Journal of Hydrodynamics最新文献

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.

Self-propelled particles are commonly found in a large number of planktonic organisms such as bacteria, fungi, and algae in nature, and researchers have taken a long interest in exploring their swimming mechanisms for more than a century. Especially in the past 20 years, with the development of computational fluid dynamics and flow display technology, as well as the need for the design of synthetic self-propelled particles and micro-swimming devices, self-propelled particles have become the forefront and hotspot of current research in the field of fluid mechanics. This paper first introduces the swimming characteristics of common self-propelled particles, leading to a classic “squirmer” type self-propelled particle model. On this basis, a systematic introduction and summary of the theoretical and numerical simulation research of “squirmer” will be conducted. Finally, the main challenges and opportunities faced by the current research will be summarized.

Riblets are a series of small protrusions formed along the flow direction, which have been extensively studied as a passive turbulent drag reduction technique. Experiments and numerical simulations have shown that well-designed riblets can significantly reduce drag in turbulent flows, making them highly promising and valuable for various applications. In this study, we focus on a scalloped riblet, which is designed by smoothly connecting two third-order polynomials, and thus the sharpness of the tip and the curvature of the valley can be well defined. We conduct direct numerical simulations of turbulent channel with smooth plate, scalloped riblet-mounted and triangular riblet-mounted walls. Width in wall units of W⁺ = 20 and height-width ratio of γ = 0.5 are selected for both riblet cases. Compared with the smooth plate case, the scalloped riblet case achieves an 8.68% drag reduction, while the triangular riblet case achieves a 4.79% drag reduction. The obtained drag reduction rate of the triangular riblet is consistent with previous experiments and simulations, and the results indicate that the scalloped riblet is more effective in reducing drag and deserves further investigation. We compare turbulent statistics of the scalloped riblet case with those of the triangular riblet case. The mean velocity profiles of riblets are similar, but both the Reynolds shear stress and second-order statistics of velocity fluctuations and Liutex are significantly reduced in the scalloped riblets controlled turbulent channel, indicating that the scalloped riblet can more effectively suppress the spanwise and wall-normal turbulent intensity near the wall. We also compare the pre-multiplied spectra of streamwise velocity and streamwise Liutex component for the three cases to investigate the energy distribution and characteristics of Liutex distribution. The Liutex vortex identification method is also utilized to analyze the instantaneous flow field, which provides insights into the flow field and could be beneficial for the further optimization of riblet.

The flow around a circular cylinder for Re = 1000 is characterized by flow separation and Karman vortex street. The typical flow features can be captured to study the correlation between fluid fields and sound fields. In this paper, the three-dimensional circular cylinder is taken as the research object, and the probes of surface fluctuating pressure and far field sound pressure are arranged every 10°. The directional diagram and the coherence of fluctuating pressure and sound pressure are analyzed. The relationship between the flow mode and hydrodynamic noise is studied by using dynamic mode decomposition (DMD). The characteristics of the dipole and quadrupole sound source term of a long span cylinder are studied. The results show that at the angles between 30°–120° and 190°–350°, the fluctuating pressure contributes more to the generation of dipole sounds. The quadrupole sound source shows three-dimensional effects, which is more obvious in a cylinder with large spanwise length.

Unsteady cavitating flow often contains vapor structures with a wide range of different length scales, from micro-bubbles to large cavities, which issues a big challenge to precisely investigate its evolution mechanism by computational fluid dynamics (CFD) method. The present work reviews the development of simulation methods for cavitation, especially the emerging Euler-Lagrange approach. Additionally, the progress of the numerical investigation of hot and vital issues is discussed, including cavitation inception, cloud cavitation inner structure and its formation mechanism, cavitation erosion, and cavitation noise. It is indicated that the Euler-Lagrange method can determine cavitation inception point better. For cloud cavitation, the Euler-Lagrange method can reveal the source of microbubbles and their distribution law inside the shedding cloud. This method also has advantages and great potential in assessing cloud cavitation-induced erosion and noise. With the ever-growing demands of cavitation simulation accuracy in basic research and engineering applications, how to improve the Euler-Lagrange method’s stability and applicability is still an open problem. To further promote the application of this advanced CFD simulation technology in cavitation research, some key issues are to be solved and feasible suggestions are put forward for further work.

This paper presents an efficient time-domain method for simulating nonlinear ship waves. The proposed method, implemented in an earth-fixed coordinate system, integrates a compact boundary element domain within a high-order spectral layer, enabling accurate modeling of both near-field and far-field ship waves. An overset mesh method and an attention mechanism are employed to track the moving ship. The effectiveness of the method is validated through simulations of Wigley and Series 60 ships sailing at various speeds. The numerical results, including the nonlinear wave run-up at the ship bow, surface pressure distribution on the hull, and the ship resistance, agree well with experimental data and published numerical results, confirming that the method is capable of accurately simulating the nonlinear ship waves.