•We studied a passive control method to stabilize the cloud cavitation instabilities using Cylindrical Cavitating-bubble Generators (CCGs).•Experimental investigations of cavitation inception, sheet ...cavitation and cloud cavitations for the hydrofoil without and with CCGs were performed.•Implemented passive cavitation control technique appeared to be quite effective method to suppress the cavitation in different cavitating flow regimes.•Using our passive control method a notable reduction in the amplitude of pressure pulsations was observed.
Cavitation often causes a destructive impact on the performance of hydraulic machinery, such as erosive wear, noise and vibrations of the framework and moving parts of marine propellers, pumps, hydraulic turbines and other equipments, which eventually leads to a degradation of overall system effectiveness. The paper reports on an experimental investigation of a passive method of flow control for different cavitation conditions: starting from the cavitation inception, including quasi-steady partial cavitation with shedding of small-scale vortical structures and finishing by unsteady cloud cavitation. The passive flow control was implemented using miniature vortex generators of a cylindrical type referred to as Cylindrical Cavitating-bubble Generators (CCGs) that were placed on the surface of a benchmark CAV2003 hydrofoil. First, we performed high-speed visualization of cavitation on the suction side of the original hydrofoil (without the control element) to find the cavitation inception point near the leading edge and to analyze the spatial structure and time evolution of partial cavities. In order to improve our understanding of the mechanism of cavitating flow unsteadiness and the effect of CCGs on the cavitation dynamics, we also applied a PIV technique to measure the mean flow velocity profiles and a hydroacoustic pressure transducer to record local pressure pulsations in the hydrofoil wake. As a result, this allowed us to determine the influence of CCGs on turbulent structure of the flow at different cavitation regimes and amplitude-frequency spectra of the pressure pulsations associated with attached cavity length oscillations for unsteady flow conditions. It was revealed that, in the case of unsteady cloud cavitation, CCGs were capable to mitigate large-scale cloud cavities. In addition, a substantial decrease in the amplitude of pressure pulsations was registered for the modified hydrofoil (with the control element). In general, CCGs appeared to be quite effective to hinder the cavitation development and to reduce the strength of side- and middle-entrant jets as the primary mechanisms of unsteady cloud cavitation.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Using high-speed visualization, we characterize two distinct mechanisms causing periodic, partial cavitation in a venturi.•The governing parameter that determines which mechanism is dominant is the ...cavitation number.•Estimates are given for the shock wave velocity in both the liquid and cavity region.
Partial cavitation dynamics in an axisymmetric converging-diverging nozzle are investigated experimentally. Shadowgraphy is used to visualize and analyze different cavitation regimes. These regimes are generated by changing the global static pressure and flow velocity independently. Cloud cavitation is the most interesting and complex regime, because the shedding of vapor clouds is caused by two different mechanisms: the re-entrant jet mechanism and the bubbly shock mechanism. The dynamics are investigated using a position-time diagram. Using such a diagram we show that for cavitation number σ > 0.95 the cavity shedding is caused by the re-entrant jet mechanism, and for σ < 0.75 the mechanism responsible for periodic cavity shedding is the bubbly shock mechanism. Both mechanisms are observed in the transition region, 0.75 < σ < 0.95. The shedding frequencies, expressed as Strouhal numbers, collapse on a single curve when plotted against the cavitation number, except for the transition region. The re-entrant jet mechanism is a pressure gradient driven phenomenon, which is caused by a temporary stagnation point at the cavity front. This leads to stick-slip behavior of the cavity. In the bubbly shock regime, a shock wave is induced by a collapse of the previously shedded vapor bubbles downstream of the venturi, which triggers the initiation of the detachment of the growing cavity. The propagation velocity of the shock wave is quantified both in the liquid and the mixture phase by means of the position-time diagram.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Partial cavitation over incipient, transitory and periodic regimes is investigated using LES.•Very good comparison to experiments is observed for vapor volume fractions, shock speed and shedding ...frequency.•LES flow field is used to investigate the conditions that lead to either the formation of re-entrant jet or bubbly shock wave.•The initiation of bubbly shock wave by the collapse-induced pressure wave from previously shed clouds is explained.
Partial cavitation over incipient, transitory and periodic regimes is investigated using large eddy simulation (LES) in the (experimental) sharp wedge configuration of Ganesh et al. (2016). The numerical approach is based on a compressible homogeneous mixture model with finite rate mass transfer between the phases. Physical mechanisms of cavity transition observed in the experiments; i.e. re-entrant jet and bubbly shock wave, are both captured in the LES over their respective regimes. Vapor volume fraction data obtained from the LES is quantitatively compared to X-ray densitometry. In the transitory and periodic regimes, void fractions resulting from complex interactions of large regions of vapor in the sheet/cloud show very good comparison with the experiments. In addition, very good agreement with the experiments is obtained for the shedding frequency and the bubbly shock wave propagation speed. In the incipient regime, the qualitative characteristics of the flow (e.g. cavitation inside spanwise vortices in the shear layer) are captured in the simulations. Conditions favoring either the formation of the re-entrant jet or the bubbly shock wave are analyzed by contrasting the LES results between the regimes. In the transitory regime, large pressure recovery from within the cavity to outside, and the resulting high adverse pressure gradient at the cavity closure support the formation of re-entrant jet. In the periodic regime, overall low pressures lead to reduced speed of sound and increased medium compressibility, favoring the propagation of shock waves. In a re-entrant jet cycle, vapor production occurs predominantly in the shear layer, and intermittently within the cavity. In a bubbly shock cycle, vapor production is observed spanning the entire thickness of the cavity. Bubbly shock wave propagation is observed to be initiated by the impingement of the collapse-induced pressure waves from the previously shed cloud. Supersonic Mach numbers are observed in the cavity closure regions, while the regions within the grown cavity are subsonic due to the negligible flow velocities.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•We point out the inappropriate classification of combining condensation shock and collapse-induced pressure wave mechanisms in the literature.•X-ray phase contrast imaging enhances significantly the ...vapour-liquid interfaces enabling a more detailed visualization of the two-phase structures.•Three different types of mechanisms responsible for large cloud shedding are revealed using experimental measurements.
The conventional high-speed images of cavitation with a set of X-ray phase contrast images reveal the presence of three different types of mechanisms responsible for large cloud shedding: re-entrant jet mechanism, condensation shock wave mechanism, and collapse-induced pressure wave mechanism. At higher cavitation numbers, the sheet cavity is relatively short and the cavity detachment is a consequence of a re-entrant jet pinching off the cavity from its leading edge. At lower cavitation numbers, the re-entrant jet plays a smaller role in the cavitation instabilities and the primary reason for periodic cloud shedding is the condensation shock mechanism where a void fraction discontinuity propagates upstream until collapsing the entire cavity. If the amount of shed vapour cloud reaches a certain extent, the collapse will emit a pressure wave strong enough to disturb the growing cavity, and subsequently make it detached from the wall. This is the third mechanism observed in the experiments. We point out the inappropriate classification of combining condensation shock and collapse-induced pressure wave mechanisms in the literature, since we identify pronounced differences between them: (i) the pressure increase across the condensation front is very weak (a few kPa) while the amplitude of collapse-induced pressure wave can be hundreds of kPa, (ii) the travelling velocity of the collapse-induced pressure wave within the cavity is much faster than the condensation shock, and (iii) the collapse-induced pressure wave does not result in an obvious discontinuity in void fraction when it propagates through the cavity, in contrary to the condensation shock.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the present work, the variations of pressure pulsation, shedding cloud cavity volume and re-entrant jet in the cavitating flow around the NACA0015 hydrofoil are captured based on experiment and ...numerical simulation at different inflow velocities. The relationship between these factors and cavitation erosion is then investigated. The findings suggest a certain correlation between pressure pulsation, shedding cloud cavity volume, re-entrant jet velocity and cavitation erosion, but the intensity and position of cavitation erosion cannot be accurately predicted only by a single factor. The Erosive Power Method is based on multifactor considerations and is more effective than the Intensity Function Method for the prediction of cavitation erosion. An improved prediction scheme based on the Erosive Power Method and considering the shedding trajectory of cloud cavity is proposed, which significantly improves the credibility of cavitation erosion prediction on the NACA0015 hydrofoil. The erosion mechanism due to shedding cloud cavity collapses at different distances from the hydrofoil surface are the key to accurately predict cavitation erosion.
Display omitted
•This work investigates the effects of various factors on cavitation erosion.•The high-speed visualization images are quantified by image processing.•The decay rate of shedding cloud cavity volume is closely related to cavitation erosion.•An improved scheme based on the shedding trajectory of cloud cavity is proposed.•The prediction results agree well with the cavitation erosion experiment.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Compressible Large-Eddy Simulation of cavitating nozzle flows featuring different cavitation regimes.•Cavitation dynamics and shedding mechanisms of cloud cavitation in a nozzle with constant ...cross-section.•Re-entrant jet and condensation shock initiated shedding.•Estimates for shock wave velocity and cavity growth velocity.•Detailed analysis of the re-entrant jet motion and velocity.
Display omitted
Cloud cavitation is related to an intrinsic instability where clouds are shed periodically. The shedding process is initiated either by the motion of a liquid re-entrant jet or a condensation shock. Cloud cavitation in nozzles interacts with the flow field in the nozzle, the mass flow and the spray break-up, and causes erosion damage. For nozzle geometries cloud shedding and the associated processes have not yet been studied in detail.
In this paper, we investigate the process of cloud cavitation shedding, the re-entrant jet and condensation shocks in a scaled-up generic step nozzle with injection into gas using implicit Large-Eddy Simulations (LES). For modeling of the cavitating liquid we employ a barotropic equilibrium cavitation model, embedded in a homogeneous multi-component mixture model. Full compressibility of all components is taken into account to resolve the effects of collapsing vapor structures.
We carry out simulations of two operating points exhibiting different cavitation regimes. The time-resolved, three-dimensional simulation results cover several shedding cycles and provide deeper insight into the flow field. Our results show that at lower cavitation numbers, shedding is initiated by condensation shocks, which has not yet been reported for nozzle flows with a constant cross-section. We analyze the cavitation dynamics and the shedding cycles of both operating points. Based on our observations we propose modifications to established schematics of the cloud shedding process. Additionally, we analyze the near-wall upstream flow in and underneath the vapor sheet and possible driving mechanism behind the formation of the re-entrant jet.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Effects of different ventilated temperatures on cavity dimensions and distributions of the flow field inside the cavity were numerically investigated using fully compressible homogeneous multiphase ...flow.•Good agreement is obtained in comparisons of the cavity shape and cavity dimensions to the experimental data.•Detailed mechanisms of the ventilated cavity evolution were provided.•Effects of temperatures on cavity dimensions, distributions of the flow field inside the cavity, and on the instability of the gas-water interface were explained.
Nowadays, with the widespread use of modern technology in underwater vehicles, particularly in naval applications, the artificial gas ejected from ventilated holes is normally heated because of engine operations. However, the dynamics of ventilated hot gas have not yet been fully studied. In this study, the cavitating flow around an axisymmetric body with different ventilated temperatures was numerically investigated. A fully compressible mixture model based on a homogeneous multiphase approach was employed. A high-order accuracy flow solver based on the numerical models of a dual-time preconditioning technique, a sharp interface-capturing scheme, and an enhanced cavitation model was used to examine the effects of temperature on the cavitating flow. First, the numerical results were validated by comparison with available experimental data of cavitating flow around a conical cavitator. Reasonable agreements on the cavity shape, cavity length, and cavity thickness were achieved. The solver was then applied in simulations to analyze the development of the ventilated cavity, including the cavity formation at the early stage, the re-entrant jet phenomenon, and the cavity shedding mechanism. In addition, the effects of ventilated temperatures on the cavity length and cavity thickness were studied. Further insight into the distributions of the flow field inside the cavity, and the instability of the gas-water interface when temperature increases were also provided.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Cavitating flow in an annular jet pump is studied by experimental and numerical method.•Pump efficiency, pressure ratio and cavitation performance are predicted well compared to experiments.•The ...main characteristics of cavity cloud in annular jet pump under different stages are captured by high speed camera.•The dynamic characteristics of cavity cloud are studied through imaging analytical method.•The physical mechanism of cavity cloud shedding in diffuser is illustrated.
Based on the experimental and numerical methods, the pump performance and inner flow details of annular jet pumps under three area ratios (cross sectional area ratio of throat and nozzle) were studied in the present paper. The cavity clouds forming at the shearing layer, recirculation center and throat inlet were captured via high speed video. The realizable k–ε turbulence model combined with mixture cavitation model was well validated by experimental results on the pump performance (pressure ratio and pump efficiency) and the static wall pressure distribution. When the annular jet pump works under the critical working condition, the pressure ratio and the pump efficiency experience a sudden drop. Simultaneously, the flow rate ratio and cavitation number keep constant regardless of the decreasing outlet pressure, since the main flow is filled with cavity clouds. Moreover, the inception and development of cavity cloud induced at the throat inlet were particularly studied in this paper. The cavitation in the throat experiences three stages (incipient, stable and unstable stage) before extending into the diffuser, in which the unstable stage signals the approaching of the critical working condition. The cavity cloud there fluctuates slowly and faintly, while it may suddenly expand over the whole throat and vanish immediately. When the cavity cloud extends into the diffuser with the closure place x/Dt<2.8 (Dt is the diameter of throat length), there is a low frequency cavity cloud surge. However, the surge disappears as the cavity cloud increases to the intermediate part of the diffuser. Additionally, based on imaging analysis method, the frequency characteristic of the cavity shedding in the diffuser was also studied. The shedding of cavity cloud in the diffuser experiences multiple periodicities when Lcav=3.25Dt (cavity length in diffuser), while there are two fundamental frequencies (58Hz and 6Hz) for Lcav=1.1Dt with the higher one corresponding to the shedding frequency. In the case of Lcav=3.25Dt, the detached cavity cloud in the diffuser is pushed downstream only by the re-entrant jet. However, the main flow plays an important role on accelerating the detached part downstream in the case of Lcav=1.1Dt.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Unsteady cavitation is an important topic due to its potential to cause huge damage to the hydraulic machinery. To control the shedding of cloud cavitation, the cavitation over a flat hydrofoil with ...an obstacle is investigated experimentally and numerically. A series of experiments around the flat hydrofoil without/with obstacle are carried out to study the evolution of cavitation. Periodic re-entrant jet and large shedding of cloud cavitation are observed in the case without obstacle, while the shedding of cloud cavitation in the case with obstacle is much weaker. Numerical simulations of the 2D unsteady cavitating flows around the hydrofoil are also performed. The transient and averaged fields of numerical simulations are presented and compared with the experimental data. The results show that in cases without obstacle, the averaged cavity length becomes longer with the decrease of the cavitation number. While in cases with obstacle, there is a range of cavitation number, in which the averaged cavity length almost keeps constant. The existence of obstacle changes the strength and direction of the transient re-entrant jet as well as the pressure distribution at the tail part of the cavity, leading to the weaker shedding of the cloud cavitation.
•Period shedding and re-entrant jet are detected in the case with/without obstacle, while the shedding is much weaker in the case with obstacle.•The thickness and the length of the cavity in the case with obstacle both decrease apparently compared with the case without obstacle.•A peak can be found in the adverse pressure gradient distribution in the case without obstacle, while it decreases obviously in the case with obstacle.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The present work investigates various swirl flow motions for the cavitation characteristics through numerical simulation of the four-phase cavitating flow in a Venturi tube. A three-dimensional ...Eulerian–Eulerian approach available in a commercial CFD software was used in conjunction with the k−ω SST scheme and Schnerr–Sauer cavitation model adopted for solid–liquid–vapor–air flows. The results of simulations are validated against the experimental data obtained in our previous study (Shi et al., 2020). Excellent predictions of flow characteristics were obtained by various solid concentrations. The results reveal that the intense swirl motions can substantially affect the movement of cavitation bubbles and micro particles. When intensive swirl motion is imposed in the process, an efficient separation of micro particles are of significant importance A higher swirl strength is of benefit to cavitation performance and degree of protection on the inner surface of throat walls from the cavitation erosion but not solid erosion. In addition, the turbulent viscosity analysis predicts weaker turbulent viscosity ratio, resulting in higher multi-factor coupling cavitation production in the divergent region. The primary and secondary swirling re-entrant jets are identified and analyzed as well. This work illustrates main features of the swirl impact on the cavitation phenomena in four-phase flows: solid–liquid–vapor–air flows. This information can strongly support the design, optimization, and application of hydrodynamic cavitation devices in the field of separation process in industrial wastewater or sludge processing.
•The impact of swirls on the separation phenomena in four-phase flows.•The intense swirl motions substantially affect the separation of micro particles.•A higher swirl strength is of benefit to separation/cavitation performances.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
PDF
