Leading-edge Protuberances Research Articles

Optimization Design of Bionic Leading-edge Protuberances on Horizontal Axis Wind Turbine Blades

Abstract The complex operating environments of horizontal axis wind turbines has made the phenomenon of blade flow separation more obvious. As a passive flow control method, the bionic humpback whale leading-edge protuberances have been shown to effectively enhance the performance of the wind turbine. This study employs a multi-objective genetic algorithm and Computational Fluid Dynamics method to analyse the impact of leading-edge protuberances on the aerodynamic performance and flow characteristics of the blades. The results indicate a significant improvement in blade performance after optimization in the wind turbine’s output power. The streamwise vortices generated by the bionic protuberances not only reduce the suction surface pressure, but also suppress the spanwise flow over the blade surface. The length of the flow separation Region is reduced from 0.410 to 0.368 radius. In the process of wind speed change, the tangential forces on the bionic blade exhibit a wave-like variation. The bionic protuberance alters the pressure coefficient of the position. Compared with the original blade, the pressure coefficient of the trough sections on both sides of the bionic blade protuberance is improved. In this study, the bionic protuberance applied to the leading-edge of horizontal-axis wind turbine blades is proposed for the first time, and holds practical value for guiding practical applications.

Investigation of the decisive factor for the effectiveness of biomimetic protuberances during the stall process

This study investigates the influence of leading-edge protuberances (LEP) on the stall process of airfoils to identify the decisive factor in the effectiveness of LEPs. Owing to its clearly defined three-dimensional structure, an LEP introduces uncertainties into its effects on the stall angle of attack and the lift coefficient curve. This poses a challenge for several airfoils when universal stall control strategies are adopted. These problems were studied by classifying 12 symmetrical baseline airfoil types based on their blade thicknesses under the condition of Re = 180 000. These include thin airfoil stalls (TS), leading-edge stalls (LS), and a combination of leading-edge and trailing-edge stalls (CS). Two representative airfoils from each category were selected to study single-protuberance airfoils. The distribution of the suction surface momentum and evolution of the spanwise vortices revealed that the streamwise vortices induced by the LEP resulted in an attached flow. However, the impact of these flow patterns varies depending on the type of airfoil used. The TS and LS airfoils experienced an increase in the stall angle and maximum lift coefficient, resulting in an overall improvement in the airfoil performance. The CS airfoils are the most heavily influenced, experiencing a decrease in the maximum lift coefficient and exhibiting phenomena such as a one-sided stall and step-by-step stall. Finally, this paper proposes for the first time that the main factor influencing the different effects of protuberances is the stall type of the airfoil. This new knowledge can serve as a valuable reference for the implementation of protuberances in practical mechanical applications.

Effect of leading-edge protuberances on swept wing aircraft performance

Stall is a complex phenomenon in aircraft that must be suppressed during flight. As a novel passive control method, bionic leading-edge protuberances (LEPs) have attracted widespread interest, particularly for delaying stall. Bionic protuberances at the leading edge of airfoils have been designed to control stall and increase the stability of unmanned aerial vehicles during operation, and it is the flow control mechanism associated with this application that is investigated in this study. First, numerical simulations are conducted to obtain the aerodynamic characteristics of original and bionic airfoils based on the S1223 large-lift airfoil. Next, the impact of the LEP amplitude is investigated. Finally, a vortex definition parameter, the Liutex vector, is utilized to analyze the influence of LEPs on vortices. The results show that bionic LEPs inspired by those on humpback whale flippers can improve the aerodynamic performance of airfoils under the extreme conditions that exist after stall, resulting in an ∼22% increase in the lift–drag ratio. LEPs are found to segment the flow field near the wing surface. The flow becomes bounded between adjacent protuberance structures, significantly inhibiting the development of flow separation and providing a drag reduction effect. This study thus provides a new approach for improving aircraft performance.

Hydrodynamic performance of pitching foils with two leading-edge protuberances

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.

Study on the Influence of Protuberance Amplitude on the Performance of Vertical Axis Wind Turbines

Abstract The performance of vertical axis wind turbine (VAWT) is mainly affected by the dynamic stall process of the blade. Inspired by the anti-stall performance of the humpback whale flipper, leading edge protuberances (LEPs) are applied to 2-blade VAWT. Based on computational fluid dynamics simulation method, the effects of LEPs amplitude on VAWT performance are investigated when blade tip speed ratio is 2.19. The results show that the larger the amplitude of the LEP, the lower the average power coefficient. When the number of protuberances is 3, the average power coefficient of the VAWT with a protuberance amplitude of 6.625mm is 1.8% higher than prototype blade. Considering the impact of increasing the chord length of biomimetic blades, the average tangential force coefficient of all biomimetic VAWTs decreases. The peak section performance of the bionic blade decreases significantly while the trough section performance increases significantly. The difference between the peak section and trough section of the LEPS plays an important role in improving the performance. This study provides a reference for the application of bionic protuberances in VAWTs.

Numerical investigation on cavitation and induced noise reduction mechanisms of a three-dimensional hydrofoil with leading-edge protuberances

In this work, a National Advisory Committee for Aeronautics 66 hydrofoil with leading-edge protuberances is designed. The large eddy simulation combined with the Schnerr–Sauer cavitation model is used to obtain a satisfactory result as compared with the experimental measurement, integrating the permeable Ffowcs Williams–Hawkings equation for cavitation-induced noise analysis. It is found that the special leading-edge geometric structure deflects the incoming flow, creating two counter-rotating streamwise vortices at the peak shoulders. These lead to upwash and downwash effects and alter the pressure distribution on the suction side. The low pressure localized in the trough facilitates the advancement of the leading-edge cavitation while severely limiting the spanwise development of the cloud cavity, shortening the cavitation evolution by about 20% and reducing the maximum cavitation volume by about 35%. Analysis using the vorticity transport equation indicates that different vorticity transport equation splitting terms play dominant roles at different stages of cavitation evolution. Although the cavitation induces disturbances in the primary vortex, the effect is limited. Acoustic simulation shows that the bionic structure can reduce the total sound pressure level by 7.8–8.3 dB. The spherical noise reduction is not as effective as expected due to the similar cavitation volume acceleration processes of the two hydrofoils. However, the pressure fluctuation caused by the collapse of the cloud cavity is reduced by cavitation suppression, which reduces the linear noise. In addition, the protuberances suppress the generation of large-scale vortex systems and transform them into smaller ones, resulting in reduced spanwise correlation and coherence of the shedding vortices. This is a critical factor in noise reduction. Finally, we hypothesize that the unstable noise reduction is related to the streamwise vortices in the trough regions. These vortices increase the momentum exchange within the boundary layer, affecting its stability and weakening the acoustic feedback loop.

Effect of protuberances on the aerodynamic performance of a wind turbine blade – a review

ABSTRACT This paper thoroughly reviews the influence of protuberances on the performance of a wind-turbine blade in terms of aerodynamic characteristics, power generation capability, and noise emission. It is observed that the protuberances are significantly useful in improving the aerodynamic performance of wind turbine blades, especially in the post-stall regime and during fluctuating wind conditions, such as wind gusts. The protuberance amplitude was found to be the dominant parameter to influence the blade performance. The amplitude and wavelength were tested in the range of 2–12%c and 2–50%c, respectively. The highest wind turbine performance was observed at small amplitude and large wavelength in the pre-stall regime and at large amplitude and small wavelength in the post-stall regime. Incorporating leading edge protuberances may increase lift and power coefficients by 50 and 55%, respectively, and reduce noise emission by nearly 16 dB. With the increase in the angle of attack, the instability noise was found to change its form. The broadband noise forms at a lower angle of attack (~2°) which converts to the tonal noise at the angle of attack, 6.5°. The protuberances substantially alter the boundary layer flow, subsequently modifying the pressure and velocity field over the suction surface of the blade. A pair of vortices forms across a protuberance, which exchanges momentum with the boundary layer flow and helps delay/eliminate the flow separation. This flow separation control is responsible for the benefits as mentioned above.

Control mechanisms of different bionic structures for hydrofoil cavitation

Cavitation limits the efficient and stable operation of rotating machinery. The exploration of control methods for hydrofoil cavitation is important for improving the performance of hydraulic machinery. The leading-edge protuberances of the humpback flipper and the spine structure of the tail fin of sailfish are two common bionic structures for cavitation control; however, the control effects of both have limitations. Accordingly, in this study, a passive control method for hydrofoil cavitation was developed by combining the two bionic structures. With the large eddy simulation method, the cavitation processes of wavy leading-edge hydrofoil, bionic fin spine structure hydrofoil, and novel bionic combined structure hydrofoil were studied under a cavitation number of σ = 0.8. The control mechanisms of the three bionic structures for the hydrofoil cavitation were investigated. The results indicated that the novel bionic combined hydrofoil realised the superposition and complementation of the control effects of the two single bionic structures and achieved a better cavitation inhibition effect, reducing the total volume of cavitation by 43 %. In addition, it enhanced the stability of the flow field and reduced the standard deviation of the pressure coefficient on the suction surface by up to 46.55 %. This research provides theoretical support for the optimisation and modification of the blades of hydraulic machinery, such as propellers and pump turbines.

Research on the aerodynamic performance of the wind turbine blades with leading-edge protuberances

The aerodynamic performance loss of wind turbine blades caused by severe flow separation is one of the main reasons for hindering the utilization of offshore wind energy. The leading-edge protuberances (LEPs) have been shown to be an effective passive control method with suppressed flow separation. This paper proposes a method for predicting the effect of the LEPs on the aerodynamic performance of the wind turbine airfoil and develops an optimized design method for the parameters of the LEPs. The GH-Bladed platform is used to verify the influence of the LEPs method on the load performance of the blades. The study found that the developed method can realize the optimization design of the aerodynamic performance for the wind turbine airfoil with LEPs, and obtain the structural parameters of the LEPs to improve the value CL/CD by 2%–11% in the post-stall region. Moreover, the average load and the standard deviation of the load for the blades with LEPs are reduced by 7% and 17% respectively near the rated wind speed of 8.7 m/s. This study realized the feasibility of the optimal design of the wind turbine airfoil with LEPs and verified the effectiveness of the LEPs technology applied to wind turbine blades.

Various Wavy Leading-Edge Protuberance on Oscillating Hydrofoil Power Performance

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–ε 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.

Application of wavy leading edge to enhance winglet aerodynamic performance

AbstractThe wavy leading edge (WLE, also known as leading edge protuberances) is a passive flow control device inspired by the humpback whale pectoral flippers. It reduces the flow of three-dimensional effects on wings and increases their aerodynamic performance at high angles of attack. Despite the numerous studies on its aerodynamic benefits, research on its possible applications is still incipient. Therefore, this article addresses an evaluation of the WLE effects on the aerodynamic performance of a winglet. A rectangular wing, a base smooth leading edge winglet, and a winglet with WLE were designed and manufactured for CFD simulations and wind tunnel measurements. The winglet with WLE increased the maximum aerodynamic efficiency, i.e. this configuration reduced the induced drag by increasing wingtip vortex dissipation at a given angle-of-attack. Such results were used in re-evaluations of the aerodynamic performance of an original agricultural aircraft initially configured with a multi-winglet device. The winglet with WLE showed to be effective at increasing the aircraft operational time and range under a simulated actual condition.

Comparative Study on the Effect of Leading Edge Protuberance of Different Shapes on the Aerodynamic Performance of Two Distinct Airfoils

This study investigated the effect of leading-edge protuberances on the aerodynamic performance of two distinct airfoils with low Reynold’s number (Re): E216 and SG6043. Three protuberance shapes, namely sinusoidal, slot, and triangular, were considered. The amplitudes (A) of protuberances considered were 0.03c, 0.06c, and 0.11c, and the wavelengths (W) were 0.11c, 0.21c, and 0.43c, where c is the chord of the airfoil. The numerical and experimental analyses were performed in the angle of attack (AoA) range of 0° to +20° at and Re of 105. The numerical investigation was performed using the commercial computational fluid dynamics package ANSYS FLUENT. The SST k-ɷ model was used to simulate turbulent flow. The experimental force measurements were conducted using a highly sensitive three-component force balance in a subsonic wind tunnel facility. The flow physics was analyzed using vorticity contours in streamwise and spanwise slices and static pressure distribution contours. The smoke flow visualization technique was used to observe flow streamlines, boundary layer separation, and reattachment over the airfoil surface. The result indicated that the triangular and slot protuberances were the most beneficial for improving poststall lift and reducing skin friction drag. The operating mechanism involved a shift in pressure distribution due to leading-edge alterations and flow energization by secondary flow emanating from the protuberances.

Experimental investigation on cavitation and induced noise of two-dimensional hydrofoils with leading-edge protuberances

The applicability of leading-edge protuberances as a passive flow control approach inspired by humpback whale flippers has attracted significant research attention in aquatic and aeronautic systems because of their influence on critical hydrodynamic and aerodynamic aspects. An experimental investigation is conducted in a cavitation tunnel under various flow conditions to determine the effectiveness of leading-edge protuberances in controlling the detrimental effects of cavitation and suppressing flow-induced noise. The experiments are carried out on four National Advisory Committee for Aeronautics airfoil 0012 hydrofoils at 7° attack angles and free stream velocities up to 10 m/s. One of the four hydrofoils is considered the baseline, while the other models have wavy leading-edge modifications with different sinusoidal protuberances. These geometry modifications are defined by the amplitudes (A) (2% and 4% of the mean chord length) and wavelengths (λ) (12.5% and 25% of the mean chord length) of the sinusoidal protuberances. Investigations of flow over hydrofoils from top and side views at various Reynolds numbers exhibit that cavitation first appears in the modified hydrofoils' troughs and is restricted to just behind the protuberance troughs for the entire cavitating flow range. These results contrast the baseline geometry, where cavitation inception occurs at the flat leading edge, and the sheet cavity expands spanwise with extensive cloud shedding. Image processing under certain conditions reveals that the protuberances reduce cavitation by 25%–60%. The analysis of the sound pressure level demonstrates that the leading-edge protuberances effectively decrease flow-induced noise at higher flow velocities when cavitation is the dominant noise source. Finally, the direct comparison of cavitating flow characteristics, quantitative cavitation measurements, and noise production analysis between the baseline and modified hydrofoils, and their comparison among the modified geometries, provides a significant reference for future modeling of potential applications employing this passive flow control technique.

Effect of the leading-edge protuberances on the aeroacoustic and aerodynamic performances of the wind turbine airfoil

Based on the restraining effect of the leading-edge protuberances (LEPs) on flow separation, this work focuses analyzing the aeroacoustic and aerodynamic performances for the wind turbine airfoil with LEPs near the outboard region of the blades, by using the experimental testing and numerical calculation methods. The associated flow patterns for the aeroacoustic and aerodynamic performances are revealed within the pre-stall and post-stall regions. The shedding of the separation vortices enhances the instability of the laminar boundary layer, generating the aerodynamic noise from the wind turbine airfoil. The LEPs suppress the generation of the tonal noise in pre-stall region by changing the distribution and reducing the intensity of the trailing edge shedding vortices. Moreover, LEPs can improve the aerodynamic performance of the wind turbine airfoil greatly by inhibiting the flow separation at a high inflow angle of attack. As a flow control method for the wind turbine blades, LEPs will help to improve the aeroacoustic performance of the blades under normal inflow wind conditions, and the aerodynamic performance under extreme wind situations where the inflow wind changes on a large scale frequently.

Hydrofoil geometry and leading-edge modifications: Influence of section profile, aspect ratio, and sweep

Modifications mimicking the leading-edge protuberances on the pectoral fin of humpback whales have been widely adopted to the designs of foils and control surfaces to delay stall and provide post-stall superior lifting characteristics. However, the efficacy of these modifications in flow control and lift augmentation depends on the wing geometry. The present work investigates the lift, drag, and flow characteristics of finite-span hydrofoils with different section profiles-both symmetric and cambered (NACA 0012, NACA 0018, NACA 634-021, NACA 2412, and NACA 4415) having twin-protuberances over their leading-edge at a Reynolds number of 2 × 105. The influence of aspect ratio (1, 2, and 3) and leading-edge sweep angle (0 deg, 15 deg, and 26.1 deg) on the hydrodynamic performance of modified foils is also investigated. The characteristic feature of twin-protuberance foil designs is the restriction of flow separation between the chordwise vortices shed from the two protuberances in certain post-stall regimes. The use of leading-edge modifications is observed to be more conducive for lift enhancement in thin foil sections, especially NACA 0012, having a smaller stall angle. Considering the other parameters, leading-edge protuberances are observed to be advantageous for hydrofoils with aspect ratios 2 and 3 at post-stall angles of attack, and for 0 deg and 15 deg sweep angles for post-stall lift enhancement. The influence of Reynolds number on the performance modifications is also investigated.

Passive control of dynamic stall in a H-Darrieus Vertical Axis Wind Turbine using blade leading-edge protuberances

Dynamic stall flow conditions affect the performance of most Vertical Axis Wind Turbine (VAWT) designs; however, the associated losses are especially relevant in smaller turbines, such as those typically used in urban environment applications. The work described in this paper experimentally evaluated the effectiveness of leading-edge protuberances in controlling the aforementioned stall behavior. A numerical study was first performed to define the leading-edge geometry to be subsequently tested in the wind tunnel. A custom experimental setup was also developed for this purpose. The wind tunnel measurements of the modified turbine showed significant performance gains over the baseline and a considerably improved self-starting behavior. The power coefficient increase was between 46% and 20% for wind velocities ranging from 5.5 m/s to 9 m/s, respectively. Nevertheless, the tip-speed ratio characteristics of the studied turbine were not meaningfully affected by the use of leading-edge protuberances.

Wake reduction in horizontal axis wind turbine: A review of advancements in techniques

Renewable energy systems have experienced exponential growth toward producing zero-carbon energy. In this context, wind turbines continue to play a significant role. To extract maximum power, wind turbines are installed in array-like formation in wind farms. This arrangement though beneficial, also leads to detrimental wind-wakes effects. The wakes reduce wind speed and induce undesirable turbulence of flow field in downstream direction. To address this issue, several innovative techniques have been proposed. This paper surveys the best methods by classifying these into passive and active techniques. The passive techniques affect the wake flow by modifying only the geometrical or operating characteristics of wind turbines such as adjustment of forward and backward sweeps, etc. The active techniques use additional surfaces/devices for wake handling such as vortex generators, leading edge protuberances, dual-rotors, and cross-axis wind turbines etc. Additionally, this paper also reviews various wake measurement methods and recommends the best suited technique.

Hydrodynamic Performance of Pitching Foils with Two Leading-Edge Protuberances

Hydrodynamic Performance of Pitching Foils with Two Leading-Edge Protuberances

LES investigation of the cavitating hydrofoils with various wavy leading edges

In the current paper, large eddy simulation is utilized to study the effect of various wavy leading edges on the control of cavitation around a modified NACA634-021 hydrofoil in the case of equal lift. Four sinusoidal wavy leading edges with different geometry parameters are numerically examined, with the baseline hydrofoil as a comparison. The numerical results are compared with available experimental data and a reasonable agreement is observed. The key flow characteristics, e.g., quasi-periodic behavior of cavity, time histories of pressure fluctuations and the effect of cavitation on streamwise vortices are compared in detail. The results show that in the case of equal lift, which is achieved by adjusting the angle of attack, no significant decrease in the volume of cavity is observed for the cases of wavy leading edges. This illustrates that none of the four different types of wavy leading edges introduced in this paper can appreciably suppress the generation and development of cavitation bubbles. However, our results indicate that the pressure fluctuations can be effectively suppressed, which is primarily caused by the compartmentalization effect of the leading edge protuberances. Meanwhile, the suppression capability decreases with an increasing amplitude-to-wavelength ratio. Further analysis shows that cavitation is the main source of affecting the development of the streamwise vortices.

Hydrofoils with Differing Leading-Edge Protuberances: CFD and Experimental Investigations

Leading-edge protuberances on the pectoral fin of humpback whales have been widely adopted to the designs of foils to provide superior lifting characteristics in the post-stall regimes. The present work investigates the lift, drag and flow characteristics of finite-span rectangular hydrofoils having different configurations of two protuberances over the leading edge with NACA 634-021 as the base design section. The results obtained from CFD analyses are validated using lift and drag measurements from experiments. The influence of using a transition-sensitive turbulence model on the results is investigated. It is observed that, in general, a foil with smaller separation between protuberances has better post-stall lift characteristics whereas that with protuberances at larger separation have better pre-stall characteristics. Depending on the separation between them, streamwise vortices are generated from the leading-edge protuberances. The two protuberances can restrict the zone of separation between them at high angles of attack. The influence of Reynolds number on the lifting performance is also investigated.