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

The tip-leakage vortex (TLV) cavitation is a challenging issue for a variety of axial hydraulic turbines and pumps from both technical and scientific viewpoints. The flow characteristics of the TLV cavitation were widely studied in the past decades, but the knowledge about the tip-leakage cavitating flow is still limited. The present paper reviews the progresses in the researches of the TLV cavitation, including the numerical methods for the TLV cavitation, the flow characteristics of the TLV, the influences of the TLV cavitation on the local flow field and the control strategies of the TLV cavitation. It is indicated that the non-condensable gas may play an important role in the development of the TLV cavitation, and this fact should be considered during a careful simulation of the TLV cavitation. It is also suggested that the development of the TLV cavitation will significantly influence the distributions of the vorticity and the turbulence kinetic energy. Due to the complexity of the TLV cavitation, it is still an open question how to suppress the TLV cavitation in a simple but effective way. Finally, based on these understandings, some advanced topics for the future work are suggested to further promote the study of the TLV cavitation, for a deeper knowledge about the TLV cavitation.

Comparative study between turbulence models in unsteady cavitating flow with special emphasis on shock wave propagation

Suppressing tip leakage vortex (TLV) cavitation in rotating machinery, such as propellers and turbines, plays a crucial role in reducing hydrodynamic noise, mitigating cavitation erosion, and enhancing operational stability. This study proposes a composite tip structure that integrates squealer rims with jet holes to suppress TLV cavitation in a synergistic manner. Large eddy simulation coupled with the Schnerr–Sauer cavitation model is employed to simulate TLV cavitating flow around the National Advisory Committee for Aeronautics 0009 hydrofoil. The results indicate that the squealer rim enlarges the tip separation vortex while reducing its intensity and inhibiting its migration toward the suction side. Meanwhile, the jet holes induce a reverse jet driven by the pressure difference between the pressure side and the tip region, significantly reducing both the streamwise and vertical velocities of the leakage flow and altering TLV evolution. At an angle of attack of 10°, the composite tip structure reduces the time-averaged tip cavity volume by 11.1%, 38.2%, and 63.7% for tip clearance widths of τ = 0.2, 0.6, and 1.0, respectively. The lift-to-drag ratio increases by 1.43% under τ = 0.2, with minor reductions observed at larger clearance widths. Furthermore, the effectiveness of the composite tip structure in suppressing TLV cavitation is evaluated across various angels of attack.

Mitigation of flow-induced pressure fluctuations in a Francis turbine operating at the design and partial load regimes—LES simulations

Numerical simulation unsteady cloud cavitating flow with a filter-based density correction model

Unsteady Large Eddy Simulation (LES) is carried out for the flow around a bluff body equipped with an underbody rear diffuser in close proximity to the ground, representing an automotive diffuser. The goal is to demonstrate the ability of LES to model underbody vortical flow features at experimental Reynolds numbers (1.01 × 106 based on model height and incoming velocity). The scope of the time-dependent simulations is not to improve on Reynolds-Averaged Navier Stokes (RANS), but to give further insight into vortex formation and progression, allowing better understanding of the flow, hence allowing more control. Vortical flow structures in the diffuser region, along the sides and top surface of the bluff body are successfully modelled. Differences between instantaneous and time-averaged flow structures are presented and explained. Comparisons to pressure measurements from wind tunnel experiments on an identical bluff body model shows a good level of agreement.

Experimental and numerical investigation on the pressure pulsation and instantaneous flow structure in a nuclear reactor coolant pump

The tip leakage vortex (TLV) cavitation mechanism of axial flow pump was investigated with the results of high speed photography and pressure pulsation measurement. The tip leakage vortex cavitation morphology and the transient characteristics of the TLV-induced suction-side-perpendicular cavitating vortices (SSPCV) were analyzed under different flow rates and different cavitation numbers which were combined with the time domain spectrum of pressure fluctuation to elucidate the relationship between the tip cavitation and pressure pulsation. The results showed that cavitation inception occurs earlier with more unstable tip leakage vortex cavitation shape under part-load flow rate condition, and the cavitation is more intense with the decrease of the cavitation number. The inception of SSPCV is attributed to the tail of the shedding cavitation cloud originally attached on the suction side (SS) surface of blade, moving toward the adjacent blade perpendicular to the suction surface, resulting in a flow blockage. With further decrease of pressure, the SSPCVs grow in size and strength, accompanied with a rapid degradation in performance of the pump. The cavitation images and the corresponding circumferential pressure distribution with the same phase showed that the lowest pressure coincides with the suction surface (SS) corner, The pressure was found to decrease along with the occurrence of the cavitation structure.

Unsteady cavitating flow usually contains various vapor structures with different length scales and significant fluid compressibility, which poses great challenges to numerical simulation. In this work, we propose a compressible Eulerian–Lagrangian method to investigate cloud cavitation inside a nozzle, with particular emphasis on the effect of compressibility on multiscale structures. The macroscopic cavity evolution is dealt with using large eddy simulation and the volume of fluid method in a compressible Eulerian framework. Microscopic bubble dynamics are described using the equation of bubble motion and the compressible Rayleigh–Plesset equation. A two-way coupling algorithm is established to bridge the two frameworks. The calculated results are validated by comparison with available experimental data. The numerical results show that the compressibility exacerbates the instability of the cavitating flow field, generates more discrete bubbles, and promotes liquid–vapor mass exchange, which further increases the overall cavitation volume. Furthermore, two distinct power laws for bubble size distribution, with exponent −4/3 for small bubbles and −10/3 for large bubbles, are obtained. Owing to the influence of the high flow velocity and high ambient pressure in the nozzle, and the wall effects on bubble dynamics, the number of large-size bubbles calculated by the numerical simulation is greater than that obtained by theoretical analysis. The formation of these bubbles is affected by shear on the liquid–vapor interface, the presence of vortex structures, and turbulence, thus differing significantly from the case of cavitation flow around a hydrofoil.

We investigated the three-dimensional characteristics in the unsteady cavitating flow over an axisymmetric body with a blunt headform. A high-speed video camera was used to visualize the cavitating flow structures. A modified PANS (Partially-averaged Navier-Stokes) turbulence model coupled with Kubota cavitation model was used to simulate the time-evolution process of the unsteady cavitating flows. The finite-time Lyapunov exponent (FTLE) and Lagrangian coherent structures (LCS) methods are applied to investigate the three-dimensional flow patterns. The results show that the numerical method is capable of simulating the unsteady cavitating flow for capturing the attached cavity with maximum-length, the cavity break-off and U-type shedding, in accordance with the qualitative features observed in the experiment. Three-dimensional characteristics are extremely impressive in the unsteady cavitating flows. The re-entrant flows originate from different circumferential positions at the closure region of the attached cavity, and move upstream at different speeds. Moreover, substantial circumferential motions in the unsteady cavitating flow are found based on Lagrangian trajectories, and it is reasonably deduced that the U-type shedding cavity is primarily induced by the effect of re-entrant flow motions consisting of moving upstream and circumferential flows as well.

Unsteady cavitating flow with high Reynolds number and significant instability commonly exists in fluid machinery and engineering system. The high-resolution approaches, such as direct numerical simulation and large eddy simulation, are not practical for engineering issues due to the significant cost in the computational resource. The objective of this paper is to provide the approach with Detached-Eddy Simulation (DES) model based on the Reynolds-averaged Navier–Stokes (RANS) equations for predicting unsteady cavitating flows. The credibility of the approach is validated by a set of numerical examples of its application: the unsteady cavitating flows around the two-dimensional (2D) Clark-Y hydrofoil and the three-dimensional (3D) blunt body. It is found that the calculated cavity shapes, cavity lengths and unsteady characteristics by DES model agree well with the experimental measurements and observations. Further analysis indicates that the turbulent eddy viscosity around the cavity and wake region is well predicted by the DES model, which results in the development of large-scale vortexes, and further cavitation instability. The DES model, which exhibits a significantly unsteady 3D behavior, is a more comprehensive turbulence model for unsteady cavitating flows.

Numerical study of flow physics in supersonic base-flow with mass bleed

Physical Modalities of Therapy in Pediatric Rheumatic Diseases

The flow and heat transfer within rotating cavities is often discussed as a conjugate problem: the temperature distribution within the cavity disks drives the large-scale flow structure within the cavity, and the cavity aerodynamics influence the heat transfer to the disks. However, most simulations of rotating cavities only consider the fluid domain in isolation. This is particularly true for turbulence resolving approaches such as large eddy simulation (LES). The large timescale disparity between the fluid time steps used in LES and the characteristic solid time-scale complicates the use of LES with conjugate heat transfer (CHT). A further issue is that an under-resolved solid domain mesh can artificially amplify higher frequency temperature fluctuations in the fluid domain. This paper addresses these challenges with a new method for LES-CHT where the low-frequency temperature fluctuation caused by the large-scale flow structure is accounted for using a multi-scale frequency domain approach. We investigate two cases: axially heated disks made of a low conductivity material and disks made from a higher conductivity material with a temperature set by radial conduction from the shroud. The formation of small-scale flow structures on both the disk and shroud is dependent on the heating configuration of the cavity — this interaction indicates that high-fidelity thermal boundary conditions should be used when simulating rotating cavities. The formation of heating induced vortical flow structures on the disk is particularly interesting, as this is unexpected from the laminar Ekman layer modelling argument usually used to consider this region of the flow.

Considering that large eddy simulation (LES) often requires more computational resources to capture small-scale flow characteristics, while the Reynolds-Averaged Navier–Stokes (RANS) methods offers better computational efficiency at the expense of accuracy. The traditional Zwart–Gerber–Belamri cavitation model (Z–G–B) has good universality but insufficiently considers the influencing factors of cavitation. This article proposes a method combining very large eddy simulation (VLES) with the Z–G–B cavitation model (VLES_Z–G–B). However, there remains a certain discrepancies between the predictions and experimental results. To address this, on the basis of the Z–G–B cavitation model, the unsteady cavitation computation method (VLES_Water_Non) is developed, considering the weak compressibility effect of the fluid and incorporating three components: the liquid, vapor, and non-condensable gas. The results indicate that the VLES_Water_Non method compensates for the deficiencies of the VLES_Z–G–B approach, and clarifying that the combined effect of re-entrant flow, side-incident reverse flow, and mainstream are the primary causes of cavity shedding. Using a robust Ω-criterion, multiscale flow features during the cavity evolution process were identified. Additionally, it was found that the vortex stretching and dilation terms are the main reason for the flow separation and formation of complex structures around the hydrofoil. The baroclinic torque term has an significant impact on the generation of vorticity during the shedding process of the cavity. This study is expected to provide a more precise scale resolution method and predictive accuracy for analyzing the internal flow mechanisms of complex fluid machinery.