Cavitation Research Articles

Effect of Vibration and Noise Measuring Points Distribution on the Sensitivity of Pump Cavitation Diagnosis

Cavitation is an essential factor in the deterioration of the hydraulic performance of centrifugal pumps. The study of cavitation fault diagnosis can help avoid or reduce the damage it causes. The vibration and noise analysis method can predict the incipient cavitation more accurately. In order to improve the accuracy of cavitation fault diagnosis, this paper studies the distribution of vibration and noise measuring points for centrifugal pump cavitation diagnosis. The research object is a centrifugal pump with an inducer and splitter blades. Vibration acceleration sensors and hydrophones were used to collect structural vibration and liquid-borne noise signals at different positions of the pump unit. Root-mean-square (RMS) and fast Fourier transform (FFT) methods were used to construct spectrums of vibration and noise signals with different NPSH and compare the sensitivity of different measuring points to the inception and development of cavitation. In addition, the SST k-ω turbulence model and Zwart cavitation model were used to study the cavitation volume distribution in the pump under different cavitation stages. By setting monitoring points at the impeller outlet, the frequency domain signal distribution of pressure pulsation was studied. The results show that the vibration acceleration measuring points at the pump inlet flange and pump axial position, and liquid-borne noise measuring point at the inlet position, are more sensitive to the diagnosis of cavitation fault. Motor current is also the basis for judging the inception of cavitation. Moreover, as the cavitation intensifies, the dominant frequency signal of the pressure pulsation in the pump is partially shifted.

Characterization of bubble dynamics in the nozzle flow of aviation fuels via computer vision tools

Bubble breakup and associated gaseous cavitation inception triggered by a single bubble injected into the flow of aviation fuel flowing in a converging–diverging nozzle are characterized by employing advanced computer vision (CV) algorithms. The obtained statistics from CV blob detection analysis provide unprecedented quantitative data from non-intrusive imaging techniques and reveal valuable insights regarding the kinematics of the bubble spatial–temporal evolution, breakup, and gaseous cavitation inception mechanisms in aviation fuels. A distinct constant velocity of the bubble before its breakup and constant group velocity of the resulting void cloud after the breakup are observed despite the continuous streamwise variation in fluid velocity in the nozzle. It is found that though the initial bubble size plays an essential role in the resulting void fraction variation after the breakup, it does not play a role in the bubble breakup location nor the resulting bubble terminal velocity before and void group velocity after the breakup. We define a unique dimensionless number, σb, that is able to distinguish between the breakup dynamic parameters for different fuels and flow regimes. Linear dependence is observed between the breakup location and σb. The obtained results shed some light on the kinematic evolution of a group of nonspherical cavities and will prove useful in modeling complex fuel cavitation mechanisms.

Performance Prediction of Composite Marine Propeller in Non-Cavitating and Cavitating Flow

The purpose of this study is to compare the performance of specific composite propellers with different ply angles from a cavitation inception speed (CIS) perspective. Composite propellers have a relatively large deformation compared to existing propellers manufactured using nickel aluminum bronze alloys. Therefore, it is necessary to understand the correlation between the stacking method of the composite materials and the propeller performance in order to design composite propellers that provide sufficient strength and generate the desired deformation. In addition, in the case of composite propellers, the deformation is closely related to the CIS because it can delay or accelerate the occurrence of tip vortex cavitation (TVC). Fluid-structure interaction (FSI) analysis of the model-scale composite propellers is performed using a coupled computational fluid dynamics (CFD)-finite element method (FEM) to examine the influence of the lamination direction on the deformation of the composite propeller. Finally, a hydroacoustic analysis of the noise generated and propagated by a composite propeller in non-cavitating and cavitating flows is conducted. The study found that some deformed parameters of the propeller affect the performance, the deformation of the composite propeller itself has no significant effect on the sound pressure level, and the volume change of cavitation has a decisive effect on the variation of the sound pressure level radiated from the composite propeller. These results can improve the feasibility, conceptual design, performance, and manufacturing methods for composite propellers.

Characteristics of cavitation onset and development in a self-excited fluidic oscillator

Hydrodynamic cavitation has been widely employed in modern chemical technology. A high-speed camera experiment is conducted to reveal the characteristics of hydrodynamic cavitation generated in one self-excited fluidic oscillator. The images obtained from the high-speed camera system are employed to describe several development stages of the hydrodynamic cavitation. The gray intensity of the images which is the volume of bubbles formed is extracted to distinguish the cavitation bubbles from the water. It is found that three regions in the fluidic oscillator could be divided according to the distance from the entrance. The inception of cavitation occurs in the region nearest the entrance. For a relatively low inlet flow rate, the whole process of cavitation could complete within the region that is the second nearest the entrance as a low pressure area appears periodically in this region. For a high inlet flow rate, the vortexes in the region farthest from the entrance are able to generate sufficient low pressures to induce the generation of cavitation. In addition, the intensity of cavitation could be reflected by the cavitation number in a self-excited fluidic oscillator.

Effect of an end-clearance width on the gap cavitation structure: Experiments on a wall-bounded axis-equipped hydrofoil

The paper deals with an experimental study of tip-clearance cavitation inception and development and its vortical structure coupled with dynamics of the main attached cavity on the suction side of a two-dimensional symmetric hydrofoil equipped with a rotation axis. The gap is formed by the end face of the model and a transparent sidewall of the test channel. The experiments were performed for attack angles of 3° and 9° and 0.4-, 0.8-, 1.75- and 3.75-mm gaps (or 1.8%–17% relative thickness) under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation occurrence, high-speed imaging was applied. The leakage flow velocity was measured inside the clearance by a modified Particle Tracking Velocimetry technique. It is shown that the mean velocity field of the leaking flow is split into eight distinctive zones where the flow direction and velocity magnitude substantially differ. Positions and extents of these zones are practically independent of the primary flow regime but are affected by the attack angle. Local velocity values of the leakage flow are unexpectedly found to be about 20% higher in the region of a gap cavity than the mean bulk velocity of the incoming flow. Cavitating cores of various vortices manifest themselves in the recorded images, showing that the vortex structure of the leakage flow associated with the tip-clearance cavitation is very complicated and the hydrofoil axis makes it even more complex by inducing new vortices and cavities. For the gaps considered, an increase of its size causes the tip-clearance cavitation to be initiated at higher cavitation numbers, i.e., this is favorable for its occurrence, while the development of the main cavity is hindered. In unsteady flow regimes, dynamics of the primary cavity on the hydrofoil suction side significantly influences the leakage flow direction and the tip-clearance cavitation evolution. Periods of oscillation cycles of the main and gap cavities coincide but the maximum size of the gap cavity is reached with a phase lag relative to the main one. At the small incidence angle, thicker gaps and unsteady flow conditions, extremely transient pressure waves are registered in the clearance, with their velocities ranging from 41 to 81 m/s.

A Venturi tube design for studying travelling bubble cavitation

Abstract Travelling bubble cavitation always appears in forms of nearly spherical travelling bubbles. These bubbles grow from the freestream/surface nuclei in the low-pressure region in the flow channel. As the inception of travelling bubble cavitation is very sensitive to the boundary layer flow, attached cavitation would be likely to appear simultaneously if the flow is not well-controlled. In this study, based on the interaction between cavitation inception and boundary layer, a Venturi tube for studying travelling bubble cavitation is designed with the aid of a computational fluid dynamics (CFD) approach. The flow channel of this Venturi tube is composed of an inlet straight pipe (10 mm × 10 mm square), a contraction section, a throat section (5 mm × 5 mm square) and two diffuser sections (diffuser 1 and diffuser 2). For visualization convenience, each cross section of the Venturi tube is square. From CFD results, no flow separation occurs near the throat section and the adverse pressure gradient is relatively small, which indicates attached cavitation may not occur. The manufactured Venturi tube is installed in a blow-down type tunnel and tested with Reynolds number at roughly 3 - 3.75 × 104 and the pressure recovery number at 2.95 - 4.41. The velocity of the Venturi inlet varies from about 6 m/s to 7.5 m/s. Experiment results shows travelling bubble cavitation can be generated successfully and pure travelling bubble cavitation inception is achieved in this Venturi tube. The formation of travelling bubbles is recorded by a Phantom VEO 710L high-speed camera with framing rate of 24000 fps.

Effect of solid wettability on three-phase hydrodynamic cavitation

In many modern industrial applications such as mineral flotation and water treatment, microbubbles have been used as an efficient aid for increasing the collision and attachment efficiencies and enhancing the recovery of valuable fine minerals. One technique to produce such micro-bubbles is to use the hydrodynamic cavitation process when gas cavities in water nucleate under low hydrodynamic pressure. To better understand the cavitation behavior in the presence of solids with different surface properties, we present an experimental study of a multi-phase system. The characteristics of hydrodynamic cavitation with a Venturi tube were experimentally investigated using acoustic detection. The effect of both particle and tube wall surface hydrophobicity and surface structure on the inception of hydrodynamic cavitation were considered. Results demonstrated that particles with lower wettability promoted hydrodynamic cavitation by efficiently trapping gas pockets that facilitate bubble nucleation. In addition, in the condition of low particle concentration (5 g/L) and fine particle size range (D50 < 20 µm), parameters including size and roughness did not play a critical role in affecting cavitation inception. Finally, tests using Venturi tubes of different surface properties showed that the wall surface behaves similarly as particles. Hydrodynamic cavitation was promoted by a more hydrophobic Venturi tube which screened the effect of particle surface properties.

ゼラチン内の気泡界面での集束超音波の後方散乱により形成されるキャビテーションクラウドに対する気泡径の影響

Cavitation cloud formation by the backscattering of focused ultrasound from a bubble interface in gelatin has been studied experimentally. A laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. The maximum size of the laser-induced bubble is about 2 mm in radius. After the laser-induced bubble collapses, a small bubble whose radius is about 0.3 mm remains around the laser focus. In the present experiment, this residual bubble is also used as a reflector bubble, and the effect of reflector bubble size on cavitation cloud formation has been evaluated. Cavitation inception and cavitation cloud formation near the reflector bubble has been observed with a high-speed camera. It is shown that the cavitation cloud grows along with the propagation axis of the incident wave. The larger the reflector bubble size becomes, the earlier the first cavitation inception near the reflector occurs. On the other hand, regardless of the reflector bubble size, the distance between the top interface of the cavitation cloud and the geometrical focus of the focused ultrasound at the maximum expansion of the cloud is almost constant. The results also show that when a residual bubble is used as a reflector, the growth rate of a cavitation cloud in the direction of the propagation axis becomes higher than that when a laser-induced bubble is used. This may be due to the difference of the interface shape of the tip of developing cavitation cloud.

気泡力学を融合したGhost Fluid法に基づく集束超音波中でのキャビテーション初生シミュレーション

気泡力学を融合したGhost Fluid法に基づく集束超音波中でのキャビテーション初生シミュレーション

A sub-grid scale cavitation inception model

Unresolved pressure fluctuations at the sub-grid scale (SGS) level of large eddy simulation (LES) or Reynolds-averaged Navier–Stokes computations affect cavitation inception predictions, as SGS low pressures are simply ignored. We present a framework to take the unresolved SGS flow into account. Representing the SGS flow as canonical turbulence, in this paper, homogeneous isotropic turbulence (HIT), the pressure fluctuations, and transport and cavitating behavior of nuclei in such turbulence can be evaluated from direct numerical simulations (DNS) and used to create a model of cavitation inception that accounts for SGS fluctuations. To accomplish this, nuclei of different sizes were transported in DNS of HIT using their pressure history to drive the Rayleigh–Plesset equation that simulates bubble dynamics. In this way, expected average cavitation frequencies were tabulated for a range of SGS Taylor scale Reynolds numbers (Reλ), nucleus size, turbulent kinetic energy dissipation rate, and mean pressure. The model uses this table to estimate the cavitation event rate in each cell of a computational fluid dynamics solution. Inception can then be predicted by comparing the total cavitation rate with the detection criterion. The model is first assessed on two cases of HIT (at Reλ = 240 and 324) by comparing the pressure statistics, which it predicts in LES runs using the SGS cavitation model against the statistics of DNS. Then, a high Reλ (1660–1880) HIT flow is simulated using LES, and cavitation events are compared against experimental data. The inception model successfully predicts the inception pressure and the cavitation rates in the flow.

Eulerian–Lagrangian modeling of spray G atomization with focus on vortex evolution and its interaction with cavitation

Internal flow is closely related to energy consumption and emissions because of its dominant role in the atomization process. The intricate three-phase flow patterns in an orifice were studied based on modified cavitation and turbulence models. Interacting vorticities and their interactions with cavitation were analyzed using vorticity identification methods and the vorticity transport equation. The paired vortex was formed before cavitation inception, and the cavitation inception occurred invariably inside the vortex core, indicating that cavitation was triggered by vortex evolution. Cavitation patterns affect the vorticity transport that further affects atomization. The internal flow data were then extracted and coupled to a hybrid break-up model with finer characteristic scales.The spray morphologies of different types of fuel at various operating conditions were obtained using diffused back illumination method to validate the proposed model. The predictions obtained from the coupled model were consistent with experimental results, exhibiting less dependence on the grid at the same time. The results of this study can supplement experimental measurements to explore the correlation between cavitation-modulated vortex transport and spray behavior.

A numerical study on the in-nozzle cavitating flow and near-field atomization of cylindrical, V-type, and Y-type intersecting hole nozzles using the LES-VOF method

Abstract In order to improve the performance of engines fueled with diesel fuel or diesel-like e-fuels so as to realize greener transportation, the V-type and Y-type intersecting hole nozzles, in which each hole is formed by the coalescence of two or three subholes, have been designed. In this article, the multiphase flow inside and outside the nozzle was numerically investigated using a volume-of-fluid large eddy simulation (VOF-LES) method to clarify the effects of the nozzle structure on the cavitating flow and primary atomization characteristics. The calculation was carried out at an injection pressure of 150 MPa and a back pressure of 0.1 MPa. Numerical results showed that unlike the L-shape pressure distribution along a cylindrical hole, for intersecting type hole nozzles, the pressure showed a stepped shape drop along the holes due to the overall convergent hole structure, which restrained the inception of cavitation. Consequently, the global loss of the flow over an intersecting type hole nozzle was lower by 24–37% than those of a cylindrical hole nozzle. Additionally, the jets emerging from the intersecting hole nozzles showed 50% wider spreading angles and 27% smaller droplet sizes than those of the cylindrical hole nozzle. Furthermore, the jets emerging from a Y-type intersecting hole nozzle showed enhanced atomization, which was found to be due to the unstable air suction near the outlets of this type of nozzle hole.

Effectiveness of hazard control through HACCP critical control points in the wet noodle production process on product quality

The cavitation phenomena should be considered during the design of hydrokinetic turbines due to cavitation could cause surface erosion, mechanical vibrations, undesirable noise and efficiency reduction in the energy transformation. Therefore, the aim of this work is to conduct a numerical comparison of the cavitation resistance of 3 hydrofoils (NACA0015, Eppler420 and S822) with the purpose of selecting the best foil for the blade cross-section design. For each hydrofoil, the traditional and the high-lift configurations were evaluated. The hydrodynamic factors, including the lift and the drag (CL and CD, respectively) coefficients were determined by using JavaFoil software for several angles of attack (α). Additionally, for the computational fluid dynamics (CFD) study in ANSYs Fluent software, the SST k-ω and the Schnerr and Sauer turbulence and cavitation models were respectively utilized to validate the values of CL and CD, and to calculate the pressure coefficient (CP). The values of CP were compared with the cavitation number (σ) for identifying cavitation on the blade and comparing the cavitation resistance of the hydrofoils studied. The methods used and the numerical results obtained were subsequently analyzed and validated using relevant experimental values available in the literature for the NACA0015 hydrofoil. CFD simulations revealed that the NACA0015 traditional hydrofoil and the high-lift configuration of the Eppler420 hydrofoil have the best resistance to cavitation inception in comparison with the traditional hydrofoils and the high-lift configurations studied, respectively.

Experimental Analysis of Flow Through Throttle Valve During Gaseous Cavitation

The multiphase flow in oil hydraulic systems has a very significant effect on the correct operation of the hydraulic system. Air can be found in various states in hydraulic systems, while free entrained air in the form of bubbles has the potential to be the most problematic. It especially affects the compressibility of the hydraulic liquid resulting in reduced stiffness of the hydraulic system. The actuators of the hydraulic mechanisms then do not achieve the fast response and the precision of movements depending on the input control signals. One possibility for the contamination of hydraulic fluid by air bubbles is through a phenomenon known as gaseous cavitation. This is a phenomenon in which gas is released when the pressure drops below the saturation pressure of the dissolved gas in the liquid. This article focuses on the experimental analysis of the flow through the throttle valve which is affected by the formation of air bubbles at the throttle edge of the valve. The regions of gaseous cavitation were observed at the different flow cross-section of the throttle valve. The throttle valve was placed into the block of transparent material to provide visualization of the individual measurements. The article is supplemented with photographs of the individual measurements showing the gaseous cavitation inception. Research background: flow cross-section, cavitation phenomenon, discharge coefficient. Purpose of the article: Effect of flow cross-section size and flow velocity on cavitation development. Methods: Experimental measurements. Findings &amp; Value added: The investigation of the gaseous cavitation inception, Visualization of the individual measurements.

Comparative study of the cavitation resistance of the traditional and high-lift hydrofoils for hydrokinetic application

The cavitation phenomena should be considered during the design of hydrokinetic turbines due to cavitation could cause surface erosion, mechanical vibrations, undesirable noise and efficiency reduction in the energy transformation. Therefore, the aim of this work is to conduct a numerical comparison of the cavitation resistance of 3 hydrofoils (NACA0015, Eppler420 and S822) with the purpose of selecting the best foil for the blade cross-section design. For each hydrofoil, the traditional and the high-lift configurations were evaluated. The hydrodynamic factors, including the lift and the drag (CL and CD, respectively) coefficients were determined by using JavaFoil software for several angles of attack (α). Additionally, for the computational fluid dynamics (CFD) study in ANSYs Fluent software, the SST and the Schnerr and Sauer turbulence and cavitation models were respectively utilized to validate the values of CL and CD, and to calculate the pressure coefficient (CP). The values of CP were compared with the cavitation number (σ) for identifying cavitation on the blade and comparing the cavitation resistance of the hydrofoils studied. The methods used and the numerical results obtained were subsequently analyzed and validated using relevant experimental values available in the literature for the NACA0015 hydrofoil. CFD simulations revealed that the NACA0015 traditional hydrofoil and the high-lift configuration of the Eppler420 hydrofoil have the best resistance to cavitation inception in comparison with the traditional hydrofoils and the high-lift configurations studied, respectively.

Numerical method to determine the cavitation inception speed of a submarine propeller based on the noise obtained from bubble dynamics

Cavitation is accompanied by increased hull vibrations, reduced propulsion performance, and increased radiated noise. This can be particularly fatal for warships and submarines. Hence, navies worldwide introduced and managed a propeller performance index called cavitation inception speed (CIS). Among the many types of propeller cavitation, the first to occur is tip vortex cavitation (TVC). In this paper, the CIS decision procedure of TVC is presented via numerical analysis in terms of noise, which is the most prominent characteristic of cavitation inception. For the CIS decision, the cavitation noise from the propeller is obtained from bubble dynamics. Cavitation is simulated with cavitation nuclei, and the input data for nuclei is obtained from computational fluid dynamics (CFD) simulations. The noise from each nucleus was modeled with a monopole assumption. Additional impulse noise due to the bubble's implosion is used as the basis for the CIS decision. The usefulness of the proposed procedure is demonstrated by applying it to three propeller geometries with different skew and rake angles. Additionally, various aspects, such as propeller stern vortex and added mass, are analyzed; the results are consistent with the CIS simulation results for each propeller.

탐욕 알고리즘 및 특이치 분해를 이용한 프로펠러 초생 캐비테이션의 위치추정

An accurate evaluation of the inception of propeller cavitation is a prerequisite for the prediction of the underwater radiated noise performance of ships. Current work treats the acoustic localization for incipient cavitation using the compressive sensing (CS) method. Orthogonal matching pursuit (OMP) based on a greedy algorithm is employed to tackle the CS localization problem owing to its low computational cost, making it appropriate for online applications. The application of singular value decomposition (SVD) is also suggested to identify the number of independent sources, which is crucial for initializing the algorithm. The established procedure is verified using experimental data acquired in a cavitation tunnel facility.

Experimental Study on the Relationship Between Cavitation and Lift Fluctuations of S-Shaped Hydrofoil

In order to study the energy loss of bi-directional hydraulic machinery under cavitation conditions, this paper uses high-speed photography combined with six-axis force and torque sensors to collect cavitating flow images and lift signals of S-shaped hydrofoils simultaneously in a cavitation tunnel. The experimental results show that the stall angle of attack of the S-shaped hydrofoil is at ±12° and that the lift characteristics are almost symmetrical about +1°. Choosingα= +6° andα= −4° with almost equal average lift for comparison, it was found that both cavitation inception and cloud cavitation inception were earlier atα= −4° than atα= +6°, and that the cavitation length atα= −4° grew significantly faster than atα= +6°. Whenα= +6°, the cavity around the S-shaped hydrofoil undergoes a typical cavitation stage as the cavitation number decreases: from incipient cavitation to sheet cavitation to cloud cavitation. However, whenα= −4°, as the cavitation number decreases, the cavitation phase goes through a developmental process from incipient cavitation to sheet cavitation to cloud cavitation to sheet cavitation to cloud cavitation, mainly because the shape of the S-shaped hydrofoil at the negative angle of attack affects the flow of the cavity tails, which is not sufficient to form re-entrant jets that cuts off the sheet cavitation. The formation mechanism of cloud cavitation at the two different angles of attack (α = +6°、−4°) is the same, both being due to the movement of the re-entrant jet leading to the unstable shedding of sheet cavity. The fast Fourier analysis reveals that the fluctuations of the lift signals under cloud cavitation are significantly higher than those under non-cavitation, and the main frequencies of the lift signals under cloud cavitation were all twice the frequency of the cloud cavitation shedding.

EVALUATION OF CAVITATION PERFORMANCE OF AN AXI-SYMMETRIC BODY WITH PUMPJET PROPULSOR

A Pumpjet (PJP) was designed for an underwater body (UWB) with an axi-symmetric configuration as part of a technology development program for design and development of pumpjet. Its propulsive and cavitation performances were predicted through CFD study. The propulsor design was evaluated for its propulsion characteristics through model tests conducted in a Wind Tunnel. In the concluding part of the study, evaluation of the cavitation performance of the pumpjet was undertaken in a Cavitation Tunnel (CT).&#x0D; In order to assess the cavitation free operational speeds and depths of the vehicle with respect to pumpjet, cavitation tests of the PJP were carried out in behind condition at CT to determine the cavitation inception numbers for rotor, stator and cowl. The model test results obtained were corrected for full scale Reynolds number and subsequently analyzed for cavitation inception speeds at different operating depths. This entire exercise facilitated the development of an innovative testing technique and a special test setup for finding cavitation performance of pumpjet propulsor. The technique was evaluated by comparative corroboration of inception position and depth obtained from CFD analysis. From the model tests it was also found that the cavitation inception of the rotor takes place on the tip face side at higher advance ratios and cavitation shifts towards the suction side as the shaft rotation rate increases whereas the stator and cowl are free from any cavitation.

Ship noise management and the ORCA class of ships

To support Transport Canada’s projects to reduce underwater noise from shipping, Defence Research and Development Canada performed a series of experiments based on the Royal Canadian Navy's ORCA-Class training vessels. Sea trials measured onboard vibrations and underwater noise to support the development of a real-time prediction of propeller cavitation inception and underwater noise using only onboard sensors and to develop a database for assessing numerical predictions of underwater noise at the design stage. Based on these trials, it was noted that the noise impact of the vessel could be significantly reduced if the propeller cavitation inception speed could be increased. A series of numerical optimizations were conducted and a low-noise design was selected. Scale models of the ship and propellers were constructed and tested in a towing tank and cavitation tunnel. While the numerical predictions (done at full scale) were very promising, the model-scale tests did not bear them out.