To improve the reliability and accuracy of large-thickness tomographic particle imaging velocimetry (TPIV) measurement data in open-channel flows, a post-processing approach has been developed. This approach integrates statistical error correction with constraints derived from physical equations. This approach begins with the application of standard deviation techniques and the normalized median filter for the statistical refinement of the raw data. It is followed by a divergence correction based on the incompressible continuity equation, ensuring the conservation of mass in the flow field. The processing procedure and parameter selection for large-thickness TPIV measurement data in open-channel flows are comprehensively discussed. The results are then compared with theoretical data and traditional methods to validate the efficacy of this post-processing approach. The results reveal a strong correlation between the post-processed 3-D velocity field data and the theoretical benchmarks, as evidenced by the agreement in basic statistical quantities, uncertainty analysis, and power spectral distribution. Furthermore, the characterization of the flow field’s coherent structures is significantly enhanced following the post-processing procedure. This investigative framework presents a robust strategy for optimizing large-thickness TPIV measurement data in open-channel flows, thereby improving both the accuracy and efficiency of 3-D flow field analysis in the context of fluid dynamics research.
Hybrid unsteady Reynolds-averaged Navier-Stokes/large eddy simulation (URANS/LES) models are progressively being applied to compressible flow study due to the capability to balance computational cost with simulation accuracy, and have demonstrated significant prospect for future applications. As a recently developed hybrid URANS/LES model, the adaptive time-scale driven (ATSD) model is particularly suited for resolving rotating turbulence of turbomachinery owing to its adaptive time-scale-driven modelling strategy. However, when applied to compressible flows, such as in compressor rotors, the ATSD turbulence model may exhibit numerical instability and insufficient predictive accuracy stemming from the strong compressible effects. To address these limitations, this study proposes a novel ATSD-ρ (ρ denotes density) turbulence model, which incorporates the following enhancements specifically designed for compressible flows: (1) An implicit eddy viscosity adjustment strategy is used to enhance numerical stability. (2) Turbulence diffusion effect of density is considered to improve prediction accuracy. (3) An optimized characterization of the resolved time scale is utilized to reflect compressible flow adaptability. This new model is validated in two canonical transonic axial compressor rotor test cases, National Aeronautics and Space Administration (NASA) Rotor 67 and NASA Rotor 37, and the results demonstrate that the ATSD-ρ model improves the prediction accuracy of rotor aerodynamic performance while enhancing the resolution of finer-scale turbulent vortical structures, thereby providing an efficient solution for engineering computation of compressible turbulence.
Pure loss of stability (PLS) is a highly nonlinear stability failure mode, which usually happens in following and stern-quartering waves. It may interact strongly with the restoring moment variation, forward speed, propulsion, steering action, and roll-yaw coupling. Reliable prediction of PLS is important for direct stability assessment in the International Maritime Organization (IMO) second-generation intact stability criteria. To examine how propeller modeling affects viscous-flow prediction of this phenomenon, free-running CFD simulations in regular stern-quartering waves were carried out for a fully appended Office of Naval Research (ONR) tumblehome model. Two propulsion treatments were compared: A resolved rotating propeller model and a body-force propeller model. Self-propulsion calculations in calm water were first performed to determine the propeller revolutions for each target speed, and subsequent wave simulations were carried out with automatic rudder control. The two models gave comparable self-propulsion results and reproduced the main trend of increasing asymmetric roll response at different Froude numbers (Fr). A detailed comparison at critical Fr = 0.275 showed that the main difference between the two propeller models was not in the overall maximum roll amplitude, but in the transient development of the instability. The discretized propeller model predicted stronger port-starboard asymmetric characteristics in propeller thrust, rudder wetted area, and rudder moments when a large starboard roll led to appendage emergence, resulting in an earlier development toward capsize. In contrast, the body-force model delayed this process and showed higher computational efficiency. The present results clarify the influence of propeller modeling on the CFD prediction of pure loss of stability and provide guidance for selecting propeller models in direct viscous-flow simulations of extreme ship motions.
The interaction between sand particles and cavitation bubbles near material surfaces under non-coaxial conditions is a common phenomenon when cavitation erosion occurs in sediment-laden water. Investigation into this phenomenon is of great significance for revealing the mechanism of cavitation erosion in sediment-laden flows. In this study, a low-voltage electric discharge device and a high-speed camera were adopted to investigate the influence of a spherical particle on the collapse characteristics of a cavitation bubble near a wall under non-coaxial conditions, including collapse morphology, collapse time, and migration behavior. The experimental results indicate that: (1) The side of the cavitation bubble surface close to the spherical particle surface exhibits delayed contraction, with the bubble surface elongated by the particle and partial surface splitting observed. (2) The collapse time of the cavitation bubble is prolonged under the influence of the spherical particle, showing a linear increase with the dimensionless parameter αε / (γpγ) (α is the angle, ε is the size ratio, and γp and γ are dimensionless distances to the particle and wall, respectively). (3) Non-coaxial particles alter the migration behavior of the cavitation bubble, including changes in the collapse direction and migration displacement. A critical condition formula for the cavitation bubble not collapsing toward the wall is proposed based on the relationship between γpγ and α. Furthermore, the variation of the cavitation bubble migration distance Z b * with the parameter αε / (γpγ) is summarized under different angles α and dimensionless distances γp, indicating that the migration of the cavitation bubble decreases as αε / (γpγ) increases. The findings of this study provide important insights into the interaction mechanisms between sand particles and cavitation bubbles in sediment-laden flows.
Semi-open centrifugal pumps often experience hydraulic instabilities under low-flow conditions, manifested as the hump phenomenon in the head-flow curve. This instability arises from tip leakage vortices (TLV) and inlet recirculation (IR), which disturb the impeller inlet and cause energy losses. Although casing treatment has been widely applied in centrifugal compressors to suppress similar instabilities, its application in centrifugal pumps remains limited. To address this gap, the present study investigates slotted endwalls as a passive flow-control strategy to stabilize internal flow and suppress low-flow instabilities in semi-open impeller pumps. Three-dimensional simulations based on the Reynolds-averaged Navier-Stokes (RANS) equations and the shear stress transport (SST) k-ω (k denotes turbulent kinetic energy, ω denotes specific dissipation rate) turbulence model were performed and validated against experimental data with good agreement. Three slot configurations, namely arc, inclined polyline, and inclined slot, were designed to assess the geometric effects on TLV and IR. Results show that all designs effectively mitigate the hump phenomenon and extend the stable operating range, with the inclined slot achieving the best performance. It effectively eliminates the hump region and enhances the efficiency at both design and high-flow rates. Further analyses reveal that slotted endwalls promote smoother reentry of leakage flow and reduce energy dissipation. Orthogonal optimization identifies slot position as the dominant factor influencing performance. These findings confirm that slotted endwall modification is an effective and practical passive design approach for improving flow stability and hydraulic efficiency in semi-open centrifugal pumps.
In open-channel flows, a notably higher spanwise vortex density near the free surface compared to other wall-bounded turbulent flows has been observed, termed the additional vortex phenomenon. Two hypotheses explain this phenomenon: The free-surface accumulation hypothesis, suggesting slower vortex dissipation in open-channel flows leading to vortex accumulation in the outer region, and the free-surface generation hypothesis, proposing that free surface restrictions on vertical motions enhance vortex generation, increasing vortex density near the surface. To elucidate the dominant origin mechanism of the additional vortex phenomenon, this study conducts direct numerical simulation (DNS) of open-channel flows and closed-channel flows at a friction Reynolds number Reτ = 500. Using the λci - criterion, the spanwise, streamwise, and vertical vortex densities variations with distance from the wall are identified and quantified for both flows, and the presence of the additional vortex phenomenon near the free surface in the open-channel DNS data is confirmed again. Furthermore, the vortex birth-death density is defined to elucidate the dominant origin mechanism of the additional vortex phenomenon. Through the analysis of the vortex birth-death density profiles, the exact birth location of the additional vortices can be pinpointed. The result reveals significant positive values for spanwise and streamwise vortices near the wall and free surface in open-channel flow, indicating substantial vortex generation, while small negative values in the central region suggest vortex dissipation. The results of the vortex birth-death density analysis support the free-surface generation hypothesis for the additional vortex phenomenon in open-channel flows.
Storm surges are natural disasters affecting the social and economic development in coastal areas of the Bohai Sea. This study used the Delft model to construct a storm surge model for the Yellow and Bohai Seas. A storm surge occurring on October 18–22, 2024, caused by non-typical weather systems was comprehensively analyzed using simulations, observations, and satellite altimetry data. Wind data from the European Centre for Medium-Range Weather Forecasts (ECMWF) 5th Generation of European Reanalysis (ERA5) dataset were used as input into the numerical model. The results showed that the simulated astronomical tide levels and storm surges were similar to the observed data. The first and second stages of the storm surge in the Yellow and Bohai Seas were caused by a strong cold air mass, and the third stage was caused by cold air and the inertial oscillation of water. A decrease in the water level on the 19th and 20th caused the accumulation of a large amount of seawater in the southern Yellow Sea. The resulting potential energy gradient caused the excess seawater to reach the Dalian and Dandong coasts and the Bohai Sea. The southerly wind combined with the astronomical tide exacerbated this non-typical flood disaster. The inertial oscillation of water and wind contributed 66.7
Under outflow conditions, water flows from the inlet/outlet to the reservoir, and the inlet/outlet experiences complex separated flow, which affects flow efficiency. In this study, proper orthogonal decomposition (POD) methods are used to identify the main characteristics of the multi-scale motions and reveal the impact of these flow structures on the separated flow within the inlet/outlet. Flow separation first appears at the upper right corner of the inlet/outlet, then extends to cover the top of the inlet/outlet, leading to reduced flow efficiency. Shear layers are present above and below the core flow, with significant fluctuating momentum exchange. The separated flow is dominated by large-scale motions, with two main features observed. On the one hand, there is a large-scale streak structure at a frequency of the Strouhal number, Sr = 0.044, causing the core flow to oscillate back and forth along the streamwise direction, accompanied by oscillations within the separation zone. On the other hand, large-scale streak structures at frequencies Sr = 0.116–0.335 induce lateral oscillations in the core flow, which manifest as up-down and left-right movements. Small-scale motions significantly influence the turbulence characteristics of the separated flow. Vortical structures exist above and below the core flow, where their growth, merging, and breakup result in high turbulence intensity, increasing the head loss in the inlet/outlet. Studying the complex separated flow within the inlet/outlet contributes to optimizing the design of inlet/outlet in pumped storage power stations.
Pendulum wave energy converters harness wave energy via an articulated power take-off (PTO) system linking the wave-absorbing buoy to the fixed base. The geometric shape design of the buoy plays a critical role in determining the overall efficiency of wave energy capture. To further enhance the conversion efficiency of such devices under real-sea conditions, this paper introduces a rapid optimization method based on free-form deformation (FFD) and frequency-domain potential flow theory. First, the CAESES-GeniE-HydroD optimization framework is established by integrating a FFD technique with frequency-domain potential flow theory, enabling parametric modeling, geometric deformation, and hydrodynamic analysis of the buoy. Subsequently, the control points defining the buoy’s geometry are adjusted using the Non-dominated Sorting Genetic Algorithm II (NSGA-II), with the objective of maximizing the pitch response amplitude operator (RAO). Through iterative global optimization, the optimal buoy shape is identified under the given design conditions. Finally, computational fluid dynamics (CFD) simulations are performed to evaluate the time domain motion response of the optimized buoy, incorporating PTO damping under irregular wave conditions. The capture width ratio before and after the optimization is compared to assess the relative improvement. The results demonstrate that, compared with the initial wave-absorbing buoy, optimizing the wave-facing surface alone increases the capture width ratio by 58.28
Based on the existing flume-experimental records of meandering compound channels and explicitly accounting for the hydraulic influence of the floodplain vegetation, a machine-learning approach, namely genetic programming, was employed to derive a compact yet highly accurate discharge prediction equation. The resulting equation was compared against existing predictors and was subsequently interrogated, together with straight-compound-channel data, to quantify the individual influences of geometric and resistance parameters. Across the entire dataset for vegetated meandering compound channels, the new formula delivers a pronounced improvement in predictive accuracy, giving the smallest mean squared error and the substantially enhanced correlation coefficient. The sensitivity analysis identifies the main-channel sinuosity (s) and bed slope (S0) as the dominant controls. The discharge decreases monotonically with increasing s, the width ratio of the main channel to the entire channel (β) and vegetation density (φ), whereas it increases with S0. When s is low, the percentage reduction in discharge attributable to the increase in s intensifies with relative flow depth (Dr). Once s exceeds 1.5, the nondimensional reduction becomes essentially depth-independent.
Within continuously and eccentrically stirred tanks, the correct prediction of fluid mixing behavior is strongly dependent on the resolution of highly complex, multi-scale hydrodynamic features. To address this challenge, this study adopts a high-fidelity computational fluid dynamics (CFD) framework, integrating the volume of fluid (VOF) method, a delayed detached-eddy simulation (DDES) turbulence model, the Q-criterion for vortex identification, and thermodynamic entropy production theory to analyze the multiphase flow of a urea-water mixture. A comparative performance evaluation between flat-blade and propeller impellers reveals that the flat-blade configuration yields superior mixing efficiency, as evidenced by lower mixing times and distinct torque profiles. Mechanistically, the flat-blade impeller generates more intense and persistent trailing vortices that enhance localized fluid entrainment and promote global macro-circulation. Furthermore, multi-scale modal analysis via proper orthogonal decomposition (POD) demonstrates that macromixing performance is predominantly governed by dominant, large-scale coherent flow structures. Ultimately, these insights clarify the interdependence between transient flow topology and mixing kinetics, offering actionable design criteria for optimizing high-performance, continuous eccentric agitation systems.
High-velocity jets entrain air as they fall through the atmosphere, enhancing energy dissipation and thus being commonly used in high dams. However, the detailed evolution of self-aeration along such free-falling jets–from non-aerated to fully aerated–remains poorly understood. This study presents systematic large-scale experiments on free-falling jets, using a phase-detection probe, to examine their lateral aeration diffusion under various initial water depths and velocities. The test results show that the streamwise evolution of air entrainment can be divided into three stages–undeveloped, developing, and fully developed–and that the aerated flow region extends with increasing initial Froude number. It was also found that the average sectional void fraction is governed primarily by the initial geometry, e.g. the initial water depth and the trajectory length, rather than by the initial velocity. Moreover, the jet break-up length exceeds eight times the initial water depth and increases with the Froude number following a power law. A turbulent diffusion coefficient is introduced to fit the complex void fraction distribution into a single metric, which is proportional to the initial water depth and velocity. Based on these insights, predictive equations for the void fraction distribution and the average sectional void fraction under various initial flow conditions are proposed. These findings advance the fundamental understanding of air-water two-phase flow in high-velocity jets.
To reduce CO2 emissions and alleviate labor shortage in the field of transportation, the modal shift should be further promoted by developing a new-type coastal ship. We have studied a multi-connected pusher-barge system (P/B) as one of the promising means of transportation. This P/B system was designed to connect at most three barges to a pusher in the longitudinal direction and to apply the operational flexibility of traditional inland P/B convoys to coastal shipping routes. The berthing time can be saved by enabling barges to join or leave the system at sea. Computational fluid dynamics (CFD) simulations were utilized to analyze maneuvering hydrodynamic forces and visualize flow fields (velocity and pressure) across different loading conditions. Based on these hydrodynamic analyses, turning and zigzag maneuvers were simulated using a mathematical model. The findings demonstrate that the system’s maneuverability is highly sensitive to the loading conditions of the barges.
Small vessels are particularly vulnerable to marine accidents due to their limited stability and frequent reliance on empirical design practices. A significant challenge in the seakeeping analysis of small vessels stems from the use of estimated damping coefficients, which are seldom validated against high-fidelity data despite their critical influence on motion predictions. To address this limitation, a hybrid simulation procedure, referred to as CFD-modified potential (CMP), is proposed to enhance the accuracy of response amplitude operator (RAO) predictions. The CMP integrates traditional linear potential-flow solvers with CFD. For seakeeping performance analysis, the target sea area is selected and wave spectra corresponding to sea states SS2, SS3, and SS4 are calculated. In the first step, six-degree-of-freedom (6DOF) motion analyses were performed for three representative small vessels under multiple wave conditions. Next, CFD simulations focusing on roll and pitch motions at the peak response wave headings and frequencies were performed to quantify damping ratios, which were then fed back into the potential-flow solver to refine the RAO predictions. Comparison with conventional potential-based analyses demonstrated that the CMP significantly improves the correlation with experiments, particularly for roll motion. Furthermore, the inclusion of pitch damping, often neglected in small-vessel design, resulted in more realistic predictions of pitch amplitudes. Overall, the CMP approach provides a robust and practical tool for improving the reliability of seakeeping performance analyses, offering a viable solution to the limitations of traditional empirical methods and contributing to the development of safer and more accurate small vessel designs.
This study systematically investigates the cavitation dynamics during the salvo water-exit of vehicles through comprehensive experiments conducted on a specially designed test platform. The research focuses on the cavitation interference characteristics and compares the motion behaviors of single and dual vehicles under typical operating conditions. Several key findings are revealed: During the salvo water-exit process, distinct asymmetrical cavitation collapse patterns were observed between consecutive vehicles. Specifically, the first vehicle exhibited a shorter cavitation collapse duration and a higher peak lateral acceleration compared with the second vehicle. Axial velocity analysis showed that the first vehicle achieved a higher maximum velocity retention rate and enhanced velocity attenuation characteristics compared with the second vehicle. Furthermore, when compared with single-vehicle water-exit, the first vehicle’s shoulder cavity reached a stable state more rapidly, and cavity shedding occurred earlier. During salvo water-exit, the second vehicle experienced more pronounced cavity collapse, with cavity length and thickness being smaller than those of a single vehicle under identical conditions. Further analysis indicated that for both single- and dual-vehicle underwater launches, cavity development at the shoulder was more restricted in the radial direction than in the axial direction. Notably, the salvo water-exit interval significantly suppressed the lateral velocity and acceleration of the vehicles, advanced the timing of the maximum axial velocity, and increased the attenuation rate during the collapse phase. These results provide valuable insights into the complex interactions between vehicles during salvo water-exit and offer guidance for the design and optimization of underwater vehicle launch systems.
This study investigated the effect of varying end conditions on vortex-induced vibration (VIV) of a vertically mounted circular cylinder at the subcritical to critical Reynolds number (Re) range (3.14×104–2.51×105). Three endplate configurations were tested: upper and lower endplates, upper endplates only, and no endplates. Results showed that end conditions have a minor impact on peak amplitude but significantly alter the vibration branches. Notably, the no endplates configuration exhibited two amplitude peaks, with endplate effects becoming more significant above a Re of 1.0×105. The frequency ratio remained within the lock-in region for all high Re tests, and with no endplates, the ratio stayed closer to unity. The effect of end conditions on hydrodynamic forces mirrored the amplitude response: Peak drag and lift forces were similar across all configurations. However, with no endplates, drag force remained substantially higher than the other conditions within a reduced velocity range of 6–8, and the lift force showed two peak values. Phase difference analysis revealed two phase jumps for both the only upper and no endplates configurations, corresponding to an expanded upper branch. These observed differences in amplitude and hydrodynamic forces were interpreted based on instantaneous pressure distributions.
Vegetation fundamentally regulates river-bar morphology. However, the morphodynamic response of bars to the progressive removal of vegetation remains insufficiently characterized. Flume experiments with two steady flows were conducted using a two-stage uprooting protocol: Stage 1 trimmed about 30
Abrupt depth transitions (ADTs) have been identified as a potential trigger for the occurrence of extreme waves. However, the nonlinear propagation of focused wave groups over ADTs has been largely overlooked in existing research. This study focuses on the nonlinear dynamics of focused wave groups as they traverse a submerged step, aiming to assess the impact of ADTs on the evolution of nonlinear wave groups. Laboratory experiments are used to validate a fully nonlinear potential flow (FNPF) model for wave group propagation over constant water depth and a submerged step. Harmonic analysis is conducted to evaluate the significance of higher harmonic elevations, while spectral analysis reveals that the second and third harmonics reach their peak values at the midpoint of the submerged step. Compared to conditions without steps, the presence of ADTs results in increased skewness and kurtosis, leading to greater asymmetry in the free surface profiles. Additionally, with the same length of submerged steps, the presence of multiple ADTs enhances reflection capabilities, thereby reducing wave energy.
To apply the lattice Boltzmann method (LBM) to violent free-surface flows in ocean engineering, rigorous validation of both numerical stability and accuracy is essential. This study quantitatively assessed the accuracy of a velocity-based two-phase LBM for gas-liquid flow simulations, with particular emphasis on wave impact pressures. Two dam-break benchmark problems–A simple dam-break and a dam-break involving a rectangular obstacle-were simulated, and the resulting pressure-time histories were compared with experimental data from previous studies. For coarse meshes, instantaneous peak pressures were generally underestimated relative to experimental measurements. As mesh resolution increased, the predicted peak pressures rose and approached the experimental values, although some sensors exhibited slight overestimation. The results demonstrate sufficient numerical stability for laboratory-scale dam-break simulations, with no nonphysical oscillations observed in the velocity field. The in-house LBM code was implemented on multi–GPU systems and enabled large-scale gas-liquid two-phase simulations with meshes up to 1932×600×600, corresponding to approximately 696 million lattice points. These findings highlight the potential of the LBM as a robust and scalable tool for high-resolution, massively parallel simulation of complex multiphase flows in ocean engineering.
This study introduces a physics-informed neural network (PINN) framework for the spatio-temporal reconstruction and prediction of turbulent boundary layer flow fields over a broad range of Reynolds numbers. By incorporating the governing Navier-Stokes (N-S) equations into the loss function, the proposed model effectively integrates physical constraints with sparse flow field data, enabling high-fidelity spatial reconstruction, resolution optimization, and temporal prediction of velocity and pressure fields. The method is validated using simulated shock-induced turbulent boundary layer data and experimental turbulent channel flow data. Results show that the PINN model accurately reconstructs flow fields with 60
