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This study investigates the three-dimensional turbulence flow properties in a meandering channel under subcritical flow conditions, specifically focusing on a few critical parameters such as three-dimensional velocity behaviour, Turbulent kinetic energy, skewness, and kurtosis. A model study has been attempted by considering the same sinuosity, Froude number, and bed material from an Indian peninsular river to understand the basic hydrodynamics of a meandering channel. Understanding these critical parameters is crucial for comprehending the complex flow behaviour and its implications in meandering channels. Experimental measurements were conducted to collect data on Velocity, Turbulent Kinetic energy, skewness, and kurtosis in all three directions. The objective was to analyse their variations and behaviour along the meandering path, providing insights into the flow dynamics and turbulence characteristics. Advanced measurement techniques, such as Acoustic Doppler Velocimeters (ADV), were utilized to obtain accurate and detailed data. The study revealed distinct patterns and trends in the velocity, Turbulent Kinetic energy, skewness, and kurtosis along the flow path of a sand bed meandering channel. These parameters were found to vary significantly in different sections, highlighting the influence of channel geometry and flow conditions. Graphical representations of the flow profiles were employed to visualize the spatial distribution and understand the variations in these turbulence properties. The findings of this study enhance our understanding of the three-dimensional turbulence flow properties in sand bed meandering channels under subcritical flow conditions. The results offer valuable information for designing hydraulic structures, flood control measures, and river restoration projects. • This study aims to understand the behaviour of three-dimensional flow parameters in a meandering channel which are responsible for the morphological variations. • The Froude number data of the Mahanadi River is collected and same Froude number, sinuosity and bed material were maintained in the laboratory setup to perform the experiment. • The graphical variations of the Velocity, TKE, Skewness and Kurtosis in three directions were presented which describes the changes in the morphology and river bank erosion. [ABSTRACT FROM AUTHOR]

The three-dimensional flow of a wavy elliptic cylinder and its wake are investigated for wavelength (λ) to hydraulic diameter (D m) ratio λ/D m = 3.43, 4.58, and 6.01. The study aims to gain insights into flow separation, flow dynamics, and three-dimensional flow topologies for the selected wavelengths. Three-dimensional flow structures are visualized through time-averaged streamlines with critical point theory. The results for λ/D m = 3.43 show spiral flow directed from the node to the saddle, counter-rotating vortices, and a wavy vortex structure. On the contrary, the flows for λ/D m = 4.58 and 6.01 respectively undergo bifurcation in stable recirculation and vortex shedding on a half-span of the cylinder wavelength. The bifurcation results in a spiral flow toward the saddle and the other spiral flow toward the node. The bifurcations of recirculation and vortex shedding are revealed for the first time. The findings contribute to an improved understanding of the insight into intricate flow physics around wavy elliptic cylinders. • The three-dimensional flow of a wavy elliptic cylinder for wavelength λ/D m = 3.43, 4.58, and 6.01. • The λ/D m = 3.43 is characterized by spiral flow, counter-rotating vortices, and wavy vortex structure. • The flow for λ/D m = 4.58 undergoes bifurcation in stable recirculation, generating spiral flows toward the saddle and node. • The flow for λ/D m = 6.01 undergoes bifurcation in vortex shedding on a half-span of the cylinder wavelength. [ABSTRACT FROM AUTHOR]

At present, research on integrating deep learning with fluid dynamics has mostly focused on two-dimensional (2D) flow fields. Shifting from the study of 2D flow fields to the study of three-dimensional (3D) flow fields is crucial for accelerating the practical applications of deep learning in the real world. In this paper, we possess a high-quality direct numerical simulation dataset: 3D inclined circular disk flow fields with Reynolds numbers between 150 and 250, and a 3D convolutional autoencoder based on this dataset has been established to explore the specific performance of deep learning techniques in the reconstruction of 3D flow fields. The proposed model achieved significantly higher accuracy than proper orthogonal decomposition in terms of feature extraction in 3D inputs. When adding mask and noise to the inputs, the model exhibited impressive robustness: It had strong denoising capability under the Gaussian noise of k = 10 , and it can also reconstruct the flow fields well with inputs covered by one-eighth of the area. Additional slicing processing was applied to the model's output results during the research to highlight the differences between 3D flow field reconstruction tasks and the 2D scenario, allowing for detailed comparative analysis. • Proposed a practical deep learning model that can directly process three-dimensional flow field data. • Established a neural network structure that is highly robust to three-dimensional flow field noise and mask. • Discovered that the most difficult regions for the model to learn emerge in the rear half of the flow field. [ABSTRACT FROM AUTHOR]

This paper introduces an adaptive sampling strategy for a multi-vehicle sensor network to explore an underwater biological system known as the pelagic scattering layer, which is a region in the water column with a high density of marine organisms that reflect sound. Ever-changing ocean flow presents a challenge to multi-vehicle coordination in this process. The presence of an ocean flow field may disrupt inter-vehicle spacing so that the group loses communication, strays from a desired formation, or ends up with reduced area of coverage, especially for robotic drifting vehicles whose motion is largely influenced by the ambient flow field. However, those vehicles may also take advantage of the vertical variation of the flow to form a desired cohesive configuration since they can tune their vertical position via depth control. A motion-control algorithm, therefore, is a key element of the adaptive sampling strategy. The paper derives a decentralized coordination algorithm to stabilize a cohesive formation in a two-dimensional flow field with an initial unknown vertical distribution. The algorithm works with a distributed extended Kalman filter that generates a local estimate of the flow from pairwise range measurements between vehicles. Another component of the sampling strategy is the modeling of organism density dynamics using estimation of the density with local optical measurements, such as from onboard cameras. The density evolution is estimated using an ensemble Kalman filter and the results are fed into the motion-coordination algorithm. Numerical simulations illustrate the efficacy of this strategy and motivate ongoing and future efforts to extend the result to a three-dimensional geophysical flow. • A decentralized depth control algorithm guides multiple drifting vehicles to varying depths, ensuring cohesive, collision-free formation in shear flow. • Buoyancy control extends to unknown shear flow using extended Kalman filter for parameter estimation, enabling formation. • Ensemble Kalman filter estimates scattering layer density from depth predictions using operator and local measurements. [ABSTRACT FROM AUTHOR]

Due to the deviation of pore water seepage in marine soft soils from Darcy flow under low hydraulic gradients, existing methods face challenges in accurately calculating the time-dependent bearing capacity of piles in such areas. This paper adopts the secondary development of Python to ABAQUS and employs a continuous coupling analysis method to establish a numerical model capable of simultaneously solving the large deformation of pile driving and non-Darcy flow. The model addresses issues such as mesh distortion during pile driving and the numerical implementation of three-dimensional non-Darcy flow. The accuracy of the model was verified through comparison with two experimental cases. Subsequently, the study analyzed how non-Darcy flow affects soil consolidation around the pile and the time-dependent bearing capacity of pile. Results indicate that when the permeability coefficient of the soil around the pile is less than 1 × 10−7 m/s, non-Darcy flow significantly delays the consolidation of soil within three times the pile diameter around the pile, thereby reducing the time-dependent bearing capacity of pile, with a greater impact observed for lower permeability coefficients of the soil. Once the soil around the pile reaches 90% consolidation, subsequent construction can proceed to shorten the project duration while maintaining safety. • Secondary development based on Python-ABAQUS. • A new numerical solution to the large deformation problem of pile driving. • Numerical implementation of three-dimensional non-Darcy flow. • Comparison with centrifuge test and axisymmetric consolidation test. • Analysis of time-varying bearing capacity of pile in coastal soft soil area. [ABSTRACT FROM AUTHOR]

In icy regions, the vibration induced by broken ice poses a hazard to risers, in addition to vortex-induced vibration (VIV). A numerical investigation has been conducted to explore the dynamic response of risers in seas with fragmented ice floes. Assuming the broken ice as a single rigid floating body bonded by particles, the flow with water and ice is considered a two-phase flow, which is established using the Smooth Particle Hydrodynamics (SPH) method based on the quasi-fluid model. Verification of the model was achieved by simulating the three-dimensional flow around a surface-piercing cylinder and VIV. Accurate predictions were made regarding hydrodynamic forces, uplift values of the free liquid surface at the front stagnation point of the cylinder, and riser motion. Furthermore, the SPH model was expanded to encompass the realm of ice-water structural interaction (IWSI) combined with the basic principle of bidirectional fluid-structure coupling, facilitating the simulation of riser vibration within ice-water flows. The results indicate that the ice load remained the principal contributor to the force on the vibrating riser, while the riser exhibited irregular motion, demonstrating a strong coupling between forces and riser movement. • The study is the preliminary implementation of the SPH solution of ice-water two phase flow. • Two-phase control equations of ice-water flow are established based on the quasi-fluid model. • A description of the broken ice SPH method based on the adhesive model is established. • The proposed SPH model is extended to the IWSI field to investigate the IIV of the riser. [ABSTRACT FROM AUTHOR]

The purpose of this study, is to investigate the aeroelastic stability and effect of vortices resulting from passing air over cylindrical shells in three-dimensional condition. Here, the Naiver-Stokes equations in the integral form are considered studying the air flow based on the finite-volume method. The ANSYS Fluent software is used to study the fluid-solid interaction effect. The shear stress transport (SST) turbulence model is utilized to simulate the turbulent viscosity. Effect of different parameters such as two cylindrical shells alignment or two cylinders diameter ratio are considered. The results indicate that the vortices can play a significant role on the aeroelastic stability of the cylindrical shell. • The aeroelastic stability of two cylindrical shells under vortex induced vibration is studied. • A three-dimensional turbulent flow is simulated by a higher order discretization method. • A strongly coupled solver between fluid and solid is used as an FSI algorithm. • Effect of shells alignment in two circular and elliptical configurations are studied. [ABSTRACT FROM AUTHOR]

The high fidelity Navier Stokes equations based CFD codes precisely take care of the turbulent and viscous effects of complex flows passing through the hydraulic turbines. However, for preliminary design synthesis and analysis purposes, this full 3D flow can be assumed as a combination of two 2D flows (meridional axi-symmetric flow and blade-to-blade or cascade flow) which is the basis of low fidelity potential flow theory for hydraulic turbines. This paper presents the utilities of high as well as low fidelity CFD analyses for obtaining a runner blade design which yields an optimum hydrodynamic performance of a prototype Francis Turbine. The complete flow domain of the Francis turbine has been considered for the high-fidelity analysis which is carried out by using ANSYS CFX 16.0 for different operating conditions. Whereas, for the low fidelity investigations, full three-dimensional flow domains of different blade rows (i.e., stay vanes, guide vanes and runner) of the Francis turbine is divided into two 2D flow domains (i.e., an axisymmetric annulus of the turbine space for meridional flow analysis and cascades at different sections from hub to shroud for blade-to-blade analysis). By using the low fidelity analysis, runner blade design parameters have been adequately varied in order to obtain a population of different promising runner blade designs. Performances of these promising designs are evaluated, and the obtained results are thoroughly analysed through the response surface analysis. An optimized hydrodynamic performance design of the runner is obtained for the target values of responses (here, swirl velocity at trailing edge of the runner (C u2) and stagnation pressure at trailing edge of the runner (P 02 /ρ)) at different sections from hub to shroud by determining the design parameters of blade profiles at different sections. The adopted multi-fidelity CFD approach is found very effective and has the potential to yield reasonably accurate hydraulic designs of the Francis turbine in considerably lesser time and cost. • Comprehensive low and high-fidelity analyses of a Francis turbine runner have been carried out. • Using low-fidelity CFD codes for new designs is reliable due to their strong control over local flow and design parameters. • Converged solutions with low-fidelity codes require a uniform distribution of local flow parameters from hub to casing. • Low fidelity codes offer faster convergence rates. • Blade-to-blade space has more influence on the turbine's hydrodynamic performance than that of the meridional space. [ABSTRACT FROM AUTHOR]

Very Large Floating Structure (VLFS) plays a crucial role in mitigating land resource scarcity in developed coastal areas. This paper introduces an innovative VLFS concept consisting of numerous concrete box-type floats interconnected by steel tie rods, designed for a 4000-m-long floating marine airport. Based on three-dimensional potential flow theory, two numerical models for the VLFS are established: the single-float model and the multi-float model interconnected by joints, where each float treated as rigid. The research explores the impact of wave direction, wave period, and real sea conditions on the hydrodynamic response of the VLFS. Additionally, this paper focus on the discrepancies in dynamic characteristics between single-float and muti-float, and investigate the special response principles of the muti-float model that closely resemble the actual hydrodynamic features of VLFS. The findings show that local deformation in the VLFS results in elevated wave loads on the structure, posing a significant challenge in designing the VLFS mooring system. Higher structural stiffness correlates with decreased mean and pulsation values of mooring cable tension. Incident waves at 0° induce local deformation on the wave-facing side of the VLFS; however, the deformation rapidly dissipates, exerting minimal impact on the floats supporting the airport runway—thus, creating an indirect wave-absorbing mechanism. When waves incident at 90°, the VLFS bends, adversely impacting the operations of marine airports and causing uneven stresses on the connectors, making them susceptible to fracture and damage. Hence, it is advisable to install marine airports parallel to prevailing wave directions. Furthermore, the hydroelastic behaviour of VLFS contribute to alleviating internal stresses within the structure. The focus of future research lies in balancing the impact of VLFS hydroelasticity on both system motion performance and structural internal stresses. The VLFS concept presented in this study performs effectively under various real sea conditions. The rationality and feasibility of the proposed VLFS concept are substantiated. The study lays the foundation and presents ideas for the future development of marine space. • A novel Very Large Floating Structure (VLFS) concept is presented. • A multi-float hydrodynamic coupled model of VLFS is established. • Comparing the hydrodynamic response of single-float and multi-float VLFS model under different wave conditions. • Summarizes the potential relationship between VLFS hydroelasticity and rigid body motion. [ABSTRACT FROM AUTHOR]

Modal decomposition is a data-driven method widely used in fluid mechanics to extract energy and dynamically significant features of fluid flow. However, traditional modal decomposition methods have failed to capture physically interpretable multi-time-scale flow characteristics for complex flow phenomena, such as cavitation with multiple states. To address this issue, this paper combines the Proper Orthogonal Decomposition (POD) and Hidden Markov Models (HMM) methods, proposing a new approach termed state-based POD, applied to three-dimensional cavitating flow. This method comprises three steps: utilizing POD to extract cavitating flow features and visualizing them as spatiotemporal trajectories, employing HMM to probabilistically model these trajectories, obtaining flow field states, and probability models representing multi-time-scale flow information, and applying POD to perform modal decomposition on the multi-time-scale flow field information. This method provides a more detailed characterization of physical phenomena and offers a novel approach to analyzing cavitating flow. • A novel method combining POD and HMM to extract physically interpretable multi-time scale modal features. • Employing HMM to probabilistically model trajectory maps, achieving high accuracy in recognizing cavitation flow states. • State-based POD enables detailed modal decomposition of multi-time scale flow field information. [ABSTRACT FROM AUTHOR]

This work presents a data-driven approach to improve the predictive accuracy of the Reynolds stresses for flows over an axisymmetric body of revolution. Sparse experimental data of velocity and Reynolds normal stress is used to learn a nonlinear eddy viscosity model represented by neural networks with the ensemble Kalman method. It was recently proposed in Zhang et al., (2023) to enhance the predictive capability of the model by introducing a neural network-based model correction in the turbulent kinetic energy transport equation. Here this newly developed technique is applied for the first time to three-dimensional engineering flows in which the complete set of tensor functions are used. The results show that the method can learn a model with good predictive capability in both velocity and turbulence kinetic energy. The predictive improvement is achieved by suppressing turbulence production in the boundary layer and wake-shear layer with the added correction term in the turbulent kinetic energy transport equation. Moreover, the learned model can improve the prediction of Reynolds normal stresses by capturing the Reynolds stress anisotropy with nonlinear tensor bases. The necessity of combining the nonlinear tensor functions and the correction in turbulence transport equations is highlighted for accurate prediction of the Reynolds stress. • Applies the ensemble Kalman method to marine engineering flows for the first time • Learns neural network turbulence models capturing TKE and Reynolds stress anisotropy • Uses sparse data of velocity and Reynolds normal stress to improve model predictions • Highlights necessity of combining nonlinear tensor functions and TKE corrections [ABSTRACT FROM AUTHOR]

Several numerical challenges exist in the analysis of water-mooring line systems which require robust, yet practical, methods to address this type of fully coupled nonlinear dynamic problems. The present study proposes a novel class of numerical techniques for the formulation and implementation of a fully coupled dynamic system which involves water flows and catenary mooring line system. In particular, the three-dimensional water flow model is replaced by a simplified multilayer shallow water system with mass exchange terms between the layers including frictional forces at the bed topography and wind-driven forces at the water free-surface. Coupling conditions between the multilayer shallow water model and the mooring line system are also investigated in the current work. As numerical solvers we implement a fast finite volume method for the multilayer shallow water equations and a nonlinear dynamic analysis for the mooring line based on elastic catenary cable elements. Efficient calculations of the interaction forces between the shallow water flow and the submerged mooring line system and associated numerical implementations are also discussed. The accuracy and computational advantages of the proposed fully coupled system are verified using a series of well-established benchmark problems and wind-driven flows over both flat and non-flat beds. The computational results obtained show high performance the developed model and demonstrate the ability of the method to simulate fully coupled dynamic water-mooring line systems. • Development of simplified models for water-mooring line systems. • Multilayer shallow water flows with mass exchange for wind-driven flows. • Numerical methods based on elastic catenary cable elements for mooring line systems. • Interaction forces between the shallow water flow and the submerged mooring line system. • Numerical assessment and verification of fully coupled dynamic water-mooring line systems. [ABSTRACT FROM AUTHOR]

Accurate prediction of slamming loads on stern impact is quite difficult due to complex air effects. In this paper, the load characteristics of a three-dimensional stern model with pitch angle are studied by a drop test. The pressure distribution and impact velocity at a drop of 250–900 mm and a pitch angle of 0–15° are measured. The validity of the measurement is verified based on the repeatable analysis. The load characteristics under the pitch angle are further analyzed. The pressure near the stern shaft has complex characteristics, such as multi-peak values and high-frequency oscillation. The vibration sources of pressure are clarified, where the cavity is the main, and the support frame is the secondary. In addition, the vibration characteristics of the load are also affected by the height and pitch angle, in which the drop height has a greater effect on the high-frequency components. The influence of the pitch angle on the load characteristics is further discussed. The load peak increases 30%–60% from 0° to 15°. Finally, the influence of impact velocity and three-dimensional flow on the pressure peak is discussed. These findings reveal the law of stern load affected by pitch angle. • Drop-test experiments are performed to clarify the oblique impact characteristics of a 3D stern model. • A complete experimental dataset including falling kinematics and slamming loads is provided for the benchmark study. • The influence of the pitch angle on the slamming pressure characteristics of a 3D stern is clarified. • The influence of the 3D effect and different drop heights on the peak pressure is revealed. [ABSTRACT FROM AUTHOR]

Double-average turbulence characteristics of combined wave-current flow are studied over a rough bed comprising hemispheres arranged in a staggered pattern with different spacing (p / r = 4, 6, and 8; p = pitch length, r = height). The instantaneous velocity data was collected by Acoustic Doppler Velocimetry for the evaluation of double-average (DA) velocity, turbulence intensity, form-induced intensity, Reynolds stress, and form-induced stresses. The smallest DA velocity was obtained for p / r = 4 at the bottom region, signifying that p / r = 4 offered the maximum resistance to the flow among the tested cases. DA Reynolds stress is decreased for the entire flow depth due to superimposed waves. The advection of momentum-flux of normal stress in stream-wise and bottom-normal directions is increased at the outer layer and decreased at the near-bed region after wave-imposition. Maximum TKE production and diffusion are obtained just above the interfacial sub-layer and the middle of the form-induced sub-layer respectively. The turbulence structure is strongly anisotropic at the bottom region for p / r = 4 and near the outer layer, the tendency towards the return to isotropic state is dominant while with the increase in roughness spacing, a decrease in anisotropy is observed. Furthermore, an increase in wave-frequency decreased the tendency for the return to isotropy at the free surface. • The double-averaged flow properties of combined wave-current flow are studied over a staggered-arranged hemispherical bed. • Turbulence properties are decreased with the increase in wave frequency and roughness spacing. • Reynolds stress anisotropy is discussed to understand the three-dimensional flow structure. • The maximum rate of energy production and diffusion is found at the interfacial and form-induced sub-layer respectively. [ABSTRACT FROM AUTHOR]

The Honji instability of in-line forced cylinder near a plane wall is investigated using Floquet analysis and direct numerical simulation using the open-source code Nektar++. The onset of the Honji instability is analyzed over a range of the gap-to-diameter ratios (e / D) from 2.0 to 0.03125, with fixed Keulegan–Carpenter number K C = 2, Stokes number β = 150, and R e = 300. As e / D decreases from 2 to 0.375, the Honji instability is primarily on the gap side of the cylinder. For 0. 25 < e / D < 0. 375 , the location of the onset of the Honji instability switches from the gap side to the top side of the cylinder. This is due to the blockage effect, which results in the two different trends in the onset of the Honji instability with decreasing e / D. For 0. 125 < e / D < 0. 25 , the favorable pressure gradient suppresses the onset of instability at the gap side. The Honji instability is observed again at the top side of the cylinder at smaller gap ratio (e / D = 0. 0625), which is due to increased flow strength at the top side. At even small gap ratio, i.e. e / D = 0. 03125 , the secondary vortexes and Honji instability together enhance the three-dimensional flow. • The secondary vortices and Honji instability are mutually enhanced at small gap ratios. • For gap ratios e/D¿0.375, the Honji instability is influenced by the blockage effect. • For gap ratios 0.375¿e/D¿0.125, the Honji instability is affected by the favorable pressure gradient. • For gap ratios e/D = 0.03125, a reversed oscillating flow is encountered. • Mode switch is observed for small gap ratio e/D = 0.03125. [ABSTRACT FROM AUTHOR]

Blind tees are commonly used in offshore production systems for the flow mixing and measurement of oil and gas products. Since subsea pipelines usually consist of multiple bends, the blind-tee effects on bend combinations should be thoroughly investigated. In this paper, numerical simulations have been conducted to investigate the effects of blind tees on the flow physics in double-bend combinations. First, three-dimensional flow characteristics are analyzed to illustrate the blind-tee effects on double-bend pipes. Then, the absolute helicity density is introduced to quantitively evaluate the mixing capacity along the bend pipes with and without blind tees. Significant differences in flow behaviours and mixing qualities are observed in different bend configurations. If a blind tee is arranged in the first bend, strong vortices are generated, and the mixing conditions are significantly improved. However, when the blind tee is in the second bend, its internal flow is negatively influenced by the upstream elbow, resulting in a poor mixing condition. Besides, the effects of the structural parameters of double-bend combinations on the flow behaviours and mixing conditions have been systematically investigated. The analysis and results of this study can provide guidance for the application of blind tees in complex piping systems. • Structural effects of bend combinations are reported. • Mixing conditions in combined bends with blind tees are quantified. • Influence of upstream structure on downstream bend flow are revealed. • Optimal design schemes for different bend combinations are recommended. [ABSTRACT FROM AUTHOR]

Hydrofoil is widely used in hydraulic machinery, the complex shear flow and wake vortex with it can cause energy dissipation of system, affecting the stable operation of the unit. This paper takes the guide vane of a Francis turbine as the research object, and adopts the SST k − ω turbulence model to simulate the complex unsteady flow in a three-dimensional hydrofoil-channel. Entropy production theory is introduced to evaluate the energy dissipation. The effects of attack angle, gap ratio and Reynolds number on the flow loss are studied. The results show that: (1) In the hydrofoil-channel, the dissipation caused by the velocity gradient always dominates the irregular flow of fluid field. So the loss caused by the turbulent dissipation term is the main source of the loss in mainstream zone, accounting for more than 95%, while the loss caused by the viscous dissipation term accounts for only about 2% of the total loss, which can be almost ignored; (2) Changing the attack angle have a significant effect on the flow separation of the suction surface and the shear effect in the wake vortex zone, causing the energy loss to fluctuate accordingly. And 2.5° is the optimal attack angle. On this working condition, the loss decreases to the minimum value of 2.2714W/K and 0.1432 m, a decrease of 57% compared to the initial value; (3) The bottom boundary of channel suppress the development of wing tip vortex, reducing gap ratio will reduce the loss caused by wake vortex to some extent. Among them, S = 0.3 is the optimal gap ratio. Under this operating condition, the loss is 2.2941W/K and 0.1447m, a decrease of 56% compared to the initial value; (4) Affected by flow separation and wake vortexes, more than 80% of energy loss occurs in the middle and downstream zones. The effects of attack angle, gap ratio and Reynolds number on the fluid field are different. Increasing attack angle and flow velocity significantly exacerbate the flow separation and shear flow, while reducing gap ratio would inhibit the adverse flow in downstream wing tip vortex zone. • Applying entropy production theory to energy dissipation in hydrofoil fluid fields. • Discovering the dominant role of velocity fluctuations in turbulence. • The effects of attack angle, gap ratio, Reynolds number on vortex structure and energy loss are discussed, and the optimal conditions are given. [ABSTRACT FROM AUTHOR]

Secondary flow significantly changes the flow structure and increases flow resistance. The pipeline motion further contributes to the evolution of secondary flow. This study aims to elucidate the characteristics of two-phase secondary flow in inclined pipelines under additional periodic motion conditions. Based on the CFD technique, three-dimensional oscillating two-phase flow fields were simulated using the VOF model and dynamic mesh method. The focus was on four common flow patterns observed in inclined pipes, namely stratified flow, slug flow, turbulence flow, and transitional flow. The impact mechanisms of swaying, heaving, and coupling motion on the structural evolution, velocity profile, and relative intensity of secondary flow were analyzed. The results indicate that additional periodic motion exhibits a significant dominant behavior on secondary flow when the amplitude exceeds 1.0 D. Heave motion has a more pronounced effect on the secondary flow structure. Additional periodic motion induces periodic evolution of secondary flow structure in stratified flow, with the evolution period aligning with the period of the additional motion. Under additional periodic motion, there is no one-to-one correspondence between secondary flow velocity and axial velocity. Additional motion significantly suppresses the axial velocity of slug flow. Furthermore, additional periodic motion significantly increases the relative intensity of secondary flow, exceeding 10%. When the amplitude is less than 1.5 D , additional motion suppresses the relative intensity of secondary flow in turbulence flow and transitional flow. • Propose a scheme to numerically simulate the three-dimensional oscillating flow field using dynamic mesh combined with UDF. • The reliability of the CFD model was verified using experimental data from the published literature. • The details of secondary flow evolution in inclined pipe under additional periodic motion are revealed. • Analyze the secondary flow behavior of four typical flow patterns under additional periodic motion conditions. [ABSTRACT FROM AUTHOR]

The propeller jet generated by a ship's propeller during berthing and unberthing from quays initiates sediment transport and causes scouring around the quay structures. Investigating the flow area caused by ship propeller jets at quays and piers is important in terms of determining the local scour at the seabed. In this study, the water jet formed by ship's propellers and sediment movements on the seabed were investigated. While medium sand is typically used as the sediment type in literature, fine-grained sand and silt were used as base materials in this study. As a result of the experiments conducted with a three-dimensional flow velocity measuring device (ADV), the measured three-dimensional velocity components and the propeller efflux velocity coefficients were redefined. By examining the time dependent scour graphs of silt and sand, their scour rates were compared, and time scales were obtained from graphs. Silt was observed to have a significantly slower scour rate compared to sand. Contrary to the findings in the literature, despite silt having a smaller median particle diameter than sand, the maximum scour depths of silt were found to be less than those of sand under the same experimental conditions. Based on the results of this study and previous studies, two equations were proposed to quantify the scour depth for sand material, and silt and sand-clay mixtures. • Usage of silt as a seabed material. • Observation of time dependent scour development with bathymeter maps. • Investigation and comparison of scour time scales of silt and sand. • Calculation of propeller seabed-scour for sand, silt and clay-sand mixture. [ABSTRACT FROM AUTHOR]

Due to the complex three-dimensional flows such as fluid separation, fluid breaking and bubble flows, accurate prediction of slamming loads on 3D ship bow is still limited. In this paper, the asymmetrical slamming load characteristics of a truncated ship bow under the combination of heel and pitch postures are clarified through drop experiment. An angle device designed in the experiment has effectively adjusted the heel and pitch posture of the test model independently. The study provides a complete dataset (water entry velocity and pressure), which provides reference for future numerical verifications. Load characteristics of bow impact under asymmetrical water entry attitudes are analysed. The results show that some air cavity and jet flows are easy to cause the multiple pressure peaks, and affect the pressure oscillation. The influence of heel and pitch angles on impact pressure is further discussed. Finally, load characteristics with different water entry velocities are discussed to further improve its application. Results find that some abnormal pressure characteristics appear in some locations, that is, the pressure decreases with the increment of water entry velocity. The research results are expected to lay a foundation for further improving the predicting accuracy of hull slamming loads. • A series of drop tests of a 3D ship bow with combined angles is performed to provide complete dataset. • Characteristics of the slamming loads on the asymmetrical impact are clarified. • The influence of heel and pitch angle on the slamming loads is discussed. • The relation of pressure peak and drop height is revealed. [ABSTRACT FROM AUTHOR]

The sea cucumber Apostichopus japonicus (A. japonicus) is a benthic invertebrate with a quasi-cylindrical body and several rows of papillae (conical prominence) on its body wall. As nutrient recyclers, A. japonicus plays a fundamental role in marine ecosystems and is one of the most important commercial aquaculture species in eastern Asia. Despite its ecological and economical importance, little is known concerning the flow characteristics surrounding A. japonicus on the organism scale. Research on the detailed flow structure at such a scale may serve as a significant step toward understanding the transport of sediment and nutrients in the vicinity of A. japonicus. This study sets up the D3Q27 multiple-relaxation-time lattice Boltzmann method to obtain the three-dimensional flow structure around an individual A. japonicus placed at the bottom of a flat-bed channel. The numerical model is validated by a water flume experiment. The flow patterns exhibit distinctive features at three different angles of attack (0 ° , 4 5 ° , 9 0 ° ). The increasing inflow velocity (0. 1 m/s , 0. 25 m/s , 0. 4 m/s) brings no fundamental change to the structure of flow fields but induces more small-scale vortices and larger velocity fluctuations. This study investigates the complex turbulent flow fields around the A. japonicus and provides new insight for future studies on the interaction between water flow and A. japonicus. • A lattice Boltzmann model was built to study flow structure around A. japonicus. • Clusters of U-shaped vortices are produced on the surface of A. japonicus. • The flow patterns exhibit distinctive features at three different angles of attack. • The influence of the incoming flow velocity was investigated. [ABSTRACT FROM AUTHOR]

Research on the flow characteristics of incompressible fluids in the field of ship and ocean engineering is important. The direct numerical simulation method can obtain all information on the flow field by solving the full-scale Navier-Stokes equations. However, it has high computational cost and low computational efficiency. By contrast, the Reynold-averaged Navier-Stokes method has significantly better efficiency and is widely used in engineering, but it is not as accurate as the direct numerical simulation method (DNS). In recent years, deep learning has developed rapidly and provided a new method for the study of fluid flow characteristics. However, current models have many problems in three-dimensional cases, at high Reynolds numbers, and with inverse problem solving. To address these problems, we propose a high-fidelity flow field reconstruction model, namely, HFRIF for incompressible fluids. The method introduces reduced-order differential equation of viscous fluid motion and low-fidelity the Reynold-averaged Navier-Stokes method (RANS) simulation data as constraints in the loss function. We apply it to the reconstruction of two-dimensional and three-dimensional flow fields around a cylinder with Reynolds numbers of 100 and 10000, respectively, and compare it to the high-fidelity flow field data. Finally, super-resolution reconstruction of the flow field and the inversion of physical equation parameters are studied under the same model parameter settings. We find that even in three-dimensional cases and at high Reynolds numbers, the model only needs low-fidelity data to realize high-fidelity flow field reconstruction. Moreover, high-precision super-resolution flow field reconstruction and high-precision parameter inversion can be achieved without changing the model hyper-parameters. • Providing a HFRIF model to optimize the accuracy of CFD simulation. • The model can realize 2D and 3D high Reynolds flow field reconstruction and parameter inversion at the same time. • The HFRIF model training strategy taking into account the training data and training time is given. [ABSTRACT FROM AUTHOR]

In the present paper, the motion of a 3D modified squid model was studied numerically. After optimization, a 3D modified squid model was built from a 3D squid model. Details of creating a 3D squid model geometry from a longfin inshore doryteuthis pealeii squid and locomotion studies at various velocities were carried out by Olcay and Tabatabaei Malazi [ The effects of a longfin inshore squid's fins on propulsive efficiency during underwater swimming. Ocean Engineering. 128, 173–182]. A Multi-Objective Genetic Algorithm (MOGA) optimization algorithm with response surface methodology (RSM) analysis was employed to optimize the geometric parameters of the 3D squid model for improving hydrodynamic performances. The k- ω SST turbulence model was employed to solve the three-dimensional flow domain. After optimizing the squid model, pressure, viscous, and total drag forces were computed for the modified squid model for Reynolds numbers ranging from 465582 to 1600000. In this study, it was assumed that the modified squid model has a funnel-like shape with different diameters, like a real squid's funnel. Propulsive efficiencies of the modified squid model were computed for funnel diameters of 0.005 m, 0.01 m, and 0.02 m. The numerical results imply that optimizing geometric parameters can significantly reduce pressure, viscous, and total drag forces for the 3D modified squid model. The design optimization of the squid model causes the total drag force to reduce nearly by 25% in the modified squid model when the Reynolds number is 1600000. • Optimization of a 3D real squid model for obtaining a 3D modified squid model. • Using a multiobjective genetic algorithm (MOGA) with response surface methodology (RSM) for optimization. • Swimming performance analysis of the modified squid model. • Propulsion efficiency study of the modified squid at various locomotion speeds. [ABSTRACT FROM AUTHOR]

The aerodynamics of realistic frigate geometries is characterized by pressure and force measurements at the base of the three aft-facing steps that make up the frigate. The base model is a Lafayette class frigate. The conditions for bistable wake flow dynamics downstream of the command post, hangar and stern, are studied by changing elements of the geometry. The wake flow dynamics were found to be bi-stable only aft of the hangar, which can be detrimental to helicopters landing or taking off from the ship's deck. The bow level in front of the command post plays a key role in the ship's aerodynamics. It was found that bi-stability was only recovered when the ship's geometry was changed, by doubling the height of the parallelepiped command post and raising the bow to the level of the T-shaped structure. The wake flow at the stern was only bi-stable when the stern section was rectangular and straight. Finally, The bi-stability of the wake flow is eliminated on the hangar by adding a flap of sufficient depth in the extension of the hangar roof. • Experimental characterization of the bi-stable wake dynamics of 3D bluff bodies with complex geometry. • Suppression of the bi-stable wake dynamics by adding flaps. • Confirmation of the existence limits of the bi-stable regime in the case of bodies with complex geometry. [ABSTRACT FROM AUTHOR]

The acquisition and identification of quantitative instantaneous whole-field turbulence in water-jet pump effected by rotation and curved boundary are huge challenges. In this research, the limitations of traditional volumetric three-component velocimetry (V3V) measurement technical has been breakthrough to obtain three-dimensional whole flow field information of water-jet pump diffuser, and its effectiveness has been verified. The spatiotemporal characteristics of multi-scale flow structure in diffuser have been discussed based statistical analysis and POD (Proper orthogonal decomposition) method. It's found that the significant back flow in blade suction surface and non-uniform separation flow in the trailing edge of pressure surface consistently are occurred. Besides, low-order modes flow structures have the coherence characteristics which means the adjacent modes flow structures have similar time evolution characteristics with specific phase differences and same the low-frequency distributed in suction surface with back flow and trailing edge of pressure surface with wake vortex. Medium transition mode and high mode carry less energy but distribute in higher frequency to promote the mixture of energy and momentum and dissipation. Flow field of diffuser have also been reconstructed under different truncation precision. Characterization and reconstruction of turbulent flows make it easier to build the connection between turbulence and boundary parameters for inverse design method of water-jet pump. [Display omitted] • Off-location calibration methods forV3V are proposed and validated. • Inhomogeneous and unsteady turbulence in diffuser measured by V3V. • Characterization and reconstruction of turbulence showing their spatial distribution and energy spectrum. [ABSTRACT FROM AUTHOR]

Estimating the space-time characteristics of a sea state is of crucial importance to a number of engineering applications, such as the ones involving three-dimensional waves interacting with marine structures. In this context, developing a technique that allows extrapolating information about the wave field utilizing only a relatively small number of records is highly impactful, as it allows minimizing the use of expensive and sophisticated measurement techniques. In this paper, a Compressive Sampling (CS) based technique is developed for extrapolating free surface displacement data. The technique relies on a directional spectrum compatible sparse representation in conjunction with formulating and solving an L 1 -norm optimization problem. Further, the accuracy of the developed technique is significantly enhanced via the use of an adaptive basis re-weighting procedure. Pertinent numerical examples demonstrate that the technique is capable of reconstructing the time history of a free surface displacement record successfully, while capturing the main features of the target frequency spectrum and of the cross-correlation function satisfactorily. [ABSTRACT FROM AUTHOR]

This paper presents an alternative and open-source methodology based on binary image analysis to measure water elevations in two-dimensional hydrodynamic experiments. The proposed methodology considers the three main stages of an artificial vision system: image digitalization, processing and analysis. In the image processing stage, binary images were obtained by intensity modulation and pseudo-color-based segmentation. The image analysis stage employs simplified morphological operations to measure water elevations at specified regions of interest in the binary images. The image processing and analysis stages were developed in scripts for the ImageJ open-source software. The applicability of the proposed methodology was verified by comparing measurements of water elevations obtained using the proposed approach and conventional wave probes during experiments of shipping water on a fixed structure. The experiments included cases where single-valued and double-valued water surfaces were observed. For all the test cases, the water elevation time series obtained using the proposed approach were in good agreement with the experiments before three-dimensional effects on flow were significant. For the case in which a double-valued water surface was observed, it was confirmed that the proposed procedure possesses the capability to measure the effective water height at specified regions of interest. [ABSTRACT FROM AUTHOR]

In this article, we describe a novel non-intrusive reduction model for three-dimensional (3D) free surface flows. However, in this work we limit the vertical resolution to be a single element. So, although it does resolve some non-hydrostatic effects, it does not examine the application of reduced modelling to full 3D free surface flows, but it is an important step towards 3D modelling. A newly developed non-intrusive reduced order model (NIROM) ( Xiao et al., 2015a ) has been used in this work. Rather than taking the standard POD approach using the Galerkin projection, a Smolyak sparse grid interpolation method is employed to generate the NIROM. A set of interpolation functions is constructed to calculate the POD coefficients, where the POD coefficients at previous time steps are the inputs of the interpolation function. Therefore, this model is non-intrusive and does not require modifications to the code of the full system and is easy to implement. By using this new NIROM, we have developed a robust and efficient reduced order model for free surface flows within a 3D unstructured mesh finite element ocean model. What distinguishes the reduced order model developed here from other existing reduced order ocean models is (1) the inclusion of 3D dynamics with a free surface (the 3D computational domain and meshes are changed with the movement of the free surface); (2) the incorporation of wetting-drying; and (3) the first implementation of non-intrusive reduced order method in ocean modelling. Most importantly, the change of the computational domain with the free surface movement is taken into account in reduced order modelling. The accuracy and predictive capability of the new non-intrusive free surface flow ROM have been evaluated in Balzano and Okushiri tsunami test cases. This is the first step towards 3D reduced order modelling in realistic ocean cases. Results obtained show that the accuracy of free surface problems relative to the high fidelity model is maintained in ROM whilst the CPU time is reduced by several orders of magnitude. [ABSTRACT FROM AUTHOR]

As one of the primary noise sources of underwater-robots, the hydrodynamic-noise of the ducted-propeller (DP) originates from various unsteady flows in the flow-field. To investigate the coupling-mechanism between the flow-field and the acoustic-field of DP, this paper establishes a three-dimensional flow field-acoustic field coupling numerical calculation model based on the vortex-acoustic equation. It combines simulation and experimental research to investigate the unsteady flow-field of the DP. The mechanism of acoustic-wave generation and the distribution characteristics of the acoustic field are studied, and the DP's propagation law of hydrodynamic-noise is analyzed based on the field-synergy theory. The results show that the DP domain's vortex structure is mainly influenced by the tensile-distortion and Coriolis-force term. However, hydrodynamic-noise generation is closely related to the vortex, turbulent pulsation, and other unstable flows in the flow-field. The radiated noise of the propeller decreases and then increases with the increase of the inlet velocity-coefficient (IVC) and is smallest at the optimal efficiency point (J = 0.6). With the increase of IVC, the synergy of the propeller flow field-acoustic field is enhanced, the energy-loss of acoustic transmission increases, and the far-field radiation noise decreases. This paper provides a theoretical reference for suppressing the hydrodynamic-noise of the DP and acoustic-optimization design of the underwater-robots. • 3D flow field-acoustic field coupling CFD-simulation using vortex-acoustic theory. • DP's propagation law of hydrodynamic noise is analyzed using field synergy theory. • Reference study for the suppression of the hydrodynamic noise of the DP. • Reference study for the acoustic optimization design of the underwater robots. [ABSTRACT FROM AUTHOR]

Wind-wave combined structures have great development potential due to larger power production that they have compared with segregated offshore wind turbines or wave energy converters. The present paper studies a combined structure of a 5-MW braceless semisubmersible floating offshore wind turbine (FOWT) and different configurations of a torus-type wave energy converter (WEC). Various configurations of the combined structure are proposed and investigated based on three-dimensional potential flow theory. Initially, the effects of the number of the WECs that are utilized in different configurations of the combined structure on the hydrodynamic performance of the semisubmersible platform are investigated in the frequency domain by comparing hydrodynamic coefficients. The hydrodynamic coupling mechanisms of the combined structure with different number of WECs are studied. Wave contours are presented and compared to study the interaction effect of surrounding fluid with the combined structure and the examined various configurations. Response amplitude operators (RAOs) and the produced power for the examined combined structures in time domain are compared and discussed, it is indicated that the combined structure with the configuration with use of three WECs has the largest produced power capacity. • The effect of the number of the WECs on the hydrodynamic performance of the semi platform are investigated. • The hydrodynamic coupling mechanisms with different number of WECs are studied. • Wave contours are presented and compared to study the interaction effect. • RAOs and the produced power in the time domain are compared and discussed. [ABSTRACT FROM AUTHOR]

During the drilling operation of a Spar platform with a tender semisubmersible platform, the motion of the floating platforms is complex and quite different from that of the separate conditions due to the influence of multibody hydrodynamics and the connecting system. This can lead to potential collision accidents. In this paper, the motion characteristics of a tender assistant drilling (TAD) system under drilling scenarios are analyzed numerically and experimentally. Based on three-dimensional potential flow theory, the numerical model of a multiple floating body system is established to obtain the hydrodynamic parameters, the first-order wave force, and the mean drift force, and then a time domain analysis is conducted. Both single-body and coupled multibody conditions are observed to analyze the motion performance of the gangway and the tension response of the connecting lines. The results show that the Spar platform affects the low-frequency motion of the semisubmersible platform more intensely when the platforms work side by side without hawser lines. The phenomenon of hydrodynamic interaction is weakened under fully coupled conditions due to this connection. In addition, the stroke of the gangway is positively related to the relative motion, which reflects the relative motion response of the platforms. • The surge-pitch coupled motion characteristics of the Spar platform and its effect on TDU are observed in detail. • The motion response of the two platforms with and without connection is discussed. • Response of hawser lines and the gangway is analyzed to study the relative motion. [ABSTRACT FROM AUTHOR]

Measurement of the wave force acting on the structure is normally achieved by the traditional load cells. In this study, a non-contact computer vision (CV)-based method is proposed, which can conveniently acquire the wave pressure and force without installing pressure sensors or load cell mounted on the structure. Based on the velocity field measured by the Optical Flow technique, the pressure field is successfully computed by solving the Poisson pressure equation, and the wave force is further determined by integrating the pressure over the surfaces of the structure. The proposed method is verified by conducting a series of laboratory experiments of wave interacting with the rectangular structure. The accuracy of the method is validated by comparing with the results measured by pressure sensors. It indicates that the CV obtained pressure and wave forces generally agree well with the sensor measured results, including the time history of wave force and the peak wave force, and the reasons for the deviation and error are also discussed. The effects of wave height and wave period on the pressure field and wave force are studied, and it is found that the Keulegan–Carpenter number is an important parameter determining the peak wave force. Finally, the limitations existed in this method are noted, including the two-dimensional simplification and air bubble effects, which need to be addressed in the future. • Proposed a computer vision (CV) based method to measure the pressure and wave force acting on a rectangular structure. • The accuracy of the CV method is validated by comparing with the results measured by pressure sensors. • The three-dimensional flow and air bubbles effects affect the accuracy of the CV obtained results. • The peak wave force is closely related to KC number, for which the relationship can be quantitatively expressed as an exponential function. [ABSTRACT FROM AUTHOR]

We perform direct numerical simulations to characterize the three-dimensional wake dynamics of long inclined circular cylinders with inhomogeneous end boundary conditions. Three Reynolds numbers, R e = 100 , 200 and 300, corresponding to the regimes of laminar flow, mode A* wake, and mode B wake, respectively, are considered to reveal the roles of the intrinsic secondary instabilities and the extrinsic end boundary effects in shaping the three-dimensional flows. At R e = 100 , the end boundary effects are felt over the entire cylinder span by inducing oblique vortex shedding, which is associated with stronger spanwise flow in the wake than a parallel shedding. The Strouhal number of the oblique shedding is related to that of the parallel shedding of straight cylinder by the cosine law, considering the combined inclination angle and oblique angle. At R e = 200 , the intrinsic secondary instability results in large-scale vortex dislocation, precluding the propagation of the end boundary effects towards further span. Nevertheless, for the inclined cases, oblique shedding is still observed within limited span from the upstream end boundary. The oblique vortices feature intact and straight vortex cores, and are related to the two-dimensional flow at R e = 200 (that are void of vortex dislocations) from the viewpoint of cosine law. Further along the span, the oblique vortices destabilizes with the formation of small-scale vortices, and the flow transitions to the typical mode A* wake. At R e = 300 , the highly three-dimensional flow near the end boundary creates disturbances that travel along the cylinder span, creating vortex dislocations for cases with low inclination angles. For high inclination angle, oblique vortex shedding is again observed over the cylinder span, and is not disrupted by vortex dislocations of either intrinsic or extrinsic causes. The present study offers renewed insights into the three-dimensional wake dynamics of inclined cylinders, and lays the foundation for the designs of long flexible engineering structures and related flow control techniques. • DNS are performed to unveil the roles of end condition, R e , and inclination on the three dimensionality of cylinder wakes. • Extrinsic end boundary effects induce oblique vortex shedding, the frequency of which can be related to parallel shedding by the generalized cosine law. • The propagation of the end boundary effects can be terminated by the occurrence of vortex dislocation at higher Reynolds numbers. [ABSTRACT FROM AUTHOR]

Numerical study of three-dimensional unsteady flow behind the double wavy (DW), singe wavy (SW), and smooth circular (SC) cylinders geometry at subcritical Reynolds number (R e = 3.0 × 10 3) using large-eddy simulations based on the finite volume method are investigated in this study. This study contributes to the understanding of the effects of geometric disturbances as a passive flow control method to reduce the hydrodynamic forces exerted by the fluid flow on the cylinders. The DW and SW cylinders with spanwise wavelength ratios as λ / D m = 3.79 − 6.82 and with fixed wave amplitude ratio as a / D m = 0.152 are investigated and compared with SC cylinder in terms of flow variables such as drag, lift coefficients, Strouhal number, vortex formation length, and vorticity contours of the cylinders. Furthermore, an investigation was also carried out to quantify the differences in the fluid flow properties behind SC and SW cylinders compared to DW cylinders. When compared to the SC and SW cylinders, the findings indicated that the DW cylinder reduces drag and lift coefficients significantly for every λ / D m. A maximum reduction in fluid forces is observed at λ / D m = 6.06 for DW cylinder; 16.63% and 94.99% reduction in mean drag and lift fluctuations respectively compared to SC cylinder. The DW cylinder also showed a longer vortex formation length than SC and SW cylinders for every λ / D m. The flow separation angle and mean streamwise velocity distributions are also investigated and compared between the SC, SW, and DW cylinders. The present study reveals that the geometric surface waviness of the cylinder can be used to minimize force coefficients at a sub-critical flow regime. The obtained conclusions could provide some references to the engineering field for the design stage of slender marine structures. • Five different cylinders, each for single wavy and double wavy, were modeled based on wavelength ratios. • Flow around the single wavy, and double wavy cylinders were numerically investigated. • Hydrodynamic coefficients and flow features were compared between smooth, single wavy and double wavy cylinders. • An optimal wavelength each for single wavy and double wavy cylinders is presented. • The double wavy cylinder achieved more reduction in hydrodynamic coefficients than single wavy and smooth circular cylinders. [ABSTRACT FROM AUTHOR]

Dynamics of the three-dimensional flow past a rectangular cylinder placed near a moving wall is studied for different values of the aspect-ratio (0.5 ≤ AR ≤ 2.0) for a constant value of the gap-ratio (G/D = 0.5) and Reynolds number (Re = 160). Spatially localized energetic coherent structures and single frequency oscillatory structures are obtained by employing proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). The structures of Mode A, Mode B and Mode C vortices are captured by these POD and DMD modes. In comparison to the streamwise and transverse velocity POD modes, energy is distributed more evenly among the spanwise velocity POD modes. The wave speed of the travelling modes has been obtained using POD. The streamwise wave speed of the dominant POD mode first increases and then decreases with an increase in AR. Energy transfer among the POD modes is faster compared to energy fluctuation in the flow. Strong nonlinearity in the flow is observed at AR = 0.5. Compared to higher values of AR , at AR = 0.5 energy is more evenly distributed among the DMD modes. The DMD modes with higher frequency have a lower spatial wavelength, which reflects in their spatial structures. • Proper orthogonal decomposition and dynamic mode decomposition techniques are employed. • Three-dimensional modes Mode A, Mode B and Mode C have been captured. • The wave speed of travelling coherent structures is obtained using POD coefficients. • Compared to higher values of AR, at AR = 0.5 energy is more evenly distributed among the DMD modes. [ABSTRACT FROM AUTHOR]

Tip vortices are observed downstream of the flow past the three-dimensional hydrofoils used in propeller, wind and tidal turbines and they largely determine the forces exerted on such structures. In this study, we present the Laser Doppler Velocimetry (LDV) measurements in the flow past three-dimensional hydrofoils namely NACA 4412, NACA 16020, NACA 66(2)415 inclined at an angle of 10° with respect to the free-stream velocity, U ∞. The three-dimensional hydrofoils have a semi-span, s = 60 mm, chord lengths, c = 48 mm, 60 mm & 80 mm which correspond to three aspect ratios, ∧ = 2.5, 2.0 & 1.5, respectively, at the chord length based Reynolds number, Re ranging from Re = 1.92 × 105 − 6.39 × 105. LDV measurements were performed at several coordinate points in zy−plane perpendicular to the stream-wise, x direction, measuring the mean stream-wise, u(z, y) and mean velocity component along the z-axis, w(z, y) together with the root-mean square of their fluctuations in the plane located at a stream-wise distance, x/c = 1.0. The swirling strength of the tip vortices are qualitatively evident from the measurements of z− velocity component field, w(z, y) for all the hydrofoils, at various aspect ratios and Reynolds numbers. It is observed that the magnitude of normalized axial velocity deficit, (u(z, y) − U ∞)/U ∞ and normalized turbulent kinetic energy, TKE(z, y) are dependent on the type of hydrofoil, their aspect ratio, ∧ and the Reynolds number, Re. TKE within the region of the wake and vortex contain about 99% of the peak magnitude of TKE observed in the x/c = 1.0 plane. The vortex center, vortex position and vortex radius are shown to be dependant on the hydrofoil shape, aspect ratio and Reynolds number. • Dynamics of the vortices observed on the leeward side of the three-dimensional hydrofoils are studied by LDV measurements at a distance of one chord length downstream of the hydrofoil. • Axial velocity component, y-component velocity and turbulent kinetic energy (TKE) are measured for NACA 4412, NACA 16020, NACA 66(2)415 hydrofoils, having three aspect ratios, at three-different free-stream velocities. • The region of the wake and vortex are identified to be the region where 99% of TKE is concentrated. [ABSTRACT FROM AUTHOR]

Axial-flow pump is widely used in water-jet propulsion, and its hydrodynamic and cavitation performance have an important effect on the performance of the vessel. Therefore, the study of pump design is very important. In this work, a coupled viscous and potential flow three-dimensional design method for impeller of axial-flow pump with pre-swirl stator is developed. The body-force model (BFM) is used to solve the effective inflow field for impeller design by using a viscous Reynolds-averaged Navier-Stokes equations (RANS) commercial software, and the lifting-surface vortex lattice method (VLM) is used to design the impeller. The feasibility of the design scheme is verified by quasi-steady numerical simulation, and the uncertainty analysis of the numerical simulation is carried out according to the uncertainty analysis method recommended by International Towing Tank Conference (ITTC). The design and verification of axial flow pump are carried out in this paper, and the results show that the design method developed in this work is feasible. In addition, the cavitation performance can be improved by adjusting the load distribution reasonably. • A coupled viscous and potential flow design method for impeller of axial-flow pump with pre-swirl stator is developed. • The body-force model is used to calculate the effective inflow based on Star-CCM + commercial solver. • A lifting-surface vortex lattice method is used to design the rotor of axial-flow pump. [ABSTRACT FROM AUTHOR]

The three-dimensional (3D) flow characteristics affected by the structural heterogeneity of a wavy cylinder varied sinusoidally along the spanwise direction at a low Reynolds number (50 ∼ 350), are studied using numerical simulations. Compared with a uniform cylinder, the transition routes and vortex shedding modes of a wavy cylinder can be revealed based on the Strouhal number–Reynolds number relationship, with new vortex structures features discovered in each mode. Unsynchronized transitions between several 3D shedding modes and the resulting hysteresis-like phenomenon attributed to different initial conditions. The extrinsic 3D effects caused by the structural wavy perturbations afford the inhomogeneous spanwise variations in flow physical quantities, such as the pressure coefficient, time-averaged separation angles, and vortex formation lengths. The effects of the Reynolds number on the wake quantities of the wavy cylinder are compared with those of a uniform cylinder, which is described by a single function form in the laminar regime and irregular features in the 3D wake-transition regime. In addition, the deviations of the time-averaged separation angles between the two-dimensional (2D) and 3D results are adopted to reveal the strengthened three-dimensionality of the flow for a wavy cylinder and its variations with shedding mode transitions. • The 3D effects of a wavy cylinder is strengthened due to the structural features. • Vortex structures of a wavy cylinder show new features during the transition. • A hysteresis-like phenomenon was discovered in the wake of a wavy cylinder. • Many flow quantities present evident spanwise heterogeneity for a wavy cylinder. [ABSTRACT FROM AUTHOR]

In this study, circular cylinders with bionic surfaces were developed to control the flow fields around cylindrical structures such as stay cables of bridges. The bionic cylinder geometries were characterized by slanted elliptical governing sections and wave surfaces. Wind tunnel experiments were conducted to identify the surface pressure distributions, aerodynamic forces, and flow field characteristics of bionic circular cylinders at a Reynolds number (Re) of 3.8 × 104. The results show that the aerodynamic forces of the bionic cylinders are sensitive to the wind attack angle (γ) and wave surface wavelength. The control effects were excellent at γ = 0°, 90°, and 180°, whereas they vanished at γ = 45° and 135° compared with a baseline circular cylinder. When the wavelength range of the bionic cylinder is 4 D –5 D , the maximum reductions in the mean drag and fluctuating lift were 27% and 93%, respectively. The vortex formation region was elongated, and the alternative vortex shedding process was suppressed. Proper orthogonal decomposition was performed to determine the coherent structure of the wake field and the boundary vorticity flux analysis demonstrates the difference in the strength of the wake vortex from the viewpoint of boundary vorticity diffusion between the fluid and wall. Spatial–temporal correlation analysis of the velocity profiles behind different cross-sections shows that the wave surface troughs contribute significantly to disrupting the flow consistency in the spanwise direction. • A bionic cylinder characterized by wave surface and slanted elliptical governing sections was investigated. • The bionic surface could reduce the mean drag and fluctuating lift of the cylinder. • The flow structures, POD modes, and boundary vorticity flux were analyzed to explore the control mechanism. • The control method could decrease the spanwise correlation of the wake velocity and enhance the characteristics of the three-dimensional flow. [ABSTRACT FROM AUTHOR]

The investigation of transient flow behaviors of gas-liquid two-phase flow under ocean conditions is vital to the application of gas-liquid transport in ocean engineering area. In this paper, the gas-liquid two-phase flow numerical model of rolling horizontal pipe is established based on CFD code combined with user-defined functions (UDF). The three-dimensional rolling flow fields are simulated using the volume of fluid (VOF) model, RNG k − ε turbulence model and additional inertia force method. First, the simulation results are compared with the experimental data, which revealed a reasonable agreement. Then, the variation in transient flow behaviors by horizontal pipe (L = 2.0 m, D = 0.035 m) with varying rolling conditions (θ m = 2 ° - 6 ° , T = 0.5–4.0 s) and flow velocities (j G = 0.1–10.0 m/s, j L = 0.1–0.5 m/s) are investigated. The results showed that the rolling motion caused agglomeration of dispersed bubbles and gas slugs into a continuous gas column, increased the cross-sectional void fraction significantly. The variation of additional inertial force and pressure drop increased with the increase of rolling angle and the decrease of the rolling period. Compared with the rolling angle, the transient flow behaviors are more sensitive to the rolling period. In addition, as the initial gas volume fraction increased, the effects of rolling motion on the transient flow behaviors decreased gradually. These qualitative conclusions may serve as a reference for gas-liquid transport simulations under rolling conditions. • A numerical model of gas-liquid two-phase flow under rolling condition is developed. • The rolling motion increased the cross-sectional void fraction significantly. • Pressure drop increases with increasing rolling angle and decreasing rolling period. • The transient flow behaviors of two-phase flow are more sensitive to rolling period. [ABSTRACT FROM AUTHOR]

In this paper, we present direct numerical simulations (DNS) of the three-dimensional flow around a circular cylinder near a stationary wall at the Reynolds number of 500. The gap (G) between the cylinder surface and the stationary wall varies in the range of 0–3.0 D where D is the cylinder diameter. We observe that the proximity wall significantly affects the flow topology around the cylinder, which is characterized by the asymmetric wake, upward gap flow, and strong interactions of the cylinder vortices with the wall-generated boundary layer. Three flow regimes are identified with variations of the fluid forces and vortex dynamics. (i) Steady regime: at G / D < 0.3, vortex shedding is suppressed in the near wake, and scattered streamwise vortices form in the far wake where the upper side shear layer meets the stationary wall. No fluctuating forces act on the cylinder. (ii) Biased unsteady regime: at 0.3 ≤ G / D < 1.5, the gap flow deflects upward and the vortices shed from the gap side are significantly suppressed. The cylinder has enlarged fluctuating drag and lift forces with identical dominant frequencies. (iii) Parallel unsteady regime: at G / D ≥ 1.5, vortices shed from both sides of the cylinder are of almost identical intensities while the gap flow is approximately parallel to the wall. The vortex dynamics approaches that of an isolated circular cylinder in unconfined flows. Flow features of each regime are illuminated through the flow fields, vorticity contours, statistics of the enstrophies and gap flow, and pressure distribution on the cylinder surface. Finally, we investigate the critical gap ratio for the onset of the vortex-shedding of the upper shear layer and discuss the underlying physics. Moreover, we observe that the occurrence of the second harmonic lift and fundamental drag frequencies are resulted from the asymmetric wake because of the proximity of the stationary wall. • Direct numerical simulations of the three-dimensional flow past around a circular cylinder near a stationary wall at Re = 500 ere were conducted. • According to the variations of the fluid forces and vortex dynamics, three flow regimes are identified. • Flow features of each regime are illuminated thoroughly. [ABSTRACT FROM AUTHOR]

We conducted three-dimensional direct numerical simulations of flow past two stationary side-by-side circular cylinders. Based on the cylinder diameter, the Reynolds number was set as Re = 500 and the center-to-center spacing ratio was s / D = 1.0–5.0. In the studied parametric space, we identified five distinct flow regimes, namely, single bluff body flow at s / D = 1.1, deflected flow at s / D = 1.2–1.8, flip-flopping flow at s / D = 2.0–2.4, hybrid flow at s / D = 2.5, and antiphase flow at s / D = 2.7–5.0. The intrinsic features of each flow were elucidated using instantaneous vorticity contours, time-averaged flow fields, three-dimensional vortical structures, time histories and spectral frequencies of the drag and lift coefficients. Furthermore, the statistics of hydrodynamic forces, vortex shedding frequency, and wake three-dimensionality within each flow regime were examined thoroughly. Finally, we discussed two critical issues, namely (a) the effects of the three-dimensional vortical structures on the hydrodynamic forces and flow behaviors where both two- and three-dimensional results are compared, and (b) cylinder length effects and underlying mechanism for the switching behavior of the gap flow. We found that (1) the existence of the three-dimensional vortical structures significantly undermines the vortex interactions by absorbing energy from the spanwise vortices, and therefore, both the hydrodynamic and wake behaviors are considerably altered. (2) An increase of the cylinder length leads to the longer switching period of the gap flow in FF regime, while the transform of the gap flow direction is strongly dependent on the phase of the gap side vortices of the two cylinders. • Three-dimensional direct numerical simulations of flow past two side-by-side circular cylinders at Re = 500 were conducted. • • Intrinsic features of each regime were elucidated using instantaneous vorticity contours, time-averaged flow fields, three-dimensional vortical structures, time histories and spectral frequencies of the forcesFive distinct flow regimes, namely, single bluff body flow at s / D = 1.1, deflected flow at s / D = 1.2–1.8, flip-flopping flow at s / D = 2.0–2.4, hybrid flow at s / D = 2.5, and antiphase flow at s / D = 2.7–5.0, were identified. [ABSTRACT FROM AUTHOR]

Tip-gap flow is inevitable between the rotor and endwall in rotating machinery, which can lead to operation instability, blockage effects, energy loss, and even cavitation in water. In this study, large eddy simulation of the tip-gap flow of a three-dimensional aerofoil in a linear cascade with a moving endwall are performed. A series of aerofoils are designed with different section parameters (chord, thickness and camber), pitch angles, and tip-gap sizes. Detailed analysis of mean flow field, turbulence statistics and tip-leakage vortex (TLV) centre trajectory is performed to determine the parameter effects. The section parameters with the main influence on TLV is found to be the gap width. Larger chords and thicknesses can effectively suppress the TLV, whereas the pitch angle has small influence. The gap size has a significant effect on the TLV evolution, and in particular, small gaps lead to completely opposite characteristics compared with large gaps by restraining the TLV close to the suction surface and endwall. Quantitative analysis of the TLV centre wandering phenomenon demonstrates the unsteady nature of TLV. Furthermore, a duct propeller is simulated with the rotor tip designed to suppress the TLV, which turns out to improve the whole hydrodynamic performance of duct propeller. • The tip-gap flow of a 3D airfoil in a linear cascade with a moving endwall is studied by using large eddy simulation. • A series of 3D airfoils are designed with different section parameters, pitch angles and tip-gap sizes. • A detailed analysis of the turbulence statistics and tip-leakage vortex kinematics is carried out. [ABSTRACT FROM AUTHOR]

This paper identifies and characterizes modes of the three-dimensional instability of the flow over a circular cylinder of diameter D near a plane moving wall with a gap height of G = 0. 1 D beyond the onset of vortex shedding through the Floquet stability analysis. The associated effects of the transitions on the hydrodynamic forces of the cylinder are examined by performing direct numerical simulations. Six distinct three-dimensional modes are identified and their characteristics are discussed in detail. It is found that the physical mechanism responsible for the synchronous mode 1 appears to be associated to an elliptic instability. The quasi-periodic mode 4 is observed to transform to a synchronous mode (mode 6) on increasing the Reynolds number, indicating that they are likely to be resulted from the same physical mechanism. For mode 6, its physical nature seems to be a hyperbolic instability. Fully three-dimensional simulations show that the wake undergoes a rapid transition to complex chaotic flow state after the most dominant mode starts to saturate, due to the strongly nonlinear interactions of many modes with different spanwise wavelengths. • The presence of a plane moving wall in close proximity to a circular cylinder fundamentally alters the dynamics and the 3D wake transitions. • Six distinct modes of 3D instability are identified, their characteristics are discussed and the underlying physical mechanisms are proposed. • The three-dimensionality of the flow is triggered by different 3D instabilities as the Reynolds number is varied beyond the onset of vortex shedding. • The wake undergoes a rapid transition to complex chaotic flow state after the most dominant instability mode starts to saturate. [ABSTRACT FROM AUTHOR]

To predict the effect of the uniform gradient stream, the hydrodynamic force acting on a translating solid is studied following the Newman method. Some new hydrodynamic force formulas that are valid in both two-dimensional and three-dimensional flow are proposed. A new and efficient velocity-vector based method is proposed to compute the added mass that is required in the new formulas. Abundant numerical examples are designed to check the correctness of the new formulas and the validity of the Newman hypothesis adopted during the proof. Comparison with classical results shows that the formula proposed by Taylor and Newman are only two special cases of the present study. Comparison with some recent studies are also studied and their rationality is analyzed. • Four formulas that predict the hydrodynamic force in non-uniform stream are proposed. • A new velocity vector based formula for evaluating the added mass is proposed. • A new concept of generalized added mass is proposed and its property is analyzed. [ABSTRACT FROM AUTHOR]

The air wake generated behind a frigate superstructure is a very complex and unsteady three-dimensional flow characterized by highly turbulent flow structures with great velocity gradients that result in flow separation over the flight deck where helicopter operations take a significant role. Naturally, this turbulent flow should be removed or, at least, reduced as maximum as possible to avoid accidents during the interaction between frigates and helicopters. This paper involves experimental and numerical analysis on a simple frigate model (SFS2) to understand and simulate the adverse effect of the turbulent flow of the air wake over the deck and tries to minimize it as with active flow control. The experimental study has been performed in the Low Speed Wind Tunnel nº1 of Instituto Nacional de Técnica Aeroespacial "Esteban Terradas'' (INTA) with Particle Image Velocimetry (PIV) to obtain the velocity field over the flight deck. The frigate has been tested for the headwind condition of 0° (wind aligned with the flight deck) with a velocity of 10 m/s. The active flow control consists on generating suction on the flight deck by a mesh of holes. Several configurations of this suction mesh have been studied according to the diameter and location of holes and suction intensity. The suction mesh configuration with the highest diameter and flow rate presents the greatest aerodynamic improvement. The low-speed area over the deck is reduced from 36% to a minimum of 3%. • Experimental and numerical investigation of airflow on a Simple Frigate Shape wind model. • Design and study of the flow control system based on the flow suction. • Comparative study of the base flow on the frigate model versus modified flow by the suction flow control system for different configurations. • Parametric study of results for different air flow rates and locations of suction. • Concordance between numerical and wind tunnel results investigated by a comparative analysis. [ABSTRACT FROM AUTHOR]

Three-dimensional direct numerical simulation is carried out to investigate the effects of the yaw angle and spanning length on the vortex structures and hydrodynamics of flow crossing a near-wall cylindrical structure. Two yaw angles (α = 0°, 45°), and three spanning length ratios (L * = 25, 50, 100) are investigated at Re = 500. Under the normal cylinder condition (α = 0°), a parallel vortex shedding pattern is observed in the wake. The vortex formation lengths are identical for all L *, whereas the spanwise correlation of the vortex shedding declines with the increasing L *, indicating the intensified three-dimensionality. A single dominant peak is found in the lift frequency spectra for each L * case. Under the yawed condition (α = 45°), the flow characteristics are fundamentally changed with negatively-yawed linear vortex shedding tubes being observed in the wake. Different from the normal cases, the vortex formation length for L * = 25 is larger than that of L * = 50 and 100, while the oblique angle of the vortex tube decreases with the increasing L *. It is shown that, at α = 45°, the three-dimensional flow features are fully developed when L * ≥ 50. The lift frequency spectra show multiple peaks while the magnitudes of the secondary peaks are significantly reduced by extending the spanning length. By comparing the results between α = 0° and α = 45°, it is found the Independence Principle (IP) is applicable for predicting the first-order statistics of the hydrodynamic forces and vortex shedding frequency, but fails in predicting the second-order force statistics. The effects of the yaw angle on the pressure and shear stress distributions of the bottom wall are also discussed. • Parallel and negatively-yawed vortex shedding patterns are observed for normal and yawed cylinder cases, respectively. • The requisite spanning length for inherently reflecting the three-dimensional flow characteristics are found. • Pressure and shear stress distributing on the bottom wall are investigated. • The validity of the Independence Principle for predicting the hydrodynamics are examined. [ABSTRACT FROM AUTHOR]

The interaction of ship and waterway is of utmost importance in inland navigation. Regulations and waterway maintenance rely on assumptions for the damage potential of propeller jets. Scours are to be avoided or repaired at an early stage. As preparation for the validation of future CFD simulations, measurements were carried out in the propeller jet using different methods. 2D3C PIV is a non-invasive method for flow measurements in a 2D plane. However, for applications with transient flows, volumetric measurements are desired. Previous Particle Tracking Velocimetry systems suffered from low resolution and difficulties with high seeding density. The system used here attempts to overcome these disadvantages with the Shake-the-Box algorithm and a new hardware design. The focus is on comparing this volumetric flow measurement technique with conventional 2D3C PIV. The test case is the wake flow of a ducted propeller in bollard pull condition. Since the tests were carried out in different facilities, the measured flow fields are not fully comparable. Nevertheless, it is shown that the method is suitable for phase-averaged measurements in addition to the advantages in three-dimensional flows. The findings from this first series of measurements are used for the further development of the system. • A PTV system with a new compact hardware design was tested in a towing tank. • Validation data for the jet behind a ducted propeller was generated. • PTV results are compared to 2D3C PIV measurements. • Findings for further development of the system could be derived. [ABSTRACT FROM AUTHOR]

The panel method is widely used in the design and analysis of marine propellers. However, this method cannot handle propellers with a blunt trailing edge due to a lack of proper models for the flow separation downstream. In this study, a numerical scheme, i.e., the three-dimensional flow separation model, is proposed to enable the panel method to be applied to propellers with a blunt trailing edge. In this model, the flow separation zone is represented as an extension of the trailing edge. The criteria of zero local lifts and zero local moments over the extension of all blade sections are used to establish the equations that determine the shape of the flow separation zone. Those equations are solved either by the Newton–Raphson method or as an optimization problem. A low-order panel method coupled with a boundary layer solver (the viscous/inviscid interaction method) is used to predict the pressure distributions and forces for the extended geometry. To validate this model, the pressure distributions of the propeller blade are compared with those from solving the Reynolds-Averaged Navier–Stokes equations. The predicted open water characteristics of a controllable pitch propeller are compared with the experimental measurements at a wide range of advance ratios with different pitches. This model requires much less computational effort while preserving high accuracy, and thus can be used reliably in designing and analyzing propellers with a blunt trailing edge. [Display omitted] • A flow separation model is proposed which enables the panel method to be applied to propellers with blunt trailing edges. • This model recovers very closely the open water characteristics from the experiment measurements of a controllable pitch propeller at a wide range of advance ratios and different pitches. • This model requires significantly less computing time and saves the efforts for meshing, compared with other methods like RANS, and proves to be a reliable, robust and efficient tool. [ABSTRACT FROM AUTHOR]

Underway replenishment is an essential component of long-term naval operations. During underway replenishment, two ships travel in close proximity at moderate forward speed. This paper examines the hydrodynamic interactions that can influence seakeeping during such operations. Presented numerical predictions include the influence of interaction effects on hydrodynamic forces for two ships in waves. A scarcity of validation material for numerical predictions prompted new towing tank experiments for two ships in waves. The experiments used semi-captive models, and the numerical code was modified to include restraining forces for specified modes. The numerical predictions and experiments show that the presence of a larger ship can significantly influence the motions of a smaller ship in close proximity. [Copyright &y& Elsevier]