Journal of Hydrodynamics最新文献

This paper employs a combined experimental and numerical approach to investigate the influence of airflow characteristics—specifically air velocity ν_a and air density ρ_a—on the evolution of water-entry cavities at low Froude numbers (Fr<13). A custom-designed test platform enables control over air density ρ_a and water-entry initial velocity V₀. The velocity V₀ influences the cavity expansion rate, which in turn determines the air inflow velocity ν_a into the cavity. Based on the experimental results, a critical condition for surface seal is proposed: ρ^*·Fr ^2.62_c =315, where ρ^*=ρ_a/ρ₀, ρ₀ is the ambient density. For constant air density, deep seal dynamics exhibit negligible direct sensitivity to airflow when the Fr < Fr_c, aligning with classical inertial theories and a 1/2-power scaling law during radial collapse. As ρ^* or Fr increases beyond the critical threshold, the cavity closure mode transitions from deep seal to surface seal. Numerical simulations, based on finite volume method, reveals that when the flow field is approximately uniform, the ratio of air flow velocity ν_a to projectile velocity ν is ~1.5. Furthermore, the airflow-induced pressure difference basically satisfies Δp = ρ_au_a²/2, driving inward splash motion. Neglecting the change in projectile velocity, there is Δp ∝ ρ_aV₀²/2. When the splash is about to close, the flow field distribution is relatively complex, and the pressure relationship no longer holds consistently. Surface seal blocks the connection between the cavity and the external atmosphere, directly impacting the internal pressure dynamics and further influencing the deep seal characteristics.

The underwater launch of an axisymmetric body involves complex cavity-structure interactions. Studying the evolution of cavity pressure around an axisymmetric body is crucial for researching its motion stability. In this work, we propose a deep neural network model for cavity pressure distribution recovery, called CPDR-net. This model can reconstruct the full-domain distribution of surface pressure based solely on the local pressure distribution. The CPDR-net model was trained using numerical simulation data with different launch depths and initial velocities, and subsequently tested on two simulation datasets under new conditions. Both training and testing datasets are obtained from the ventilated cavitating flow over an underwater axisymmetric vehicle. Results demonstrated that CPDR-net can accurately predict the pressure distribution along each longitudinal line of the axisymmetric body and provide the pressure evolution over time for each point on the surface. Thus, we can obtain the evolution of surface pressure distribution throughout the entire voyage process based on the CPDR-net model. The findings from this study may provide a valuable reference for subsequent research on underwater launches.

The floating horizontal-axis tidal turbine (FHATT) stands out as the most commercially viable tidal energy device. This paper reviews recent literature on FHATT and summarizes experimental and computational fluid dynamics (CFD) methods employed in FHATT research. Based on this foundation, the coupling effects of wave and platform motion (pitch/roll) on FHATT hydrodynamic performance were investigated through flume experiments and CFD simulations. The variations of the power coefficient (C_P) and thrust coefficient (C_T) are analyzed under different platform motion periods, amplitudes, wave periods, and wave heights. The results demonstrate that under the coupling of waves and pitch motion, C_P, C_T exhibit dual-frequency oscillations based on the pitch period, with oscillation amplitudes increasing with both pitch frequency (wave frequency) and pitch amplitude (wave height). Within the working conditions of this study, the maximum mean output power under the coupling of pitch motion and waves increases by 26.1%. The maximum fluctuation amplitude of C_P reaches 349.8%. When waves and roll motion are coupled, wave parameters dominate, while the influence of roll motion can be ignored. Moreover, the hydrodynamic fluctuations induced by waves and platform motion can couple with each other. This coupling effect not only amplifies the fluctuation amplitude of hydrodynamic coefficients but also has the potential to offset each other. These findings provide insights into the structural design and system control of FHATT, serving as valuable references for FHATT development.

Vortex-induced vibration (VIV) of cylindrical structures is a critical fluid-structure interaction (FSI) phenomenon in ocean engineering. Simulating VIV accurately can be computationally expensive. This study presents a graphics processing unit (GPU)-accelerated simulation model for VIV utilizing the immersed boundary lattice Boltzmann method (IB-LBM), aiming to reduce computational costs while preserving accuracy. The program is developed using machine learning library JAX, which enables parallelism on GPU and multi-GPU platforms. The model incorporates multi-GPU parallelization and multi-block grid refinement strategies to enhance computational efficiency. Validation against existing high-fidelity simulation data demonstrates good agreement. Performance tests show significant speed-ups with GPU acceleration compared to traditional CPU-based approaches. These results underscore the potential of the developed simulator as an efficient and reliable tool for in-depth parametric studies and practical engineering analysis of VIV, facilitating more rapid design iterations and risk assessments for offshore structures.

In order to explore the advantages of the triangular weir fishway and the influence of the angle of the triangular weir fishway on fish migration, this paper simulates the internal flow field and free surface of the fishway by using large eddy simulation (LES) and volume of fluid (VOF) method, respectively, and analyzes the hydraulic characteristics of the triangular weir at the three angles, where for the analysis of turbulent structure, omega (Ω) eddy identification is also used. And based on the analysis results, the length of the pool chamber and the height of obstacles were changed to obtain a more stable flow regime. The results show that the main flow is obvious, the turbulent kinetic energy distribution is regular and small in value, and the angle has an effect on the return flow. The vortex structure mainly existed in the mainstream area with high flow velocity, the air-liquid interface and the porosity, and was distributed transversely. 90° triangular weir had the largest vortex structure, and 105° triangular weir had the smallest. 75° triangular weir fishway could effectively attenuate the energy of the main stream, and the area of high velocity flow was small, which made the migratory conditions more favorable. Adding obstructions to the mainstem area can reduce flow velocity and turbulent kinetic energy. The triangular weir structure can improve the flow structure of the fishway, and the different angles and water depths provide a variety of flow conditions for a variety of fish migrations, providing new ideas for the fishway.

Vibration of flexible pipelines in the marine environment affects the flow characteristics of the transported materials inside the pipelines, which is related to transportation efficiency and energy consumption, thereby necessitating further investigation. In this study, the flow characteristics of particle-liquid two-phase flow transported upward in flexible pipelines are investigated based on the computational fluid dynamics-discrete element method (CFD-DEM). Typical forms of vibration including standing wave vibration and traveling wave vibration are employed and compared with a stationary pipeline. Results reveal that particles in the upward-traveling-wave vibrating pipeline still mainly distribute in the middle of the pipeline, while particles in the standing-wave vibrating pipeline exhibit periodic transverse aggregation near the pipe wall, and the fluctuations of particle concentration and particle z-direction velocity over time in each cross section of the pipeline are more obviously suppressed. When the propagation direction of the vibration wave changes from the same direction as the particle transport to static and then to the opposite direction, its hindering and regulating effect on the particles gradually increases, and the pipeline pressure drop gradually decreases.

This study compares the “cavitation to no-cavitation” transition processes and the resulting pressure oscillation effects of water, liquid oxygen and liquid methane in an adjustable cavitation Venturi tube through back pressure experiments. The modal characteristics of cavitation images and the frequency domain characteristics of vibrations were analyzed using the proper orthogonal decomposition (POD) and fast Fourier transform (FFT). By observing with a transparent Venturi tube and high-speed photography, combined with one-dimensional homogeneous flow simulation, the evolution laws of cavitation length and downstream pressure oscillation during the processes of changing the pressure ratio p_r and the throat area A_t were studied. The evolution laws of cavitation length and flow pattern about working parameters were obtained, revealing the correlation between cavitation and vibration. Under low pressure ratios, the oscillation of bubble collapse dominated by the returning jet causes the tail of the cavitation area to be lengthened, resulting in an increase in downstream amplitude. Increasing the pressure ratio can effectively control the cavitation length and reduce the amplitude of downstream pressure oscillation. Reducing the throat area by moving the plug cone increases the cavitation length, and the downstream oscillation amplitude first increases and then decreases, with an oscillation maximum value existing.

This paper investigates the coupling characteristics of variable mass tank sloshing and ship motion. A full nonlinear numerical model of variable mass tank sloshing-external wave-ship motion coupling is established. Firstly, the coupled motion characteristics of tank sloshing and simplified hull in beam sea are compared, the time history of dimensionless roll angle agrees well with experimental results, and the accuracy of the numerical model is verified. Secondly, the effects of wave excitation and ship speed on the coupled characteristics of Korea Research Institute of Ships and Ocean Engineering very large crude carrier (KVLCC) tank sloshing and ship motion are discussed, and the influence of liquid filling rate on ship heave motion and pitch motion is discussed emphatically. The results show that during the tank filling, the sloshing pressure in the tank increases steadily, and the growth rate is positively correlated with the filling rate. At the same time, the ship pitch motion is less affected by tank sloshing, while the ship heave motion is evidently affected by tank sloshing.

Wake-induced vibration (WIV) is common in engineering and may cause structural damage. The application of bionic devices in vibration control has attracted much attention, but most studies focus on isolated system and pay insufficient attention to the response under wake interference, which limits their application in engineering. Therefore, it is crucial to better understand the interaction between flow and bionic devices under wake interference. Inspired by the tail fin structure of fish tail, this paper studies the hydrodynamic characteristics of an elastically mounted cylinder with a tail fin in the wake of an inverted D-section cylinder. By changing the distance between the two centers of the circle and the length of the tail fin, the amplitude, frequency response and the mechanism of wake-induced vibration are explored. The results indicate that as the spacing ratio (L / D) increases, the WIV of the cylinder with a longer tail fin is excited. When WIV occurs, the phase difference (Φ) between the hydrodynamic coefficient and the transverse displacement experiences two jumps, and the lock-in bandwidth is wider compared with the single cylinder. As the tail fin length increases, the transverse lock-in frequency ratio yosu (f_yosu / f_n) decreases. Additionally, vortex shedding from the upstream inverted D-section cylinder (UIDC) affects the surface pressure distribution and vortex shedding characteristics of the downstream cylinder with a tail fin (DCTF). An analysis of the energy transfer reveals that the direction of transmission between the energy transfer and cylinder motion affects the amplitude response.

This study examines the vortex-induced vibrations (VIV) and hydrodynamic characteristics of a rotating circular cylinder with two degrees of freedom (2DOF). The effects of varying mass ratios (M^*), rotation rates (α) and reduced velocities (U^*) on the cylinder’s vibration responses, hydrodynamic properties and near-wake structures are explored. The results indicate that the cylinder rotation significantly suppresses vibrations in the cross-flow direction, with the inhibitory effect intensifying with the increasing rotation ratio α. Additionally, contrary to the non-rotating cylinder, in which f ^*_x = 2f ^*_y , the rotating cylinder shows congruence in vibration frequencies in both the X - and Y - directions. In the motion trajectory, with the non-rotating cylinder exhibiting an “8” pattern, the rotating cylinder traces a single closed-loop shape. The mass ratio notably influences the displacement amplitude, trajectory, and phase portrait. Higher mass ratios (M^* = 6, 8) suppress the growth of time-averaged displacements in both the X - and Y - directions and the incidence of “lock-in” phenomena in the Y direction. Particularly, as M^* = 2, α = 1.0, 1.5, the displacement in the Y - direction is notably higher than that under other conditions. The higher mass ratio also modifies the motion trajectory of the cylinder, shifting it from a closed-loop droplet shape to a single closed loop, with the frequency ratio between lift force and cross-flow displacement transitioning from 3:1 to 1:1. Vortex structure analysis reveals patterns such as “2P”, “P+S”, and “2S”, with high mass ratios showing a U-shaped mode and an extended “2S” region in the wake structure.