•Cavitation around a twisted hydrofoil is simulated by LES.•Cavitating vortical flow is analyzed by Euler–Lagrangian viewpoints.•Five groups of LCSs due to sheet/cloud cavitation transition are ...identified and discussed.•The relationship between velocity circulation and cavitation shedding dynamics is discussed.
In the present paper, large eddy simulations combined with the Zwart cavitation model are conducted to simulate the transient cavitating turbulent flow around a Delft Twisted hydrofoil. Numerical results show a reasonable agreement with the available experimental data. A three dimensional Lagrangian technology is developed to provide an alternative method for the analyses in cavitating flow, which is based on Lagrangian viewpoint. With this technology, the track lines of re-entrant and side-entrant jets are straightforwardly displayed and clearly indicate that collisions of the mainstream, the re-entrant jet and the side-entrant jet play an important role in the primary and secondary shedding. The evolution of U-type structures and the interactions between cavitation and vortices are well captured and discussed in detail from the Eulerian viewpoint. The numerical results show that during the stage of attached cavity, the topology of the cavity leaves an important influence on the vortex structure. Once the cavity is cut off, the vortex structure evolution will affect significantly the local cavitating flow. Further analysis demonstrates that the lift acting on U-type structures, which is induced by velocity circulation around U-type structures, significantly affects the formation and the development of U-type structures. Lagrangian Coherent Structures (LCSs) obtained with the three dimensional Lagrangian technology are used to reveal the influence of U-type structures on local flow patterns and it shows that there is a close relationship between the local flow separation and vortex structures. Our work provides an insight into the interactions of cavitation-vortex in the cavitating flow around a twisted hydrofoil and demonstrates the potential of 3D LCSs in the analyses of cavitating flow.
Five groups of LCSs are identified and discussed as the sheet/cloud cavitation shedding occurs periodically. Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The objective of this paper is to investigate the unsteady cavitation behaviors and corresponding cavity-induced vibrations. Experimental results are presented for NACA66 hydrofoils made of stainless ...steel and POM Polyacetate for various cavitation regimes. A simultaneous sampling technique is applied to analyse the transient cavitating flow structure and corresponding structural vibration characteristics. The results showed that the maximum vibration amplitude keeps relative small for the inception and sheet cavitation, increases dramatically for the cloud cavitation and declines for the supercavitation. As for the cloud cavitation regime, the trend of the vibration velocity goes up with the growth of the attached cavity, accompanied with small amplitude fluctuations, and decreases with large amplitude fluctuations. At the moment the cloud cavity breaks from the primary attached cavity, the vibration velocity hits the peak. As for the effect of hydroelastic response, the main vibration frequency for POM Polyacetate hydrofoil is larger than that for stainless steel hydrofoil due to the larger angle of attack caused by the twist deformation. The cavitation pattern for POM Polyacetate hydrofoil appears to be fragmentized, corresponding to the chaotic hydroelastic response of the hydrofoil, which is mainly attributed to the disturbance caused by the deformation and cavity-induced vibration.
•The cavitating flow-induced vibration associated with the cloud cavitation behaviors is analyzed.•A high-speed camera and a laser Doppler vibrometer is combined via a simultaneous sampling technique is applied.•Effect of hydroelastic response on cavitation development is investigated.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Ventilated cavitating flow structure is studied by a high-speed camera and a time-resolved particle image velocimetry (TR-PIV).•The large eddy simulation (LES) model is proposed to simulate the ...unsteady ventilated cavity shedding process.•A map is constructed to depict the various ventilated cavitation regimes.•The mechanism of vortex shedding behaviors is illustrated.
The objective of this paper is to investigate ventilated cavitating flow structures with special emphasis on vortex shedding dynamics via combining experimental and numerical methods. In the experiments, the high-speed video and time-resolved particle image velocimetry (TR-PIV) technique are used to observe ventilated cavitating patterns, and to measure the flow velocity and vorticity fields. The numerical simulation is performed by CFX with large eddy simulation (LES) model to capture the unsteady cavity shedding process, and the corresponding velocity and vorticity dynamics. The results show that the flow patterns can be classified into two principally different categories: structures mainly with vortex shedding (namely Bénard–Kármán vortex street; Bénard–Kármán vortex street with vortex filaments and Aligned vortices) and relatively stable structures (namely Aligned vortices with Re-entrant jet; Re-entrant jet and Stable supercavity). For the structures mainly with vortex shedding, the Strouhal number St corresponding to vortex shedding frequency and ratio h/λ corresponding to vortex streets are significantly different in variable ventilated cavitating regimes: St and ratio h/λ increase with enhancement of gas entrainment coefficient Qv for the Bénard-Kármán vortex street, and then St declines gradually for Bénard-Kármán vortex street with vortex filaments and Aligned vortices, but ratio h/λ declines dramatically for the above both patterns. In addition, the influences of Qv on the velocity and vorticity distributions have also been investigated. The proper orthogonal decomposition (POD) analysis of PIV measurements is used to characterize the coherent large-scale flow unsteadiness of velocity fields. It demonstrates that the ventilated cavitation plays an important role in the first mode pairs to mainly affect the vortex shedding in the wake. Moreover, the vorticity transport equation is applied to illustrate the influence of ventilated cavitation on the vorticity distribution. It can be found that the associated vortex dilatation term and baroclinic torque term are important mechanisms for the complicated change of vortices.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•The turbulent cavitating flow around a hydrofoil is simulated by LES.•The three equation method is used to assess the LES results in cavitating flow.•The effect of verification and validation in ...cavitating flow is discussed.•The vortex structure and spectrum characteristic during cavity shedding is clarified.
This study uses implicitly filtered Large Eddy Simulation with a homogenous cavitation model to investigate the transient turbulent cavitating flow around a Clark-Y hydrofoil with emphasis on Verification and Validation (V&V). The numerical results indicate that the present simulation can predict the time evolution process of the periodic cavity shedding and agree reasonably with the available experimental data. This study presents the first practical application of LES V&V in a transient cavitating flow. The three-equation method is used to assess the LES error magnitudes in unsteady cavitating flow. The results show that a noticeable difference can be observed between the modeling error and numerical error, and the former has a larger magnitude than the latter in cavitating flow. It is demonstrated that the unsteady cavitation has a big impact on the LES errors. Additionally, compared with non-cavitating flow, the unsteady cavitation increases the values and fluctuation amplitudes of numerical, modeling, and total errors. Grid requirement for modeling the cavitating flow has been discussed from the viewpoint of LES V&V. The numerical results also reveal that the periodic cavity shedding causes the complex and turbulent flow feature by vortex and spectrum analyses, and this has a great influence on the simulation accuracy.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•A two-way coupling Eulerian-Lagrangian strategy is presented to assess cavitation erosion risk on the Delft twisted hydrofoil.•The distribution and intensity characteristics of the high impulsive ...pressures are analyzed quantitatively in detail.•The erosion sensitive zones at different stages of one cloud cavitation cycle are determined and their formation mechanism is clarified.
Cavitation damage is a major threat to the life span of fluid machinery and has been a hot research issue in engineering for a long time. The current work aims to show the ability to assess cavitation erosion risk on the twisted hydrofoil using a hybrid Eulerian-Lagrangian solver. The volume of fluid method is used for the liquid-vapor interface of resolved vapor structures, while the discrete bubble model is utilized to track micro-scale unresolvable bubbles. A two-way coupling approach is introduced to enable the transition between resolved cavities and bubbles based on their volume, relative location, and shape. A Lagrangian erosion model considering the effect of asymmetric bubble collapse is proposed to predict cavitation erosion risk. Compared with the experimental result, the current model can accurately identify the high erosion risk regions on the hydrofoil surface. The distribution and intensity of the high impact pressures emitted from bubbles is quantitatively evaluated. In addition, the erosion sensitive zones at different stages of the one cloud cavitation cycle are determined and the hydrodynamic mechanisms of aggressive collapse events are analyzed in detail. The results reveal that the potential erosion risk is highest in areas where primary shedding occurs, which causes the macroscopic cavity to roll up. Bubbles with high impact pressures are mainly focused around the edges of the shedding cavity. The secondary shedding contributes to erosive structure in a limited middle angle of attack area, such that when the primary shedding U-shaped structure evolves and begins to collapse, it becomes less erosive, and only isolated points of erosion is found. Further investigation demonstrates that this is due to a transformation from U-shaped vortex to O-shaped vortex, moving the bubbles gradually away from the hydrofoil surface, thereby reducing the cavitation erosion risk.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The effect of vortex roll-up on the instability of TVC is discussed.•The coherent structures in the tip vortex cavitating flow are identified by the 3D-POD analysis.•A modal turbulence transport ...theory is introduced to measure the modal contribution to the instability of TVC.
The instability mechanism of tip vortex cavitation (TVC) is investigated in the present paper, which is found to be significantly influenced by the roll-up of secondary vortices. The three-dimensional proper orthogonal decomposition (3D-POD) analysis is utilized to identify the coherent structures around the TVC, and a modal turbulence transport theory is introduced to further measure the modal contribution to the TVC instability. According to the dynamic evolution of coherent structure identified by the 3D-POD analysis, the fluctuating component is gradually introduced into the tip vortex together with the roll-up of secondary vortices. Moreover, the dynamic evolution of coherent structure is always accompanied by a significant distribution of modal turbulence kinetic energy (TKE), implying that the TVC instability is promoted along with the merging of secondary vortices into the tip vortex. A deeper understanding is gained with the assistance of the modal turbulence transport theory. It is found that the modal Reynolds stress introduced by the vortex roll-up is the trigger for the local TVC instability, because it can significantly enhance the energy input from mean flow to fluctuating flow.
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The results of a mesh sensitivity study for hydroacoustic simulations of a NACA 662−415 section elliptic wing in cavitating conditions are presented. The results of flow field and acoustic field ...analyses were compared using two distinct methodologies for the acoustic examination: the fully impermeable approach and the fully permeable approach. From the flow field analysis, a quadratic convergence for both lift and drag was obtained with numerical uncertainties below 2.5% for all cases. The deviation from the experiment found for the lift was below 6%. For the acoustic results, the impermeable approach exhibited robust convergence for all noise terms, with p values ranging from 1 to 3 and numerical uncertainties below 6% for all noise source terms and below 1.5% for the global noise term. However, for the permeable approach, the convergence of surface terms notably increased due to the amplification of the noise signal amplitude. Nevertheless, although this had an impact on the uncertainties of the global noise term, the convergence remained acceptable. Overall, the flow and acoustic results were consistent, and the noise comparison with the experiments was satisfactory.
•Hydro-acoustic optimization of propellers requires a multi-disciplinary approach.•Developing numerical methods to predict hydro-acoustic propeller performance is crucial.•Cavitating conditions affect predicted noise convergence directly.•Impermeable approach yields better noise convergence than permeable method.•Both approaches show acceptable comparison with experimental results.
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The Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) are employed to analyze the coherent structure of cavitating flow around ALE 15 hydrofoil. The snapshot data sequence is ...obtained from the numerical simulations by means of LES approach and modified Schnerr-Sauer cavitation model. Under cavitation number σ = 2.3, the cavitating flow around ALE 15 hydrofoil sheds at the short side while it almost remains stable at the long side. The eigenvalue distribution of DMD is symmetrical along the real axis of the plane, and the eigenvalues appear as conjugate pairs. The DMD method can accurately extract the frequency characteristics, and results show that the decomposed Mode at St = 2.224 is related to the shedding frequency of cloud cavitation, which agrees well with the shedding frequency of 2.208–2.805 in experimental measurement. The POD method can effectively analyze the major structure of high energy, in which the first four modes contain over 60% total energy of flow field. In comparison of POD, the DMD is more effective to decompose the complex flow field into uncoupled coherent structures with specific dynamic modes and corresponding frequencies.
•The cavitating flow around ALE 15 hydrofoil is calculated by LES.•Dynamic mode decomposition and proper orthogonal decomposition are used to decompose the cavitating flow.•The coherent structure and corresponding frequency of cavitating flow is revealed.•The decomposed results are compared to the flow field characteristics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The cavitating flow-induced vibration associated with the cloud cavitation behaviors is analyzed.•A hybrid coupled FSI model is applied to simulate the cavitating flow around a flexible hydrofoil.•A ...simplified one-dimensional model is adopted and calibrated against numerical results.•The physical mechanism of the cavitation-excited pressure pulsation is clarified.
The objective of this paper is to investigate the two-phase flow structure of cloud cavitation over a flexible hydrofoil with particular emphasis on cavitating flow-induced vibration and the cavity shedding dynamics via combined experimental and numerical studies. The experimental studies are conducted in a closed-loop cavitation tunnel using a simultaneous sampling technique and the numerical investigations are performed using a hybrid coupled fluid structure interaction model. Results are presented for a modified NACA66 hydrofoil made of POM Polyacetate and good agreement can be obtained between the numerical and experimental results. The predicted cavitation behaviors, including the cavity growth, break-off and collapse, agree fairly well with the experiment. The vibration velocity goes up with the growth of the attached cavity and the break off of the cloud cavity induces some fluctuations of the vibration velocity. For the cavity shedding dynamics, the cavity partially collapses with the vibration and breaks into several medium-scale cloud cavities accompanied with complex interaction between the counter-rotating vortexes. Good agreement can be obtained between the predicted and measured vibration amplitudes, with the dominant flow-induced vibration frequency in accordance with the cavity shedding frequency and other two vibration frequencies corresponding to the natural frequencies of the first bending and twisting mode respectively. The one-dimensional analytical model is applied to better illustrate the physical mechanism of the cavitating flow induced pressure pulsation. The results show that the transient cavitating behaviors lead to the periodic pressure fluctuation on the hydrofoil. The one-dimensional model can track the main features of the pressure fluctuations, which are mainly derived from the cavity volume acceleration.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Fast Fourier Transform(FFT), Bispecturm and Dynamic Mode Decomposition (DMD) are used to detect the frequency 4 characteristics.•Pressure field around the hydrofoil and force coefficients are ...governed by the acceleration of vapor cavity.•Application of Proper Orthogonal Decomposition (POD) displays the large-scale coherent structures among cavity 7 shedding evolution.
In this paper, the unsteady cavitating flow around a symmetrical twisted hydrofoil is investigated numerically. Cavitating flow characteristics are analyzed in terms of dynamical behaviors and temporal/spatial fluctuations of the cavities along the hydrofoil. At the midsection of the foil, sheet cavity firstly grows with nearly constant speed until the occurrence of the reverse flow at the closure line. During the reverse phase the flow moves upstream, and the sheet cavity keeps growing turbulently with low vapor content at the closure area. Fast Fourier Transform (FFT), Bispecturm and Dynamic Mode Decomposition (DMD) analyses show the existence of harmonics of the shedding frequency, of which the double and triple frequency are captured but only the fundamental frequency dominates the cavitating field. Pressure fluctuations around the hydrofoil and force coefficients are governed by the acceleration of vapor cavity. Application of Proper Orthogonal Decomposition (POD) displays the large-scale coherent structures among cavity shedding evolution. The sheet cavity growth, the main cavity shrinking and developing into a pair of root-like cavity structure and the sector cavity structure are captured by the first mode.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
