Cavitation is arguably a highly turbulent phenomenon in the liquid flow system. The cavitating flow around a mini cascade was carried out to investigate the turbulent characteristics and pulsation mechanisms. The results demonstrate that cavitation can significantly affect the turbulence velocity fluctuation and turbulence anisotropy, and intensively alter the local turbulent energy. To better provide an understanding of fundamental mechanisms dictating time-averaged pulsating energy, the inhomogeneity of the local concentration of pulsating energy at the vapor-liquid interface and the turbulent vortex core involves different fundamental mechanisms are expounded thoroughly through the ability of the time-averaged turbulent kinetic energy and the time-averaged pulsating entropy. The pulsating energy of cavitating flow around the mini cascade is basically obtained from the time-averaged flow while the surrounding dissipative mechanisms are driven by the diffusion and dissipation terms. Further, the new definition of viscous diffusion term is derived based on the resolved turbulent kinetic energy, which can also clearly delineate the diffusion effect of turbulent kinetic energy produced by the molecule viscosity. Finally, the turbulent kinetic energy and pulsating enstrophy transport mechanisms inside the shedding vortex are revealed as significant characteristics of the interaction between vortex dynamics and turbulence-cavitation.
In this research, the cavitating flow around a NACA0015(National Advisory Committee for Aeronautics) hydrofoil obtained by the large eddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the re-entrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism and the temporal/spatial evolution are revealed. POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the lift and drag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. Besides, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Further, the vortex force and pressure gradient are reconstructed using the first 4 modes, 17 modes, and 160 modes, which indicates that the low-order POD modes without the impact of small-scale structures and noise can clearly capture the fundamental aspects of the flow field.
Water wheels used for power generation are applied to tailwater and ultra-low head sites. In this research, the VOF method and the standard k-ε turbulence model are utilized to predict the performance and transient flow fields of water wheels. The numerical results show a reasonable agreement with the experimental data. This work aims at improving the performance and increasing the internal fluid stability of the water wheel, based on the entropy production approach to research the detailed distribution of energy loss in the water wheel for power generation under the clearance effects between blades and hub. Under the same rotational speed, it is indicated that by setting appropriate clearance, the performance of the water wheel can be elevated by 8.7%, targeted elimination of vortical flow, improving flow adaptability, attenuating to a great extent of the backwater phenomenon, and reducing the fatigue damage of the hub and blade. Further, the interaction mechanism of vorticity–pressure which will induce irreversible energy loss of the water wheel under different clearance effects is investigated. Therefore, this research indicates that the entropy method can provide a theoretical reference and engineering guidance for the targeted optimization of water wheels.
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