Leading-edge Protuberance Research Articles

Vibration control of the straight bladed vertical axis wind turbine structure using the leading-edge protuberanced blades

This study investigates the impact of structural responses resulting from alterations in aerodynamic characteristics caused by incorporating leading-edge protuberance (LEP) blades in vertical-axis wind turbines (VAWTs) under various wind speeds. The current experimental investigation was conducted on a scaled-down straight-blade VAWT with multiple wind speeds at a fixed pitch angle and uniform blade configuration. The unsteady structural excitation caused by the decay of vortices due to the different aerodynamic loads and its effect on the VAWT structure is examined in great detail. The presence of counter-rotating vortices from the LEP profile, however, enhances the aerodynamics of the blades at higher angles of attack. It is crucial to understand the structural characteristics resulting from changes in aerodynamics. The paper presents the effects of the LEP on the structural excitation of the VAWT by analyzing the acceleration and displacement data obtained from the static structure of VAWT for various operational wind speeds. The results showed decreased acceleration and displacement amplitudes for the VAWT with LEP at medium and higher wind speeds. Significant aerodynamic and structural damping was observed in the VAWT with LEP blades. The results obtained were in close agreement with the time and frequency domains of the measured displacement and acceleration from the structure. The bidirectional vibration control was observed for the VAWT with LEP, which was found to have a reduction ratio of 70 %.

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.

A study on separation control mechanisms of tandemly arranged wing with linear leading edge and leading edge protuberance

A study on separation control mechanisms of tandemly arranged wing with linear leading edge (LLE) and leading edge protuberance (LEP) in low Reynolds number was conducted. In this study, to investigate the separation control mechanism, we carried out the visualization experiment using smoke-wire method, the streamwise and spanwise velocity measurements above the upper surface of the backward wing by using hotwire anemometer, and the numerical simulation. From the results of flow visualization experiment, it was found that the high lift coefficient obtained by tandemly arranged wing with LLE and singly and tandemly arranged wing with LEP were caused by reducing the flow separation region. From the results of velocity measurements and numerical simulation, it was found that in the case of wing with LLE, the turbulence generated by forward wing, which had mainly streamwise component with peak frequency of 230Hz and broadband frequency over than 30Hz, controled the flow separation. Whereas in the case of wing with LEP, regardless of the wing arrangement, separation control was brought by the separation bubble that caused by the longitudinal vortices generated by the LEP. It was also found that the turbulence from forward wing affected the position of the valley part where the separation bubble and the strong negative pressure were formed.

A study on unsteady fluid-dynamic force characteristics of a pitching wing with leading edge protuberance

A study on unsteady fluid-dynamic force characteristics of a pitching wing with leading edge protuberance

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.

Effects of Reynolds Number and Protuberance Amplitude on Twin-Protuberance Airfoil Performance

A stall occurs in the flow around an airfoil when the angle of attack reaches a critical value, leading to a dramatic drop in airfoil performance. Recently, a passive leading-edge protuberance control method, inspired by the fin of a humpback whale, has demonstrated obvious advantages in improving airfoil stall. Hence, this study experimentally and numerically investigates the aerodynamic characteristics of an airfoil with twin protuberances at the leading edge. First, the aerodynamic characteristics were investigated for six Reynolds numbers via wind-tunnel experiments, obtaining the hysteresis caused by the test sequence. Then, numerical simulations were performed using a transition shear stress transport turbulence model, and the numerical accuracy was verified. Finally, the influence of structural parameters on stall behavior was revealed by changing the amplitude of the protuberance. The results indicate that the twin-protuberance airfoil produces step-by-step and one-side stall phenomena. The influence of vortices on the movement of separation bubbles on the lower surface is presented. The critical angle of attack for stall increases with increasing Reynolds number; its value for deep stall increases by more than 2 deg. As the protuberance amplitude is reduced, the influence on the airfoil flow is weakened, and the first stall angle increases.

Effect of solidity on the dynamic behaviour of the Darrieus turbine with leading-edge protuberance

The dynamic behaviour of the straight–type Darrieus turbine with leading-edge protuberance (LEP) was analysed under various solidity ratios at several tip speed ratios through experiments. The Darrieus turbine is a type of Vertical Axis Wind Turbine (VAWT) which uses wind energy to generate electricity. This type of turbine was subjected to vortex-induced and buffeting types of vibrations. These vibrations were more sensitive to the number of blades and tip speed ratios. Based on the experimental measurements, the results revealed that, at a low tip speed ratio, the four-bladed turbine exhibits lesser vortex-induced vibrations than those of the three and five-bladed turbines. However, at a high tip speed ratio, the three-bladed configuration operates well against the vortex-induced vibrations. In the case of buffeting, a three-bladed turbine diminishes the dynamic oscillations at both low and high tip speed ratios, whereas the four and five-bladed turbines induce dynamic oscillations at slightly higher amplitudes. However, the amplitude of buffeting is smaller than those of vortex-induced vibrations.

Effects of Wavy Leading-Edge Protuberance on Hydrofoil Performance and Its Flow Mechanism

This paper examines the effects on a Clark-y three-dimensional hydrofoil of wavy leading-edge protuberances in a quantitative and qualitative way. The simulation is accompanied by a hybrid RANS-LES model in conjunction with Zwart-Gerber–Belamri model. Detailed discussions of the stable no-cavitating, unsteady cavitating flow fields and the control mechanics are involved. The force characteristics, complicated flow behaviors, cavitation–streamwise vortex interactions, and the cavitating flow instability are all presented. The results demonstrate that protuberances acting as vortex generators produce a continuous influx of boundary-layer vorticity, significantly enhancing the momentum transfer of streamwise vortices and therefore improving the hydrodynamics of the hydrofoil. Significant interactions are described, including the encouragement impact of cavitation evolution on the fragmentation of streamwise vorticities as well as the compartmentation effect of streamwise vorticities binding the cavitation inception inside the troughs. The variations in cavitation pressure are mainly due to the acceleration in steam volume. In summary, it is vital for new hydrofoils or propeller designs to understand in depth the effects of leading-edge protuberances on flow control.

A Review on Modeling of Bionic Flow Control Methods for Large-Scale Wind Turbine Blades

Due to complicated working conditions, the normal operating large-scale wind turbine blades are often suffering from some inevitable problems, i.e., friction adhesion, flow separation and acoustic noise, which may significantly affect the aerodynamic performance of the blades and thus the wind turbine system. Therefore, effective measurements must be taken to solve these issues. Correspondingly, several novel bionic flow control methods by mimicking shark skin, whale fin and owl wing, i.e., riblet, leading-edge protuberance and trailing-edge serration, have been recently studied, and good progresses have been made in terms of effectiveness, analysis and mechanism. However, these potential techniques are unable to be widely applied within wind energy community due to the lack of reasonable modeling methods, clearly reflecting the effect of bionic structures on the flow field around, which results in incapability to carry out further optimal design of bionic blade. To this end, this review paper first concentrated on a summary of the control mechanisms of three bionic techniques. Based on this, some feasible ideas of model buildup were proposed. Finally, the flow analyses around the typical blade airfoils were chosen as case studies to verify the feasibility and accuracy of these simulation methods.

Aerodynamic performance improvements of a vertical axis wind turbine by leading-edge protuberance

A detailed numerical study has been carried out to investigate the effects of leadingedge protuberance as a novel flow-separation control-technique for vertical-axis-windturbine (VAWT). The aerodynamic performance of a stationary protuberanced blade and an associated H-type Darrieus VAWT made of three protuberanced blades were investigated using unsteady Reynolds-Averaged-Navier-Stokes (URANS) and Implicit Large Eddy Simulation (ILES) methods. The current study involves a comprehensive set of five different types of sinusoidal leading-edge protuberances with three different wavelengths and amplitudes. It is found that this passive flow-control-method can favourably change lift and drag at stall and post-stall regions, which is of high importance for VAWTs. Static stall variation can be changed from leading-edge-stall type to trailing-edge-stall type. This is attributed to a “bi-periodic” phenomenon over the blade suction side and a reduced turbulence kinetic-energy production. Protuberance amplitude was found to be more important than the wavelength in improving aerodynamic performance. Of all the tested cases, the protuberance with amplitude of 1% chord-length (c) and wavelength of 2.5%c behaves the best. The leading-edge protuberance can significantly increase VAWT power-coefficient at low tip-speed-ratios, which are dominated by post-stall conditions. The dynamic stall was delayed and the coherent spanwise wavy flow-structures on the blade suction-side were observed.

Aerodynamic load control on a dynamically pitching wind turbine airfoil using leading-edge protuberance method

The aerodynamic loads of wind turbine blades are substantially affected by dynamic stall induced by the variations of the angle of attack of local airfoil sections. The purpose of the present study is to explore the effect of leading-edge protuberances on the fluctuation of the aerodynamic performances for wind turbine airfoil during dynamic stall. An experimental investigation is carried out by a direct force measurement technique employing force balance at a Reynolds number Re = 2 × 105. The phase-averaged and instantaneous aerodynamic loads of the pitching airfoil, including the baseline and the wavy airfoil, are presented and analyzed. The phase-averaged results indicate that the effects of dynamic stall for the wavy airfoil can be delayed or minimized compared to the baseline airfoil, and the negative damping area of the wavy airfoil is significant decreased in full-stall condition. These effects of leading-edge protuberances are more notable at a higher reduced frequency. For the instantaneous aerodynamic loads of the wavy airfoil, there is an observable reduction in fluctuations compared with baseline case. Furthermore, spectral analysis is applied to quantitatively undercover the nonstationary features of the instantaneous aerodynamic loads. It is found that the leading edge protuberances can reduce the harmonics of the aerodynamic force signal, and enhance the stability of the aerodynamic loads under different reduced frequencies. In conclusion, leading-edge protuberances are found effective to reduce the fluctuation characteristics of the aerodynamic loads during the dynamic stall process, and help to improve the stability and prolong the service life of the wind turbine blades.

Flow control on wind turbine airfoil affected by the surface roughness using leading-edge protuberance

In recent years, leading-edge protuberances have attracted plenty of attention as a passive method to manipulate flow separation on wind turbine blades. In this paper, an experimental investigation is carried out to explore the effect of leading-edge protuberances on the aerodynamic performance of wind turbine airfoils with and without surface roughness. A direct force measurement technique with the application of three-component balance is applied, and a hot-wire anemometer is used to measure the boundary layer flow to understand the governing mechanism for the flow regulation of the leading-edge protuberances. The current results confirm that the surface roughness leads to a decreased lift-to-drag ratio and an increased aerodynamic fluctuation of the baseline airfoil, which may cause severe losses in wind power capture. For the wavy airfoil, the leading-edge protuberance has facilitated the suppression of flow separation and the improvement of the airfoil aerodynamic performance in the poststall region. The results of the velocity measurement indicate that the leading-edge protuberances weaken the vortex shedding on the airfoil, which has a significant impact on the fluctuation of the aerodynamic force. For airfoils with a rough surface, the vortex shedding energy close to the trailing edge is evidently weakened for the wavy airfoil, which contributes effectively to the suppression of the shedding of the separated vortex and improve the stability of the aerodynamic force, compared to the baseline case. The effectiveness of leading-edge protuberances in the improvement of the performance stability may facilitate the service life of the blade and the ability of wind power capture.

Suppression of hump characteristic for a pump-turbine using leading-edge protuberance

The application of wave guide vanes with bio-inspired leading-edge protuberances to the hump characteristic of a pump-turbine is examined in this study. Numerical simulation with a shear-stress transport turbulence model is used to calculate the three-dimensional flow in a pump-turbine in pumping mode. Three tubercle amplitudes of 0.02c, 0.04c, and 0.08c (c is chord length), and three spanwise wavenumbers (2/s, 4/s and 8/s, s is the length of span) for guide vanes are especially considered. The results obtained show that the simulated performances of original guide vanes are found to be in good qualitative agreement with experimental data, supporting the validation of the computational fluid dynamics method. For different wave guide vanes with leading-edge protuberances, it is shown that the hump characteristic of the pump-turbine in pumping mode is effectively improved. This is due to improved flow fields below the tongue in view of entropy production and vector field. The energy loss can be clearly compared through the entropy distribution for different locations of the guide vanes, and it is improved for the wave guide vanes with bio-inspired leading-edge protuberances. For current pump-turbine, the optimal amplitude and wavenumber are found to be around 0.04c and 4/s.

Experimental and theoretical investigations on the effect of a single leading-edge protuberance on airfoil performance

Geometrical modification of airfoils by leading-edge protuberances has attracted plenty of attention as a passive separation control method. In this research, the effect of a single leading-edge protuberance with different amplitudes on the performance of a two-dimensional airfoil (NACA 634-021) is investigated through wind tunnel experiment and theoretical analysis. Force measurement experiment shows that all the modified airfoils demonstrate a special two-step stall performance. When the first step of stall occurs, the lift coefficient of the modified airfoils drops to a medium value between the pre-stall and post-stall lift coefficients of the baseline airfoil and keeps nearly constant within a range of AOA (angle of attack). Tuft visualization reveals that during the first step of stall, the flow pattern on different sides of the protuberance is distinctly different, respectively, showing leading-edge separation and trailing-edge separation, which are compartmentalized by the attached flow originated from the peak of the protuberance. The flow control mechanism is contributed to the combined effect of the early occurrence of the leading-edge separation around the protuberance root, the inherent hysteretic performance, and the flow compartmentalization effect. A theoretical model based on the lifting line theory, which takes the amendment of the hysteresis and compartmentalization effects into consideration, is proposed to analyze the performance of the modified airfoils. The sectional and overall performances by theoretical analysis are highly consistent with the tendency of the experiment results. Based on the theoretical model, the spanwise circulation gradient induced by the protuberance is considered to be responsible for the earlier separation around the protuberance roots at pre-stall AOAs. Further investigation on the effect of the geometrical parameters of the protuberance reveals that the amplitude-to-wavelength ratio plays a dominant role on the performance of the modified airfoils. The special flow phenomenon induced by the single leading-edge protuberance discovered in this research and the proposed mechanism provide an essential point of view for fully understanding the combined mechanism of multiple protuberances.

Effect of the Reynolds Number on the Performance of a NACA0012 Wing with Leading Edge Protuberance at Low Reynolds Numbers

A study on the improvement of the NACA0012 wing performance in the low Reynolds number (Re) region of 10,000 ≤ Re ≤ 60,000 using a leading edge protuberance (LEP) was conducted. In this study, we focused on the effects of varying Re on the effectivity of the LEP. Wind tunnel tests and numerical simulations were performed to measure the wing performance and to visualize the flow structures around the airfoil. In the higher angle of attack region of α > 10∘, LEP causes the lift to increase independently of Re. Furthermore, in the lower angle of attack region, i.e., α ≤ 10∘, with Re ≤ 20,000, the lift coefficient also increases when using the LEP. The separation control is caused by the streamwise vortices generated by LEP which provides the momentum from the freestream flow into the separation region near the valley section, and the mechanism was basically the same at all Reynolds numbers and angle of attacks used in this study.

Effect of a Single Leading-Edge Protuberance on NACA 634-021 Airfoil Performance

Leading-edge protuberances on airfoils or wings have been considered as a viable passive control method for flow separation. In this paper, the aerodynamic performance of a modified airfoil with a single leading-edge protuberance was investigated and compared with the baseline NACA 634-021 airfoil. Spalart–Allmaras turbulence model was applied for the numerical simulation. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foil decreased and the decline of the lift coefficient became mild. The poststall performance of the modified airfoil was improved, while the prestall performance was declined. Asymmetric flows along the spanwise direction were observed on the modified airfoil, and the local region around one shoulder of the protuberance suffered from leading-edge separation at prestall angles of attack, which may be responsible for the performance decline. At poststall angles of attack, the attached flows along the peak of the protuberance with a sideward velocity component would help improving the total performance of the airfoil. Experimental visualization methods, including surface tuft and smoke flow, were performed, and the asymmetric flow pattern past the protuberance was successfully captured. This specific phenomenon may be largely related to the formation of the biperiodic condition and other complicated flow patterns induced by multiple leading-edge protuberances. The formation mechanism and suppression method of the symmetry breaking phenomenon should be investigated more deeply in the future to guide the practical application of this passive control method.

Two-Step Stall Characteristic of an Airfoil with a Single Leading-Edge Protuberance

An experimental investigation has been conducted to examine the influence of a single leading-edge protuberance on the performance of a baseline NACA airfoil. Static aerodynamic forces of the airfoils have been measured as the angle of attack increased and decreased. A hysteresis loop of the lift coefficient around the stall angle is observed for the baseline airfoil, with a sharp decline of the lift coefficient during angle-of-attack increase and a delayed recovery during angle-of-attack decrease. It was discovered for the first time that both the decline and the recovery of the lift coefficient of the modified airfoil with a single leading-edge protuberance included two steps. When the first step happened, the lift coefficient reached an intermediate value between the maximum value and the poststall value of the baseline airfoil, and it remained almost constant within a range of angles of attack. Surface tuft visualization revealed that, during the first step of stall, one side of the modified airfoil was stalled with leading-edge separation, whereas the other side remained in a nonstall condition. When the second step of stall happened, both sides of the modified airfoil were stalled, except for an attached flow on the protuberance peak. This one-sided stall phenomenon indicated that one important role of the leading-edge protuberance might be confining the local stall region from extending spanwise, which is similar to the effect of a wing fence.

Experimental study of flow separation control on a low-Re airfoil using leading-edge protuberance method

An experimental study of flow separation control on a low-Re c airfoil was presently investigated using a newly developed leading-edge protuberance method, motivated by the improvement in the hydrodynamics of the giant humpback whale through its pectoral flippers. Deploying this method, the control effectiveness of the airfoil aerodynamics was fully evaluated using a three-component force balance, leading to an effectively impaired stall phenomenon and great improvement in the performances within the wide post-stall angle range (22°–80°). To understand the flow physics behind, the vorticity field, velocity field and boundary layer flow field over the airfoil suction side were examined using a particle image velocimetry and an oil-flow surface visualization system. It was found that the leading-edge protuberance method, more like low-profile vortex generator, effectively modified the flow pattern of the airfoil boundary layer through the chordwise and spanwise evolutions of the interacting streamwise vortices generated by protuberances, where the separation of the turbulent boundary layer dominated within the stall region and the rather strong attachment of the laminar boundary layer still existed within the post-stall region. The characteristics to manipulate the flow separation mode of the original airfoil indicated the possibility to further optimize the control performance by reasonably designing the layout of the protuberances.

Flow past a delta wing with a sinusoidal leading edge: near-surface topology and flow structure

The near-surface flow structure and topology on a delta wing of low sweep angle, having sinusoidal leading edges of varying amplitude and wavelength, are investigated using a stereoscopic technique of high-image-density particle image velocimetry at a Reynolds number of 15,000. Identification of critical points, in conjunction with surface-normal vorticity and velocity, provides a basis for determining the effectiveness of a given leading edge. At high angle of attack, where large-scale three-dimensional separation occurs from the wing with a straight leading edge, an amplitude of the leading-edge protuberance as small as one-half of one percent of the chord of the wing can substantially alter the near-surface topology. When the amplitude reaches a value of four percent of the chord, it is possible to completely eradicate the negative focus of large-scale, three-dimensional separation, in favor of a positive focus of attachment. Moreover, alteration of the near-surface topology is most effective when the ratio of the wavelength to amplitude of the sinusoidal leading edge is maintained at a small value.