Journal of Hydrodynamics最新文献

The random displacement model (RDM) can efficiently simulate particle transport processes, which are difficult to observe, incorporating stochastic and hydraulic parameters. In recent decades, it has been used in many domains, including environments, hydraulics, and ecology. However, the results exhibit significant uncertainties arising from the model resolution, hydrodynamic accuracy, intrinsic characteristics of particles, and boundary conditions. The objective of the present study is to comprehensively interpret the RDM from theory to application, and emphasize essential considerations for users in different domains. The study also provides several application strategies for the model, based on several practical RDM cases. Determining the turbulent diffusivity and velocity profiles in complex flow field is a critical step to precisely simulate particle movement. Furthermore, the physical and biological properties of passive and active particles require fundamental investigation to extend the applicability of the model. Existing studies suggest that flexibly coupling the RDM with other numerical models customized to the characteristics of distinct problems will substantially expand the utility of the RDM and could yield innovative approaches for addressing previously intractable issues.

Tip vortex cavitation (TVC) is a critical phenomenon in propeller and turbine machinery. While much of the existing research on TVC has focused on its inception, the mechanisms driving its continuous growth remain under-explored. In this study, we propose a comprehensive theoretical model that integrates both gas diffusion and free nuclei entrainment to better understand the slow growth of tip vortex cavity. The efficacy of this model is validated by comparing its predicted temporal evolution of cavity size with experimental data, under both nuclei-depleted and large nuclei-injection conditions. Additionally, the model is used to further examine the individual effects of nuclei content and size on tip vortex cavity growth. Results reveal that, in sub-saturated nuclei flow, two critical equilibrium values for cavity size are identified: one determined by the balance of dissolved gases inside the cavity and the surrounding fluid, and the other by the balance between dissolved gases inside the cavity and the surrounding gas nuclei. The cavity stability size gradually shifts from the first to the second critical value as the gas nuclei content increases. However, since the model does not consider the destabilization mechanism of the cavity, the cavity may destabilize before reaching the second critical value. Meanwhile, the cavity growth rate increases significantly with increasing gas nuclei size. This work not only provides a comprehensive explanation for the experimental observations, but also provides new insights into the hysteresis phenomenon observed in TVC.

Cavitation performance is a critical hydrodynamic characteristic of ship propellers, and it has been a key focus in naval architecture research. This study introduces a hybrid multiscale Euler-Lagrange model for unsteady propeller cavitation simulations, incorporating the effects of water quality. A uniform mixture model is used for macroscopic cavity simulation. Under the Lagrangian framework, the dynamics and motion of nuclei and bubbles are resolved. Comparisons with experimental data and numerical results from traditional cavitation models show that the multiscale model accurately predicts cavitation on propeller blades and reproduces certain tip vortex cavitation phenomena. The model’s applicability is validated across different advance coefficients and cavitation numbers, further confirming its robustness in simulating propeller cavitation. Additionally, the study explores the distribution of nuclei and emphasizes the advantages of the multiscale approach in capturing tip vortex cavitation. This research provides a strong foundation for investigating the comprehensive effects of water quality on propeller cavitation and offers promising avenues for future studies in this area.

Vortices play a fundamental role in fluid dynamics, but mathematically defining them remains elusive. While many vortex identification methods are scalar-valued, vortices are inherently rotational, vector-based phenomena. Liutex, as a vector quantity, addresses these limitations by accurately capturing the local rotational characteristics of fluid elements while remaining independent of shear influences. This unique property makes Liutex particularly well-suited for vortex identification and the quantitative analysis of turbulent flows. This paper explores the statistical analysis of Liutex in various turbulence regimes and proposes an objective Liutex-based vortex identification method. The objective method is rooted in the statistical properties of Liutex. Furthermore, the paper investigates the performance of Liutex-based subgrid models in large eddy simulation (LES). The effectiveness of these models is evaluated by comparing their performance in different flow conditions, such as decaying homogeneous isotropic turbulence and turbulent channel flows, against conventional models. Results demonstrate that the inclusion of Liutex significantly enhances the ability of subgrid models to accurately capture flow structures. Importantly, the new model maintains the same form regardless of whether strong or weak shear is present, ensuring robustness and consistency in both vortex identification and turbulence modeling. These findings highlight the significant potential of Liutex to improve turbulence modeling in both theoretical and practical contexts, with ongoing research aimed at further refining its theoretical foundations and expanding its application in more complex flow scenarios.

Ice jams, which are prevalent in rivers of cold regions, can escalate into severe flooding disasters. Understanding variations in ice jam thickness is crucial, generating significant scholarly interest in developing accurate computational methods. Current models primarily rely on mechanical equilibrium equations to estimate ice jam thickness, representing a significant advancement in theoretical research. However, these models often overlook critical factors such as the cohesion of ice jams and the distribution of equilibrium stress across the river’s width, which can undermine their accuracy. This study introduces an enhanced model that incorporates these aspects, thereby improving the mathematical rigor. Validated against empirical data from natural rivers, the proposed model demonstrates strong agreement with observed values. This research not only refines the theoretical framework for calculating ice jam thickness but also improves the prediction and management of ice jam evolution and related disasters in cold regions.

Vortex dynamics and wall shear stress induced by cavitation bubble oscillations under the influence of particles are numerically investigated with the aid of a compressible two-phase flow solver in OpenFOAM. The generation, evolution, and disappearance of vortexes for three jet behaviors are discussed in detail. The variations of wall shear stress induced by the jets and the vortexes during the bubble oscillations are analyzed. Numerical results reveal that vortex dynamics is related to bubble evolution and jet behavior. After the jet pierces the bubble, an annular vortex with the bubble as the vortex core is generated by the bubble collapse. For the bubble that collapses near the wall, it generates multiple small vortexes near the wall and causes discontinuities in the wall shear stress. For the bubble that collapses away from the wall, it does not generate vortexes near the wall, and its oscillations cause the wall shear stress to vary periodically with different periods. In addition, the wall shear stress at the axis of symmetry increases abruptly when the jet hits the wall.

Axial flow pumps are widely used in water conservancy, petrochemical and agricultural industries. Efficient operation is crucial for energy conservation and emission reduction. Improving efficiency under severe conditions requires studying the internal flow of axial-flow pumps, particularly at low flow rates where backflow vortices form near the impeller inlet. This study investigates the unsteady flow characteristics of backflow vortices at different flow rates in an axial-flow pump. Results show that backflow vortices form when the flow rate decreases to 0.59Q_d. As the flow rate further declines, the backflow vortex progresses upstream, contracts, and rebounds. The flow rate range is divided into three stages: Stage I with no backflow vortex, stage II with initial vortex development extending upstream and relatively fragmented, and stage III with vortex contraction and rebound forming a more coherent structure. Besides, backflow vortices induce significant pressure fluctuations and velocity oscillations with the primary frequency being 0.5 f_b. They exhibit a three-dimensional spiral motion involving changes in axial length, self-rotation, and revolution around the pump axis, with an angular velocity of approximately half the impeller’s rotational speed. This work enhances insights into backflow vortex behaviors, which is essential for optimizing pump design and improving operational stability in challenging environments.

Hydrophilic revetments serve various pedestrian needs, including walking and wading, but they also confront serious wave overtopping challenges. Pedestrians on these revetments may face instability, falls, or even accidents leading to slipping into the sea due to overtopping flow. This study numerically investigates the hydrodynamic processes of overtopping flow impacting human bodies on a vertical hydrophilic revetment. A human sliding instability model tailored to wave overtopping conditions is developed, analyzing the human instability risks under different orientations (frontal, lateral and oblique), movement postures (stationary, slow walking and brisk walking), and somatotype (children, adults 1 and 2). The findings indicate that pedestrians facing overtopping flow at a 45° angle experience the maximum impact force. While brisk-walking pedestrians generally encounter lower impact forces, the effect of concomitant inertial force doubles their instability risk compared to when stationary. Notably, the instability risk for children is approximately 5.7 times that for adult one when facing wave overtopping. This research can provide a scientific foundation for designing and managing hydrophilic spaces in coastal areas.

A shallowly submerged hydrofoil often induces disturbances on the free water surface by generating numerous vortex structures, leading to phenomena such as wave breaking and droplet splashing. These phenomena involve various physical mechanisms. In this study, the third-generation vortex identification technique, Liutex, is employed to perform a detailed analysis of the vortex structures generated by the hydrofoil near the free surface. It is observed that these coherent vortex structures strongly entrain surrounding fluid, resulting in air entrainment and bubble sweep-down phenomena. We analyze the bubble dynamics in terms of bubble number density, volume distribution, and number distribution, revealing the dynamic characteristics of bubbles under the influence of vortex structures. Additionally, by tracking the vortex structures, two distinct forms of air entrainment are identified. The analysis of bubble motion using Liutex demonstrates the evolution and distribution patterns of bubble sizes in the turbulent flow field. The results indicate that the third-generation vortex identification technique, Liutex, effectively explains the mechanisms behind free surface breaking induced by the shallowly submerged hydrofoil.

Partial cavity oscillation is characterized by large-scale cavity shedding and intense pressure pulsations, causing severe damage to hydraulic machines. An incompressible homogeneous two-phase mixture numerical model is employed to investigate the effect of local active oscillatory surfaces on cavitating flows. The surfaces are positioned close to the cavity closure and simulated using an oscillatory velocity boundary to encumber the re-entrant jets. The results show that at high oscillation frequencies, the cavity shedding frequency synchronizes with the excitation frequency, indicating the presence of a lock-in mechanism. Meanwhile, the mean value and the fluctuation amplitude of the vapor volume are significantly reduced, indicating that the cavitation intensity and unsteady behaviors have been effectively weakened by the local active oscillation. Additionally, the high-pressure pulsation region diminishes, and the maximum pulsation amplitude declines. A simplified model manifests that the total vaporization and condensation rates of the entire domain exhibit a periodic variation in response to pressure pulsations. The dynamic mode decomposition (DMD) analysis reveals that small vortices shed from the main flow and dissipate under the influence of local oscillation. This study demonstrates that local active oscillatory surfaces are effective in inhibiting cloud cavitation.