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Pipeline vibration induced by fluid flow can cause pipeline fatigue damage, which seriously endangers the safety of the operation. This paper aims to clarify the dynamic variation law of pipeline vibration induced by gas–liquid two-phase flow in the elbow and the influence of different gas–liquid ratios on the dynamic evolution of two-phase flow patterns. The volume of fluid (VOF) method is used to capture the flow pattern characteristics to explore the complex information of dynamic flow pattern evolution. The vibration characteristic mechanism corresponding to the evolution of the flow pattern is explored based on the time and frequency domain, and the origin of the exciting force of the elbow is explored according to the momentum balance equation. The simulation results show that the VOF method can well capture the characteristics of slug flow developed by the interphase instability mechanism. After phase space reconstruction, the dynamic evolution mechanism of the flow pattern is complex and the chaotic characteristics of slug flow are strong. The evolution of the flow pattern is related to the increase in wave height. The superficial velocity of the liquid phase is more sensitive to the flow pattern's formation mechanism than the gas phases. The amplitude of the wave strongly depends on the Vsg and Vsl. There is a strong correlation between the main exciting force pulsation and momentum flux pulsation.

The wake field has three-dimensional flow separation characteristics, affecting various ship performances. The research on the characteristics of the wake field can provide theoretical guidance for the formation of new ship design methods and provide an understanding of flow mechanisms for improving ship performance. To explore the flow mechanism of the wake field, particle image velocimetry was used to carry out the detailed flow measurement of bulk carriers. First, the experimental uncertainty and convergence are analyzed. Then, the spatial distribution characteristics of the time-averaged field, instantaneous field, and turbulent flow statistics are discussed in detail, and a criterion for discriminating turbulent anisotropy is proposed. The results show that the vortex structure significantly affects flow characteristics, and the axial velocity contours present a U-form distribution with prominent “hook-like” features. Compared with the time-averaged velocity field, the instantaneous velocity field is chaotic and has multiple additional vortex structures, and the velocity contours and streamlines have prominent non-smooth characteristics. The wake field has an apparent double vortex structure, and the aggregation of many small vortices forms the bilge vortex. The instantaneous rotation characteristics of vortices in the wake field are highly time-dependent and fluctuate with time. The turbulent kinetic energy, the root mean square of fluctuation velocity, and the Reynolds stress have a U-form distribution. The U-form region is concentrated in the area with a large gradient. The wake distribution is in a state of turbulent anisotropy, and the kinetic energy change layer and low kinetic energy region have a low turbulent anisotropy.

Aquatic organisms in their natural environment have soft bodies and flexible mobility. Clarifying the generation, evolution, and dissipation of vortices and jets during turning maneuvers is crucial for understanding the propulsion principle of aquatic species, which, in turn, provides guiding value for fish-like propulsion device design. In this study, time-resolved particle image velocimetry is used to explore the kinematic and dynamic characteristics of Misgurnus anguillicaudatus while turning. The results showed that M. anguillicaudatus maintained the wavy movement of its trunk by bending different body parts. Pressure gradients that are weaker and stronger than the surrounding environment were formed at the peaks and troughs, respectively, resulting in a thrust mechanism dominated by suction. The body fluctuation and relative fluid motion served to form a vortex. The connection of the separation line of the saddle point to the focus in this process creates an unstable flow structure that accelerates vortex dissipation. Jets are formed between the reverse vortices; the thrust jets provide forward power for turning maneuvers, and the side jets provide turning torque. As the jets and tail are situated at angles to one another, only part of the jet-generated kinetic energy provides power for the fish to swim. Additionally, proper orthogonal decomposition is utilized for objectively filtering high-frequency spatial noise in complex fish wake data. The flow field reconstructed via the mode selection of an appropriate order can be used to clearly show the evolution characteristics of large-scale coherent structures.