In this paper, the S-CLSVOF (simple coupled level-set and volume of fluid) and BM (bifurcation model) models are used for the accurate prediction of the flow in an open pump intake with a vertical pipe. The continuous wavelet transform, which is suitable for the vortex detection is applied to the pressure signals near both air-entrained and subsurface vortices. Low-frequency with long duration for air-entrained vortex due to the vortex wandering and broadband with short duration for floor vortices (dominant in subsurface vortices) due to the generation of extreme strong vortex are observed. The vortex motion mechanism is revealed by the analysis of the transport equation of the vertical vorticity's enstrophy. Different from previous results in which the tilting effect controls the vortex motion, the stretching effect is found to be dominating when it is large enough. When going through the bell mouth, the air-entrained vortex's vorticity changes the sign and strengthens the vortex with the same sign. On the plane near the bell mouth, three vortex patterns including co-rotating pair, merging and counter-rotating pair are observed. The onset criterion of the vortex merging at a/b = 0.29~0.32 is found to be applicable to the present case. The counter-rotating pair is found to be more stable. Air-entrained vortex serves as an amplification of the strong vortices generated from subsurface.

Purpose This study aims to develop and validate a new cavitation model that considers thermodynamic effects for high-temperature water flows. Design/methodology/approach The Rayleigh-Plesset equation and B-factor method proposed by Franc are used to construct a new cavitation model called thermodynamic Zwarte-Gerbere-Belamri (TZGB) by introducing the thermodynamic effects into the original ZGB model. Furthermore, the viscous term of the Rayleigh-Plesset equation is considered in the TZGB model, and the model coefficients are formulated as a function of temperature. Cavitating flows around the NACA0015 hydrofoil under different water temperatures (25 degrees C, 50 degrees C and 70 degrees C) at the angle of attack of 5 degrees are calculated. Findings Results of the investigated temperatures show good agreement with the available experimental data. Given that the thermodynamic and viscosity effects are included in the TZGB model and the model coefficients are treated as a function of temperature, the TZGB model shows better performance in predicting the pressure coefficient distribution and length of cavity than the original ZGB cavitation model and other models do. The TZGB model aims to determine the thermodynamic and viscosity effects and perform better than the other models in predicting the mass transfer rate, particularly in high-temperature water. Originality/value The TZGB model shows potential in predicting the cavitating flows at high temperature and the computational cost of this model is similar to that of the original ZGB model.

This study aims to research the prediction performance of a bifurcated adaptive DDES (BADDES) model in different turbulent flows with rotation and curvature

Purpose: This study aims to research the prediction performance of the bifurcation approach with different base models in different kinds of turbulent flows with rotation and curvature.Design/methodology/approach: The k−ω and Shear-Stress Transport (SST) k−ω models are modified by using the complete eddy viscosity coefficient expression, and the latter is modified by using two sets of model coefficients. The two bifurcation models were tested in three cases: rotating channel flow with system rotation, Taylor–Couette flow with wall rotation and curvature effect and swirling flow through an abrupt axisymmetric expansion with inlet swirling flow.Findings: In these flows, the bifurcation approach can significantly improve the prediction performance of the base model in the fluctuation velocity. The deviation of the BSkO model is slightly superior to the BkO model by about 2% in the Taylor–Couette flow. The prediction effect of the root-mean-square (RMS) velocity of the BSkO model increases by about 4–5% as the number of grids increases about 2.37 times, and the best is the Large Eddy Simulation (LES) grid used. Finally, compared with the SST k−ω model, the average iteration time of the SST with curvature correction (SST-CC), bifurcation k−ω (BkO) and bifurcation SST k−ω (BSkO) models increased by 27.7%, 86.9% and 62.3%, respectively.Originality/value: This study is helpful to understand further the application of the bifurcation method in the turbulence model.

This study aims to investigate the critical state of the saddle-shaped unstable region of the axial-flow pump and propose a suitable criterion for identifying this state. The bifurcation SST $k–\omega$ model considered the rotation effect is used in the present work and verified in the numerical calculation of a water jet pump. Then, it is used to simulate the critical state of the axial-flow pump. Results show that the leading-edge separation vortex generates at 0.6Q_d, while the head declines only at 0.55Q_d. Therefore, using the inflection point of the head-flow curve as the critical state criterion is unsuitable. In addition, the fixed monitoring point is unsuitable for identifying the critical state due to the insensitivity to the amplitude, main frequency, and periodicity changes at the critical state. Finally, to identify the critical state, it is essential to arrange a monitoring point at the leading edge of the blade suction near the shroud, which should rotate with the impeller. The critical state criterion is that the main frequency position of the pressure fluctuation signal is offset at the monitoring point, and the amplitude is increased by 10 times.