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Rewetting is the establishment of water–surface contact that occurs during quenching of high temperature surfaces by water jet impingement. Rewetting is an unexpectedly complex phenomenon that has been reported to occur at surface temperatures significantly higher than the superheating limit of water. The presence of intermittently wet and dry episodes, and in particular the occurrence of so-called explosive boiling, is one of the theories to explain the contact of water with high temperature surfaces. However, there is a lack of experimental data in the literature to prove the presence of explosive boiling and intermittent wetting due to the small duration and scale of the rewetting phenomenon. In this study, recordings of the jet stagnation zone during rewetting are provided at a frame rate of 81 kfps. The high-speed recordings show a flashing regime consisting of intermittent (dry) bubble-rich and (wet) bubble-free periods at frequencies up to 40 kHz when the rewetted surface temperature exceeds the water superheat limit. As far as the authors know, these are the first direct observations of intermittent dry–wet periods occurring in the jet stagnation zone during quenching by water jet impingement. The dependency of the flashing frequency on initial surface temperature is quantified. A correlation between the size of the rewetting patch and the flashing frequency is found. Finally, a hypothesis to explain the role of water subcooling in maintaining the water–surface contact at surface temperatures well above the superheating limit of water is presented.

The vertical volute centrifugal pump is widely used in the long-distance water division project. Flow separation and vortex phenomenon can be observed in the vaned diffuser under near stall operating conditions. This kind of unsteady flow structures would result in the instability of the pump unit which affect the safety and reliability of the unit operation. In this paper, the unsteady flow patterns and induced pressure fluctuation in a vertical volute centrifugal pump with vaned diffuser were simulated based on the SAS turbulence model and refined mesh under near stall conditions. The research results show that the SST turbulence model combined with the selected grid can be validated in the CFD uncertainty analysis during the steady calculation based on the ITTC relevant procedures. The performance results predicted by the SAS turbulence model show quite good agreement with the experimental data, and the prediction errors were less than 5%. The 3D flow matching between the flow angle at the impeller blade outlet and the incidence angle of the guide vane at different spans would induce the stall inception of the pump. The flow patterns of the pump under near stall conditions were revealed combined with vorticity and streamline distributions. When the pump unit was operated under near stall conditions, the main frequency changed from the blade passing frequency 7fn to 0.9fn. It was found that the main frequency was mainly affected by the large-scale vortices in the vaned diffuser, and the main vortices in the vaned diffuser appear at every 0.9 time of the period of one revolution of impeller.

The aim of the present investigation is to simulate and analyze the tip leakage flow structure and instantaneous evolution of tip vortex cavitation in a scaled axial-flow pump model. The improved filter-based turbulence model based on the density correction and a homogeneous cavitation model were used for implementing this work. The results show that when entering into the tip clearance, the backward flow separates from the blade tip near the pressure side, resulting in the generation of a corner vortex with high magnitude of turbulence kinetic energy. Then, at the exit of the tip clearance, the leakage jets would re-attach on the blade tip wall. Moreover, the maximum swirling strength method was employed in identifying the TLV core and a counter-rotating induced vortex near the end-wall successfully. The three-dimensional cavitation patterns and in-plain cavitation structures obtained by the improved numerical method agree well with the experimental results. At the sheet cavitation trailing edge in the tip region, the perpendicular cavitation cloud induced by TLV sheds and migrates toward the pressure side of the neighboring blade. During its migration, it breaks down abruptly and generates a large number of small-scale cavities, leading to severe degradation of the pump performance, which is similar with the phenomenon observed by Tan et al. [35].

The cavitation shedding flow around a 3-D Clark-Y hydrofoil is simulated by using an improved filter-based model (FBM) and a mass transfer cavitation model with the consideration of the maximum density ratio effect between the liquid and the vapor. The unsteady cloud cavity shedding features around the Clark-Y hydrofoil are accurately captured based on an improved FBM model and a suitable maximum density ratio. Numerical results show that the predicted cavitation patterns and evolutions compare well with the experimental visualizations, and the prediction errors of the time-averaged lift coefficient, drag coefficient and Strouhal number St for the cavitation number σ = 0.8, the angle of attack at α = 8° Reynolds number Re = 7 × 105 are only 3.29%, 2.36% and 9.58%, respectively. It is observed that the cavitation shedding flow patterns are closely associated with the vortex structures identified by the Q-criterion method. The predicted cloud cavitation shedding flow shows clearly three typical stages: (1) Initiation of the attached sheet cavity, the growth toward the trailing edge. (2) The formation and development of the re-entrant jet flow. (3) Large scale cloud cavity sheds downstream. Numerical results also indicate that the non-uniform adverse pressure gradient is the main driving force of the re-entrant jet, which results in the U-shaped cavity and the 3-D bubbly structure during the cloud cavity shedding.

The unsteady cavitation evolution around the Clark-Y hydrofoil is investigated in this paper, by using an improved filter-base model (FBM) with the density correction method (DCM). To improve the prediction accuracy, the filter scale is adjusted based on the grid size. The numerical results show that a small filter scale is crucial for the unsteady simulations of the cavity shedding flow. The hybrid method that combines the FBM and the DCM could help to limit the overprediction of the turbulent viscosity in the cavitation region on the wall of the hydrofoil and in the wake. The large value of the maximum density ratio, ρl/ρv, clip promotes the mass transfer rate between the liquid phase and the vapor phase, which results in a large sheet cavity length and the vapor fraction rise inside the cavity. The cavity patterns predicted by the improved method are verified by the experimental visualizations. The time-average lift, the drag coefficient and the primary oscillating frequency St for the cavitation number σ = 0.8, the angle of attack, α=8α, at a Reynolds number Re = 7×10 are 0.735, 0.115 and 0.183, respectively, and the predicted errors are 3.29%, 3.36% and 8.93%. The typical three stages in one revolution are well-captured, including the initiation of the sheet/attached cavity, the growth toward the trailing edge (TE) with the development of the re-entrant jet flow, and the large scale cloud cavity shedding. It is observed that the cloud cavity shedding flow induces the vortex pairs of the TE vortices in the wake and the shedding vortices. The positive vorticity vortex of the re-entrant jet and the TE vortices interacts and merges with the negative vorticity vortex of the leading edge (LE) cavity to produce the shedding flow.

The complex flow field caused by the dynamic stall can affect the operational stability of hydrodynamic machinery. In this paper, the NACA0009 blunt trailing edge hydrofoil is used as the object of study, and the dynamic stall characteristics of the hydrofoil are investigated by using the transition model and the dynamic mesh method. It is found that the hydrofoil deep stall calculated by the transition model is delayed compared to that calculated without the transition model. The hydrofoil dynamic stall can be divided into four stages, initial stage, development stage, stall inception stage and deep stall stage. In the initial stage and the development stage, the lift and drag characteristics are influenced by the shedding vortex. In the stall inception stage and the deep stall stage, the lift and drag characteristics are influenced by the leading edge separation vortex and the trailing edge vortex. The increase of angular velocity and Reynolds number of the dynamic hydrofoil delay the onset of the deep stall while accelerating the boundary layer transition. The research in this paper has a certain guiding effect for the safe and stable operation of hydrodynamic machinery.

Impeller hub corner separation flow (IHCS) has a significant influence on energy conversion of the bulb tubular pump, and its unsteady characteristics are investigated with CFD-based method. The generation mechanism and power loss characteristics of IHCS are investigated by the entropy production method and pressure fluctuation analysis. The main cause can be attributed to the large transverse pressure gradient near the hub at the trailing edge of the impeller, which is aggravated by the circumferential movement trend and the diffuser reverse flow, while the IHCS is significantly weakened with increased flow rate. The undesirable flow behavior is more likely to cause a significant increase in energy loss near the hub region compared to that in the rim region. The relative vortex stretching induced by the velocity gradient is the main cause of the horn-like vortex (HLV), and its intensity and resulting energy loss tend to decrease along the vortex trajectory. The HLV changes the dominant frequency of the pressure fluctuations in the nearby flow field, the value of which increases from 1 f r (the blade passing frequency) to 2 f r with increased amplitude, mainly due to the lower pressure regions on the impeller suction surface (SS) and HLV vortex core. Due to the effect of rotor-stator interaction (RSI), the HLVs generated between two adjacent impeller blades are cut into several sections by the diffuser vanes and propagate and dissipate along the mainstream direction.

Boundary layer transition is one of the key factors affecting the hydrodynamic characteristics of hydrofoil. In order to explore the capability of a transition model coupled to different turbulence models in the prediction of hydrofoil boundary layer transition, the flow field around the NACA0009 blunt-edge hydrofoil at 0°angle of attack was calculated using γ-Reθt transition model coupled with SST, SST-DES, SST-SAS, SST-DDES turbulence models. The characteristics of the hydrofoil boundary layer at ReL=2*106 are analysed. The experimental data and numerical calculation results are compared. The results show that the four turbulence models differ quite a bit in simulating hydrofoil boundary layer transition at the same calculation conditions. The flow parameters of the hydrofoil boundary layer calculated by the transition model coupled with SST-DDES are closer to the experimental results.

In order to improve the efficiency of high-pressure liquid recovery and utilization in the desalination industry, a forward curved blade turbine in the desalination energy recovery integrated machine is investigated as the research object in this article. The optimization research of the flow components is completed, and the transient pressure pulsation characteristics of the turbine before and after optimization are analyzed. The numerical simulations are performed using Ansys CFX software. The study found that the volute pressure pulsation is mainly affected by the rotor-stator interaction, and its amplitude is proportional to the distance from the impeller. The intensity and range of the pressure fluctuation in the impeller are mainly affected by the flow separation effect, among which the separation vortex and secondary vortex cause low-frequency pulsation in the flow channel. The pressure pulsation amplitude of the optimized geometry is reduced compared with the initial geometry under various operating conditions. The dead water area of the flow field in the impeller flow channel is reduced, and the low work capacity caused by the low pressure on the leading edge pressure surface of the blade under small flow conditions is improved. This enhancement is conducive to increasing the pressure bearing capacity of the blade and reducing the possibility of blade damage due to uneven force. The optimization contributes to the stable operation of the turbine.

Background The concept of osteoarthritis (OA) heterogeneity is evolving and gaining renewed interest. According to this concept, distinct subtypes of OA need to be defined that will likely require recognition in research design and different approaches to clinical management. Although seemingly plausible, a wide range of views exist on how best to operationalize this concept. The current project aimed to provide consensus-based definitions and recommendations that together create a framework for conducting and reporting OA phenotype research. Methods A panel of 25 members with expertise in OA phenotype research was composed. First, panel members participated in an online Delphi exercise to provide a number of basic definitions and statements relating to OA phenotypes and OA phenotype research. Second, panel members provided input on a set of recommendations for reporting on OA phenotype studies. Results Four Delphi rounds were required to achieve sufficient agreement on 11 definitions and statements. OA phenotypes were defined as subtypes of OA that share distinct underlying pathobiological and pain mechanisms and their structural and functional consequences. Reporting recommendations pertaining to the study characteristics, study population, data collection, statistical analysis, and appraisal of OA phenotype studies were provided. Conclusions This study provides a number of consensus-based definitions and recommendations relating to OA phenotypes. The resulting framework is intended to facilitate research on OA phenotypes and increase combined efforts to develop effective OA phenotype classification. Success in this endeavor will hopefully translate into more effective, differentiated OA management that will benefit a multitude of OA patients.