Characteristics Of Hydrofoil Research Articles

Study on the formation and characteristics of unstable sheet cavitation on hydrofoils

Hydrofoils, as fundamental components of hydraulic machinery, directly influence the performance of such machinery. This study conducted an analysis of the cavitation characteristics of hydrofoils at a + 4° angle of attack under various cavitation numbers using numerical simulation and experimental research methods. The focus of the research was to explore the phenomenon of unstable sheet cavitation and its causes, as well as to reveal the characteristics of the re-entrant jet. The large eddy simulation method was employed to calculate the cavitation morphology under three different cavitation numbers. The method is highly consistent with the experimental results in simulating the small-scale detachment at the tail of unstable sheet cavitation and the large-scale shedding of cloud cavitation. The study found that the detachment of unstable sheet cavitation is closely related to the re-entrant jet, which exhibits transient and abrupt characteristics during the unstable sheet cavitation phase. Furthermore, by applying FFT processing to the distribution of maximum reverse velocity and the spatiotemporal changes of Ux on characteristic lines, eigenfrequency of the detachment of unstable sheet cavitation were identified. The research results indicate that cavitation mainly show as sheet cavitation when the cavitation closure point does not exceed the zero-slope point. Beyond this point, it transitions to cloud cavitation. This study provides new insights into the cavitation phenomenon of hydrofoils and offers quantitative research on the phenomenon of unstable sheet cavitation.

Numerical Study on the Influence of Plasma Actuation on the Cavitation Characteristics of Hydrofoil

Numerical Study on the Influence of Plasma Actuation on the Cavitation Characteristics of Hydrofoil

Investigation of dynamic characteristics of hydrofoil with leading-edge tubercles

The convex structure of a humpback whale flipper is included in the airfoil design to investigate the control principle of the dynamic stall process. The effect of two different types of tubercles on the dynamic stall process of airfoils was investigated. The distribution of dynamic hysteresis loops and pressure fields with streamlines was used to explain the properties of the dynamic flow field and the emergence of dynamic stall vortices. The results demonstrate that the airfoils with tubercles have stronger hysteretic characteristics in the pre-stall region, and the ζ CM of mod-1 is increased by 51.33%, indicating stronger torsional hydroelastic stability. In the post-stall region, mod-2 is always in a condition of significant flow separation, which impairs the torsional hydrodynamic stability of the airfoil and has the potential to cause flutter. These findings suggest that leading-edge tubercles offer a negligible advantage in reducing airfoil dynamic stall, and their significance warrants further investigation.

Study on Noise Attenuation Characteristics of Hydrofoil with Specific Cavitation Number

Study on Noise Attenuation Characteristics of Hydrofoil with Specific Cavitation Number

Pressure fluctuation and cavitation noise characteristics of hydrofoil at different temperatures

The current work investigates the cavitating flow over a NACA0015 hydrofoil at temperatures of 285.65K and 305.95K using experiment and simulation. The results show that higher-temperature liquid flow can significantly decrease the periodic fluctuation frequency of the attached cavity, reduce the scale of shedding cavitation clouds, and suppress the collapse of discrete small bubbles downstream. It also inhibits the instability of the attached cavity, effectively mitigating the energy impact and the cavitation noise caused by collapse, especially the high-frequency region of cavitation noise. The simulation results indicated that higher-temperature liquid flow can reduce the range and the intensity of vortex structures in the flow field. At different temperatures, the strict periodic pulsation characteristics of cavitation noise are consistent with the attached cavity in the spectral-temporal domain. Although the cavitation states corresponding to the high sound intensity regions are different, they both have wideband characteristics. Especially over multiple cycles, the cavitation states corresponding to these high sound-intensity regions are consistent. The research results provide valuable references for predicting cavitation development state solely through noise signals.

Effect of Tip Clearance Size on the Cavitation Flow and Noise Characteristics of the NACA0009 Hydrofoil

The noise characteristics of NACA0009 hydrofoil with tip clearance are discussed. Select SST k- ω Turbulence model and Schnerr Sauer cavitation model simulate tip leakage cavitation flow. The results show that the simulation results are in good agreement with the original records, and the tip clearance reduces the energy characteristics of the hydrofoil. With the expansion of tip clearance model, TLV and cavitation compressive strength gradually decrease. In addition, the study of far-field noise shows that the nonspecific curve of the total noise coefficient of far-field noise shows a disc shaped symmetric structure. Compared with the original hydrofoil, when the tip clearance is 20 mm, the far-field dipole noise value at 170 ° angle is reduced by 17.68%. Tip clearance reduces offline and far-field noise. Far field noise decreases with the increase of tip clearance model.

Effects of vortex generator on the hydrodynamic characteristics of hydrofoil and horizontal axis tidal turbine

Tidal turbine blades are prone to flow separation in the boundary layer under high speed or high angle of attack, which will reduce energy efficiency and even the stall damage of the blades. This paper proposes introducing the flow control theory of vortex generators (VGs) to tidal turbines and studying the influence of VGs on the hydrodynamic characteristics of the tidal turbine blades. First, a numerical study is performed to investigate the effects of VGS on the hydrodynamic performance of the National Advisory Committee for Aeronautics (NACA) 4418 hydrofoil. The impact of different parameters, such as VG arrangement, spacing, height, and length, on the hydrodynamic performance of hydrofoil is studied by the computational fluid dynamics method. The results show that VGs can effectively suppress the flow separation and improve the maximum lift coefficient of the hydrofoil. The influence of VGs on flow separation characteristics of horizontal axis tidal turbines is studied by the CFD method. The results show that the flow separation of turbine blades mainly occurs at the root part of the suction surface, and the flow separation region expands radially as the flow velocity increases. VGs can effectively reduce the flow separation area on the suction side of turbine blades by suppressing the flow separation effect. Compared with the turbine blades without VGs, the power coefficient of turbine blades with VGs is increased by up to 5%. The flume experiment verifies the accuracy of the simulation results.

Bionic investigation of a dolphin head-based hydrofoil with emphasis on energy performance and flow characteristics

Tidal energy, regarded as clean and renewable energy source, has received widespread attention from scholars. As the basic structure of tidal power generator, the energy performance and flow characteristics of hydrofoil have a significant impact on the power generation efficiency. In this paper, a new dolphin head-based bionic hydrofoil is proposed based on the special shape of the dolphin's head, then three structures of dolphin bionic hydrofoils are obtained through corresponding modifications. Based on the accuracy verification of the numerical method, the energy performance and flow characteristics of the bionic hydrofoils are investigated. The results reveal that compared with original dolphin hydrofoil (ODH), the lift coefficients of deflected dolphin hydrofoil (DDH) under different attack angles are increased by 35.51%, 22.75%, 19.44%, 20.80%, 25.98% and 32.86%, respectively. The pressure distribution and flow characteristics around bionic hydrofoils are analyzed, and the results indicate that the pressure on the hydrofoil surface affects the lift and drag coefficients. The vortex distribution of the hydrofoil proves that DDH has a more stable flow pattern. The lift and drag coefficients is increased with the rise of inlet velocity, which provides potential value for the selection of tidal energy equipment under different application scenarios. The enhancement of the energy performance and flow characteristics prove that bionic hydrofoil will be effective in promoting energy utilization of tidal power generation and hydro turbine.

Research on shipping energy-saving technology: Hydrofoil amphibious vehicle driven by waterjet propulsion

The shipping industry is one of the crucial sources of carbon emissions. With carrying capacity both on land and water, the hydrodynamic characteristics of amphibious vehicles are essential indicators of energy saving and emission reduction. The hydrodynamic characteristics of an amphibious vehicle driven by a single waterjet propulsion are investigated in this study, together with the impact of the NACA0012 hydrofoil mounted at the bow and stern. Based on the RANS and body force methods, the hydrodynamic calculation models of the bare hull and waterjet-driven hull are established. All CFD calculations are based on STAR-CCM+. Based on the towing experiment, an uncertainty analysis is conducted to confirm the precision and legitimacy of the numerical calculation approach. The thrust deduction characteristics of vehicle hulls with different hydrofoil models are analyzed respectively. Small or negative thrust deductions always occur at medium and high speeds. Combined with the analysis of vehicle attitude, flow field, and pressure distribution, the inherent thrust deduction characteristics of the amphibious vehicle and the influence of hydrofoils are studied. Then, the hydrodynamic characteristics of different configurations are compared. The lift characteristics of hydrofoils and their influence on the vehicle's attitude and resistance are studied. The change in the performance of the stern hydrofoil caused by the waterjet propulsion is analyzed. Results show that the hull with two hydrofoils has the best drag-reduction effect when Fr<1.43, and the highest drag-reduction rate is 25.7%.

Cavitation flow and broadband noise source characteristics of NACA66 hydrofoil with a V groove on the suction surface

In the present work, a V groove on the suction surface is proposed to investigate the cavitation flow and noise sources of a NACA66 hydrofoil. The SST k-ω turbulence model and broadband noise source model is employed to analyze the flow pattern and noise distribution. Proper orthogonal decomposition (POD) is then used to investigate the coherent structure of cavitation flow around the hydrofoil. The results show that, under cloud cavitation conditions, the groove improves the energy characteristics of the hydrofoil and increases the lift-to-drag ratio by 2.95%. During the sheet cavitation growth phase, the groove can suppress the development of low-pressure, low-velocity regions on the hydrofoil surface, thus suppressing cavitation near the leading edge of the hydrofoil. During the cavitation collapse stage, the groove reduces the pressure impact of cavitation collapse on the hydrofoil. The groove extends a cavitation cycle and reduces the average cavitation volume during a cavitation cycle by 17.46%. Results indicate that during a cavitation cycle, the groove reduces the average dipole noise on the hydrofoil surface by 5.07% and the average quadrupole noise on the middle longitudinal section by 6.86%. POD results show that the first five orders of the modes capture more than 90% of the energy of the flow field. The primary frequency of the modal coefficients of Mode 1 and Mode 2 is the cavitation frequency, and the other frequencies are multiples of the cavitation frequency. The groove significantly affects Mode 2, which represents a change in velocity distribution in the flow field characteristics.

Research on pressure reconstruction of cavitation hydrofoil surface based on compressed sensing

The development of hydrofoil cavitation has obvious unsteady characteristics that induce complex pressure variation on the hydrofoil surface. In this work, an algorithm for the pressure reconstruction of the foil based on compressed sensing (CS) technology combined with proper orthogonal decomposition (POD) and particle swarm optimization (PSO) is proposed. Based on the dataset produced by numerical simulation, the pressure field reconstruction with the algorithm is demonstrated to have good accuracy, generalization, and robustness. Firstly, the impact of the number of truncated POD modes and measuring points on the error of the algorithm reconstruction is researched that the error shows a downward trend with the rise in these two elements. Then the sensitivity of the algorithm reconstruction to the hydrofoil characteristics is investigated. It is found that the reconstruction results are more accurate than other stages when the foil is attached to the cloud cavitation and the reconstruction accuracy of the algorithm is proportional to the stability of the flow field. Finally, it is shown that the algorithm can be applied for the reconstruction of different flow parameters of the same model. The placements of the measuring points learned by the algorithm are more likely to be trained to the relatively unstable region.

A Study on Vortex-Induced Vibration Characteristics of Hydrofoils considering High-order Modes

A Study on Vortex-Induced Vibration Characteristics of Hydrofoils considering High-order Modes

Tip Vortex Cavitation and Induced Noise Characteristics of Hydrofoils

Tip vortex cavitation is one of the most classical themes in fluid mechanics. Although many experimental and theoretical studies have been performed, unsolved problems still remain. In particular, the trailing vortices at the tip of the hydrofoil directly affects the hydrodynamic and acoustic performance of submerged objects such as the marine propeller, rudder and various foil-shaped appendages of the ship. In this study, the experimental results from the measurements of the vortex cavitation from the tip of two different three-dimensional hydrofoils are presented. Experiments have been carried out in Chungnam National University-Cavitation Tunnel (CNU-CT). By high speed imaging technique, the development process of vortex cavitation is observed in detail. Based on the high-speed images, physical features of the cavity inception and the swirling motion of the tip vortex cavity flow are examined. In addition, the induced noise characteristics in the vortex development process are examined by unsteady pressure measurements. The forces exerted on the hydrofoil were also measured using a dynamometer with a view to verify the scaling relation between the inception cavitation number and the non-dimensional parameters namely, the coefficient of lift, CL and the Reynolds number, Re. The results further shed light on the cause of the intense noise induced by tip vortex cavitation.

Low-frequency oscillation characteristics of flow for NACA66 hydrofoil under critical stall condition

Hydrofoil is the core unit of energy conversion for axial-flow pumps and hydro-energy turbines. The dynamic characteristics of hydrofoil significantly affect the operation stability of pumps and turbines, especially under the critical stall condition. In this paper, the unsteady flow field of NACA66 hydrofoil under critical stall condition at Reynolds number 0.75 × 10E6 is investigated by IDDES method. The low-frequency oscillations (LFO) under critical stall condition characterized by two typical frequencies with Strouhal number (St) 0.031 and 0.065 are reported in pressure pulsation and lift coefficient respectively. It is found that the merging phenomenon of separation vortices are the main reason of the wave peak of LFO with St 0.031, while the phenomenon of generation and shedding of separation vortices exist at the moment corresponding to the wave trough. The reason of the LFO of lift coefficient with St 0.065 is found to be the long-short switching of streamwise coverage of the leading edge separation vortex. The change of streamwise coverage is corresponding to the movement of the reattachment point of the separation area. The revelation of LFOs flow mechanism in hydrofoil under critical stall can provide reference for its unsteady flow control and applications.

LES study of unsteady cavitation characteristics of a 3-D hydrofoil with wavy leading edge

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.

Investigation of turbulence-induced quadrupole source acoustic characteristics of a three-dimensional hydrofoil

In order to investigate the turbulence-induced acoustic characteristics of hydrofoils, the flow and sound field for a model NH-15-18-1 asymmetric hydrofoil were calculated based on the mixed method of large eddy simulation (LES) with Lighthill analogy theory. Unsteady fluid turbulent stress source around the hydrofoil were selected as the inducements of quadrupole sound. The average velocity along the mainstream direction was calculated for different Reynolds numbers [Formula: see text]. Compared to experimental measurements, good agreement was seen over a range of [Formula: see text]. The results showed that the larger the [Formula: see text], the larger the vortex intensity, the shorter the vortex initial shedding position to the leading edge of the hydrofoil, and the higher the vortex shedding frequency [Formula: see text]. The maximum sound pressure level (SPL) of the hydrofoil was located at the trailing edge and wake of the hydrofoil, which coincided with the velocity curl [Formula: see text] distribution of the flow field. The maximum SPL of the sound field was consistent with the location of the vortex shedding. There were quadratic positive correlations between the total sound pressure level (TSPL) and the maximum value of the vortex intensity [Formula: see text] and velocity curl, which verified that shedding and diffusion of vortices are the fundamental cause of the generation of the quadrupole source noise.

Study of hydrofoil boundary layer transition using different turbulence models

Boundary layer transition is one of the key factors affecting the hydrodynamic characteristics of hydrofoil. In order to explore the capability of a transition model coupled to different turbulence models in the prediction of hydrofoil boundary layer transition, the flow field around the NACA0009 blunt-edge hydrofoil at 0°angle of attack was calculated using γ-Reθt transition model coupled with SST, SST-DES, SST-SAS, SST-DDES turbulence models. The characteristics of the hydrofoil boundary layer at ReL=2*106 are analysed. The experimental data and numerical calculation results are compared. The results show that the four turbulence models differ quite a bit in simulating hydrofoil boundary layer transition at the same calculation conditions. The flow parameters of the hydrofoil boundary layer calculated by the transition model coupled with SST-DDES are closer to the experimental results.

Flow field and cavitation characteristics of hydrofoils coated with hydrophilic and hydrophobic polymers

Tidal power turbines take advantage of tidal energy to generate renewable hydropower. Since the tidal turbines are fixed in the ocean, it is common to paint the blade and the structure of tidal energy generator with antifouling coating to prevent marine organisms from attaching to them. In this research, hydrophilic and hydrophobic coatings which are thought to be useful as countermeasures to prevent marine organisms’ adhesion are studied. We focused on the influence of the (hydrophilic and hydrophobic) coatings on the cavitation and flow field characteristics. The hydrophilic coated foil restrained the cavitation inception and growth compared to the hydrophobic coated foil from our experiment. And then, FFT was carried out on the pressure fluctuation measured in each coating foil, and the absolute value of the pressure fluctuation amount and the difference in the fluctuation period were clarified. And as another characteristic of the coated foils, the flow field near the coating surface was investigated. The velocity distribution near the foil’s surface was measured using Laser Doppler Velocimetry (LDV). In this experiment, a flat plate with or without the hydrophilic and hydrophobic coatings were used. As a result, differences in the boundary layer thickness and the velocity near the wall were revealed.

Cavitation on hydrofoils with sinusoidal leading edge

Cavitation characteristics of hydrofoils with sinusoidal leading edge were examined experimentally at a Reynolds number of 7.2 × 105. The hydrofoils had an underlying NACA 634-021 profile and an aspect ratio of 4.3. The sinusoidal leading edge geometries included three amplitudes of 2.5%, 5%, and 12% and two wavelengths of 25% and 50% of the mean chord length. Results revealed that cavitation on the leading edge-modified hydrofoils existed in pockets behind the troughs whereas the baseline hydrofoil produced cavitation along its entire span. Moreover, cavitation on the modified hydrofoils appeared at consistently lower angles of attack than on the baseline hydrofoil.

An experimental study on flow separation control of hydrofoils with leading-edge tubercles at low Reynolds number

Hydrodynamic characteristics of hydrofoils with leading-edge tubercles were experimentally investigated in a water tunnel at a Reynolds number of Re=1.4×104. Particle image velocimetry measurements and particle-streak visualizations reveal that the tubercles improve flow separation behaviour. In particular, hydrofoils with larger wave amplitudes and smaller wavelengths tend to perform significantly better in flow separation control. Cross-stream flow measurements indicate that streamwise counter-rotating vortex pairs are generated over the tubercles and mitigate flow separation. Analysis confirms that the tubercles function as vortex generators, due to their comparable heights relative to the boundary layer thickness. The vortex pairs meander and interact with adjacent flows, causing the flow separation behaviour to be occasionally unstable, thus leading to variable flow separation region sizes. This suggests that measures may have to be taken to ensure the stability of the counter-rotating vortex pairs for more persistent and predictable improvements.