Journal of Hydrodynamics最新文献

In the present paper, the unsteady cavitating turbulent flow over the twisted NACA66 hydrofoil is investigated based on an modified shear stress transfer k - ω partially averaged Navier-Stokes (MSST PANS) model, i.e., new MSST PANS (NMSST PANS) model, where the production term of kinetic energy in the turbulence model is modified with helicity. Compared with the experimental data, cavitation evolution and its characteristic frequency are satisfactorily predicted by the proposed NMSST PANS model. It is revealed that the interaction among the main flow, the reentrant jets, and sheet cavitation causes the formation of the primary shedding cavity near the mid-span and the secondary shedding cavity at each side of the twisted hydrofoil, and further induces the remarkable pressure gradient around shedding cavities. Along with the development of the primary and the secondary shedding cavities, the great pressure gradient associated with large cavity volume variation promotes the vortical flow generation and the spatial deformation of vortex structure during cavitation evolution, and results in the primary and the secondary U-type vortices. Further, dynamic mode decomposition (DMD) analysis is utilized to confirm the interaction among the main flow, the main reentrant jet and two side reentrant jets, and cavitation. These results indicate that the proposed NMSST PANS model is suitable to simulate the complicated cavitating turbulent flow for various engineering applications.

The presence of particles and the shock waves generated by the cavitation bubbles can significantly affect the safety and the performance of hydrodynamic machineries. In the present paper, the shock waves generated by cavitation bubble collapsing near the particle are numerically investigated based on the OpenFOAM together with the numerical schlieren for the shock wave identifications. The numerical results reveal that the stand-off distance is one of the paramount factors affecting the interactions between the particle and the shock waves. Several different kinds of shock waves (e.g., bubble-inception, jet formation, particle reflected and jet-split shock waves) are observed during the bubble collapsing near the particle. For stand-off distance smaller than 0.5 or larger than 1.1, the maximum pressure at particle surface generated by the bubble growth can surpass those of the collapse stage.

This paper investigates the sloshing phenomena in a spherical liquid tank using the moving particle semi-implicit (MPS) method, a crucial study in fluid dynamics. Distinct from previous research focused on rectangular or LNG tanks, this work explores the unique motion patterns inherent to spherical geometries. The accuracy of our in-house MPS solver MLParticle-SJTU is validated against experimental data and finite volume method (FVM). And the MPS method reveals a closer alignment with experimental outcomes, which suggests that MPS method is particularly effective for modeling complex, non-linear fluid behaviors. Then the fluid’s response to excitation at its natural frequency is simulated, showcasing vigorous sloshing and rotational motion. Detailed analyses of the fluid motion are conducted by drawing streamline diagrams, velocity vector diagrams, and vorticity maps. The fluid’s motion response is explored using both time-domain and frequency-domain curves of the fluid centroid, as well as the sloshing force.

Aquatic vegetation is a vital component of natural river ecosystems, playing a crucial role in maintaining ecological balance, providing habitat and improving water quality. However, the presence of vegetation results in increased resistance in vegetated channels compared with non-vegetated channels, rendering traditional sediment movement predictions inadequate for the latter. Consequently, the concept of a vegetation influence factor, denoted by C_Dah, has been proposed by previous researchers to represent the effect of vegetation on sediment movement in watercourses. In this study, we focus on exploring the vegetation resistance coefficient (C_D) among the vegetation influence factors, evaluating two different calculation methods for vegetation resistance coefficient, and presenting two expressions through genetic algorithm analysis to predict the incipient flow velocity of sediment in vegetated watercourses. The predicted values from the new formulae show excellent agreement with measured data, highlighting the high accuracy of the proposed methods in predicting the incipient flow velocity of sediment. Our results provide a solid theoretical basis for understanding the influence of aquatic vegetation on sediment particle movement.

To reveal the cavitation forms of tip leakage vortex (TLV) of the axial flow pump and the flow mechanism of the flow field, this research adopts the partially-averaged Navier-Stokes (PANS) model to simulate the cavitation values of an axial flow pump, followed by experimental validation. The experimental result shows that compared with the shear stress transport (SST) k - ω model, the PANS model significantly reduces the eddy viscosity of the flow field to make the vortex structure clearer and allow the turbulence scale to be more robustly analyzed. The cavitation area within the axial flow pump mainly comprises of TLV cavitation, clearance cavitation and tip leakage flows combined effect of triangular cloud cavitation formed. The formation and development of cavitation are accompanied by the formation and evolution of vortex, and variations in vortex structure also generate and promote the development of cavitation. In addition, an in-depth analysis of the relationship between the turbulent kinetic energy (TKE) transport equation and cavitation patterns was also conducted, finding that the regions with relatively high TKE are mainly distributed around gas/liquid boundaries with serious cavitation and evident gas-liquid change. This phenomenon is mainly attributed to the combined effect of the pressure action term, stress diffusion term and TKE production term.

The scenario simulation analysis of water environmental emergencies is very important for risk prevention and control, and emergency response. To quickly and accurately simulate the transport and diffusion process of high-intensity pollutants during sudden environmental water pollution events, in this study, a high-precision pollution transport and diffusion model for unstructured grids based on Compute Unified Device Architecture (CUDA) is proposed. The finite volume method of a total variation diminishing limiter with the Kong proposed r-factor is used to reduce numerical diffusion and oscillation errors in the simulation of pollutants under sharp concentration conditions, and graphics processing unit acceleration technology is used to improve computational efficiency. The advection diffusion process of the model is verified numerically using two benchmark cases, and the efficiency of the model is evaluated using an engineering example. The results demonstrate that the model perform well in the simulation of material transport in the presence of sharp concentration. Additionally, it has high computational efficiency. The acceleration ratio is 46 times the single-thread acceleration effect of the original model. The efficiency of the accelerated model meet the requirements of an engineering application, and the rapid early warning and assessment of water pollution accidents is achieved.

In this study, the interaction between 3-D bedforms and submerged rigid vegetation has been investigated. Various laboratory experiments were conducted to study the distribution of flow velocity, Reynolds shear stress, turbulent kinetic energy, and skewness coefficients for a constant density of vegetation. Results showed that the velocity profile in the pool section deviates from those in the upstream section of the pool. It has been found that the dip parameter varied between 0.6H and 0.9H depending on various factors including bed roughness, vegetation distribution, and pool entrance/exit slopes. However, scattered vegetation in the pool and differences in slopes created non-uniform flow conditions. Also, in the wake region behind each vegetated element, flow velocity reduced significantly, and small-scale eddies are formed, causing increased perturbations. By decreasing the entrance slope and bed roughness, relatively uniform flow and weaker turbulence was resulted, but the random distribution of vegetated elements counteracted this balance and intensified turbulence. With the decrease in the pool entrance slope, the contribution of sweep event decreased and the contribution of ejection event increased.

Fishway research is important for mitigating the fragmentation of river habitats caused by hydraulic projects. The vertical slit fishway is a broadly used fishway type because of its high efficiency and adaptability to water levels. However, the resulting vortex current disrupts the fish passage hence directly affecting fish migration. This study aims to accurately capture the vortex structure in the fishway and analyze the effect of vortex elements (vortex structure, vortex intensity, etc.) on fish. We conducted an analysis of the 3-D current flow field in the fishway through the utilization of an experimental model and the large eddy simulation (LES) method. Moreover, we captured the vortex information in the fishway at different flow rates using the Liutex vortex identification method and investigated the effect of the vortex on fish migration. The results revealed that the structures inside the fishway pool occupy most of the room, however, the areas with higher vortex strength were primarily located in the vortex near the vertical seam and the mainstream, the vortex strength inside the fishway gradually increases with increasing flow, suppressing fish migration. Fish experienced significantly increased resistance when encountering strong vortices. This suggests that the vortex may act as a physical barrier to fish migration. These findings highlight the potential negative effects of vortex on fish movement and reiterate the importance of understanding vortex dynamics for aquatic environmental management. As an effective tool for identifying vortices in fluid flow, the Liutex method demonstrates features of vortex within the fishway, thereby providing important insights into the interaction between fluid dynamics and aquatic organisms.

The tip-clearance flow in a pump-jet propulsor exerts great impacts on the fluctuating pressures and resultant unsteady forces, which are important sources of structural vibrations and radiated noise underwater. The blade geometry close to the tip is an important factor determining the vortex strength in the tip-clearance flow. In the open-water condition, the effects of raking the rotor tips on the duct-surface fluctuating pressures and the resultant unsteady forces acting on different components of the propulsor are investigated via physical model experiments and the numerical solution of Reynolds-averaged Navier-Stokes (RANS) equations coupled with the SST k - ω turbulence model. The measured and simulated results of hydrodynamic pressures are consistent to each other, and the simulated flows help better understand why the fluctuating pressures change with the tip geometry. The strong fluctuations of duct-surface pressures are caused by intensive tip separation vortices. The duct-surface pressure fluctuations are effectively reduced by using the rake distribution near the tip towards blade back side and, for the combination of the five-bladed rotor and the seven-bladed stator, the resultant unsteady horizontal (and vertical) forces acting on the duct and stator are also reduced; while increasing rake leads to negative effect on pressure fluctuations and unsteady horizontal (and vertical) forces acting on all the components of the propulsor.

The traditional high-level Green-Naghdi (HLGN) model, which uses the polynomial as the shape function to approximate the variation of the horizontal- and vertical-velocity components along the vertical direction for each-fluid layer, can accurately describe the large-amplitude internal waves in a two-layer system for the shallow configuration (h₂ / λ ≪ 1, h₁ / λ ≪ 1). However, for the cases of the deep configuration (h₂ / λ ≪ 1, h₁ / λ = O(1)), higher-order polynomial is needed to approximate the variation of the velocity components along the vertical direction for the lower-fluid layer. This, however, introduces additional unknowns, leading to a significant increase in computational time. This paper, for the first time, derives a general form of the HLGN model for a two-layer fluid system, where the general form of the shape function is used during the derivation. After obtaining the general form of the two-layer HLGN equations, corresponding solutions can be obtained by determining the reasonable shape function. Large-amplitude internal solitary waves in a deep configuration are studied by use of two different HLGN models. Comparison of the two HLGN models shows that the polynomial as the shape function for the upper-fluid layer and the production of exponential and polynomial as the shape function for the lower-fluid layer is a good choice. By comparing with Euler’s solutions and the laboratory measurements, the accuracy of the two-layer HLGN model is verified.