Collapse Of Cloud Cavitation Research Articles

Effect of micro vortex generators on cavitation collapse and pressure pulsation: An experimental investigation

The cloud cavitation phenomenon has destructive effects on high-speed hydraulic machinery. It is vital to develop effective methods to control cloud cavitation. In this study, the dynamics of cavitation on the NACA0015 hydrofoils with and without the micro vortex generators (micro-VGs) were investigated experimentally. The current work focuses on the effect of micro-VGs on cavitation collapse and pressure pulsation. The results show that the cavitation collapse on the smooth hydrofoil involves two stages: the local cavity collapse caused by the first re-entrant jet as well as the shedding and collapse of cloud cavitation caused by the second re-entrant jet, which are related to the two peaks of pressure obtained by the sensors at 60% of chord length on the smooth hydrofoil. These characteristics have also been well identified by the proper orthogonal decomposition method because there are significant burrs in low-order modes and numerous short-time pulse signals in the corresponding time fluctuation terms. Micro-VGs induced the micro vortex flows and caused the vortex cavitation, improving the cavity stability in the midstream of the foil. Thus, there is no local cavitation collapse during the growth of the attached cavity and only one peak of pressure exists in a cavitation cycle.

Eulerian-Lagrangian Study of Bubble Collision on Pressure Distribution due to Cloud Cavitation Collapse

Eulerian-Lagrangian Study of Bubble Collision on Pressure Distribution due to Cloud Cavitation Collapse

Numerical study on cavitation–vortex–noise correlation mechanism and dynamic mode decomposition of a hydrofoil

The large eddy simulation model coupled with the modified Schnerr–Sauer cavitation model has been used to numerically simulate the unsteady cavitation and noncavitation flow of the three-dimensional NACA66 (National Advisory Committee for Aeronautics) hydrofoil under different operating conditions. The results show that the magnitude of the cavitation number plays a decisive role in the hydrofoil cavitation quasiperiodic phenomenon. The cavitation number of 1.25 is used as a typical working condition for analysis. Using the Ffowcs Williams–Hawkings acoustic analogy approach accompanied by the vorticity transport equation splitting, the growth and shedding of cavitation also lead to the growth and shedding of the vortex structure. The cavitation–vortex interaction is mainly influenced by the vortex stretching term and vortex dilatation term and amplitude of them are larger than 500. The baroclinic torque term may be responsible for generating vorticity during the cloud cavitation collapse and has a lower amplitude about 200. The cavity volume acceleration is the main influencing factor of the low-frequency pressure fluctuation around the cavitating hydrofoil. Moreover, the NACA66 hydrofoil surface-pressure data are collected for dynamic mode decomposition to locate the hydrofoil surface noise sources. The alternate high and low amplitude regions in the mode results overlap highly with the cavitation transformation regions. The cavity transformation and pressure fluctuations are the main reason for the generation of periodic low-frequency noise source regions on the hydrofoil surface. Moreover, the corresponding frequencies of each order mode are linearly correlated with the cavitation shedding frequency of 5.70 Hz. Combined with the results of the multiple mode comparisons, it can be inferred that the hydrofoil suction surface under the cavitation effect will generate quasiperiodic waves starting from upstream and moving downstream.

Computational study of the collapse of a cloud with12500gas bubbles in a liquid

We investigate the process of cloud cavitation collapse through large-scale simulation of a cloud composed of 12500 gas bubbles. A finite volume scheme is used on a structured Cartesian grid to solve the Euler equations, and the bubbles are discretized by a diffuse interface method. We investigate the propagation of the collapse wave front through the cloud and provide comparisons to simplified models. We analyze the flow field to identify each bubble of the cloud and its associated microjet. We find that the oscillation frequency of the bubbles and the velocity magnitude of the microjets depend on the local strength of the collapse wave and hence on the radial position of the bubbles in the cloud. At the same time, the direction of the microjets is influenced by the distribution of the bubbles in its vicinity. Finally, an analysis of the pressure pulse spectrum shows that the pressure pulse rate is well captured by an exponential law.

The characteristics of unsteady cavitation around a sphere

The unsteady cavitation typically includes three processes: the growth of cavitation, the instability and shedding of cavitation, and the collapse of cloud cavitation and the regeneration of attached cavitation. In this paper, the unsteady features of the cavitating flow past a sphere are investigated. A detached eddy simulation turbulence model and a transport equation cavitation model are used to model the cavitating flow. The numerical results give the unsteady process of cavitation at different cavitation numbers (0.36 < σ < 1.0), which cover the cavitation state from inception to supercavitation. When the cavitation number 0.8 < σ < 1.0, the flow instability belongs to the single frequency mode; when the cavitation number σ < 0.8, the flow instability becomes the dual-frequency mode. We analyzed the Strouhal (St) number based on the length of the cavitation and found that the St numbers are stable around 0.5 and 0.2 in the dual-frequency mode. In this mode, the high frequency corresponds to the frequency of the large-scale cavity shedding caused by repeated re-entrant jets. The low frequency, caused by the combination of wake flow and cavitation, is close to the natural frequency of the sphere. In addition, the cavity leading edge position and cavity morphology are analyzed in details. Some of the numerical results are compared with existing experimental data.

Multilevel Control Variates for Uncertainty Quantification in Simulations of Cloud Cavitation

We quantify uncertainties in the location and magnitude of extreme pressure spots revealed from large scale multiphase flow simulations of cloud cavitation collapse. We examine clouds containing 50...

Image post-processed approaches for cavitating flow in orifice plate

A comprehensive investigation on cavitating flow and cavitation-induced erosion was performed experimentally in an orifice plate system. Three image post-processed approaches were applied to analyze the test data, in order to obtain the cavitation characteristics. The cavitating flow pattern was studied by high speed images. In one cavitation developing period, there could be three distinct cavitation clouds, whereas the second one is not fully developed. The first image post-processing approach was applied to obtain the mean value and standard deviation distribution, which indicate the erosion area may cover almost all the cavitation developing route and the most vulnerable erosion area locates near the cavitation collapse site. It is coincides with the erosion tests analyzed through the pit-count algorithm approach. The cavitation circulation frequency was invested via PSD analysis approach. It shows that the frequency linearly decreasing with decreasing cavitation number. Additionally, the cavitation intensity effect on cavitation erosion was quantitatively studied based. It is found that the damages are strongly enhanced when increasing the flow velocity. Moreover, the growth rate of eroded pits number is actually stepwise instead of linear (similar to our previous work in a venturi tube), which supports the idea that the cloud cavitation collapse is the primary reason for erosion. The present approaches applied here shows good potential ability of investigating cavitating flows and can be utilized for other apparatus.

Cloud cavitation vortex shedding inside an injector nozzle

The development and collapse of cloud cavitation and its link to surface erosion within a transparent test single-orifice nozzle operating with a closed Diesel fuel hydraulic circuit, has been characterized using high-speed imaging. Data have been obtained for a range of cavitation and Reynolds numbers under fixed lift positions. Post processing of a large number of images acquired with short exposure time (1μs) allowed the elucidation of the distinct flow phenomena associated with the highly transient two-phase flow. At the inlet of the flow orifice, the vapour cloud was found to occupy the largest part of the nozzle hole cross-section. Coherent vortical structures of a hairpin shape have been detected to onset at the closure region of this vapour cloud and shed downstream in a fully transient manner. The effect of the operating parameters on the temporal and spatial characteristics with regards to the emergence and collapse of the hairpin vortices has been quantified. It has been established that the cavitation-vortex shedding was taking place in a periodical manner, characterized by a Strouhal number.

Large eddy simulation of turbulent attached cavitating flow with special emphasis on large scale structures of the hydrofoil wake and turbulence-cavitation interactions

In this paper, the turbulent attached cavitating flow around a Clark-Y hydrofoil is investigated by the large eddy simulation (LES) method coupled with a homogeneous cavitation model. The predicted lift coefficient and the cavity volume show a distinctly quasi-periodic process with cavitation shedding and the results agree fairly well with the available experimental data. The present simulation accurately captures the main features of the unsteady cavitation transient behavior including the attached cavity growth, the sheet/cloud cavitation transition and the cloud cavitation collapse. The vortex shedding structure from a hydrofoil cavitating wake is identified by the criterion, which implies that the large scale structures might slide and roll down along the suction side of the hydrofoil while being further developed at the downstream. Further analysis demonstrates that the turbulence level of the flow is clearly related to the cavitation and the turbulence velocity fluctuation is much influenced by the cavity shedding.

Large Scale Simulation of Cloud Cavitation Collapse

We present a high performance computing framework for large scale simulation of compressible multicomponent flows, applied to cloud cavitation collapse. The governing equations are discretized by a Godunov-type finite volume method on a uniform structured grid. The bubble interface is captured by a diffuse interface method and treated as a mixing region of the liquid and gas phases. The framework is based on our Cubism library which enables the efficient treatment of high-order compact stencil schemes that can harness the capabilities of massively parallel computer architectures and allows for processing up to 1013 computational cells. We present validations of our approach on several classical benchmark examples and study the collapse of a cloud of O(103) bubbles.

Experimental investigation of unsteady characteristics of ventilated cavitation flow around an under-water vehicle

The objective of this article is to investigate the flow structure of the ventilated cavitation around an under-water vehicle. In the experiments, a high-speed camera system is used to observe cavity evolution of the unsteady cavitation flow, the velocity field is measured by the particle image velocimetry technique, and dynamic pressure measurement systems are used to measure pressure fluctuations under different cavitation numbers. We seek to investigate the mechanism of the re-entrant flow and shock wave phenomenon during the cavity evolution. The study concludes that the ventilated cavity is insufficient to overcome the re-entrant jet intrusion and the re-entrant jet moves upstream straightly as well as curvilinearly. Then, the vortex structure rotating clockwise forms on the vehicle surface and the cavity area with low velocity represents the vortex. The re-entrant jet rolls back after the re-entrant jet reaches the front of the cavity. The experimental results also show that the pressure signals at different instants destabilize on the vehicle surface; fluctuant pressure peak is detected at the closure region of the cavity and pressure peak increases as the cavitation number is decreased. Surface pressure fluctuations travel on the vehicle surface from the collapse of cloud cavitation. The variation in shedding frequency for different cavitation numbers is discussed at the end of the article.

Assessment of improved Schnerr-Sauer model in cavitation simulation around a hydrofoil

To enhance the capability of Schnerr-Sauer model in simulating cavitating flows, in this study, we developed an improved Schnerr-Sauer model based on the Rayleigh-Plesset equation and homogeneous flow assumption. The model considers the effects of turbulent fluctuation and noncondensable gas. The unsteady cavitating flow over a two-dimensional Clark-Y hydrofoil was numerically investigated combined with the SST k-ω turbulence model, in which the turbulence eddy viscosity is corrected. We obtain the time-averaged lift and drag coefficients, the cavity shape evolution at cloud cavitation, and its x-velocity profiles from these calculations. Compared with available experimental data in the literature, the time-averaged lift and drag coefficients predicted from the improved Schnerr-Sauer model agree better with the experimental results than with those of the original. In addition, the modified model can accurately predict the characteristics of the time-averaged lift and drag coefficients during different cavitation conditions. Moreover, the improved model can better simulate the inception, growth, shedding, and collapse of cloud cavitation than the original model. The overall results prove the reliability and accuracy of the improved Schnerr-Sauer model in cavitating flow simulations over a hydrofoil.

High intensity focused ultrasound lithotripsy with cavitating microbubbles

In the medical ultrasound field, microbubbles have recently been the subject of much interest. Controlling actively the effect of the microbubbles, a novel therapeutic method has been investigated. In this paper, our works on high intensity focused ultrasound (HIFU) lithotripsy with cavitating microbubbles are reviewed and the cavitation detection method to optimize the HIFU intensity is investigated. In the HIFU lithotripsy, collapse of the cloud cavitation is used to fragment kidney stones. Cloud cavitation is potentially the most destructive form of cavitation. When the cloud cavitation is acoustically forced into a collapse, it has the potential to concentrate a very high pressure. For the control of the cloud cavitation collapse, a novel two-frequency wave (cavitation control [C-C] waveform) is designed; a high-frequency ultrasound pulse (1-4 MHz) to create the cloud cavitation and a low-frequency trailing pulse (500 kHz) following the high-frequency pulse to force the cloud into collapse. High-speed photography showed the cavitation collapse on the stone and the shock-wave emission from the cloud. In vitro erosion tests of model and natural stones were also conducted. In the case of model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high- and low-frequency waves over either wave alone. For the optimization of the high-frequency ultrasound intensity, the subharmonic acoustic pressure was examined. The results showed relationship between the subharmonic pressure from cavitating bubbles induced by the high-frequency ultrasound and eroded volume of the model stones. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The method has the potential to provide a novel lithotripsy system with small fragments and localized cavitating bubbles on a stone.

Large eddy simulation of a sheet/cloud cavitation on a NACA0015 hydrofoil

A single fluid model of sheet/cloud cavitation is developed and applied to a NACA0015 hydrofoil. First, a cavity formation model is set up, based on a three-dimensional (3D) non-cavitation model of Navier–Stokes equations with a large eddy simulation (LES) scheme for weakly compressible flows. A fifth-order polynomial curve is adopted to describe the relationship between density coefficient ratio and pressure coefficient when cavitation occurs. The Navier–Stokes equations including cavitation bubble clusters are solved using the finite-volume approach with time-marching scheme, and MacCormack’s explicit-corrector scheme is adopted. Simulations are carried out in a 3D field acting on a hydrofoil NACA0015 at angles of attack 4°, 8° and 20°, with cavitation numbers σ = 1.0, 1.5 and 2.0, Re = 10 6 , and a 360 × 63 × 29 meshing system. We study time-dependent sheet/cloud cavitation structures, caused by the interaction of viscous objects, such as vortices, and cavitation bubbles. At small angles of attack (4°), the sheet cavity is relatively stable just by oscillating in size at the accumulation stage; at 8° it has a tendency to break away from the upper foil section, with the cloud cavitation structure becoming apparent; at 20°, the flow separates fully from the leading edge of the hydrofoil, and the vortex cavitation occurs. Comparisons with other studies, carried out mainly in the context of flow patterns on which prior experiments and simulations were done, demonstrate the power of our model. Overall, it can snapshot the collapse of cloud cavitation, and allow a study of flow patterns and their instabilities, such as “crescent-shaped regions.”

Dynamics of the Cloud Cavitation and Cavitation Erosion.

It is well known that a cloud cavitation is one of the most destructive forms of cavitation. The set of equations for the motion of a spherical bubble cloud is formulated. Behavior of bubble clouds are simulated numerically when the surrounding pressure is decreased from 50 kPa to a 10 kPa and then increased to 50 kPa. To study the collapse of cloud cavitation more strictly, the internal phenomena of bubble and the compressibility of liquid are considered in the governing equations. An inward propagating shock wave is formed during the collapse of bubble cloud and the shock wave is focused in the center region of the cloud. This makes a violent bubble collapse, which causes a high emitted pressure from the bubbles, which is several hundreds times larger than the single bubble collapse. Moreover, relationship between the cloud collapse and cavitation erosion is studied.