Hydrodynamic Flow Field Research Articles

Refined Simulation Study of Hydrodynamic Properties and Flow Field Characteristics around Tandem Bridge Piers under Ice-Cover Conditions

Ice cover is a common phenomenon in rivers in cold regions during the winter freeze-up period, leading to the formation of unsteady bypass structures around underwater piers. To reveal the variation law of the flow field around a pier under ice, a numerical calculation method is proposed to obtain the spatial and temporal characteristics of the fluid flow environment around the pier. The verification of flow conditions and convergence showed that the numerical model constructed in this study is reliable and can meet research requirements. The simulation results showed that the ice-cover condition considerably impacted the extent of a scour hole, and in the horizontal plane Z of −0.02 m, the lateral influence of the scour hole was approximately 2.6 times the diameter of the pier, which was approximately 42% wider than that of a scour hole under open-flow conditions; in the area on the side of the pier, there was a peak in longitudinal section y/D of −0.6, and the relative turbulence intensity was 0.4 and 0.51 under open-flow and ice-cover conditions, respectively, indicating that ice cover made the peak more significant in the area.

Rotational dynamics of a disk in a thin film of weakly nematic fluid subject to linear friction

Dynamics at low Reynolds numbers experiences recent revival in the fields of biophysics and active matter. While in bulk isotropic fluids it is exhaustively studied, this is less so in anisotropic fluids and in confined situations. Here, we combine the latter two by studying the rotation of a disk-like inclusion in a uniaxially anisotropic, globally oriented, incompressible two-dimensional fluid film. In terms of a perturbative expansion in parameters that quantify anisotropies in viscosity and in additional linear friction with a supporting substrate or other type of confinement, we derive analytical expressions for the resulting hydrodynamic flow and pressure fields as well as for the resistance and mobility coefficients of the rotating disk. It turns out that, in contrast to translational motion, the solutions remain well-behaved also in the absence of the additional linear friction. Comparison with results from finite-element simulations shows very good agreement with those from our analytical calculations. Besides applications to describe technological systems, for instance, in the area of microfluidics and thin cells of aligned nematic liquid crystals, our solutions are important for quantitative theoretical approaches to fluid membranes and thin films in general featuring a preferred direction.

Motility and pairwise interactions of chemically active droplets in one-dimensional confinement.

Self-propelled droplets serve as ideal model systems to delve deeper into understanding the motion of biological microswimmers by simulating their motility. Biological microorganisms are renowned for showcasing a diverse array of dynamic swimming behaviors when confronted with physical constraints. This study aims to elucidate the impact of physical constraints on swimming characteristics of biological microorganisms. To achieve this, we present observations on the individual and pairwise behavior of micellar solubilized self-propelled 4-cyano-4'-pentyl-biphenyl (5CB) oil droplets in a square capillary channel filled with a surfactant trimethyl ammonium bromide aqueous solution. To explore the effect of the underlying Péclet number of the swimming droplets, the study is also performed in the presence of additives such as high molecular weight polymer polyethylene oxide and molecular solute glycerol. The capillary confinement restricts droplet to predominantly one-dimensional motion, albeit with noticeable differences in their motion across the three scenarios. Through a characterization of the chemical and hydrodynamic flow fields surrounding the droplets, we illustrate that the modification of the droplets' chemical field due to confinement varies significantly based on the underlying differences in the Péclet number in these cases. This alteration in the chemical field distribution notably affects the individual droplets' motion. Moreover, these distinct chemical field interactions between the droplets also lead to variations in their pairwise motion, ranging from behaviors like chasing to scattering.

Deep-learning-assisted triboelectric whisker for near field perception and online state estimation of underwater vehicle

Agile near field perception remains a challenge for underwater vehicles, which could significantly enhance their capability of online state estimation. Harbor seals have evolved a whisker array that can accurately measure and identify environmental information in their surroundings. Inspired by the "smart" whiskers of harbor seals, the study has designed a deep learning-assisted bionic underwater triboelectric whisker sensor (UTWS) to passively perceive diverse hydrodynamic flow fields, including omni-directional steady flow and wakes induced by bluff body upstream. The device mainly comprised an elliptical whisker shaft with a high-aspect-ratio of 0.403, four flexible triboelectric sensing units mimicking the nerve in the follicular sinus complex, and a flexible corrugated joint imitating the surface skin of marine organisms' cheeks. The UTWS demonstrated impressive advantages of rapid response times of 21 ms, high sensitivity of 1.16 V/m.s−1, high signal-to-noise-ratio of 61.66 dB. By implementing deep learning analytics to process the multi-channel signals, the underwater vehicle equipped with the UTWS can accomplish online velocity estimation proficiently, with an approximate root mean square error of approximately 0.093 in the verification case. Thus, this UTWS-based, deep-learning-assisted perception could become a promising tool for integration with underwater vehicles in the local navigation tasks.

Numerical Study on the Hydrodynamic Coefficients and Flow Field Characteristics of Underwater Manipulator

Numerical Study on the Hydrodynamic Coefficients and Flow Field Characteristics of Underwater Manipulator

Numerical investigation on the influence of free surface on the hydrodynamic and wake characteristics of submarine

In the process of movement, the submarine inevitably involves free surface navigation, such as floating observation, suction power generation, and rescue. Under various circumstances, the interaction between the submarine and the free surface leads to complex flow fields and wakes, which affects its hydrodynamic performance. In this study, a three-dimensional numerical model of a submarine considering the influence of free surface is established. The volume-of-fluid model with an artificial compression term is used to capture the gas–liquid interface, and the unsteady flow field and hydrodynamics are predicted using the shear stress transport k–ω turbulence model. Based on the analysis of mesh convergence and numerical reliability, the hydrodynamic performance and wake flow field characteristics of submarines under different submergence depths were studied. The results show that the free surface has a significant influence on the resistance, wave wake, wake field, and vortex structure of the submarine, which is closely related to the submergence depth. Compared with the infinite submergence depth, the total resistance of the submarine near the free surface increases by 159.2%, mainly due to the pressure resistance. The surface wave system generated by the interaction between the free surface and the hull will directly affect the distribution of the wave surface wake, the wake flow field, and the vortex structure. As the submergence depth increases, the free surface effect gradually weakens, and it can be ignored when the submergence depth is more than 4 times the diameter of the hull.

Analysis of scale effect on the propulsion discrepancy of a four-waterjet system

This paper focuses on the scale effect on the propulsion discrepancy and the correlating flow characteristics of a high-speed ship with four waterjets. The propulsion discrepancy of a four-waterjet-propelled ship in model test is observed in previous study (Gong et al. Ocean Eng. 2019, 180, 211–222). Current study further performs a systematic analysis of the scale effect on several hydrodynamic coefficients (net thrust, lateral/vertical forces and torque) and flow field details (velocity, TKE and pressure). It is noted that the net thrust discrepancies are amplified in model-scale simulations and tests due to the scale effect, and the lateral/vertical force discrepancies of waterjet units are gradually decreasing with the increase of advanced speed in both scales. Stable hydraulic performance of the inner and outer pumps is observed although there are minor differences in the inflow quality, while stronger turbulence intensity exists near the channel faces in full scale due to the induction effect of the impeller.

An Experimental Study of the Effects of Asymmetric Pitching Motion on the Hydrodynamic Propulsion of a Flapping Fin

Aquatic organisms have evolved exceptional propulsion and even transoceanic migrating capabilities, surpassing artificial vessels significantly in maneuverability and efficiency. Understanding the hydrodynamic mechanisms of aquatic organisms is crucial for developing advanced biomimetic underwater propulsion vehicles. Underwater tetrapods such as sea turtles use fins or flippers for propulsion, which exhibit three rotational degrees of freedom, including flapping, sweeping, and pitching motions. Unlike previous studies that often simplify motion kinematics, this study employs a specially designed experimental device to mimic sea turtle fins’ motion and explore the impact of pitching amplitude, asymmetric pitching kinematics, and pausing time on lift and thrust generation. Force transducers and particle image velocimetry techniques are used to examine the hydrodynamic forces and flow field, respectively. It is found that boosting the fin’s pitching amplitude enhances both its lift and thrust efficiency to a certain extent, with a more pronounced effect on thrust performance. Surprisingly, the asymmetrical nature of the pitching angle’s pausing time within one flapping cycle significantly influences the lift and thrust characteristics during sea turtle swimming; extending the pausing time during the forward and upward flapping process improves lift efficiency; and prolonging the pausing time during the downward flapping process enhances thrust efficiency. Furthermore, the mechanism for high lift and thrust efficiency is revealed by examining the vortices shed from the fin during different motion kinematics. This research contributes to a more comprehensive understanding of the fin’s hydrodynamic characteristic, providing insights that can guide the design of more efficient biomimetic underwater propulsion systems.

The comparative transient prediction on hydrodynamic characteristics and flow field properties of pump-jets with accelerating and decelerating ducts

Pump-jet holds a pivotal position in various marine applications, underscoring the need for comprehending their transient behavior for the purpose of design enhancement and performance refinement. This paper employs Reynolds-averaged Navier–Stokes equations method in conjunction with Detached Eddy Simulation model. The study delves into the ramifications of accelerating and decelerating ducts, distinguished by camber f and attack angles α, on transient hydrodynamic characteristics. The hydrodynamic characteristics are investigated numerically, after the validation of the numerical methodology by comparing simulation outcomes against experimental results. Subsequently, the study delves into propulsion characteristics, followed by an exploration of time-domain and frequency-domain data transformed through fast Fourier transform to analyze thrust fluctuations and pulsating pressures. Additionally, a detailed examination of pressure distribution and velocity field is provided, aiming to dissect the mechanisms through the variations in f and α influence the flow field. Findings suggest that the outlet velocity of accelerating ducts significantly surpasses the inlet velocity, a behavior contrasted by decelerating ducts. Notably, the patterns of accelerating and decelerating ducts resulting from alterations in f exhibit consistent characteristics with those brought about by changes in α. However, several opposite characteristics surface in transient flow field due to the distinct modifications in the duct profile. Furthermore, by considering vorticity magnitude distribution and vortices, a comparative analysis elucidates the effects of varying f and α on rotor and stator trailing vortices. This contributes to understanding the flow instability mechanism under differing duct configurations. It is evident that changes in f and α exert significant influence on both performance and flow field.

Intensification of low-grade phosphate ore purification using a hydrocyclone with venturi-style inlet

The accumulation of large quantities of low-grade phosphate ore throughout the year occupies land and pollutes the environment. Addressing the challenge of purifying this ore to produce high-grade phosphate is essential for alleviating the shortage of phosphorus resources, making it a pressing research topic. Herein, we propose a hydrocyclone with a venturi-style inlet that combines multiple morphologies to purify low-grade phosphate ore. This innovative design addresses issues with conventional hydrocyclones by reducing short-circuit flow and eliminating circulation flow. Additionally, the new hydrocyclone tackles the overflow entrainment problem of large-density P particles and reduces the possibility of lower-density Si-Mg particles escaping through the underflow pipe, leading to intensified separation performance. The hydrodynamic multiphase flow field characteristics of different hydrocyclone were investigated via CFD simulation to elaborate the reason for the performance intensification. The venturi-style inlet architecture improved the tangential velocity and enhanced the spiral flow field, expanding the length and range of the locus of zero vertical velocity (LZVV) in the primary separation area. This designed hydrocyclone could purify low-grade phosphate ore at lower expenditure and higher performance, with separation efficiency increased from 76% to 92% and precision promoted from 70% to 87% compared to conventional one. This work presented a cheap and efficient technology for purifying low-grade phosphate ore and demonstrated comprehensive potential for the recycling of mineral resources.

Hydrodynamic Performance and Flow Field Characteristics of Tidal Current Energy Turbine with and without Winglets

In order to gain a more comprehensive understanding of the influence of winglets on the hydrodynamic performance and flow field characteristics of tidal current energy turbines, two different shapes of winglets are designed, and numerical simulation results for turbines with and without winglets are compared and analyzed. The results show that both shapes of winglets can improve the energy conversion efficiency, and the winglets with a cant angle (60°) are more effective than the flat (0°) winglets; the winglets transfer the tip vortices to the winglet tips and weaken the tip vortices, increase the pressure coefficients of the cross-section in the tip region, and inhibit the three-dimensional flow phenomenon in the tip region; the winglets will make the wake axial velocity deficit larger in the near-wake region, and with the axial distance increases, the axial velocity of the wake flow with winglets recovers faster than that without winglets; winglets can make the vortex pairing and breaking of the turbine tip vortex faster, which can also be observed by the change in the turbulent kinetic energy (TKE).

Comparison of dimensionality reduction techniques for multi-variable spatiotemporal flow fields

In the field of fluid mechanics, it is a potential consensus that nonlinear dimensionality reduction (DR) techniques outperform linear methods. However, this conclusion has been obtained based on simple fluid phenomena and an incomplete evaluation system of dimensionality reduction algorithms. In this study, we use an improved evaluation system of DR methods to compare and evaluate the performance of four DR methods, including two linear techniques: Principal Component Analysis (PCA) and Independent Component Analysis (ICA), and two non-linear techniques: Isometric Mapping (ISOMAP) and Locally Linear Embedding (LLE). The four methods are applied to analyze a complex hydrodynamic flow field with cavitation by considering the joint features of multiple variables. Results show that LLE can capture redundant features that do not contribute to understanding the characteristics of flow fields, while ISOMAP is more suitable for handling datasets with multiple scales than LLE. PCA and ISOMAP can successfully capture the characteristics and evolution of flow fields. In addition, 3D supplementary information can assist ICA in improving the problem of identifying unstable flow field states.

CFD Prediction of Ship Propeller-induced Underwater Radiated Noise

As the negative impact of man-made underwater radiated noise (URN) on marine life is recognized by the scientific community, guidelines and initiatives are proposed by port authorities and international organizations for monitoring and reducing URN from ship operations. In medium to large-size ships, propellers are often the dominating URN source as compared to on board machinery and flow turbulence. Within this paper, URN from cavitating flow around a ship propeller in behind-hull condition is predicted by a high-fidelity CFD solver, coupled with an acoustic analogy method. The noise sources are hydrodynamic flow fields around propeller and rudder models, simulated by CFD with a multi-phase flow model. Far-field acoustic noise is calculated by the permeable Ffowcs-Williams Hawkings (FWH) acoustic analogy method. Unsteady cavitation patterns and URN levels from the numerical predictions are validated against experimental results from a cavitation tunnel test. The propeller blade shape is optimized for minimum cavity volume by parametric design variations and performance evaluations based on steady CFD simulations with a simplified hull wake model. The URN predictions from unsteady CFD-FWH simulations demonstrate that the cavity volume reduction for the optimized blade design can lead to considerable decrease of low-frequency tonal noise.

Comparative study on the wake dynamics of pump-jet and ducted propeller based on dynamic mode decomposition

A comparative study on the wake dynamics of a pump-jet propulsor (PJP) and a ducted propeller (DP) is conducted to investigate the effects of the pre-swirl stator and corresponding stator–rotor interaction on the wake evolution and destabilization mechanisms of a PJP system. The flow field analysis, vortex structure identification, and dynamic mode decomposition (DMD) analysis are carried out based on the numerical results obtained from delayed detached eddy simulations. The numerical hydrodynamic loading and flow field of the PJP are compared with experimental results, and they are in good agreement. Compared with the DP, the stator trailing vortices of the PJP interact with the rotor trailing vortices as well as the hub vortex, accelerating their diffusion and viscous dissipation. The pre-swirl stator triggers the generation of secondary vortices and moderates the spiral behavior of tip leakage vortices, which dominates the wake instability of PJP. The DMD analysis revealed that the wake field evolution is primarily characterized by the different mode structures at blade passing frequency and its multiples, especially in the PJP due to its strong stator–rotor interaction. The modal energy decays faster in the PJP wake field owing to its more turbulent and earlier instability. The hub vortex plays an important role in the wake dynamics of the DP.

Flow Field Pattern and Hydrodynamic Characteristics of a Grid Device Made with Various Grid Bar Spacings at Different Inclination Angles

The grid is a crucial component in constructing grid-type bycatch reduction devices. The grid’s structural characteristics and orientation significantly impact the hydrodynamic characteristics and efficacy of the separation device. Therefore, it is essential to thoroughly understand the grid device’s hydrodynamic characteristics and flow field to optimize its structure. Thus, this study used CFD numerical simulation and flume tank experiments to investigate the effects of inclination grid angles and grid bar spacing on hydrodynamic forces and flow fields around a circular grid. The results indicated that the hydrodynamic forces acting on the circular grid increased with higher flow velocity and inclination grid angle, decreasing with smaller grid bar spacing. Flow velocity acceleration zones were observed at the upper and lower ends of the grid and between the grid bars. Additionally, upwelling and vortices were present at the back of the grid. It was found that an increase in the inclination grid angle accelerated the vortex and wake effects.

Shear stress-induced delamination method for the mass production of Ti3C2Tx MXene nanosheets

MXene nanosheets are considered advantageous for functional materials, but current delamination methods to prepare MXene nanosheets have many limitations including high cost, small production scale, low efficiency, and deteriorated structure integrity of obtained nanosheets. Here, we propose a simple, efficient, and scalable shear stress-induced delamination (SSID) strategy to boost the production of single-/few-layered Ti3C2Tx MXene nanosheets. Molecular dynamics simulation indicates that the pan mill-type grinding discs create a strong hydrodynamic flow field, which exerts gigantic shear stress to substantially delaminate the multilayered MXene stacks into homogeneously dispersed MXene nanosheets. Furthermore, shear stress generated from vigorous water flow has limited fragmentation effect, ensuring large lateral size and good structure integrity to the obtained MXene nanosheets as evidenced by the morphological and structural characterizations. Compared to conventional delamination methods, this novel SSID strategy exhibits great advantages in terms of efficiency, scalability and the properties of resultant MXene nanosheets, which opens up great opportunity for the scalable production and commercialization of high-performance MXene-based materials.

Study on the effect of inflow direction on the hydrodynamic characteristics of underwater manipulators

When operating in water, an underwater manipulator is subjected to different directions of flows. Thus, its hydrodynamic performance and flow-field evolution vary. Herein, the hydrodynamic coefficients and flow field characteristics of an underwater manipulator with different incoming flow directions in a subcritical range are studied. The effects of the different inflow directions on the hydrodynamic coefficients and the flow field evolution of the sectional shape, spacing, and arrangement order are numerically investigated. Results indicate that the hydrodynamic coefficients of the upstream circular section decrease at γ = 60° as the angle of the inflow direction increases. The root-mean-square value of the lift coefficient of the downstream elliptical section increases by a factor 1.5 with an increase in the inflow angle. When the elliptical section is upstream, the working attitude of the underwater manipulator is relatively stable. The research conclusions of this work will provide help for the precise positioning and fishing of underwater manipulators in the ocean.

Hydrodynamic interactions in ion transport-Theory and simulation.

We present a hydrodynamic theory describing pair diffusion in systems with periodic boundary conditions, thereby generalizing earlier work on self-diffusion [B. Dünweg and K. Kremer, J. Chem. Phys. 99, 6983-6997 (1993) and I.-C. Yeh and G. Hummer, J. Phys. Chem. B 108, 15873-15879 (2004)]. Its predictions are compared with Molecular Dynamics simulations for a liquid carbonate electrolyte and two ionic liquids, for which we characterize the correlated motion between distinct ions. Overall, we observe good agreement between theory and simulation data, highlighting that hydrodynamic interactions universally dictate ion correlations. However, when summing over all ion pairs in the system to obtain the cross-contributions to the total cationic or anionic conductivity, the hydrodynamic interactions between ions with like and unlike charges largely cancel. Consequently, significant conductivity contributions only arise from deviations from a hydrodynamic flow field of an ideal fluid, which is from the local electrolyte structure as well as the relaxation processes in the subdiffusive regime. In the case of ionic liquids, the momentum-conservation constraint additionally is vital, which we study by employing different ionic masses in the simulations. Our formalism will likely also be helpful to estimate finite-size effects of the conductivity or of Maxwell-Stefan diffusivities in simulations.

Design, Hydrodynamic Analysis, and Testing of a Bio-inspired Movable Bow Mechanism for the Hybrid-driven Underwater Glider

Hybrid-driven Underwater Glider (HUG) is a new type of underwater vehicle which integrates the functions of an Autonomous Underwater Glider (AUG) and an Autonomous Unmanned Vehicle (AUV). Although HUG has the characteristics of long endurance distance, its maneuverability still has room to be improved. This work introduces a new movement form of the neck of the underwater creature into HUG and proposes a parallel mechanism to adjust the attitude angle and displacement of the HUG’s bow, which can improve the steering maneuverability. Firstly, the influence of bow movement and rotation on the hydrodynamic force and flow field of the whole machine is analyzed by using the Computational Fluid Dynamics (CFD) method. The degree of freedom, attitude control range and movement amount of the Movable Bow Mechanism (MBM) are obtained, and then the design of MBM is completed based on these constraints. Secondly, the kinematic and dynamic models of MBM are established based on the closed vector method and the Lagrange equation, respectively, which are fully verified by comparing the results of simulation in Matlab and Adams software, then a Radial Basis Function (RBF) neural network adaptive sliding mode controller is designed to improve the dynamic response effect of the output parameters of MBM. Finally, a prototype of MBM is manufactured and assembled. The kinematic, dynamics model and controller are verified by experiments, which provides a basis for applying MBM in HUGs.

파수-주파수 변환을 이용한 선체 벽면 압력으로부터 선박 추진기 압축성 압력 추출

Underwater radiated noise (URN) is a compressible pressure component that propagates in the far-field from sources. Therefore, compressible pressure separation is required as a first step in developing a far-field URN monitoring technique based on hull surface pressure. In this study, a numerical method of decomposing surface total pressure induced by an underwater propeller into compressible and incompressible pressure was proposed using wavenumber-frequency transformation (WFT). First, a compressible multiphase Delayed Detached Eddy Simulation (DDES) of the hydrodynamic flow field was computed. Acoustic pressure caused by cavitation from the underwater propeller was predicted using the Ffowcs Williams–Hawkings (FW–H) integral equation and compared with the measured data to validate the numerical simulation. Finally, the compressible pressure was extracted from the total surface pressure on the stern part by using the WFT decomposition technique. In the high-frequency range, the compressible pressure appeared to be relatively high compared to the incompressible pressure in this study.