, sheet cavitation and cloud cavitation. As the σ decreases, the cavity area increases. And the the corresponding time and position of cavitation patterns also changed significantly. In the instantaneous velocity field of an oscillating hydrofoil, the low velocity zone (LVZ) and the mainstream zone (MZ) are observed. The LVZ corresponds to the cavitation region and changes with the change of cavitation morphology. Meanwhile, the LVZ has a large velocity gradient. The velocity gradient in the MZ is larger along the direction parallel to the incoming flow, and the velocity tends to increase first and then decrease gradually downstream. With the decrease of cavitation number, the area of LVZ increases. And the distribution of LVZ is more uneven, which is mainly caused by oscillation motion and uneven water vapor mixing.

Experimental and numerical simulation investigation of cavitation phenomenon during bubble pulsation process

The current research on the cavitation phenomenon in the underwater explosion process is focused on the shock wave phase. In this study, it was found that there was also significant cavitation during bubble pulsation. Underwater explosion experiments were carried out at the bottom of a sheet using 2.5 g of TNT, which also showed that: (a) Cavitation lasts for the same duration as the bubble contraction time, (b) A cavitation closure pressure, the peak of which is greater than the bubble pulsation pressure, is produced by the closing of the cavitation zone. A tensile cut-off model was introduced to simulate the cavitation phenomenon during bubble pulsation, and the formation mechanism was investigated. The numerical simulation results show that: (a) The cavitation phenomenon is visible when the sheet above the explosive and the explosion distance are between 0.93 and 1.17 times the radius of the largest bubble, or between 0.69 and 1.85 times the maximum radius of the bubble when the sheet below the explosive, (b) The cavitation region and cavitation closure pressure increase and decrease with increasing explosion distance, peaking at a value close to one times the theoretical maximum bubble radius, and (c) The cavitation zone shrinks and the cavitation closing pressure drops as sheet thickness increases.

Modeling the distribution characteristics of vapor bubbles in cavitating flows

Dispersed vapor bubbles are the dominant rheology in cloud cavitation, and their size distribution is directly associated with cavitation noise and erosion. However, the numerical resolution of large numbers of dispersed bubbles remains a challenge. In this work, we establish a new cavitation model based on the population balance equation (PBE) that can predict the size distribution and spatiotemporal evolution of bubbles within cloud cavitation under different cavitation numbers. An expression for the phase transition source term without empirical parameters is derived based on the bubble size distribution (BSD) function, enabling the coupling of mass transfer in the governing equations with the PBE cavitation model. The cavitation model is solved alongside the Eulerian homogeneous mixture flow. The mass transfer between water and vapor, and the bubble coalescence and breakup under turbulent flows, are modeled to determine the BSD. The numerical model is carefully validated through comparisons with experimental results for cavitation flows on a wedge-shaped flat plate, and good agreement is achieved with respect to the pressure distribution, void fraction, and BSD. This confirms that our proposed cavitation model can accurately predict the void fraction and BSD within the cloud cavitation region.

Study of the Mechanism of Cavitation in Inner-Hole Rotating Cavitators

To investigate the mechanism of cavitation, a mathematical model of the cavitation region and an expression for the gas volume fraction in the inner hole of a rotating cavitator are developed. Then, based on the realizable k-ε turbulence model and the Schner & Sauer cavitation model, the flow field is simulated numerically. With the help of pressure and streamlined distribution contours, velocity contours, and gas phase contours, the three stages of the cavitation process are simulated by the cavitation evolution model. Finally, the rate of heat generation is used as the experimental characterization of cavitation, and the cavitation mechanism is confirmed by experiments involving rotation speed and pressure. The results indicate that the cavitation effect can be measured by the generation of cavitation heat and that the influence of the rotation speed on the cavitation effect is more significant than that of the inlet pressure.

Mixed-flow pump cavitation characteristics extraction based on power spectrum density through pressure pulsation signal analysis

Mixed-flow pump is an important machine for transporting fluids, which has the advantages of both centrifugal pump and axial flow pump, but it is prone to cavitation, endangering efficiency and safe operation. A 20 m × 8.5 m high-precision hydraulic mechanical stand and a vertical mixed-flow pumping station device were built for the study of cavitation signals. The detection of pressure pulsation signal is a direct measurement of the cavitation region, and the decay of information is less. In this study, the time domain and frequency domain characteristics of water pressure pulsation signal are analyzed by using fast Fourier transform method. The power spectrum density (PSD) is used to extract the characteristics of each cavitation stage, especially the full cavitation stage with random signal characteristics. The results show that PSD method reflects more information and is more sensitive than traditional time–frequency domain analysis. The characteristic power spectrum (PS) value of the cavitation of the mixed-flow pump machinery can be given through the feature extraction of the PS, which plays a great role in the effective identification and prevention of cavitation.

Analysis of inner flow and outflow characteristics of different scramjet elliptical holes

ABSTRACT The elliptical hole has the potential to promote jet breakup and improve atomization and mixing quality. The flow behavior in the nozzle and the outflow characteristics have an important influence on the atomization and mixing process of the downstream fuel. Therefore, the inner cavitation flow processes and outflow characteristics for elliptical holes for scramjet engine at injection pressures of 3 MPa and 5 MPa were investigated using the Volume of Fluid (VOF) method and Large Eddy Simulation (LES). The numerical findings demonstrated that the cavitation region and vortex structures of elliptical holes mainly appeared on the long axial plane. The cavitation intensity of elliptical holes was weaker than that of the circular hole. Compared with the circular hole, the gas phase volume fraction of elliptical hole with the aspect ratio of 4 was reduced by about 22.1 and 21.1% respectively under 3MPa and 5MPa injection pressure. The application of elliptical holes inhibited the generation of cavitation and had large mass flow rate and flow coefficient. The mass flow rate of the elliptical hole with the aspect ratio of 4 was the largest, and the mass flow rate increased by about 5.1% compared with that of the circular hole. Elliptical holes showed a smaller velocity coefficient and a larger area coefficient when cavitation existed. The elliptical hole with the aspect ratio of 4 held the smallest velocity coefficient and largest area coefficient, with the difference of 12.1% and 18.9% respectively from the circular hole. Another important point was that elliptical holes enhanced the turbulent disturbance at the hole outlet, which facilitated the breakup of downstream jet.

Behaviors of cavitation bubbles driven by high-intensity ultrasound

In a multi-bubble system, the bubble behavior is modulated by the primary acoustic field and the secondary acoustic field. To explore the translational motion of bubbles in cavitation liquids containing high-concentration cavitation nuclei, evolutions of bubbles are recorded by a high-speed camera, and translational trajectories of several representative bubbles are traced. It is found that translational motion of bubbles is always accompanied by the fragmentation and coalescence of bubbles, and for bubbles smaller than 10 μm, the possibility of bubble coalescence is enhanced when the spacing of bubbles is less than 30 μm. The measured signals and their spectra show the presence of strong negative pressure, broadband noise, and various harmonics, which implies that multiple interactions of bubbles appear in the region of high-intensity cavitation. Due to the strong coupling effect, the interaction between bubbles is random. A simplified triple-bubble model is developed to explore the interaction patterns of bubbles affected by the surrounding bubbles. Patterns of bubble interaction, such as attraction, repulsion, stable spacing, and rebound of bubbles, can be predicted by the theoretical analysis, and the obtained results are in good agreement with experimental observations. Mass exchange between the liquid and bubbles as well as absorption in the cavitation nuclei also plays an important role in multi-bubble cavitation, which may account for the weakening of the radial oscillations of bubbles.

Research on the Performance of a Centrifugal Aviation Fuel Pump Based on Response Surface Methodology

The performance of centrifugal aviation fuel pumps can be crucial in determining aircraft efficiency and safety. Here, we delve into the intricate interplay between flight altitude and flow rate on pump performance. Employing the central composite design method, we characterized the relationship between these parameters and the pump’s head and efficiency. Our analysis, underpinned by response surface methodology, revealed distinct optimal operating conditions at 10,000 m and 12,000 m flight altitudes, pinpointed at flow rates of 1.1016 Q and 1.1222 Q, respectively. Moreover, the emergence and growth of cavitation regions with increasing flow rates underscore the challenges in pump operation. Significantly, we identify rotor–stator interactions as a dominant influence on pressure fluctuations, especially near the impeller exit. These findings not only elucidate the operational dynamics of aviation fuel pumps but also pave the way for advanced pump designs and optimizations.

Study on the Influence of Thermodynamic Effects on the Characteristics of Liquid Nitrogen Cavitating Flow around Hydrofoils

Cryogenic cavitation exhibits complexities primarily represented by the coupled interactions of thermodynamic effects, vortices, and cavities during the cavitation process. To further investigate this coupling mechanism, this study employed the DDES turbulence model and Sauer–Schnerr cavitation model to perform unsteady numerical simulations of liquid nitrogen cavitation flow around the NACA0015 Hydrofoil. Numerical validation of the model utilized a symmetrical Hord hydrofoil. The results reveal that the upstream development of the recirculation flow under inverse pressure gradients is the fundamental cause of the detachment in the primary cavitation region. At a cavitation number of 0.616, thermodynamic effects noticeably suppress the formation of cavities and alter the range of adverse pressure gradients, consequently influencing the detachment behavior in the primary cavitation region.

Real-time cavitation monitoring based on the entropic analysis of ultrasound RF signals

The violent inertial cavitation activity generated in high intensity focused ultrasound (HIFU) treatment has a risk of damaging healthy tissues around the target area, so that it is necessary to monitor the degree of cavitation during the treatment. Entropy, as a non-model-based statistical parameter, can effectively characterize the effect of cavitation on the structure of scatters within tissues. Thus, a real-time cavitation monitoring system was established based on the entropy analysis of ultrasound radio frequency (RF) signals, and the degree of cavitation was evaluated through entropy images. Based on the two-dimensional average filtering method, we achieved real-time monitoring of cavitation through the reconstruction of entropy image and the binarization of cavitation regions, breaking the inherent limitation of current systems that can only perform cavitation monitoring between HIFU pulses. The results showed that the entropy image retained more frequency components, which could determine the time and location of cavitation earlier and display cavitation areas. Moreover, as the acoustic pressure increased, the advance of cavitation indication in the entropy images became earlier. This method might provide a feasible evaluation scheme for improving real-time cavitation monitoring during HIFU treatment and was expected to become an effective reference index for imaging in HIFU surgery.

Numerical studies on applications of cavitation models in water hammer-induced cavitating flows in pipelines

Hydraulic transients caused by a fast-closing needle valve and accompanying water hammer-induced cavitation-vortex flow are numerically investigated using the shear stress transport (SST) k–ω model with a pressure–density equation considering the weak compressibility of water and the new cavitation models based on the Rayleigh–Plesset equation that takes into account the surface tension and kinematic viscosity effect on the bubble radius change rate, respectively. The predicted pressure evolutions exhibit reasonable agreement with the experimental data, and the interfacial surface tension-based model is the most accurate in predicting transient pressure evolutions, followed by the kinematic viscosity-based model and the Singhal cavitation model, based on three proposed error indices with minimum of 0.0515, 0.1337, and 0.0035. The wave speed in the upper part of the cavitation trailing edge is higher than that in the lower part, resulting in the evolution of inclination angle of the trailing edge. According to the convective transport mechanism of vorticity, the change rate of vorticity in non-cavitation regions is mainly dominated by the stretching–twisting term and the turbulent fluctuation–diffusion term. In cavitation regions, the dilatation–shrink term makes significant contributions to the change rate of vorticity, and the baroclinic-torque term mainly acts near the edge of the cavitation region.

ОПРЕДЕЛЕНИЕ РЕЖИМОВ УЛЬТРАЗВУКОВОЙ КАВИТАЦИОННОЙ ОБРАБОТКИ МОТОРНОГО ТОПЛИВА

The article considers an example of ultrasonic cavitation created by a piezoelectric emitter. To identify the modes of ultrasonic cavitation treatment, the Qsonica sonicators Q-700 laboratory unit was used. The aim of the work is to determine the modes of ultrasonic treatment of motor fuel to create a cavitation region. Cavitation areas were determined by the area of destruction of aluminum foil samples, 0.008-0.009 mm thick. A sheet of aluminum foil was placed in the test medium parallel and perpendicular to the tip at different distances h, l. In order to identify the intensity of the resulting cavitation, cavitation noise was isolated from the signal of a hydrophone immersed in motor fuel at different distances from the radiator and connected to a multimeter model UT61E. The intensity of cavitation was estimated by the change in the resistance of the medium, R. The following types of motor fuel were used for processing: commercial diesel fuel, biodiesel based on ginger oil, mixed diesel fuel (containing 20% biodiesel). After the study, the foil was dried and the area of destruction was determined. It has been established that the following parameters influence the creation of cavitation – the distance from the radiating surface, intensity, amplitude and time of ultrasonic treatment of the sample.f= f (l, h, I, A, t). Four cavitation areas are defined: the area of active or optimal cavitation, the area of the beginning of cavitation, the area of incipient cavitation, the area of absence of cavitation. Depending on the amplitude and intensity, 6 operating modes are highlighted. It is established that the processing of the used motor fuel should be carried out on the 3 cavitation mode of operation (A =21-40%, I= 12.5-24.8 W / cm2) of the laboratory installation in the used technological volume or in larger volumes with reflective surfaces. It has been found that the cavitation noise in biodiesel is lower than in diesel fuel, since it is more viscous. In viscous liquids, cavitation is less developed and, consequently, the ultrasound absorption coefficient is lower for them. In this regard, it is necessary to design devices for media with a higher viscosity based on the optimal distance from the radiator to the bottom.

Analysis of cavitation and shear in bellows pump: transient CFD modelling and high-speed visualization

Pneumatic drive bellows pumps (PDBPs) are a crucial fluid control component in the semiconductor industry. However, when dealing with specific chemical mechanical polishing (CMP) slurries, agglomeration of particles can occur, leading to scratches, particle residues, pits, and other defects on the wafer surface. The leading cause of agglomeration has been proposed to be cavitation and excessive shear in PDBPs. A high-fidelity multiphase computational fluid dynamics (CFD) model, which fully resolves the response of check valves under hydrodynamic and other external loads, was established to evaluate cavitation behaviour and shear distribution in PDBPs. Moreover, a test rig was built to visualize cavitating flow patterns in PDBPs and verify the accuracy of the CFD model. Among different cavitation regions captured by high-speed visualization, the region near the inner leading edge of the end-ring of the inlet valve is dominant, with high intensity, long duration, and complex morphology evolution. Detailed initiation and development process of cavities in the dominant region are analysed numerically. The shear rate in different domains of PDBPs is estimated, and the inlet valve domain stands out from the other. The strongest shear rate is observed near the leading edge of the end-ring. Furthermore, an analysis of the relationship between cavitation and shear indicates that the high-intensity vortices generated owing to strong shear account for the inception of cavitation. This study contributes to understanding inner flow details in PDBPs and provides a guideline for further optimization of similar pumps.

Investigating the performance of the modified two-component pressure approach in capturing column separation with large vapour cavities

This paper investigates the performance of the modified two-component pressure approach (proposed model), recently proposed by the authors, in capturing column separation with large vapour cavities. The proposed model calculates both the cavitating and waterhammer flow regions using a single set of equations that governs unsteady flow in open channels. Comparing the results with experimental data, and the results obtained from the discrete gas cavity model (counterpart model) reveals that: (1) the proposed model can accurately capture the evolution of large vapour cavities in pipe systems; (2) the model provides more accurate results compared to the counterpart model; (3) under some circumstances the counterpart provides inaccurate results as it fails to account for the energy loss in the cavitating flow zone while the proposed model was shown to resolve the issue. Finally, the results exhibit the superiority of the proposed model over the counterpart model and the conventional open-channel-based models.

Experimental study on lubrication characteristics and leakage inhibition of oil-lubricated seal with herringbone grooves

Experimental research was conducted on mechanical seals with pressurized lubricated-oil, focusing on lubrication and leakage inhibition of herringbone-grooved face seals. Through the visualization experiments and axial displacement tests, this study evaluated the performance of the inner-diameter-side (IDS) and middle-diameter-side (MDS) herringbone grooved seals based on the measured cavitation region, leakage rate, friction torque, surface temperature of stator, and axial displacement of the primary ring. The experiment results show that, increasing the width of the seal dam can reduce the hydrodynamic effect and cavitation development of the liquid film but enhance the hydrostatic effect, maintain a small leakage range, the maximum leakage rate is less than 19 mL∙min−1. Zero-leakage can be achieved in MDS due to cavitation suction. The axial displacement curves indicate that the runout amplitude of IDS is smaller than that of MDS, the maximum axial displacement difference is about 25 µm, the stability of IDS is better than MDS. Comparisons of surface temperature and friction torque demonstrates that the lubrication of MDS is better than that of IDS.

Numerical study on the lubrication performance of oil films in textured piston/cylinder pairs

The piston/cylinder pair is one of the most important friction pairs in the axial piston pump; reducing the friction of it can improve the axial piston pump efficiency. In this paper, the Reynolds equation coupled with the Half–Sommerfeld boundary condition is applied to predict the dynamics of the textured piston/cylinder pairs, and the characteristic function is used to predict the optimal geometry parameters. The analysis results show that the combination of the shearing coefficient and the characteristic function can predict the cavitation region and the critical inclination angle. The results of this paper provide theoretical guidance for the surface texture design on piston/cylinder pairs.

Kinetics of Ultrasonic Dissolution of Metal Oxide Powder for Different Spatial Combinations of the Cavitation Region and Eckart Acoustic Flow

Kinetics of Ultrasonic Dissolution of Metal Oxide Powder for Different Spatial Combinations of the Cavitation Region and Eckart Acoustic Flow

Experimental and Numerical Study on Cavitation Effects in Centrifugal Pumps

Experimental and numerical investigations of the centrifugal pump performance at non-cavitating and cavitating flow conditions were carried out in the present study. Experiments were performed by applying a vacuum to a closed-loop system to investigate the effects of the net positive suction head available (NPSHa), flow rate, water temperature and pump speed on the centrifugal pump performance. Accordingly, many of the important parameters concerning cavitation phenomenon were calculated. Also, the noise which is accompanied by cavitation was measured. Numerical analysis was implemented for two phase flow (the water and its vapor) using a 2-D simulation by ANSYS FLUENT software to investigate the internal flow of centrifugal pump under cavitating conditions. It was observed that with decreasing NPSHa, the values of the pump head, flow rate and efficiency initially remain constant, but with further reduction in NPSHa these parameters will decrease. Also, it was found that at 3% head drop the percentage drop of the flow rate is less than 2% whereas the percentage drop of the efficiency is greater than 3%. Numerically, it was noticed that the cavitation regions appear at the leading edge of suction side of the impeller blades which represents the lowest pressure area inside the computational domain of the centrifugal pump.

Development of efficient method of generating reactive oxygen species by expanding cavitation region using ultrasound focus scanning in the direction of ultrasound propagation

Sonodynamic therapy is a minimally invasive high-intensity focused ultrasound (HIFU) based therapy that combines a sonosensitizer and reactive oxygen species (ROS), which are produced by acoustic cavitation. In this paper, we experimentally examined the effectiveness of a proposed method of scanning the ultrasound focus in the direction of HIFU propagation for continuously generating cavitation clouds to expand the region of ROS generation and increase the amount of ROS. First, the continuous generation of cavitation bubbles was observed using a high-speed camera, and it was confirmed that bubbles could be generated even with a short-duration HIFU, which is difficult to generate bubbles by itself. Subsequently, we evaluated the area and amount of ROS generation by imaging sonochemiluminescence. Comparing the irradiation at 1 point with that at 7 points, the amount of ROS per acoustic energy increased by about 1.5 times, suggesting that it is an efficient method for ROS generation.

Evaluation of a modified URANS prediction of unsteady cavitating flow around a hydrofoil by comparing with LES results and experimental results

The research makes the analysis of cavitating flow field around hydrofoil NACA0009 through both experiments and numerical simulations. We previously proposed an interface effect-based correction model (IE model) for turbulent viscosity correction of cavitation flow, which performed well for hydrofoil cavitation simulation at small angles of attack. This research makes a further evaluation of the IE model. Firstly, the cavitation experiments of the NACA0009 blunt trailing edge hydrofoil with a 10-degree angle of attack are carried out on a closed test platform. The images of quasi-periodic shedding of partial cavitation are obtained. Secondly, the standard k-ε turbulence model, the IE model, and the large eddy simulation (LES model) are used to reproduce the cavitation flow around the hydrofoil. The results show that the standard k-ε model over-predicts the turbulent viscosity of the flow field, resulting in the cavitation region stably attached to the leading edge of the hydrofoil, which can't simulate the unsteady characteristics of the cavity bubbles. The LES model successfully reproduces the process of cavitation structure quasi-periodic shedding off. The IE model modifies the turbulent viscosity in the high shear flow region so that the reentrance flow can reach the leading edge of the hydrofoil and cut the attached cavity off. IE model can properly simulate the macro features of the cavity and the shedding frequency. In each stage of cavitation periodic shedding, the results of the IE model and the LES model on the pressure distribution and velocity field are consistent well. In addition, the calculation time cost of the IE model is basically the same as that of the standard k-ε model, only 0.14 times the LES model costs. The IE model is more suitable for cavitation evaluation in engineering applications.