Journal of Hydrodynamics最新文献

Suspended vegetation in rivers, lakes, reservoirs and canals can change flow structure, which will in turn affect the sediment transport and cause the variation of water ecological environment. In order to study the characteristics of bend flow through suspended vegetation, three-dimensional numerical simulations are carried out by using the multi-relaxation-time lattice Boltzmann method (MRT-LBM). The drag force induced by vegetation is added in the velocity correction in the equilibrium distribution and a hybrid format combined bounce and specular reflection scheme is applied in the solid-fluid boundaries. After the validation of this model, six cases are designed to conduct the numerical simulations according to the root depth and the arrangement of vegetation. The simulated results show that the suspended vegetation can redistribute the flow structure in curved open channels. After the arrangement of suspended vegetation, the main flow moves to the side without vegetation, and the distribution of velocity tends to be balanced when vegetation is arranged on the entire cross section, the range of circulating current is reduced from the whole cross section to the local position without vegetation, however, the circulating current can still exist in the curve where the suspended vegetation enters less than half of the water depth. In addition, it can also be concluded that the suspended vegetation can affect the lateral gradient of flow velocity, and the bed shear stress in the curved channel.

This study aimed to investigate the previously unexplored effects of ice cover and submerged vegetation on flow structure. Experiments were undertaken under both open channel and ice-covered flow conditions. The bed material consisted of three non-uniform sands. The findings revealed that when vegetation patches were present on the bed and an ice cover was present, the velocity profiles exhibited a distinctive pattern with two peak values. Turbulent kinetic energy (TKE) also exhibited two peaks, one above the vegetation bending height and another at the sheath section, with a decreasing trend towards the ice cover. Furthermore, quadrant analysis showed that when the flow surface is covered by an ice cover, the contributions of inward and outward events increased compared with those observed in an open channel flow. In most cases, these contributions surpassed the sweep and ejection events. The findings enhance our understanding of vegetation’s response to diverse surface conditions and have practical implications for river management and environmental engineering.

Numerical investigations of floating platforms with different outer column inclined angles under two operating conditions of regular wave and irregular wave are presented in this paper. A coupled aero-hydrodynamic computational fluid dynamics in-house solver FOWT-UALM-SJTU is applied for the calculation. First, the validation for wave and wind generation are conducted to determine mesh distribution strategy. Based on these, the hydrodynamic motion response, aerodynamic performance and wake flow are analyzed to explore the impact of inclined angle. Conduct spectral analysis on the motion response under wave action, discuss the aerodynamic attack angle and inflow wind velocity along the blade spanwise direction in detail, reveal different trends in wake development and recovery. The results show that for the regular wave condition with the increase of inclined angles, the equilibrium position of surge motion is constantly rising, while pitch is decreasing. The maximum root mean square (rms) value occurs at angle = 30°, compared with the original OC4 FOWT, the rms in power and thrust increase 0.35%, 0.71%. And there are two low regions of attack angle and high regions of axial inflow velocity, corresponding to aerodynamic loads. The spectral analysis indicates that the natural frequency of pitch motion will increase with inclined angle. Besides, from the middle to far region of wake flow, the velocity recovery of FOWT with inclined angle will become faster, which is beneficial for downstream turbines to enhance more wind energy.

Water entry problems represent complex multiphase flows involving air, water, and structure interaction, occurring rapidly in rough seas, and potentially effecting structural integrity of floating structures. This paper experimentally investigates asymmetric slamming loads acting on a 3-D elastic wedge section. The specimen, featuring two different bottom plates (stiffened and unstiffened), each 4 mm thick, aims to assess the effect of structural stiffness on dynamic loads. The experiments are conducted at different drop heights of 25 cm and 50 cm and varying heel angles from 5° to 25°. The paper describes the experimental conditions, including wedge geometry, material properties, and the test plan. The study explores the influence of heel angle on impact acceleration, revealing an increase in peak acceleration with a higher inclination angle, particularly in the vertical direction. Additionally, the hydrodynamic pressure resulting from asymmetric slamming is presented. The pressure results analyzed and compared at different locations along the length of the wedge. The experimental findings indicate that, despite the leeward side (stiffened) experiencing a smaller hydrodynamic load, the heel angle significantly affects pressure results on the windward side (unstiffened), leading to a more pronounced dynamic response. The time history of pressure results emphasizes the effect of elastic vibrations, particularly noticeable on the unstiffened bottom plate. This study contributes to a deeper understanding of asymmetric slamming on aluminum structures, facilitating the enhancement of mathematical models and the validation of numerical simulations.

The load rejection transient process of bulb turbine units is critical to safety of hydropower stations, and determining appropriate closing laws of guide vanes (GVs) and runner blades (RBs) for this process is of significance. In this study, we proposed a procedure to optimize the co-closing law of GVs and RBs by using computational fluid dynamics (CFD), combined with the design of experiment (DOE) method, approximation model, and genetic optimization algorithm. The sensitivity of closing law parameters on the histories of head, speed, and thrust was analyzed, and a two-stage GVs’ closing law associating with a linear RBs’ closing law was proposed. The results show that GVs dominate the transient characteristics by controlling the change of discharge. Speeding GVs’ first-stage closing speed while shortening first-stage closing time can not only significantly reduce the maximum rotational speed but also suppress the water hammer pressure; slowing GVs’ second-stage closing speed is conducive to controlling the maximum reverse axial force. RBs directly affect the runner force. Slowing RBs’ closing speed can further reduce the rotational speed and the maximum reverse axial force. The safety margin of each control parameter, flow patterns, and pressure pulsations of a practical hydropower station were all improved after optimization, demonstrating the effectiveness of this method.

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.