A propulsion system based on tandem hydrofoils is studied experimentally and numerically. An experimental measurement system is developed to extract hydrodynamic loads on the foils and capture their ...twisting deformation during operation. The measured data allowed us to assess the efficiency of the propulsion system as a function of travel speed and stroke frequency. The numerical simulation of the propulsion system is also presented and involves 3D, full-scale fluid–structure interaction (FSI) computation of a single (forward) foil. The foil is modeled as a combination of the isogeometric rotation-free Kirchhoff–Love shell and bending-stabilized cable, while the hydrodynamics makes use of the finite-element-based arbitrary Lagrangian–Eulerian variational multiscale formulation. The large added mass is handled through a quasi-direct FSI coupling technique. The measurement data collected is used in the validation of the FSI simulation, and excellent agreement is achieved between the predicted and measured hydrodynamic loads and foil twisting motion.
This study aimed to numerically investigate the power extraction performance of an oscillating hydrofoil with a time-varying camber in the swing arm mode. The effects of the swing arm length (R), ...swing arm amplitude (H0), pitching amplitude (θ0), reduced frequency (f*), and flexure amplitude (α) on the average power coefficient (CP‾) and efficiency (η) were investigated. The results revealed that, with an increase in α, the force coefficients were improved, which is beneficial for improving the power extraction capability. At α = 15%, η of the flexible hydrofoil was 38.5%; this represents an increase of approximately 40.5%, as compared to rigid hydrofoils. Further, R and H0 of the swing arm caused variations in the vertical amplitude of hydrofoils, thereby altering the motion velocity and effective angle of attack and further affecting the evolutions of the flow fields and vortices, pressure distribution, and force coefficients. Moreover, the leading-edge vortex was found to separate from the hydrofoils gradually in advance with an increase in θ0, whereas the increase in f* decreased the synchronization between the directions of the force coefficient and motion velocity. Furthermore, it was found that the optimal f*, which could reach peak values of CP‾ and η, increased with θ0.
•The energy extraction of a flexible hydrofoil in swing arm mode was studied.•The flexible hydrofoil improves the energy extraction power and efficiency.•The vertical amplitude and heaving velocity were altered by swing arm parameters.•The optimal reduced frequency increased with pitching amplitude.•The high frequency decreases the synchronization between the force and velocity.
•Cavitating flow around a 3-D Hydrofoil with Wavy Leading Edges (WLE) is investigated using the LES approach in OpenFOAM.•Comparison of cavity features of wavy leading edges (WLE) hydrofoil with ...straight leading edge (SLE) one.•Force coefficients, re-entrant jet and separation are assessed during the cloud cavity evolution around WLE and SLE hydrofoils.•The mechanisms of the laminar separation bubble (LSB) and low-pressure zone behind the WLE hydrofoil are illustrated.•Role of vorticity stretching and vorticity dilatation with the presence of cavity for the WLE and SLE hydrofoils are investigated.
The present study seeks to conduct numerical investigations of the cavitating flow characteristics around a sinusoidal wavy leading edge (WLE) 3-D hydrofoil underlying a NACA 634–021 profile with an aspect ratio of 4.3. Cavitational and non-cavitational characteristics of hydrofoils are numerically examined at a chord-based Reynolds number of 7.2 × 105. The sinusoidal leading edge geometries include two WLE amplitudes of 5% and 25% and two WLE wavelengths of 25% and 50% of the mean chord length. We examined the cavitating flow around the hydrofoils in different cavitation numbers, namely σ = 0.8 and σ = 1.2. The flow over the protuberances of the WLE hydrofoil is considered at varying chord lengths and a constant angle of attack α = 6°, where significant spanwise variations in all flow properties, in contrast to the straight leading edge (SLE) hydrofoil, were observed. Large eddy simulation (LES) and Kunz mass transfer models are employed to simulate the dynamic and unsteady behavior of the cavitating flow. Besides, the compressive volume of fluid (VOF) method is used to track the cavity interface. Simulation is performed under the two-phase flow solver —interPhaseChangeFoam— of the OpenFOAM package. Compared to the SLE hydrofoil, we provided an exhaustive report of the time-averaged and instantaneous fluid dynamic characteristics of the cavitating flow around the sinusoidal leading edge hydrofoil, i.e., pressure, velocity, and vorticity fields, as well as lift and drag coefficients, and turbulent kinetic energy are reported. Furthermore, detailed analyses of the instantaneous cavity leading edge and flow separation treatment, vortical structure of the flow, vorticity stretching and dilatation, details of the spanwise flow, the formation of a low-pressure zone behind the WLE hydrofoil, streamwise velocity fluctuation, and evolution of the cavity dynamics through a complete cycle are reported. Results show that early development of the laminar separation bubble (LSBs) on the suction side of WLE hydrofoil prevents significant flow separation. Furthermore, the WLE cases exhibit a significantly reduced level of unsteady fluctuations in aerodynamic forces at the frequency of periodic vortex shedding.
•By GCI evaluation method to study grid independence and discrete error of hydrofoil.•Numerical simulation of the hydrofoil using various turbulence models.•Simulation results consistent with the ...experiments are obtained through the DCMFBM turbulence model.•Revealing the mechanism of hydrofoil cavities instability and shedding.
To improve the accuracy of hydrofoil unsteady cavitation simulations, we investigate grid irrelevance and discrete error using the GCI evaluation method to determine the optimal number of grids. Numerous turbulent viscosity correction approaches are used to improve the turbulence model, and numerical simulations are conducted in conjunction with unsteady cavitation of the hydrofoil with a 3° angle of attack, as well as application evaluation. The results indicate that the DCMFBM model produces the most accurate unsteady cavity shape of the hydrofoil surface. The details of the cavity's primary and secondary shedding are captured during the cavitation process. The DCMFBM turbulence model with a density correction index of 10 and a filter parameter of 0.15c has the highest simulation accuracy. The flow structures of sheet cavitation, transition state cavitation, cloud cavitation, and cavitation shedding are analyzed. The mechanism of hydrofoil cavitation instability and shedding is revealed, providing a theorem.
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•Evolution of flow patterns and vibratory response are well predicted and analyzed.•Cavitation affects the foil deformation and the hydroelastic characteristics.•The hydroelastic response leads to a ...complex pattern of the cavitation and vortexes.
The objective of this paper is to investigate the hydroelastic response of a flexible NACA66 hydrofoil in cavitating flows by combined experimental and numerical studies. Experimental results are presented for rigid/flexible NACA66 hydrofoils fixed at α0=8° for subcavitating (σ=8.0) and cavitating flows (σ=1.4). The high-speed video camera and Laser Doppler Vibrometer are applied to investigate the flow patterns and vibration characteristics. The multiphase flow is modeled with the incompressible and unsteady Reynolds Averaged Navier–Stokes (URANS) equations. The k−ω SST turbulence model with the turbulence viscosity correction and the Zwart cavitation model are introduced to the present simulations. The results showed that the cavitation has significant effect on the foil deformation and the unsteady characteristics of the hydroelastic response. The bending deformation is enhanced when the cavitation occurred. Meanwhile, the hydroelastic response has also affected the cavitation development and the vortex structure interactions. The cavity shedding frequency and vortex shedding and interacting frequency for the flexible hydrofoil are higher than that for the rigid hydrofoil. Compared to the periodic development of the hydrodynamic coefficients for the rigid hydrofoil, the hydrodynamic load coefficients of the flexible hydrofoil fluctuate more significantly, and the chaotic response of the flexible hydrofoil is mainly attributed to the disturbance caused by the flow-induced flutter and deformation of the foil. The evolution of the transient cavity shape and the corresponding hydrodynamic response can be divided into three stages: During the development of the attached cavity, the partial sheet cavity is formed and develops with the lift and drag coefficients increasing, while the maximum attached cavity formed on the suction side of the flexible hydrofoil is larger than that of the rigid hydrofoil, which is caused by the increase of the effective angle of attack due to the twist deformation. During the vortex structure interaction and cavity shedding process, the hydrodynamic loads for the flexible hydrofoil fluctuate because of the foil deformation, leading to a more complex cavitation pattern. During the residual cavity shedding and partial sheet cavity formation process, the cavities, together with the counter-rotational vortex structures, shed downstream totally and are followed by the formation of partial sheet cavities in next period, which is in advance for the flexible hydrofoil due to the larger effective angle of attack.
Tidal current turbine is one of the innovative and emerging technologies of marine renewable energies because it offers constant and predictable energy source that can be very beneficial, especially ...for commercial scale production of electrical power. Hydrofoils (HF) are essential elements of tidal current turbine (TCT) and should be properly designed as they play a vital role in improving the turbine output and providing adequate resistance to the blade structure. In connection with the hydrofoil designs, it is noteworthy that the primary objectives in their designs are to increase the coefficient of lift and to reduce the coefficients of drag and pitching moment, thus delaying the cavitation phenomenon. In this paper, the technology developments of the hydrofoil designs used in the horizontal axis TCT industry are reviewed, including the hydrodynamics design and the mechanical structure design. Besides, an up-to-date review and the newest achievements of marine TCT technologies with their developing histories are further explored. Included are also reviews on the numerical models used to assess the performance of TCT and optimization methods applied to design the hydrofoils. This in turn significantly contributes to a better knowledge on the recent designs of TCT hydrofoils for the researchers working in the marine turbine energy domain. Such information could also have important implications in the design of more sophisticated hydrofoils for the exploitation in diverse tidal current energy technologies for reaching a sustainable future.
•Up-to-date review tidal current technologies projects.•Current state-of-research on recent design of hydrofoils for the horizontal axis current turbine (HATCT).•A comprehensive review on the numerical models used in the design of TCT including the hydrodynamic and structural models.
To mitigate the adverse effects of cavitation on the performance degradation and chemical corrosion of hydraulic machines, a jet hydrofoil physical model based on the structure of a guide vane model ...of a high-pressure mixed-flow hydraulic turbine was established. A density correction turbulence model based on filters was adopted to numerically simulate the cavitation characteristics on the surface of the jet hydrofoil. The results demonstrate that when the jet speed is 12 m/s and the jet angle is −15°, the scheme exhibits the best cavitation suppression effect among all combinations of jet angles and speeds. Cavities near the jet structure are prematurely collapsed after being pressurized, and the cavitation suppression effect in the low-pressure area at the trailing edge is also satisfactory. The time-averaged bubble volume fraction reaches its minimum at 2.17%, with the time-averaged bubble volume fraction being 27.3% of the prototype hydrofoil. At flow speeds of 2.4 m/s and 7.2 m/s, small-angle jet fluid accumulated will stick to the closed cavities on the surface of the hydrofoil and cause them to rupture, while the jet fluid at the jet angle of 60° and speeds of 7.2 m/s and 12 m/s will directly cut off the bubbles.
•DCMFBM model is used to simulate cavitation characteristics on hydrofoil.•Analysis of jet angle and velocity impact on cavitation characteristics.•High-speed jet at small angles most effectively suppresses cavities.
•Investigating the cavitation of jet hydrofoils with different chord length positions.•The mechanism of the influence of the jet on the reflux jet was studied.•Bionic jet can improve the lift-to-drag ...ratio of hydrofoil.•Hydrodynamic performance and cavitation inhibition are best at the jet position 0.6c.•The return jet on the jet hydrofoil is mainly driven by the clockwise rotation of the bubble.
To suppress the cavitation phenomenon of the fluid machinery under high-velocity flow conditions, we designed the shark gill slit jet structure based on the principle of bionics. The selected hydrofoil is an expanding symmetric structure with chord length of 100 mm and span length of 80 mm. a bionic jet with a velocity of 4.836 m/s is placed on the suction surface of the hydrofoil. The jet direction is consistent with the mainstream direction, and numerical simulations are performed for the jet hydrofoil with different chordwise jet positions. The results show that when the chordal jet position is 0.6c, the time-averaged vacuole volume fraction is the smallest, which is reduced by 46% percent compared with the prototype hydrofoil. The lift-to-drag ratio of the jet hydrofoil is improved compared to the prototype hydrofoil. The pressure fluctuations on the suction surface continue to decrease as the longitudinal position of the jet is shifted back, and the shock waves generated by the cavitation collapse of the cloud are continuously suppressed. The return jet on the jet hydrofoil is driven mainly by the clockwise rotational motion of the cavity.
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