Journal of Hydrodynamics最新文献

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.

To evaluate the safety of the bulb tubular turbine, the dynamic hydraulic characteristics of a hydropower station system during the load rejection process are studied through numerical simulations and a prototype test. In the developed model, a dynamic grid technology (DGT) controls the closure of the guide vane and the blade, whilst the moment balance equation and the user-defined function (UDF) provide the runner’s rotation speed. The 3-D transient simulation method can well predict the rotation speed and mass flow curves in the state of load rejection. The simulation outcomes of the system performance are basically consistent with the measurement data of the prototype. As observed, the runner is subjected to the reversely increased torque and axial force, the system is in a braking phase, and the maximum speed peaks at 144.6% of the rated speed. Moreover, the internal flow of the runner is greatly affected by the closure of the guide vane, and the draft tube forms an eccentric spiral vortex rope. It breaks downstream, aggravating the instability of the draft tube. Overall, the transient characteristics span for the first five seconds, demonstrating the importance of establishing an efficient governing controller. The obtained results are useful for designing the turbine’s flow channel with a double regulating function and comprehending the turbine’s transient characteristics.

In this paper, the principal decomposition of the velocity gradient tensor [∇v] is discussed in 3 cases based on the discriminant ∆: ∆ < 0 with 1 real eigen value and a pair of conjugate complex eigen values, ∆ > 0 with 3 distinct real eigen values, and ∆ = 0 with 1 or 2 distinct real eigen values. The velocity gradient tensor can also be classified as rotation point, which can be decomposed into three parts, i.e., rotation [R], shear [S] and stretching/compression [SC], and non-rotation point, we defined a new resistance term [L], and the tensor can be decomposed into three parts, i.e., resistance [L], shear [S] and stretching/compression [SC]. Example matric are also displayed to demonstrate the new decomposition. Connections of principal decomposition between 3 different cases, and between Resistance and Liutex will also be discussed.

This study investigates turbulent particle-laden channel flows using direct numerical simulations employing the Eulerian-Lagrangian method. A two-way coupling approach is adopted to explore the mutual interaction between particles and fluid flow. The considered cases include flow with particle Stokes number varying from St = 2 up to St = 100 while maintaining a constant Reynolds number of Re_τ = 180 across all cases. A novel vortex identification method, Liutex (Rortex), is employed to assess its efficacy in capturing near-wall turbulent coherent structures and their interactions with particles. The Liutex method provides valuable information on vortex strength and vectors at each location, enabling a detailed examination of the complex interaction between fluid and particulate phases. As widely acknowledged, the interplay between clockwise and counterclockwise vortices in the near-wall region gives rise to low-speed streaks along the wall. These low-speed streaks serve as preferential zones for particle concentration, depending upon the particle Stokes number. It is shown that the Liutex method can capture these vortices and identify the location of low-speed streaks. Additionally, it is observed that the particle Stokes number (size) significantly affects both the strength of these vortices and the streaky structure exhibited by particles. Furthermore, a quantitative analysis of particle behavior in the near-wall region and the formation of elongated particle lines was carried out. This involved examining the average fluid streamwise velocity fluctuations at particle locations, average particle concentration, and the normal velocity of particles for each set of particle Stokes numbers. The investigation reveals the intricate interplay between particles and near-wall structures and the significant influence of particles Stokes number. This study contributes to a deeper understanding of turbulent particle-laden channel flow dynamics.

The combined effect of cavitation and silt abrasion presents a great challenge and threat to secure the operation and the efficiency of hydraulic machineries working in sediment-laden fluid. The present paper critically reviews the current research progress on the interaction mechanisms of the bubbles and the particles. Firstly, the analytical models including boundary treatment methods for predicting the jet dynamics of the bubble collapse near particles are demonstrated. Secondly, the bubble collapsing dynamics, jet dynamics and shock wave characteristics near particles are revealed both experimentally and numerically. Finally, the bubble-particle-wall system is investigated with a focus on microjets.

The biomimetic hydrophobic surface is a potentially efficient underwater drag reduction method and the drag reduction mechanism of this kind of surface comes from the interfacial slippage. For now, it is a hotspot to grasp the slippage characteristic and explore slippage enhancement strategies. This paper not only summarizes our numerical simulation and experimental results of slippage characteristic at the solid-liquid interface (SLI) of hydrophobic surfaces (HS) and the gas-liquid interface (GLI) of superhydrophobic surfaces (SHS) in recent years, but also introduces some innovative methods that can effectively improve the gas film stability and drag reduction effect of SHS. First, we used the molecular dynamics (MD) simulation method to figure out the effect of the solid-liquid interaction strength, the system temperature and the shear rate on the slippage of SLI, and expound their action mechanism from molecular scale. Then, by MD and multibody dissipative particle dynamics (MDPD) method, the slippage behavior at the GLI was studied under the influence of the microstructure size and the flow driving velocity. We proposed a new kind of hybrid slip boundary condition model to describe the slippage characteristic on GLI. In addition, we found through experiment that a three-dimensional backflow will appear on the GLI under the interfacial adsorption of surfactants, and the backflow direction will reverse with the change of GLI morphology. Finally, we put forward the wettability step structure and gas injection method to enhance the stability and drag reduction effect of the gas film on SHS.