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•The cavitation evolution around the baseline and biomimetic hydrofoils is studied by LES approach.•Far-field cavitation noise for both hydrofoils is compared by permeable FW-H ...method.•The noise reduction mechanism of wavy leading edge is revealed.
This paper investigates the noise reduction performance of biomimetic hydrofoils with wavy leading edge and the corresponding mechanisms. We employ Large Eddy Simulation (LES) approach and permeable Ffowcs Williams-Hawkings (PFW–H) method to predict cavitation noise around the baseline and biomimetic hydrofoils. The results show that the wavy leading edge can effectively reduce the high-frequency noise, but has little effect on the low-frequency noise. Further analyses and discussions deal with the noise reduction mechanisms. The main source for the low-frequency noise is the cavity volume acceleration, while the wavy leading edge has little effect on it. The high-frequency noise sources, related to the surface pressure fluctuations and the turbulence characteristics, are significantly suppressed by the wavy leading edge, thus decreasing the high-frequency noise intensity. Our investigation indicates that the wavy leading edge has great prospects for cavitation noise reduction technique.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Delayed Detached Eddy Simulation (DDES) turbulence model was coupled with a homogeneous cavitation model to analyze the tip-leakage vortex (TLV) cavitating-flow characteristics in a waterjet ...pump. The numerical results agree well with experimental data. The results show that the vortex evolution in the waterjet pump has three stages, which is similar to that around a hydrofoil, but the vorticity variations in the waterjet pump are more complicated. The relative-vorticity-transport equation was then applied to find the reason for the differences between the vorticity variation observed in the waterjet pump and that around a hydrofoil. The results indicate that the drastic fusion process of the TSV cavity and the TLV cavity in the waterjet pump resulted in the formation of triangular cavitation region near the blade tip that is difficult to reproduce by stationary hydrofoil simulation. This fusion process caused the local variation of fluid volume and further affected the vorticity transport. The entropy-production evaluation method considering the phase transition was then used to analyze the dissipation losses in the complex cavitation region. The results indicate that the drastic fusion process of the TSV cavity and the TLV cavity significantly influenced the entropy production rate distributions and enhanced the disturbance of the flow field. In addition, severe phase transition occurs in the drastic fusion region accompanied by huge phase-transition losses.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Stable operation is a challenge for hydropower stations with multi-turbine hydraulic coupled division systems with a common penstock. In this paper, the serious power fluctuations in a power station ...with such a division system are analyzed. The fluctuations occur in many conditions without any movement of the regulating system. The mathematical analysis illustrates that pressure fluctuation is responsible for power fluctuations. The computational fluid dynamics (CFD) method provided by ANSYS is used to study the flow pattern in penstocks. The vortex caused by the irrational structure of trifurcation is the reason for pressure fluctuations. Several methods are proposed to optimize the flow stability and three cases are simulated based on these measures. The calculation results of three cases prove the effectiveness of these measures. The measure of setting the new guide plate is finally conducted, and the output power fluctuations vanish. The flow calculation plays a role in the analysis and optimization of the hydraulic system of the hydropower unit. Some rules are summarized from the cases and are helpful in the design of trifurcation in a division system with a common penstock.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
•The cavitating flow around a tip vortex is simulated with LES.•The reason for the discrepancy between the predicted and observed TVC is discussed.•An Euler-Lagrangian cavitation model is proposed to ...improve the accuracy of TVC.
Numerical simulation of tip vortex cavitation (TVC) remains a challenging task in a variety of applications, such as axial turbines and pumps as well as marine propellers. Although it is well known that TVC is highly sensitive to gas content, be it dissolved or not, numerical models do not consider this aspect so far. As a result, numerical simulations usually underestimate the development or the intensity of TVC. In the present paper, we propose an new Euler-Lagrangian cavitation model based on Rayleigh-Plesset (R-P) equation, taking into account the non-condensable gas. In this model, the Euler method is used to solve the global flow field and the Lagrangian method is used to track the migration of non-condensable gas bubbles into the vortex core. Based on the simplified R-P equation, the connection between local gas concentration and its effect on cavitation is modeled and the mass source terms in the original Schnerr-Sauer (S-S) cavitation model are modified. We applied the new cavitation model to a simplified case study, made of an elliptical NACA-16020 hydrofoil and compared the results with experimental observation. We obtained a significant improvement of the TVC prediction. Our work illustrates the major role of the gas content in sustaining cavitation downstream of the hydrofoil through an efficient attraction of nuclei, fueled by the low pressure induced by the vortex flow.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•A two-way coupling Euler-Lagrange method is presented to investigate the multi-scale characteristics of unsteady cavitating flow.•A microbubble generation model is proposed to explain the formation ...mechanism of cloud cavitation.•The breaking ocean wave theory is introduced to statistical analysis the inner structure of cloud cavitation.
The inner structure and the formation mechanism of cloud cavitation around a twisted hydrofoil was numerically investigated using a multi-scale method. A homogeneous mixture model was used to capture the macroscopic vapor structures, while the microbubble behavior was tracked by Lagrangian model. The spatial and temporal evolution of the multi-scale cloud cavitation is analyzed with special emphasis on the microscopic bubble dynamics. A microbubble generation model was proposed to explain the source of the numerous bubbles inside the cloud cavity. The shear layer at the liquid-vapor interface was found to be an important cause of the bubble formation. Furthermore, a statistical analysis of the large number of bubbles inside the cloud cavity showed two distinct bubble size spectra, N(D), which is similar to the theory for breaking ocean waves. Two mechanisms were established to explain this phenomenon. The first is when the attached cavity pinches off and rolls up which creates small bubbles conforming to Ns(D) ∼ D-1.26 with the bubble generation controlled by shearing effects. The second is when the vortex structures absorb bubbles into their core, where the low local pressure allows them to grow resulting in larger bubble size satisfying Nb(D) ∼ D-2.8. When the U-shaped cloud collapses, it breaks into a mass of tiny bubbles with a bubble size spectrum conforming to Ns(D) ∼ D-1.26 as well as a significantly greater number of larger bubbles that conform to Nb(D) ∼ D-2.2, which is thought to be due to both vortex and turbulent fragmentation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Cavitating flow around a hydrofoil is simulated by a compressible solver in OpenFOAM.•A new dynamic cubic nonlinear sub grid-scale model is applied in cavitating flow.•The pressure wave ...characteristics and its origin are demonstrated.•The difference of the re-entrant jet and the condensation shock are compared.
Cavity shedding mechanisms, such as those due to re-entrant jets (RJ) and condensation shocks (CS), are important but challenging topics in cavitating flows. This study investigates unsteady cavity shedding around a NACA66 hydrofoil using a self-defined compressible cavitation solver based on OpenFOAM with a dynamic cubic nonlinear subgrid-scale model. The predictions give satisfactory agreement with experimental data for the quantitative pressure evolution and the cavity shedding behavior induced by the re-entrant jet and the pressure wave. Moreover, the numerical data provides deeper insight into the pressure wave characteristics (e.g. propagation speed, wave intensity, shock Mach number, etc.) during cavity contraction. Multi-perspective analyses demonstrate that the pressure wave is a condensation shock which is responsible for the attached cavity abruptly disappearing. Additionally, the different influences of the re-entrant jet and the condensation shock on the local flow patterns are compared in detail in terms of the duration, average motion velocity, cavity behavior, formation mechanism and pressure intensity. The results show the condensation shock is characterized by fast propagation speed, short duration and high pressure pulse, which differs from the re-entrant jet features. This research provides an improved understanding of the cavity shedding mechanism around a cavitating hydrofoil and demonstrates the effectiveness of the current compressible cavitation solver.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Noises induced by sheet and tip leakage vortex (TLV) cavitating flows are studied with the FW-H equation.•Relationship between cavitation noise and the evolution of cavities is ...analyzed.•Pseudo-thickness and pseudo-loading terms of cavitation noise are analyzed and compared.
In the present paper, cavitation noise induced by sheet cavity and tip leakage vortex cavity (TLVC) together generated by a NACA0009 hydrofoil are numerically investigated with Ffowcs Williams-Hawkings (FW-H) equations. Large eddy simulation (LES) combined with Schnerr-Sauer cavitation model is utilized and a satisfactory result is obtained as compared with the referenced experimental measurements. It is found that even though the volume of TLV cavity is much smaller than that of sheet cavity, their acoustic levels are comparable to each other. A close relationship is indicated between cavity evolutions and their radiated noises, which vary periodically for both sheet and TLV cavities. Moreover, intense sound pulses of pseudo-thickness and pseudo-loading noise are observed especially during the collapse process. It is indicated that the rise in far field pressure and action of the dilatation term inside the cavities may trigger the collapse of both kinds of cavities in the flow, resulting in sound impulses. It is worth mentioning that in agreement with previous investigations, high pressure distribution and jets at the ends of sub-TLVCs are noted to have a close relationship with the collapse of sub-TLVCs. Noise generated by sheet cavity is higher than that generated by TLV cavity at the dominant Strouhal number of 0.74, with relatively high values of both pseudo-thickness and pseudo-loading noises at the dominant Strouhal number. While noise of TLV cavitating flow has acoustic energy concentrated at either low Strouhal numbers or high Strouhal numbers for the two terms, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•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
In the current paper, large eddy simulation is utilized to study the effect of various wavy leading edges on the control of cavitation around a modified NACA634-021 hydrofoil in the case of equal ...lift. Four sinusoidal wavy leading edges with different geometry parameters are numerically examined, with the baseline hydrofoil as a comparison. The numerical results are compared with available experimental data and a reasonable agreement is observed. The key flow characteristics, e.g., quasi-periodic behavior of cavity, time histories of pressure fluctuations and the effect of cavitation on streamwise vortices are compared in detail. The results show that in the case of equal lift, which is achieved by adjusting the angle of attack, no significant decrease in the volume of cavity is observed for the cases of wavy leading edges. This illustrates that none of the four different types of wavy leading edges introduced in this paper can appreciably suppress the generation and development of cavitation bubbles. However, our results indicate that the pressure fluctuations can be effectively suppressed, which is primarily caused by the compartmentalization effect of the leading edge protuberances. Meanwhile, the suppression capability decreases with an increasing amplitude-to-wavelength ratio. Further analysis shows that cavitation is the main source of affecting the development of the streamwise vortices.
•The cavitating hydrofoils with various wavy leading edges are studied by LES.•The cavity volume is not effectively suppressed by wavy leading edges in the case of equal lift.•The pressure fluctuations can be effectively suppressed by wavy leading edges.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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