The energy harvesting performance of symmetrical flapping hydrofoil varying with thickness, shape and position of pitching axis was numerical investigated via immersed boundary method at a Reynolds ...number of 1100. The foil performs the given sinusoidal heaving and cosine-curvilinear pitching motions. The center of the hydrofoil is the intersection of the chord line and vertical line at maximum thickness. α is the ratio of the length between the leading edge and the center to the length of chord. β is the ratio of the length between the upper edge and the center to the length of chord. xp is the length between the leading edge and the pitching axis. It is found that enhancing the thickness of the foil can modestly increase the energy harvesting efficiency in the range of 0.03≤β≤0.06, however, it is in contrast in the range of 0.06≤β≤0.105. It is also found that the higher the value of α, the higher the extracting energy when α≤0.4. The optimal foil thickness is β = 0.06. The optimal pitching axis is located at the foil center when α≤0.35, and it is xp=0.35l when 0.35<α≤0.50.The highest energy harvesting efficiency of 41.71% is achieved when α=0.4.
•Active flapping foil is simulated using immersed boundary method.•Outline of foil on energy harvesting efficiency were investigated.•The highest energy harvesting efficiency of 41.71% is recorded.
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•The hydrofoil-flap arrangement has a positive effect on the blade performance.•Experimental tests were carried out for two different hydrokinetic turbine rotors.•CFD analyses were ...conducted through 6-DoF user-defined functions.•The CP value increases about 20.53% when a hydrofoil-flap arrangement is used.•The rotor optimization is crucial for reducing the energy transformation costs.
In this work, the efficiency of two horizontal-axis hydrokinetic turbines, whose blades were designed with and without multi-element hydrofoil cross-sections, has been numerical and experimentally investigated for tip speed ratio (λ) values ranging between 2.5 and 9.0 to compare the experimental rotor performance with numerical results. The Eppler 420 hydrofoil was used for the design of the blades applying the blade element momentum (BEM) theory. The variation of the power coefficient curve of the turbines was analyzed by using computational fluid dynamics (CFD) and experimental tests through ANSYs Fluent software with six-degrees of freedom (6-DoF) user-defined function (UDF) method and an open hydraulic channel, respectively. Numerically, for the turbine with a multi-element hydrofoil and without a multi-element (traditional) hydrofoil, maximum power coefficients (CPmax) of 0.5050 and 0.419 (at a λ value equal to 7.129 and 6.739, respectively) were obtained. It is worth noting that a reasonable agreement between the numerical and the experimental results was achieved. In this regard, the blade with a multi-element hydrofoil has a positive influence on the hydrokinetic turbine performance; therefore, it can be used for power generation in river or marine systems.
Fishes school to swim more efficiently while not much of fish schooling is understood from the perspective of fluid–structure interactions. To understand the benefits of fish schooling, we ...investigate thrust, efficiency, and wake structure of a two-hydrofoil system where the upstream hydrofoil is forced pitching (active) and the downstream hydrofoil is free pitching (passive). The streamwise and lateral separations between the two hydrofoils are 0.109c and 1.0D, respectively, where c is the chord length and D is the diameter of the leading edge. The active hydrofoil pitches with normalized amplitudes A∗=0.55 – 0.80 and normalized frequencies StD=0.23–0.33. It is found that the upstream active hydrofoil undergoes up to 38% thrust enhancement in the presence of the downstream passive hydrofoil while the downstream passive hydrofoil achieves the same thrust as the upstream active hydrofoil under certain conditions. The maximum combined thrust and efficiency achieved for the two-hydrofoil system are 95% and 180% higher than those for a single isolated hydrofoil. These are some reasons for fish swimming in school. Three distinct flow structures (vortex impingement flow, vortex trapping flow, and vortex splitting flow) are identified in the A∗– StD domain, where the vortex trapping flow provides the greatest thrust and efficiency enhancement.
This paper introduces an algorithmic method for the design and resistance prediction of hydrofoil vessels for fast ferry applications, using only design speed and mass as input parameters. A database ...of resistance estimates has been created and shared online. Furthermore, a simple model is introduced for the extension of the results to off-design speeds.
The efficacy of the models is demonstrated through three case studies. Firstly, the importance of accounting for the power requirement during take-off is investigated, concluding that a simple assumption of constant speed yields accurate energy estimates for routes exceeding approximately six nautical miles.
Case study number two entails a comparative analysis of hydrofoil vessel resistance to that of slender catamarans across a wide range of vessel sizes and speeds. The results reveal a potential for reducing the energy requirement of passenger fast ferries by 30% to 50% through the use of hydrofoils.
The paper concludes with an investigation into the influence of surface roughness on hydrofoil resistance, indicating significant deteriorating effects even at moderate roughness levels.
•An algorithmic hydrofoil design method.•A database of hydrofoil resistance estimates.•A comparison of the resistance of hydrofoils and slender catamarans.•A study of the importance of take-off energy.•An investigation into the importance of roughness.
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 (CP‾) and heaving response. The peak value of CP‾ is attained when σ = 2; this is attributed to the satisfactory synchronization between the flow velocity and heaving motion of the hydrofoil. Initially, CP‾ 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.
Numerical simulations have been used in this paper to study the propulsion device of a wave glider based on an oscillating hydrofoil, in which the profile of the pitching and heaving motion have been ...prescribed for the sake of simplicity. A grid model for a two-dimensional NACA0012 hydrofoil was built by using the dynamic and moving mesh technology of the Computational Fluid Dynamics (CFD) software FLUENT and the corresponding mathematical model has also been established. First, for the sinusoidal pitching, the effects of the pitching amplitude and the reduced frequency were investigated. As the reduced frequency increased, both the mean output power coefficient and the optimal pitching amplitude increased. Then non-sinusoidal pitching was studied, with a gradual change from a sinusoid to a square wave as the value of β was increased from 1. It was found that when the pitching amplitude was small, the trapezoidal pitching profile could indeed improve the mean output power coefficient of the flapping foil. However, when the pitching amplitude was larger than the optimal value, the non-sinusoidal pitching motion negatively contributed to the propulsion performance. Finally, the overall results suggested that a trapezoidal-like pitching profile was effective for the oscillating foil of a wave glider when the pitching amplitude was less than the optimal value.
•Subtle vortical wave, hairpin vortices and pressure disturbance are captured by LES.•Cavitation makes super large-scaled vortices in wake and abnormal pressure spectrum.•Shedding mechanism is not ...accurate by re-entrant jet but also by turbulence effect.•Counterintuitive force jump is caused by pressure rising induced by cavity collapse.•Vorticity mechanisms are analyzed to clarify cavitation effects on vortex structure.
The intrinsic 3D vortical structures of turbulent cavitating flow are difficult to be measured in experiment. In the present work, the compressible large eddy simulation (LES) was performed to investigate the cavitating flow around NACA66 hydrofoil, with a special emphasis put on the laminar-turbulent transition, the turbulence-cavitation interaction and the dynamics features. The mesh is constructed with sufficient resolution to capture the coherent structure and energy spectrum of turbulence, with the prerequisites of y+, Δx+ and Δz+ fulfilled to resolve at least 80% of the turbulence kinetic energy. The LES captured the subtle coherent structures of laminar-turbulent transition, including the evolution of pressure disturbance, 3D vortical wave and hairpin vortices. It manifests that, the cavity shedding produces super large-scaled vortices in the wake, and the Batchelor's pressure spectrum of conventional mono-phase turbulence is no longer met in turbulent cavitating flows. It is found that the conventional cavity shedding mechanism by re-entrant jet is not accurate if strong turbulence is present. The sweeping effect of the jet front expands the turbulent fluctuation along the cavity surface which eventually makes it broken. The various vorticity transport mechanisms are analyzed to clarify the role of cavitation effects in the evolution of the vortical structures. The counterintuitive drastic oscillation of lift and drag is studied, finding that the pressure is focused during the cavity collapses and a violent pressure rise is induced. By formulating approximate one-dimensional analytic formulas, the far-field pressure disturbance is found to be in strict accordance with the second-order time derivative of cavity volume.
Inspired by the tubercles on humpback whales, the ‘tubercle-effect’ first identified by (Fish and Battle, 1995; Fish and Lauder, 2006) had been studied extensively to understand their influence on ...overall efficiency for airfoils and wings. Pitching and/or heaving motion of foils produces net thrust inspired from fin-based locomotion of aquatic animals, and is often being proposed as a greener means of propulsion having reduced acoustic signature. With this aim in mind, the present work investigates the effect of using tubercles on pitching motion of foils. The hydrodynamic characteristics of finite aspect ratio pitching foils having leading-edge modifications in the form of two protuberances are investigated using experiments. The thrust and efficiency with and without protuberances are investigated for pure pitching motions at reduced frequencies (1.8 – 3.2) and pitching amplitudes (7.5°- 20°) for Reynolds number 2 × 104. The experimental results are compared with inviscid linear theory calculations which over-predict the mean thrust, while the amplitude of the lift force and its variation with reduced frequency is similar. The performance improvements in terms of thrust and efficiency for the modified design are restricted to ≥15° pitching amplitudes at higher reduced frequencies. The wake vorticity patterns obtained using Computational Fluid Dynamics investigations show the influence of the leading-edge vortex, which gets modified when the protuberances are used. The transition from 2S wake to 2P wake with the increase in the amplitude of oscillation is captured. The reversal of von Kármán Vortex Street with the pitching frequency and thus the transition from drag to thrust generation is observed for both the baseline and modified foil designs. The results indicate that protuberances over the leading-edge of hydrofoils can influence the hydrodynamics during pitching motion and can be effectively employed for performance tailoring based on the geometry and motion characteristics. The simplified setup used here is also meant to serve as a basis towards future research including verification and validation work for numerical solvers.
•The influence of leading-edge protuberances on a hydrofoil in pure pitching motion is studied.•Experimental and numerical studies are performed for the assessment of thrust, efficiency, and flow characteristics.•At the Reynolds number 20000, the modified design produces higher thrust and lift only at amplitudes greater than 15 deg.•The protuberances have a positive effect on thrust and efficiency at higher reduced frequencies.•The change in 2S to 2P wake patterns with the increase of the pitching amplitude is observed from CFD investigations.•For the modified hydrofoil, the leading-edge vortex pattern changes from the crest to the trough of the protuberances impacting the wake and propulsion performance at different Strouhal numbers.
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