Numerical Study of Unsteady Cavitating Flow in an Inducer with Omega Vortex Identification

To accurately capture the behaviors of cavitation and reveal the unsteady cavitating flow mechanism, a condensate pump inducer is numerically analyzed in a separate numerical experiment with LES at critical cavitation number sind,c under the design point. Based on the new Omega vortex identification method, the correction between the flow structures and cavities is clearly illustrated. Besides, the pressure fluctuations around the inducer are analyzed. Special emphasis is put on the analysis of the interactions between the cavities, turbulent fluctuations, and vortical flow structures. The Omega vortex identification method could give an overall picture of the whole cavitating flow structures to present a clear correlation between the vortices and cavities. The results show that the shear cavitation dominant the cavitation characteristics under the design point. The pure rigid rotation region mainly concentrates at the edge of the cavities while the other sheet-like cavities near the casing walls are characterized by strong turbulence fluctuations. Besides, based on the analysis of the correlation between the cavities and flow structures, the rotating cavitation under the design point may mainly attribute to the interaction between the tip leakage vortex cavitation and the next blade.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

Les Investigations of Periodic Cavitation Shedding with Special Emphasis On Three-Dimensional Asymmetry in a Scaled-Up Nozzle Orifice

The periodic shedding of cloud cavitation in a nozzle orifice has a significant influence on the flow field and may have destructive effects. Most of the existing research on the shedding of cloud cavitation in an orifice is based on experimental visualization with focus on the 2D motion of the re-entrant jet and the shedding mechanism. However, the actual cloud cavitation shedding in an orifice is a complex 3D process. Some limited signs of three-dimensionality and asymmetry in cylindrical orifices have been detected recently, but the 3D shedding characteristics remain unclear. In this paper, the cavitation regimes and periodic shedding process in the scaled-up nozzle orifice used by Stanley experiment were simulated with Large Eddy Simulation (LES). The re-entrant jet and periodic shedding mechanism, as well as the shedding frequency, were analyzed from 2D and 3D perspectives. The main results show that the simulated cavitation regimes and the 2D periodic shedding mechanism agree fairly well with the experimental observations, but more 3D features are revealed. By analyzing the 3D shedding process and the three-dimensionality caused by the inclination of the closure line, the three-dimensional asymmetric shedding mode with phase difference p is revealed. Based upon this finding, the shedding frequency and Strouhal number are calculated. The corresponding relationships between shedding frequencies and the frequency peaks of the power spectrum density (PSD) for pressure fluctuations are also confirmed. These results extend the understanding of the unsteady cavitating flow within nozzle orifices from 2D to 3D patterns.

Lagrangian Analysis of Unsteady Partial Cavitating Flow Around a Three-Dimensional Hydrofoil

This study employs an incompressible homogeneous flow framework with a transport-equation-based cavitation model and shear stress transport turbulence model to successfully reproduce the unsteady cavitating flow around a three-dimensional hydrofoil. Cavity growth, development, and break-off during the periodic shedding process are adequately reproduced and match experimental observations. The predicted shedding frequency is very close to the experimental value of 23 ms. By monitoring the motions of the seeding trackers, growth-up of attached cavity and dynamic evolution of U-type cavity are clearly displayed, which indicating the trackers could serve as an effective tool to visualize the cavitating field. Repelling Lagrangian Coherent Structure (RLCS) is so complex that abundant flow patterns are highlighted, reflecting the intricacy of cavity development. The formation of cloud cavities is clearly characterized by the Attracting Lagrangian Coherent Structure (ALCS), where bumbling wave wrapping the whole shedding cavities indicates the rotating transform of cavities and stretching of the wave eyes shows the distortion of vortices. Generation of the re-entrant jet is considered to be not only associated with the adverse pressure gradient due to the positive attack angle, but also the contribution of cloud cavitating flow, based on the observation of a buffer zone between the attached and cloud cavities.

Numerical investigation on cavitation instability and flow-induced vibration of liquid rocket engine inducer

The objective of this paper is to numerically investigate the unsteady cavitating flow around a four-blade inducer, with focus on the cavitation instability and the flow-induced vibration characteristics. In the numerical simulation, the modified rotation/curvature correction turbulence model and the Zwart cavitation model are used for the simulation of the flow field. The tightly coupled algorithm is adopted for the precise prediction of the fluid-structure interaction, including the calculation of the hydrodynamic loads based on the multiphase fluid dynamics and the computation of the structural displacement via the Finite Element Method (FEM). The results showed that good agreement has been obtained between the experimental and numerical results. The fluctuation of cavity volume is the main cause of the change in the head of the inducer, and the backflow vortex cavitation has little effect on that at this flow condition. The backflow vortex cavity develops and rotates with the blades of the inducer, but with a much lower rotational velocity than that of the blades. The flow-induced vibration of the inducer caused by the unsteady cavitating flow mainly manifests as a first-order bending mode. The backflow vortex cavitation has a significant impact on the vibration of both the blades and the guide-water cone. Besides, a cavitation auto-oscillation at the inlet of the inducer has also been detected based on the phase correlation analysis.