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The strong increase in anthropogenic underwater noise has caused a growing intention to design quieter ships given that ship propellers are one of the dominating noise sources along the worldwide shipping routes. This creates an imminent demand for deeper knowledge on the noise generation mechanisms of propeller cavitation. A cavitating, oscillating two-dimensional NACA0015 hydrofoil is analyzed with hydrophone and high-speed video recording as a simplified and manipulatable representative of a propeller blade in a ship's wake field for the identification of major influencing parameters on the radiated noise. A pneumatic drive allows the application of asymmetrical temporal courses of the angle of attack, a novel amendment to the widely reported sinusoidal setups. Three different courses are tested with various cavitation numbers. The combination of a moderate angle increase and a rapid decrease is found to generate significantly higher pressure peaks compared to symmetrical angular courses. Considering that the rapid change of the angle of attack caused by the inhomogeneous wake field behind the hull is the core of the cavitation occurrence, the understanding of its influence may contribute to the design of quieter ships in the future while still allowing for the necessary high propeller efficiency. [ABSTRACT FROM AUTHOR]

In the present two-dimensional numerical study, we investigate the roles of geometrical parameters of a hydrofoil (shape/curvature of the leading and trailing edges and thickness) and kinematic parameters (phase difference between heave and pitch (ϕ)) on the propulsive performance of different-shaped hydrofoils oscillating at maximum angles of attack up to α max = 30 ∘ . The study was carried out at a fixed non-dimensional maximum heave to chord ratio h ∘ / C = 0.75 , Strouhal number S t = 0.25 , and Reynolds number R e = 5000 . Our findings reveal that hydrofoil performance and stability improve with leading and trailing edge curvatures but decline as thickness increases. By analyzing the near-wake structure, we establish that even minimal flow separation increases power consumption while moderate flow separation enhances thrust. Over the range of different-shaped hydrofoils at different α max and ϕ , maximum propulsion efficiency occurs for those parameters for which there is a small degree of flow separation but with no roll-up of a separating vortex. In comparison, maximum thrust generation occurs when there is a moderately strong flow separation but without induction of a significant amount of fluid around the trailing edge. These insights offer valuable knowledge for understanding fish propulsion efficiency and have applications in designing autonomous underwater vehicles (AUVs) and micro-air vehicles (MAVs). [ABSTRACT FROM AUTHOR]

Parametric models have been widely used in pertinent literature for reconstructing, modifying and representing a wide range of airfoil and/or hydrofoil profile geometries. Design spaces corresponding to these models can be exploited for modeling and profile-shape optimization under various performance criteria. Accuracy requirements, along with the need for modeling local features, often lead to high-dimensional design spaces that hinder the process of shape optimization and design through analysis. In this work, we propose a shape-informed dimensional reduction approach that attempts to tackle this deficiency by producing low-dimensional latent design spaces that can be efficiently used in shape representation and optimization. Furthermore, geometric moments are introduced in an attempt to cost-effectively capture analysis-relevant information that is generally expensive to produce. Specifically, geometric integral properties, although intrinsic features of the shape, are quite commonly related to performance indicators employed in performance optimization and therefore provide a cost-effective physics-informed component in the description of the design in the latent space. To this end, we employ the generalized Karhunen-Loève expansion to produce a shape- and physics-informed subspace retaining the highest possible geometric variance and robustness, that is, a lack of invalid designs. At the same time, a series of shape discretizations, encoding the foil's shape profile, are examined with regard to their effect on the resulting latent space's quality and efficiency. Our results demonstrate a significant reduction in the dimensionality of the original design space while maintaining a high representational capacity and a large percentage of valid geometries that facilitate fast convergence to optimal solutions in performance-based optimization. [ABSTRACT FROM AUTHOR]

The Partially Averaged Navier–Stokes (PANS) method is assessed with various values of the control parameters ( f k = 0. 3 –1.0, f = 1. 0) by performing unsteady cavitating flows around a NACA0015 hydrofoil in a surrogate fluid of fluoroketone. Available experimental data of the cavity evolution and pressure are utilized to validate and evaluate the computational method. The results show that decreasing the control parameter f k can help to avoid the overestimations of the turbulence viscosity near the rear region of the cavity and can resolve more scales turbulence structure. The control parameter f k = 0. 4 yields good predictions on cavitation shedding dynamics behavior and pressure distribution. Furthermore, the temperature around the hydrofoil undergoes a strong evolution that is contributed by the local evaporation and condensation processes. Interestingly, there are significant unsteady characteristics along the chordwise and spanwise directions of the hydrofoil. Finally, the thermal effect on cavitating flows is associated with the physical properties of fluid media. Evaporative cooling effects are more pronounced at high temperature and subsequently suppress the intensity of cavitation. [ABSTRACT FROM AUTHOR]

In the current paper, a particular leading-edge form called Step-Shaped Leading-Edge (SSLE) is studied in continuous and non-continuous forms along the span of an infinite wing based on N A C A 63 4 − 021 , via the RANSE scheme. The computed lift and drag coefficients are in good agreement with experimental data up to an angle of attack of 21 ∘ , i.e. the stall angle of the baseline model. This study shows that although the continuous SSLE lowers the stall angle at 12 ∘ , the non-continuous SSLE increases it up to 25 ∘ . The flow visualisation over the studied models has clearly illustrated how the SSLE alter the flow pattern over a hydrofoil. The model with non-continuous SSLE has lift and drag coefficient diagrams similar and even better in some angles of attack than hydrofoils with sinusoidal leading edge. Furthermore, to bring order into the chaos and score different leading-edge forms, a quantitative and visualising rating technique is proposed. [ABSTRACT FROM AUTHOR]

In this paper, the physics of external flow past a superhydrophobic SD7003 hydrofoil is studied at Reynolds number of 1 0 5 . We used Navier's slip length to model the superhydrophobic surfaces, and investigated the effect of different slip lengths on hydrofoil performance by means of RANS (Reynolds-averaged Navier–Stokes) simulations. The slip length boundary condition is imposed on upper, or/and lower side(s), to study the effect of each superhydrophobic region on lift coefficient. By increasing the slip length (l s), it was expected to see lift enhancement and drag reduction. However, the lift enhancement trend had some exceptions. The unexpected trends were explained by studying the location of laminar separation and reattachment for different angles of attack (AOA) and slip lengths. For example, the lift coefficient of a full superhydrophobic hydrofoil at AOA = 2° does not change monotonically against slip length. By increasing the slip length, separation and reattachment points are shifted towards the trailing edge and at slip length 200 μ m, there is separation with no reattachment which results in lift reduction. By further increasing the slip length, the separation is shifted further down and eventually there would be no separation n\and flow becomes fully attached. • Hydrofoils are main components of propulsion system for different under water vehicles. • Flow past SD7003 hydrofoil was investigated for three cases, i.e. superhydrophobicity on suction side, pressure side and both sides. • Increasing the slip length, results in drag reduction. • Increasing the slip length, results in lift enhancement with some exceptions. [ABSTRACT FROM AUTHOR]

Tidal energy is an important renewable energy that uses the potential energy created by the rising and falling of ocean tides to generate electricity. The tip leakage vortex (TLV) and cavitation caused by the tip leakage flow have adverse effects on the energy conversion efficiency and stable operation of the tubular turbine. The selection of the tip clearance shape is critical. Therefore, the effects of four different combinations of asymmetrical tip clearance shapes on the energy and cavitation characteristics of hydrofoils are studied in this paper. The results show that the TLV remains unchanged when tipwall and endwall are sinusoidal and cosine curve shaped, respectively. When tip clearance is the combination of a flat tipwall and a sinusoidal endwall, the lift-drag ratio decreases by 10.72 %. The high shear stress region of tipwall near leading edge decreases. The flow resistance of the leakage flow decreases, and the leakage flow becomes more serious. The cavitation volume is 2.5 times that under the original flat tip clearance. When tip clearance is the combination of a flat tipwall and a cosine endwall, and the lift-drag ratio increases by 9.67 %. The shear stress increases, the leakage flow decreases. The swirling strength around the center of the vortex core is weakened. The cavitation volume is 3.81, with a decrease of nearly 30 %. A proper asymmetrical tip clearance can effectively improve the energy performance and cavitation characteristics of hydrofoils. This study provides theoretical support for the design and optimization of hydraulic machine with tip clearance. [ABSTRACT FROM AUTHOR]

The generation of hydro-mechanical resonance is related to the transition of the boundary layer and the development of vortex shedding. The application effect of suction control in hydrodynamics is equally deserving of consideration as an active control technique in aerodynamics. This study examines how suction control affects the flow field of the NACA0009 blunt trailing edge hydrofoil using the γ transition model. Firstly, the accuracy of the numerical method is checked by performing a three-dimensional hydrofoil numerical simulation. Based on this, three-dimensional hydrofoil suction control research is conducted. According to the results, the suction control increases the velocity gradient in the boundary layer and delays the position of transition. The frequency of vortex shedding in the wake region lowers, and the peak value of velocity fluctuation declines. The hydrofoil hydrodynamic performance may be successfully improved with a proper selection of the suction coefficient via research of the suction coefficient and suction position on the flow field around the hydrofoil. The lift/drag ratio goes up as the suction coefficient goes up. The boundary layer displacement thickness and momentum thickness are at their lowest points, and the velocity fluctuation amplitude in the wake region is at its lowest point as the suction coefficient Cμ = 0.003. When the suction slots are at the leading edge, the momentum loss in the boundary layer is minimal and the velocity fluctuation in the wake zone is negligible. [ABSTRACT FROM AUTHOR]

The finite-time Lyapunov exponent (FTLE) method is a mature and practical method for analyzing the characteristics of Lagrangian coherent structures. It can be used for studying the severe impacts of cavitation on the coherent structure of flow. The reduced-order modeling (ROM) method has also significant advantages in extract key features of flow structure. This study analyzed the cloud cavitation flow structure of National Advisory Committee for Aeronautics (NACA)0015 hydrofoil. The backward FTLE and ROM were combined, and a comparison was made between the low-order modes of FTLE structure and the FTLE obtained from the low-order modes. The results indicate that the two methods have effectively captured the main coherent structural features of cloud cavitation flow fields. The main characteristic structures captured by the FTLE obtained from the low-order modes of the flow field are much clearer. The first two coherent structures of the FTLE obtained from the low-order modes of the flow field decompose the FTLE of the velocity field into three distinct parts: the leading-edge structure of the hydrofoil, the reflux structure in the middle of the hydrofoil, and the wake region of the hydrofoil. It is proved that the combination of FTLE and ROM can provide a new perspective and means for the analysis of turbulent structures. [ABSTRACT FROM AUTHOR]

The evolution of transient flow structures and mass transport in cavitating flow around a pitching hydrofoil is investigated qualitatively and quantitatively, and the interaction between cavitation patterns and vortices is elucidated from Lagrangian viewpoint. First, turbulence effects are estimated by the density-corrected k–ω model to account for the local compressibility of the multiphase flow at Reynolds number Re = 6.4 × 10 5 . Then, the formation and evolution of vorticity structures during the whole pitching cycle are analyzed using Lagrangian averaged vorticity deviation method. By comparing the flow structures and hydrodynamic properties at varying angles of attack, the cavitating flow is divided into two distinct stages, namely multi-scale cloud cavitation phase from α + = 10 ° to α − = 8 ° , and traveling sheet cavitation phase from α − = 8 ° to α + = 10 °. Specifically in cloud cavitation, the formation of the cavitation pattern is closely related to the development of the main vortex. Furthermore, the quantitative analysis method based on Lagrangian flow network is developed to deeply analyze the transport and mixing processes. Importantly, the coherence ratio and the mixing parameter are proposed as transport indicators to precisely quantify the spatial connectivity behavior. Finally, the correlations between vapor fraction, codelength, global coherence ratio and global mixing parameter are evaluated. As the conclusion, it is shown that Lagrangian methods are powerful tool for both qualitative and quantitative analysis, and the results obtained could provide a key and important understanding of the flow structure and changing mechanism between cavitation and vortices in marine hydro and propulsion systems. [ABSTRACT FROM AUTHOR]

Numerical investigation of the cavity dynamics around a composite hydrofoil with a blunt trailing edge in the cloud cavitating flow is carried out using a tightly coupled fluid–structure interaction method. The hydrofoil is made of a carbon-fiber-reinforced polymers with a ply angle of − 4 5 ∘ (CFRP −45). The results of a stainless-steel hydrofoil with the same geometry and conditions are used as a reference. Simulation results have been validated carefully against experimental data. Several fundamental mechanisms are dictated through simulation results and mode decomposition, including the multistage shedding process, the influence of the bend–twist coupling effect on cavity behaviors, cavitation–vortex interaction, and kinematics of coherent structures. The main reason for the generation of a secondary re-entrant jet is that the primary cloud cavity collapse leads to high pressure, which spreads to the residual sheet cavity closure and then induces a high-pressure gradient. The negative bend–twist coupling effect causes the CFRP −45 hydrofoil to exhibit a smaller cloud cavity scale and non-uniform re-entrant jet strength in the spanwise direction compared to the stainless-steel hydrofoil. Modal decomposition via proper orthogonal decomposition and dynamic mode decomposition indicates that the dominant coherent structures in the cloud cavitating flow include the large-scale cloud cavity, rotating structures due to the re-entrant jet, attached cavity, and small-scale vortex in the wake. The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to complex cloud cavitating flow around a composite hydrofoil. [ABSTRACT FROM AUTHOR]

Fish schools are capable of simultaneous linear acceleration. To reveal the underlying hydrodynamic mechanism, we numerically investigate how Reynolds number R e = 1000 – 2000 , Strouhal number S t = 0.2 – 0.7 , and wavelength λ = 0.5 – 2 affect the mean net thrust and net propulsive efficiency of two side-by-side hydrofoils undulating in anti-phase. In total, 550 cases are simulated using immersed boundary method. The thrust increases significantly with the wavelength and the Strouhal number, yet only slightly with the Reynolds number. We apply a symbolic regression algorithm to formulate this relationship. Furthermore, we find that mirror-symmetric schooling can achieve a net thrust more than ten times that of a single swimmer, especially at low Reynolds numbers. The highest efficiency is obtained at S t = 0.5 and λ = 1.2 , where St is consistent with that observed in the linear-accelerating natural swimmers, e.g., Crevalle jack. Six distinct flow structures are identified. The highest thrust corresponds to an asymmetric flow pattern, whereas the highest efficiency occurs when the flow is symmetric with converging vortex streets. [ABSTRACT FROM AUTHOR]

In this study, a parametric analysis was conducted for the design of a suggested dual flapping hydrofoil turbine. A Navier–Stokes‐based computational fluid dynamics code was utilized for the analysis while varying the pitch angle, reduced frequency, separation distance, and phase difference. The smallest pitch angle of 60° and the reduced frequency of 0.1 among the given ranges of the parameters were fixed for the first parametric analysis. After an assessment, the 90° front‐lead with the longest distance of 6c and the 90° rear‐lead with the shortest distance of 2c were chosen for further parametric analysis based on a suggested performance indicator and a margin for improvement. It was derived from a subsequent parametric analysis that a 70° pitch angle, 0.12 reduced frequency, and a 4c distance for the 90° front‐lead was the final optimum with 59.48% efficiency and 55.44% fluctuation according to the performance indicator as well as the system length and power balance of dual hydrofoils. [ABSTRACT FROM AUTHOR]

This is a new installment in the series of publications that describe the mathematical modeling of the Floating Hybrid Generator Systems Simulator (FHYGSYS) tool. This work presents an improved mathematical model of the turbines of the floating hybrid system—consisting of an "OC3-Hywind" wind turbine and two marine current turbines—presented by the authors in previous publications. In this third installment, the modeling of the three turbines of the floating hybrid system is described using the Blade Element Momentum (BEM) theory. This modeling allows one to replace the one based on the One-Dimensional theory used in previous installments. For the operation of modeling with BEM, it has been considered necessary to implement a continuous feedback control system. In this case, two PID (proportional–integral–derivative) controllers have been implemented in each of the turbines. The first controls the torque on the turbine generator and the second controls the collective pitch angle of the blades. The results obtained are presented and validated through a code-to-code comparison with simulations carried out with FASTv8 under the same conditions and with the operating results of marine current turbines that exist in the literature. This improvement in the mathematical model offers the possibility of implementing other types of controllers that allow for the testing of different strategies of the floating hybrid control system, with the aim of maximizing energy production while ensuring the structural stability of the floating hybrid system. [ABSTRACT FROM AUTHOR]

This paper presents an examination of the primary applications of solar energy as the main power source in the maritime sector, focusing on recent developments. A comprehensive review of the existing literature, including journal articles, proceedings, and patents, is conducted to identify three prominent areas for advancing solar energy-powered boats: maritime drones, sporting boats, and short-range touristic vessels. Maritime drones primarily serve as small autonomous boats for research, conservation, or military operations. On the other hand, sporting boats include nautical and energy design competitions involving students and enthusiasts. In terms of commercial interest, there is a growing demand for environmentally friendly and low-noise boats suitable for tourist activities, particularly in protected areas. Furthermore, specific and illustrative cases are explored in a dedicated section. Lastly, potential future perspectives are discussed and elucidated. [ABSTRACT FROM AUTHOR]

In this study, the modified Sauer cavitation model and Kirchhoff-Ffowcs Williams and Hawkings (K-FWH) acoustic model were adopted to numerically simulate the unsteady cavitation flow field and the noise of a threedimensional NACA66 hydrofoil at a constant cavitation number. The aim of the study is to conduct and analyze the noise performance of a hydrofoil and also determine the characteristics of the sound pressure spectrum, sound power spectrum, and noise changes at different monitoring points. The noise change, sound pressure spectrum, and power spectrum characteristics were estimated at different monitoring points, such as the suction side, pressure side, and tail of the hydrofoil. The noise characteristics and change law of the NACA66 hydrofoil under a constant cavitation number are presented. The results show that hydrofoil cavitation takes on a certain degree of pulsation and periodicity. Under the condition of a constant cavitation number, as the attack angle increases, the cavitation area of the hydrofoil becomes longer and thicker, and the initial position of cavitation moves forward. When the inflow velocity increases, the cavitation noise and the cavitation area change more drastically and have a superposition tendency toward the downstream. The novelty is that the study presents important calculations and analyses regarding the noise performance of a hydrofoil, characteristics of the sound pressure spectrum, and sound power spectrum and noise changes at different monitoring points. The article may be useful for specialists in the field of engineering and physics. [ABSTRACT FROM AUTHOR]

The hydrofoil harnesses wave energy and converts it into thrust. In this paper, we present the results of the first experimental study investigating the dynamic behavior of a fully passive foil with spring-loaded pitch and heave in regular waves. Our study shows that the real-time load signal is multi-harmonic with strong superposition, directly proving the robust energy harvesting performance due to the restoring springs. By interpreting the hydrofoil's pose and path from an image sequence captured underwater, we conclude the dynamic evolution of the fully passive hydrofoil interacting with regular waves. The hydrofoil's dynamics exhibit asymmetric surge, pitch, and heave in a motion cycle. Furthermore, we employ a pixel capturing algorithm with self-correction utility to quantify the hydrofoil's forward displacement from the image sequence of the moving carriage. These findings provide valuable insight into the performance and potential of hydrofoils for marine propulsion. [ABSTRACT FROM AUTHOR]

The influence of sweep angles on cavitation characteristics and mechanisms of a wedge-section hydrofoil is investigated experimentally and numerically. Four hydrofoils with sweep angles of 0° (straight), 30°, 45°, and 60° are considered across a range of cavitation numbers from 2.00 to 0.48 and angle of attack (AOA) of 0°, 5°, 10°, and 15°. Two high-speed cameras are used to visualize the cavitation flow in a high-speed cavitation tunnel. The numerical simulation is conducted using unsteady Reynolds-averaged Navier–Stokes equations through OpenFOAM. At an AOA of 0° and 5°, vortex cavitation first appears in the wake region of all models at a cavitation number of 0.98. However, at higher AOA values of 10° and 15°, tip-vortex cavitation (TVC) begins first for the straight foil, and this is followed by sheet and wake cavitation. In contrast, the swept foil does not succumb to TVC. Instead, as the sweep angle increases, sheet cavitation develops into root leading-edge vortex cavitation (LEVC). The inclination angle of the LEVC from the leading edge is observed to be between 6° and 15°, depending on the sweep angle, and it is independent of the cavitation number and AOA. The wake vortex changes from eddy vortex shedding at the wake region of the straight hydrofoil into two root trailing-edge vortices as the sweep angle increases. The swept hydrofoil reduces the average cavity volume by more than 45% compared with the straight foil. The lift coefficient of the straight hydrofoil increases as sheet cavitation is generated and reaches a maximum value of 0.6 when cavitation covers the suction side of the hydrofoil before dropping sharply when it extends to the wake region. However, there is only minor deterioration in the lift coefficient of the 60° swept-angle foil when the cavitation occurs. The drag coefficient reduces when cavitation forms for both foils. However, the drag coefficient of the swept hydrofoil is lower than that of the straight foil. These findings offer valuable insight into the design and optimization of foils for various applications where cavitation affects their performance and stability. [ABSTRACT FROM AUTHOR]

Data from Large-Eddy Simulations on a grid consisting of 3.8 billion points are utilized to reconstruct the acoustic far field of a propeller–hydrofoil system, using the Ffowcs-Williams and Hawkings acoustic analogy. Results demonstrate that the lowest frequencies, around the shaft frequency, are dominated by the sound coming from the surface of the downstream hydrofoil, especially in its normal direction. In contrast, at the blade frequency and higher frequencies, the acoustic signature of the upstream propeller gains the lead, especially in the upstream and downstream directions. Minima of the sound pressure levels correlate in all cases with the spanwise direction of the hydrofoil. Despite the dramatic change of the flow conditions for growing incidence angles of the hydrofoil, characterized by a substantial rise of the turbulent fluctuations on its suction side, no similar increase in the acoustic pressure in the far field occurs at large incidence angles, even at the onset of separation phenomena, which are able to affect the highest frequencies only. [ABSTRACT FROM AUTHOR]

In the present paper, the restricted cavitation bubble dynamics near a symmetric Joukowsky hydrofoil are researched theoretically and experimentally. Using Kelvin impulse theory, the Joukowsky transformation, and the circle theorem, a theoretical model for restricted bubble dynamics is established to analyze the collapse jet characteristics. The validity of this model is then verified using high-speed photographic experiments. The velocity and direction of the collapse jet at specific position angles are quantitatively analyzed. Furthermore, the spatial characteristics of the Kelvin impulse direction near the symmetric Joukowsky hydrofoil are revealed by theoretical results. The main conclusions include the following: (1) the new theoretical model is proven to be effective in predicting the direction of the collapse jet for a restricted bubble near a symmetric Joukowsky hydrofoil. (2) As the distance between the bubble and hydrofoil increases, the collapse jet direction changes from pointing toward the nearest wall to pointing toward the center of the hydrofoil. (3) The variation rate of the Kelvin impulse direction for the restricted bubble is very sensitive to the bubble position near the two ends of the symmetric Joukowsky hydrofoil. [ABSTRACT FROM AUTHOR]

In this work, supervised Machine Learning (ML) techniques were employed to solve the forward and inverse problems of airfoil and hydrofoil design. The forward problem pertains to the prediction of a foil's aerodynamic or hydrodynamic performance given its geometric description, whereas the inverse problem calls for the identification of the geometric profile exhibiting a given set of performance indices. This study begins with the consideration of multivariate linear regression as the base approach in addressing the requirements of the two problems, and it then proceeds with the training of a series of Artificial Neural Networks (ANNs) in predicting performance (lift and drag coefficients over a range of angles of attack) and geometric design (foil profiles), which were subsequently compared to the base approach. Two novel components were employed in this study: a high-level parametric model for foil design and geometric moments, which, as we will demonstrate in this work, had a significant beneficial impact on the training and effectiveness of the resulting ANNs. Foil parametric models have been widely used in the pertinent literature for reconstructing, modifying, and representing a wide range of airfoil and hydrofoil profile geometries. The parametric model employed in this work uses a relatively small number of parameters, 17, to describe uniquely and accurately a large dataset of profile shapes. The corresponding design vectors, coupled with the foils' geometric moments, constitute the training input from the forward ML models. Similarly, performance curves (lift and drag over a range of angles of attack) and their corresponding moments make up the input for the models used in the inverse problem. The effect of various training datasets and training methods in the predictive power of the resulting ANNs was examined in detail. The use of the best-performing ML models is then demonstrated in two relevant design scenarios. The first scenario involved a software application, the Design Foil Assistant, which allows real-time evaluation of foil designs and the identification of designs exhibiting a set of given aerodynamic or hydrodynamic parameters. The second case benchmarked the use of ML-enabled, performance-based design optimization against traditional foil design optimization carried out with classical computational analysis tools. It is demonstrated that a user-friendly real-time design assistant can be easily implemented and deployed with the identified models, whereas significant time savings with adequate accuracy can be achieved when ML tools are employed in design optimization. [ABSTRACT FROM AUTHOR]

Cavitation and cavitation erosion often occur and seriously threaten the safe and stable operation of hydraulic machinery. However, during the operation of hydraulic machinery, the cavitation flow field is often difficult to contact andmeasure, and the shedding and development characteristics of cavitation floware unknown. This paper uses theDetached Eddy Simulation (DES) turbulence model and Zwart-Gerber-Belamri (ZGB) cavitationmodel to conduct numerical research on the cavitation flow of a twisted hydrofoil and verifies the effectiveness of numerical simulation by comparing it with experimental results. Then, based on the fractal dimension method, the number and fractal dimension of themain cavitation clusters in two different views are analyzed. The research results show that with the continuous flow of fluid, the cavitation at the tail of the hydrofoil periodically falls off, and the number of cavitation bubbles shows a strong randomness between 1 and 2. The fractal dimensionmethod calculates that the frequency of cavitation bubbles falling off is 30 Hz. The fractal dimension of cavitation also fluctuates periodically. The fractal dimension of cavitation in the side view fluctuates between 1.30-1.38, and the fractal dimension of cavitation in the top view fluctuates between 1.31-1.42. The fractal dimension of the cavitation is closely related to the size and shape of the cavitation. By analyzing the relationship between the cavitation development process and the fractal dimension in the side view, we found the relationship between the development of cavitation in a cycle and the change of fractal dimension, which is of great significance to understanding the characteristics of cavitation flow through the image of the cavitation flow field. [ABSTRACT FROM AUTHOR]

Recently, lots of oscillating targets inspired from motions of some insects and birds have been applied extensively to many engineering applications. The aim of this work is to reveal the performance and detailed flow structures over the pitching corrugated hydrofoils under various working conditions, using the SST γ − R e ∼ θ transition model. First of all, the lift coefficients of a smooth oscillating airfoil at different reduced frequency and pitching angles show a good agreement with the experiments, characterized by the accurate prediction of the light and deep stall. For the pitching corrugated hydrofoils, it shows that the mean lift coefficient increases with the pitching magnitude, but it has an obvious drop at high reduced frequency for the case with large pitching amplitude, which is mainly induced by the pressure modification on the surface with smooth curvature, depending on the oscillation significantly. In addition, the mean drag coefficient also indicates that the drag turns into the thrust at high reduced frequency when the pitching amplitude exceeds to the value of 10°. Increasing the reduced frequency delays the flow structure and leads to the deflection of the wake vortical flow. The Reynolds number also has an impact on the hydrofoil performance and wake morphology. Furthermore, regarding the shape effect, it seems that hydrofoil A (consisting of two protrusions and hollows and the aft part with smooth curvature) achieves the higher lift than hydrofoil B (comprising several protrusions and hollows along the surface), specially at high reduced frequency. Although the frequency collected from two hydrofoils remains nearly the same near the leading edge and in the wake region, the high sub-frequency is evidently reduced for hydrofoil B in second and third hollows, due to the relatively stable trapped vortices. Then, the wake transition from the thrust-indicative to drag-indicative profile for hydrofoil B is also slower compared with hydrofoil A. Finally, it is observed that with the increase of the thickness, the lift/drag ratio decreases and the slow wake transition is detected for the thin hydrofoil, which is associated with the relatively low drag coefficient. [ABSTRACT FROM AUTHOR]

The objective of this paper is to understand the effects of waves and vaporous cavitation upon the hydrodynamic and hydroelastic responses of a flexible surface-piercing hydrofoil, adding to the subcavitating results presented in Part 1. In general, the presence of a sufficiently large vaporous cavitation bubble facilitated the formation of a ventilated cavity, substantially reducing the angle of attack or speed required to induce fully ventilated flow, relative to subcavitating conditions. A new co-analysis procedure was used to isolate synchronous hydrodynamic modes and structural operating deflection shapes. Significant dynamic load amplification occurred when the resonant frequencies of the first twisting and second bending modes coalesced in both fully wetted and partially cavitating flows. The presence of waves did not diminish the effect of frequency coalescence, but did encourage intermittent lock-in with both leading edge cavity shedding and trailing edge vortex shedding at certain speeds. Partial cavity shedding typically had a negligible impact on the power spectral densities of structural motions because of incoherent cavity shedding. However, lock-in between the cavity shedding frequency and modal coalescence frequency led to shifting of the primary frequency peak, as well as amplified harmonics and interactions between the cavity shedding frequency and vortex shedding frequency. The transition from partially cavitating to fully ventilated flow caused sudden and large drops in the mean hydrodynamic loads and deformations, as well as substantial reductions in the intensity of the fluctuations. [ABSTRACT FROM AUTHOR]

Hydrofoils made of metal alloys were broadly used on high-speed boats in the past. Nowadays, much lighter hydrofoils made of composite materials are finding increasingly more applications on sailing yachts and powerboats. However, these hydrofoils are usually rather flexible, and their design requires computationally demanding analysis, involving hydroelastic calculations. In this study, exploratory high-fidelity simulations have been carried out for surface-piercing hydrofoils in unsteady conditions with help of a computational fluid dynamics solver for fluid flow coupled with a finite element solver for the foil structure. To model unsteady foil deformations, the morphing mesh approach was utilized, and the volume-of-fluid method was applied for multiphase flow simulations. The computational setup, as well as verification and validation study, is described in this paper. Three hydrofoils of different stiffness, including a perfectly rigid foil, were simulated in both calm water conditions and regular head waves. Representative examples of foil deflections and wave patterns, as well as timedependent structural and hydrodynamic characteristics, are presented. [ABSTRACT FROM AUTHOR]

The paper features a computational fluid dynamics study of a flapping NACA0015 hydrofoil moving with a combination of sinusoidal heaving and pitching. Several kinematic configurations are explored, varying sequentially pitch and heave amplitude, Strouhal number and phase angle, in an attempt to determine the influence of each parameter on the propulsive performance. To optimize efficiency the angle of attack should assume the highest value that also avoids the arise of the leading edge vortex generated in the dynamic stall state. At low Strouhal number optimum is reached at high heave amplitudes, which correspond to the configurations minimizing the hysteresis in the (C y , C x) plane. The same outcome in terms of hysteresis minimization has been verified to occur when optimal phase shift was considered. Differently, when the Strouhal number and the angle of attack become higher, to exploit efficiently the lift increment owed to dynamic stall it emerged the necessity of adopting low heave amplitude to improve separation resistance, avoiding the occurrence of deep stall. [ABSTRACT FROM AUTHOR]

A Karman vortex is a phenomenon of fluid flow that can cause fluctuation and vibration. As a result, it leads to fatigue damage to structures and induces safety accidents. Therefore, the analysis of the shedding law and strength of the Karman vortex is significant. To further understand the laws of turbulent Karman vortex shedding and strength, this study conducts a numerical vorticity simulation of a Karman vortex at the trailing edge of a hydrofoil based on the two-dimensional simplified model of the NACA0009 hydrofoil under different Reynolds numbers. Combined with image segmentation technology, the fractal characteristics of a turbulent Karman vortex at the trailing edge of a hydrofoil are extracted, the number and total area of vortex cores are calculated, and the fractal dimension of the vortex is obtained. The results show that the fractal dimension can characterize the change in vortex shape and strength under different Reynolds numbers, and that the fractal analysis method is feasible and effective for the shedding analysis of a turbulent Karman vortex. [ABSTRACT FROM AUTHOR]

The impetus of this study is to provide an in-depth insight into the unsteady hydrodynamic characteristics of the cavitating flow, effects of the wavy leading edge (WLE) on the noise suppression mechanism due to a cavity cloud formation, which contains condensation, detachment, collapse, spanwise flow, streamwise velocity fluctuation, and shedding phenomenon. NACA 634-021 hydrofoil was considered with WLE having a wavelength of 25% and an amplitude of 5% of the mean chord length and was compared to a straight-leading-edge (SLE) hydrofoil at cavitation numbers of σ = 0. 8 and a chord-based Reynolds number of 7. 2 × 1 0 5 . Counter-rotation vortices were produced between the peaks of the WLE hydrofoil by destroying the horseshoe vortex and delaying the tail vortex, changing the frequency. Here, the hydrodynamic forces have also been discussed in addition to the noise. The results showed that the leading-edge vortex formation and flow separation dynamics fundamentally differed between the SLE and the WLE hydrofoil. The main difference between the WLE and SLE hydrofoil turbulent flow is the formation of counter-rotating streamwise vortices pairs. We solved the cavitating flow using the large eddy simulation (LES) approach, as well as the Kunz mass transfer model, which is performed under the framework of the OpenFOAM package. [ABSTRACT FROM AUTHOR]

This article numerically investigates the taper ratio influence on the performance of three-dimensional (3-D) cavitating hydrofoils moving steadily under a free water surface. The paper modifies and expands upon a previously developed iterative boundary element method (IBEM) to solve this problem. The fluid is assumed to be inviscid and incompressible and to have irrotational flow. All variables and equations have been made non-dimensional to achieve a consistent numerical scheme and very quick convergence. The IBEM iteratively solves the hydrofoil problem and free surface problem separately through the effects they have on each other via their potential values. Both the 3-D hydrofoil surface and the free surface have been modeled using a constant strength source and constant strength doublet panels. The method’s results were first validated against those regarding a tapered wing. Later, the model was applied to a tapered hydrofoil to investigate the effects of taper ratio on cavitating hydrofoil performance. The taper ratio has been found to cause a decrease in the drag coefficient on a cavitating hydrofoil, thereby causing an increase in the lift-drag ratio in an unbounded flow domain. The taper ratio also causes a slight improvement in the lift-drag ratio of the cavitating hydrofoil when moving under a free surface. [ABSTRACT FROM AUTHOR]

This work describes the effects of waves on the hydroelastic response of a hydrofoil in fully wetted and ventilated flows. In the absence of vaporous cavitation (described in Part 2 of this paper series), shallow long-period non-breaking waves delayed ventilation inception because velocity fluctuations prevent the formation of a stably separated region of flow at the foil's leading edge. Aerated von Kármán vortex shedding occurred from the blunt trailing edge, producing vortex-induced vibration of the hydrofoil at a near-constant Strouhal number. Regular waves led to near sinusoidal oscillations of the load and deformations at the wave encounter frequency, while the mean response and the dynamic response at other frequency peaks corresponding to hydrodynamic and structural modes remained mostly unaffected. Significant dynamic load amplification was observed at a submerged aspect ratio of 2 for cases with low angles of attack because of coalescence between the second and third wetted structural modes; at high angles of attack, the amplitude of the load fluctuations and flow-induced vibrations reduced because energy was diverted away from the coalescence frequencies to the nearby vortex shedding frequency. In both calm water and wave conditions, transition from fully wetted to fully ventilated flow resulted in sudden and significant reduction in the load coefficients, as well as foil deflections. An impulse-like signature was observed in the time-frequency spectra during these transitions. In many of the cases, transition to fully ventilated flow also led to substantially reduced amplitudes in the load and deformation fluctuations. [ABSTRACT FROM AUTHOR]

A tidal turbine can benefit from exquisitely designed morphing blades with a flexible trailing edge by mitigating up to 90% of the load fluctuation in harsh ocean environments, which reduces the overall cost of tidal energy. However, existing fluid–structure interaction (FSI) methods of resolving flow-induced deformation of the blades is computationally expensive, which poses an important challenge to effective morphing blade design. This paper presents a novel static FSI tool based on deep learning to cost-efficiently analyze the fluid–structure coupling of a hydrofoil. Specifically, adopting a convolutional neural network (CNN) to predict the fluid force and finite element method (FEM) to solve the solid structure response, a new CNN-FEM framework with an iterative scheme for solving the FSI problem is developed to achieve equilibrium between the fluid and structural forces. The new framework is used to predict the elastic deformation of the flexible blade section of the hydrofoil to demonstrate its effectiveness in the FSI evaluation. Comparison of the results to those produced by commercially developed software (i.e., Ansys Workbench) shows that this method yields extremely close prediction results of average equivalent stress and an accuracy of more than 92%. Moreover, it is 100 times more computationally efficient than the commercial Ansys Workbench software, requiring less than 3s for one-way FSI calculation. Taking advantage of this cost-effectiveness, the CNN-FEM can be used to achieve the accurate prediction of the deformation characteristics of the flexible hydrofoil under various flow scenarios that lay a foundation for advanced morphing blade design in the future. [ABSTRACT FROM AUTHOR]

The fully passive activation of a flapping hydrofoil turbine is an emerging scheme for increasing the power efficiency of these devices and simplifying the system complexity owing to the use of a pitching controller. In this study, a dual-flapping hydrofoil turbine (DFHT) of which the front hydrofoil oscillates upstream and the rear hydrofoil oscillates downstream with respect to the corresponding axis with a new coupling mechanism is suggested to realize fully passive activation. The pitch and arm angular motions of the front and rear hydrofoils are simply linked by chain connections so that the turbine can start independently. A mechanical spring is adopted in order to enhance the recovery force to the middle position. It is found from an experimental evaluation in a circulating water channel that the coupling mechanism activates the DFHT fully passively with a nearly 90-degree phase difference at the middle positions where the power peak exists. Subsequently, zero-power zones are removed from the total power curve. An increase in the spring constant increases the flapping frequency and the self-starting velocity of the DFHT. Eventually, fully passive activation could contribute to improving the power efficiency, and the 90-degree phase difference could be utilized to reduce power fluctuations. [ABSTRACT FROM AUTHOR]

In order to understand the mechanism of cavity shedding and evolution, turbulent cavitating flows of the twisted hydrofoil were numerically investigated using the k-ε turbulence model and the ZGB cavitation model. The results of the numerical calculation and the experimental method are basically consistent, which confirms the feasibility of the numerical calculation model. This study has obtained the following conclusions. Firstly, the cavity shedding can be summarized into six stages, and the cavity shape, pressure and velocity field at different stages are displayed, analyzed and compared in detail. Secondly, the shedding of cavity and its evolution are mainly caused by the re-entrant jet and side-entrant jet, in which the former provides the kinetic energy and the latter plays the role of guiding the direction. Thirdly, under the convective shearing action of the re-entrant jet and the main flow, a strong vortex located in the mid-back edge of the hydrofoil is formed, which promotes the transformation of the cavity shape into a U-shaped structure. [ABSTRACT FROM AUTHOR]

This paper reports an experimental study of tip vortex flowfield and cavitation inception of a tip-loaded hydrofoil. Vortex strength, wandering, and turbulence statistics are characterized using stereo particle image velocimetry (SPIV) in a water tunnel facility, at a chord Reynolds number of 1.3 × 10 6 . Cavitation physics are characterized using high-speed videography and dual-hydrophone acoustic cavitation measurements. The loading of the rectangular planform hydrofoil has a maximum at 65% span, 56% greater than that at the root, i.e., the hydrofoil loading is representative of non-elliptically loaded open propellers. Acoustic cavitation inception is quantified and is observed to precede visual cavitation onset using unaided and high-speed imaging. Measurements reported here show that vorticity fluctuations are nearly of the same magnitude as the ensemble vorticity. Instantaneous measurements of vorticity at the trailing edge, 1 2 -chord downstream, and one-chord downstream positions are reported. Their peak magnitudes are located adjacent to the ensemble vortex center and are between four and five times the ensemble mean. The fluctuating vorticity measurements, taken in conjunction with high-speed video observations, provide insight into the hydrodynamic conditions responsible for intermittent cavitation events. The reported measurements elucidate instantaneous and mean turbulence physics associated with vortex cavitation and can provide a validation basis for numerical simulations. [ABSTRACT FROM AUTHOR]

Cavitation inception in tip leakage flows remains a challenging topic in the engineering field, as the effect of tip gap width on inception is unclear. The present study is devoted to an analysis of the effect of gap width on tip leakage cavitation inception by using full-wetted numerical simulations. Numerical results show that the cavitation inception number is strongly related to the dimensionless gap width t, which is defined as the ratio of tip gap width to the maximum hydrofoil thickness, and the reason behind it is explained by the specific flow structures. The cavitation inception number of suction side (SS) sheet cavitation decreases gradually with the increase of dimensionless gap. The cavitation inception numbers of tip leakage vortex (TLV) and tip separation vortex (TSV) increase first and then decrease with the increase of the gap, reaching the maximum at t=0.2 and t=0.3 respectively. The main reason is that in the gap range of 0.2-0.3, TLV and TSV cores have the highest vorticity and the lowest pressure. [ABSTRACT FROM AUTHOR]

This paper presents a simplified mathematical model of a pumped hydrofoil (PH)—a surfboard elevated above water surface and connected to a tandem of hydrofoils by a strut. The PH is operated by a rider who stands on the surfboard and produces swinging up-and-down motions resulting in forward propulsion of the device. In the present paper, the description of the vertical motion of the PH is reduced to a linear oscillator excited by an oscillating mass coupled with the requirement that the weight is supported by dynamic lift of the foil(s). The inertial and damping influence of the hydrofoil(s) is accounted for by expanding unsteady lift force on the foil(s) in terms of kinematic parameters. The restoring term of the oscillator is associated with the phenomenon of automatic stabilization of shallowly submerged hydrofoils. The latter effect manifests itself in that when a hydrofoil approaches free surface, its lift decreases, and when it moves away from free surface, its lift increases. The analytical solution of the pumping foil mass-spring type forced oscillations equation allows one to calculate the flapping motion of the foil(s) and, thereafter, the period-averaged thrust generated by the PH. The resulting speed has been estimated on an assumption that the device enters its cruising mode when the thrust becomes equal to the drag, the latter comprising viscous, wave, and induced drag components. The model under discussion allows one to relate the main parameters of the system to its performance and, hopefully, provides further insight into the pumped hydrofoil phenomenon, its design methodology, and operation strategy. The review part of the paper focuses on two aspects of the problem: hydrodynamic behavior of the hydrofoil(s) in proximity to free water surface and their propulsion due to oscillations. [ABSTRACT FROM AUTHOR]

In order to improve the hydrodynamic performance of flapping hydrofoils and solve the problem of insufficient hydrodynamic force in plain river network areas, in this study, we consider the more realistic swing of fish tails and propose an arc flapping method, the coupled motion of which has three degrees of freedom: heave, pitch, and lateral displacement. Two flapping methods, positive arcs and negative arcs, were derived on the basis of the lateral displacement direction. By using the finite volume method (FVM) and overlapping grid technology, a numerical simulation was conducted to compare and analyze the pumping performance of three types of flapping hydrofoil, namely, linear, positive arcs, and negative arcs, in order to further provide guidance for the structural optimization of bionic pumping devices. The results showed that the wake vortex structures of the three flapping modes all had anti-Kármán vortex streets, but the wake vortex of linear flapping deflected upward, and the wake vortex of positive arc flapping tended to be further away in the flow field. In one cycle, thrust was always generated by the positive arc flapping hydrofoil and the linear flapping hydrofoil, but the thrust coefficient curve of the positive arc flapping hydrofoil was more stable than that of the linear flapping hydrofoil, and the peak value was reduced by 46.5%. In addition, under the conditions of a flow rate of 750 L · s − 1 and an average head of 0.006 m, the pumping efficiency of the positive arc flapping hydrofoil reached 35%, thus showing better pumping performance than the traditional linear flapping hydrofoil under conditions with ultra-low head. [ABSTRACT FROM AUTHOR]

The unsteady RANS equations for a two-dimensional hydrofoil were solved using ANSYS Fluent to model and simulate the hydrofoil at a constant Reynolds number, Re, of 2 × 105 and a fixed reduced frequency, f*, of 0.14. The simulations were performed by varying parameters, such as the number of deflectors N, tilt angle of the deflectors β, and vertical spacing of the deflectors J* = J/c, to determine the effect of the upstream deflector's position on the hydrofoil's performance. The results demonstrated that the deflector was effective at redirecting the separated flow away from the edges, which was then amplified downstream before colliding with the leading edge of the oscillating hydrofoil to increase power extraction. The performance of the oscillating hydrofoil was highly reliant on all three studied parameters. The hydrofoil with two deflectors (N = 2) displayed marginally superior power extraction capability compared to the hydrofoil with a single deflector (N = 1). Furthermore, the hydrofoil with the rightward inclined deflector at a low tilt angle (−5° ≥ β ≥ −10°) exhibited relatively better power extraction performance than the others. The best deflector design increased the hydrofoil's cycle-averaged power coefficient by approximately 32% compared to a hydrofoil without a deflector. The vortex structures revealed that the flow evolution and power extraction performance were dependent on the size, robustness, and growth rate of the leading edge vortex (LEV) as well as the timing of LEV separation. The power extraction efficiency of an oscillating hydrofoil increased in the mid downstroke and upstroke due to the formation of a more robust LEV when the hydrofoil–deflector interaction was advantageous, but it dropped in the wing reversal due to the early separation of the LEV when the hydrofoil–deflector interaction was counterproductive. [ABSTRACT FROM AUTHOR]

The effects of unsteady stream on the hydrodynamic behavior of a flexible hydrofoil in the semi-passive mode are investigated in this study with the goal of improving power extraction. Specifically, the influences of flow velocity amplitude (λ), the frequency ratio (σ) between hydrofoil and flow velocity, and phase difference (ε) between the flow velocity and pitching motion of the hydrofoil are examined. The results reveal that σ has a significant effect on lift force and heaving velocity, thus severely affecting the average power coefficient ( C P ‾) and heaving response. The peak value of C P ‾ is attained when σ = 2; this is attributed to the satisfactory synchronization between the flow velocity and heaving motion of the hydrofoil. Initially, C P ‾ increases with ε until ε = 90°; then, it decreases as ε further increases. The equilibrium position of the heaving response is sensitive to both ε and λ. The relative relationship between the flow velocity and pitching motion is modified by ε , altering the evolution of the lift force, heaving velocity, vortex, and pressure distribution. Through comprehensive adjusting σ and ε according to λ , the hydrofoil encounters minimum incoming flow near the dead point. This is the key to improve power extraction. [ABSTRACT FROM AUTHOR]

Large-eddy simulations are reported on a system consisting of a marine propeller and a downstream, semi-infinite hydrofoil, carried out on a cylindrical grid of about 3.8 × 109 points. The results are compared with those of an earlier study, considering a similar hydrofoil of infinite spanwise extent, to shed light on the influence of the end effects on the wake flow. The comparisons show good agreement between the two cases at conditions of no incidence of the hydrofoil. However, as its incidence angle grows, end effects become important. Accounting for the limited spanwise extent of the hydrofoil results in the generation of a couple of streamwise-oriented vortices from the port and starboard edges of its tip, a reduced spanwise elongation of the propeller wake, and lower turbulent stresses on the suction side of the hydrofoil, where the massive separation phenomena characterizing the infinite hydrofoil at large incidence angles are missing. In the wake of the overall system, the peak values of turbulent stresses are produced in the region of shear between the vortex shed from the pressure side edge of the tip of the hydrofoil and the tip vortices from the propeller. The latter vortices roll around the former, resulting in an intense interaction between them. In contrast, downstream of the infinite hydrofoil, the highest turbulent stresses are achieved within its wake, due to its shear with the elongated wake of the propeller. [ABSTRACT FROM AUTHOR]

We conducted a large eddy simulation (LES) of cavitating turbulent flow around a Clark-Y11.7% hydrofoil at an angle of attack of 8 deg. We focused on the improvement of the reproducibility of hydrofoil performance under cavitating conditions when considering the unsteadiness of the flow field sufficiently by LES. A homogeneous cavitation model was adopted. The subgrid-scale model for LES was the one-equation dynamic model. The unsteady phenomena such as cavity oscillation and cloud cavity shedding accompanied by the reentrant jet were simulated. Large-scale spanwise vortices were also captured, which occurred intermittently from the onset of the sheet cavity. Then, the flow inside the sheet cavity was de-accelerated and the streamlines went around the sheet cavity. Streamline curvature formed a separation bubble behind the sheet cavity accompanied by a low-pressure region. Thus, the sheet cavity length was sufficient and the reproducibility of the pressure distribution around the hydrofoil and lift coefficient were improved. [ABSTRACT FROM AUTHOR]

This paper is presented to compare various wavy leading-edge protuberances on oscillating hydrofoil performance and power efficiency. The unsteady turbulent 3D flow simulations were carried out by using the StarCCM+ software. The 3D hydrofoil with a straight at the leading-edge is a NACA0015 section with a chord of 0.24 m and an aspect ratio of 7. Four new types of hydrofoils are proposed with wavy leading-edge protuberance. The RANS equations with the realizable k-e turbulence model are used to predict the turbulent flow around the hydrofoil under different conditions. In order to validate, a comparison of the numerical results of the forces coefficients and power efficiency of non-protuberance oscillating hydrofoil are shown in good agreement with experimental data. Then, four new profiles on the leading-edge of the hydrofoils are simulated and many results of the pressure distribution, vorticity contour, streamline velocity, and power coefficient for five hydrofoil types are presented and discussed. It is concluded that the hydrofoil of Type-M can achieve constantly higher efficiency of over 46% by employing appropriate heave and pitch amplitudes. [ABSTRACT FROM AUTHOR]

Submerged vanes are the hydrofoils which generate the helical currents in the flow due to the difference in pressure between the approaching flow side and the downstream side of vanes and are placed obliquely with the flow, with angles ranging from 10° to 40°. Previous studies have been done on the rectangular shaped submerged vanes but only a few studies have been reported for the submerged vanes with non-rectangular shapes. The present study aims to optimize the parameters of tapered vanes and their effect on flow structure around the vanes through numerical modelling. Numerical modelling for the present study was done in ANSYS-CFX software using the K-ω turbulence closure model to simulate the vortical flow. It was observed that maximum strength of secondary currents was obtained for angle of attack, sweep angle and relative vane height (ratio of vane height to depth of flow) of 17°, 10° and 0.48, respectively. It was also observed that in the proximity of the tapered vane, secondary currents are dominated by vortex-lift while in far-reaches, potential lift prevails. It was observed that transverse velocity was maximum for a sweep angle of 10°. Comparing the optimal rectangular vane (with angle of attack of 30) with the tapered vane (with angle of attack of 17), it was observed that the rectangular vane has a tendency to generate higher transverse velocities and hence may act as a sediment diverter to counter sediment movement while the tapered vane has a tendency to generate vortical structures over a larger distance, hence may act as a sediment managing device. [ABSTRACT FROM AUTHOR]

Hybrid cavitation model is developed using both interface tracking model and homogeneous fluid model. The target of the analysis is cavitating turbulent flow around ClarkY11.7% hydrofoil at angle of attack of 2 and 8 degrees, and the one-equation dynamic model is used as the subgrid-scale model. The end of the tracked sheet cavity interface is closed without interfering with the progress of the re-entrant jet. The separated flow and its low-pressure region behind the sheet cavity are simulated as the streamlines go around the sheet cavity, increasing lift coefficient. On the other hand, separated flow promotes the progression of the re-entrant jet, leading to underestimation of the area that is always covered by the sheet cavity. These results suggest that the reproducibility of cavity near the front of the re-entrant jet should be considered in addition to the streamlines to improve the accuracy of quantitative reproductiono f lift characteristics. [ABSTRACT FROM AUTHOR]

This study presents a novel index to assess the thrust propulsion performance of flapping-hydrofoil, and identifies the order of priority of motion parameters that affect the thrust pulsation amplitude. The flapping motion is simulated using the Udf (user-defined motion) feature in the commercial software Fluent, and the impacts of three flapping motion parameters on flapping thrust are investigated by manipulating these parameters. The results of the numerical simulation indicate that there is a positive correlation between the heave amplitude and the thrust pulsation amplitude. Additionally, the thrust pulsation amplitude is interactively affected by the pitch amplitude, the phase difference between horizontal and heave motion, and the heave amplitude. The Levenberg-Marquardt Algorithm is employed to fit the function that relates the thrust pulsation amplitude with the three motion parameters. The coefficients obtained from the fitting function indicate the impacts of the interaction effect, as well as the effect of each motion parameter on the pulsation amplitude. [ABSTRACT FROM AUTHOR]

In this research, the cavitating flow around a NACA0015 (National Advisory Committee for Aeronautics) hydrofoil obtained by the large-eddy simulation method is analyzed using the proper orthogonal decomposition (POD) theory. Various fundamental mechanisms have been investigated thoroughly, including the reentrant jet behavior, pressure gradient mechanism, vortex dynamics, and dynamic properties of the hydrofoil. The influence of the vortex dynamics, pressure mechanism, and temporal/spatial evolution is revealed. The POD decomposition indicates that the first four dominant POD modes occupy 97.4% of the entire energy. Based on the vortex force field extracted from the first four single POD modes, it is found that the lift-and-drag characteristics in the cavitating flow are determined by the specific spatial distribution of mode vortex structures. In addition, the coupling of velocity pulsations and pressure fluctuations is carried out to obtain the POD modal pressure gradient field, which reveals that the pressure gradient has a close connection with the cavity evolution. Furthermore, the vortex force and pressure gradient are reconstructed using the first four modes, 17 modes, and 160 modes, which indicates that the low-order POD modes without the impact of small-scale structures and noise can clearly capture the fundamental aspects of the flow field. [ABSTRACT FROM AUTHOR]

This article presents the findings from a numerical study on hydrokinetic turbines for application in isolated and low populated areas. A numerical code for the determination of the performance of horizontal axis hydrokinetic turbine is developed based on the blade element momentum method, optimized and validated. It is intended to investigate hydrofoils which are easy to manufacture and to maintain. These hydrofoils include hydrofoil and the Joukowski J 0015, GO 410, flat plate and NACA 0024 (reference) hydrofoils. The effects of the chord and twist angle distributions are investigated, and their influence on the hydrodynamic performance is evaluated. The results showed that the linear twist angle distribution along the blade showed the best performance. The power coefficient for the Joukowski J 0015-, Gottingen GO 410- and NACA 0024-based rotor blades is, respectively, 0.512, 0.506 and 0.513 indicating the good performance of the Joukowski hydrofoil. The combination of the Joukowski J0015 hydrofoil with linear chord and twist distributions showed the best performance. The flat plate hydrofoil-based rotor with constant chord and linear twist angle distributions showed a reasonable power coefficient of 0.324 and can be a viable solution considering facility of manufacturing and maintenance. [ABSTRACT FROM AUTHOR]

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