Leading Edge Serrations Research Articles

On the acoustic optimality of leading-edge serration profiles

Leading-edge serrations are studied extensively as a way of reducing leading-edge noise and have been shown to be able to reduce leading-edge noise significantly. Previous experiments showed that different serration geometries have different noise reduction capabilities. However, the optimal serration geometry has not been known. Consequently, there are no guides that can be used at the design stage of serrations. In this paper, by performing an asymptotic analysis, we show that in order to achieve greater noise reduction in the high frequency regime (k1h ≫ 1, where k1 denotes the dimensionless streamwise hydrodynamic wavenumber and h half of the dimensionless root-to-tip amplitude of serrations), the serration profile cannot have stationary points. Therefore, piecewise smooth profiles free of stationary points are more desirable. Moreover, we show that greater noise can be achieved in the high frequency regime by using serrations that are sharper around the non-smooth points. The underlying physical mechanisms of these findings are discussed. Based on these findings, a new type of serration profile is proposed, and analytical model evaluations confirm its improved acoustic performance in the frequency range of interest. At low frequencies, a slight deterioration may be expected, but this is often negligible. To verify the conclusion drawn from the analysis, we perform an experimental study to investigate the acoustic performance of this new serration design. The results show that it is indeed superior than conventional sawtooth serrations. For example, a remarkable 7 dB additional noise reduction is observed in the intermediate frequency range with no perceivable noise increase elsewhere. The trends predicted by the analysis are well validated by the experiment. It is expected that these findings can serve as an essential guide for designing serrations, and lead to more acoustically optimized serration geometries.

Bio-Inspired Aerodynamic Noise Control: A Bibliographic Review

It is well-known that many species of owl have the unique ability to fly silently, which can be attributed to their distinctive and special feather adaptations. Inspired by the owls, researchers attempted to reduce the aerodynamic noise of aircraft and other structures by learning their noise reduction features from different viewpoints and then using the gained knowledge to develop a number of innovative noise reduction solutions. Although fruitful results have been achieved in the bio-inspired aerodynamic noise control, as far as the authors know, comparatively little work has been done to summarize the main findings and progresses in this area. In this bibliographic survey, we systematically review the progresses and trends of the bio-inspired aerodynamic noise control, including the macroscopic and microscopic morphological characteristics of the owl wing feathers, the noise measurements on both flying birds in the field and prepared wings in the wind tunnel, as well as theoretical, numerical and experimental studies that explored the feasibility, parameter influence, aerodynamic effects and underlying mechanisms of the four main bio-inspired noise reduction techniques, i.e., leading edge serrations, trailing edge serrations, fringe-type trailing edge extensions and porous material inspired noise reduction. Finally, we also give some suggestions for future work.

Reducing Wing Observability to Radar using Microserrations at the Leading Edge

Serrations are found in nature on the leading edge (LE) of bird wings and whale flippers. These serrations reduce flight noise and improve aerodynamic performance. This has inspired LE serrations to be incorporated in the design of aircraft wings and turbine blades. LE of wings can contribute significantly to observability of aircraft by radar due to LE diffraction of horizontally polarized incident electromagnetic (EM) waves. Serrations at the wing LE can also be used to reduce observability to radar under such circumstances. In the present study, the method of equivalent currents is used to analyse the effect of microserrations on an infinitesimally thin metallic delta wing. It is shown that microserrations at the wing LE can be effectively used to control backscatter due to diffraction of EM waves at the wing LE. LE serrations, in general, work by introducing a randomness in the interaction of wing LE with incident fluid flow or EM field.

The Hydrodynamic Noise Suppression of a Scaled Submarine Model by Leading-Edge Serrations

High hydrodynamic noise is a threat to the survival of underwater vehicles. We investigated a noise suppression mechanism by putting leading-edge serrations on the sail hull of a scaled SUBOFF model, through numerical calculation and an experimental test. We found that the cone shape of leading-edge serrations can decrease the intensity of the adverse pressure gradient and produce counter-rotation vortices, which destroy the formation of the horseshoe vortex and delay the tail vortex. To achieve the optimum hydrodynamic noise reduction, we summarized the parameters of leading-edge serrations. Then, two steel models were built, according to the simulation. We measured the hydrodynamic noise based on the reverberation method in a gravity water tunnel. The numerically calculated results were validated by the experimental test. The results show that leading-edge serrations with amplitudes of 0.025c and wavelengths of 0.05h can obtain hydrodynamic noise reduction of at least 6 dB, from 10 Hz to 2 kHz, where c is the chord length and h is the height of the sail hull. The results in our study suggest a new way to design underwater vehicles with low hydrodynamic noise at a high Reynolds number.

A theoretical study of serrated leading edges in aerofoil and vortical gust interaction noise

Serrated leading edges are one of the most promising passive aerodynamic control methods for the reduction of aerofoil–turbulence interaction noise. To elucidate the possible physical mechanisms, the current paper studies the simplified set-up with aerofoil–vortical gust interaction and proposes an analytical model by incorporating Fourier transform into the Wiener–Hopf method. The proposed model suggests that the serrations operate on the incident vortical gusts as convolution, which leads to the innovative concept that models serrations as transfer functions in the wavenumber domain. Overall, the current theoretical study could provide a unique insight of the inherent aerodynamic noise control mechanisms of leading-edge serrations.

The suppression of hydrodynamic noise from underwater sonar domes by flow control

Hydrodynamic noise is one of the three major noise sources of underwater vehicles. The sonar dome is a device placed in front of the ship and the submarine to absorb the flow fluctuation and to reduce the hydrodynamic noise, so that the sonar inside the dome is not affected by the external fluid. However, with the increase of the velocity of ships and submarines, cavitation can usually form in the bulge of the sonar domes, which will bring high level of noise to the sonar. The internal self-noise of the sonar dome mainly comes from two areas: the leading-edge stagnation point and the transition zone of boundary layer. In the paper, we designed the leading-edge serrations and dimples in the leading-edge and transition areas of the sonar dome respectively to reduce the movement resistance and prevent the separation of the boundary layer. The research on leading-edge serrations and dimple technology is carried out by using theoretical analysis, numerical calculations. The results show that the leading-edge serrations and dimples can add energy from the outer flow into the boundary layer; the cavitation phenomenon can be delayed. The hydrodynamic noise has been suppressed by about 20dB.

Noise reduction effect of leading-edge serrations on axial fan

Noise reduction effect of leading-edge serrations on axial fan

Aeroacoustic study of a wavy stator leading edge in a realistic fan/OGV stage

The effect of sinusoidal serrations applied to the leading-edge of the vanes of a realistic fan stage is investigated using high-fidelity numerical simulations. The CFD solver PowerFLOW based on a hybrid lattice-Boltzmann/very-large-eddy-simulation model is used to compute the unsteady flow and radiated noise of the 22-in source diagnostic test fan rig of the NASA Glenn Research Center. A computational model validated for three different geometries of the outlet guide vanes with straight leading edge is used. A subset of validation results is reported to prove the capability of the solver to accurately predict the influence of the stator geometry on the far-field noise. Different sinusoidal leading edge serrations are investigated for a radial and a swept stator and the same rotor and operating conditions. The influence of the serrations on the acoustic far-field and noise power level is reported in relation to the statistical properties of the velocity fluctuations in the wake of the rotor. Some noise reductions are obtained when the undulation amplitude and wavelength are large enough compared to the integral scales of the impinging turbulence fluctuations.

Improvement of the aerodynamic performance of vertical axis wind turbines with leading-edge serrations and helical blades using CFD and Taguchi method

The flow performance of vertical-axis wind turbines (VAWTs) is significantly deteriorated by dynamic stall at low tip-speed-ratios (TSR). Dynamic stall occurs periodically during the rotation of turbine blades and generates flow separations formed at leading-edges, which results in tremendous fluctuations in torque and essential reduction in power outputs. The target of this study is to apply a combination of CFD and Taguchi method to optimize the aerodynamic performance of a typical three-bladed VAWT at a low TSR of 2. Leading-edge wavy serrations and helical blade structures are implemented as the design parameters and the performance of these designs are compared with a corresponding baseline straight blade design of a traditional H-type VAWT. A comprehensive simulation study is carried out to obtain detailed flow characteristics. The Taguchi method is applied to find out the optimal combination of the selected factors such as amplitude and wave length of the serrations and twist angle of helical blade. A modified additive model is used to include the interaction effects among the selected factors. The optimal combination is found to appear in the case with an amplitude of 2.5%cb (cb is the blade chord length) and a wavelength of cb/6 for leading-edge serration as well as a blade twist angle of 60°. The simulation results show that the flow separations and torque fluctuations are significantly suppressed due to the passive flow control strategies implemented in the optimized VAWT model. The power output of the optimized wind turbine is found to increase by 18.3% in comparison to the baseline model. In addition, the force fluctuations are observed to be significantly reduced by employing the helical design, which is beneficial for improving the lifespan of turbines. The results derived from this study indicate a better VAWT design for the rural and built environments with relatively insufficient wind energy.

Aerofoil broadband noise reductions through double-wavelength leading-edge serrations: a new control concept

Aerofoils operating in a turbulent flow generate broadband noise by scattering vorticity into sound at the leading edge. Previous work has demonstrated the effectiveness by which serrations, or undulations, introduced onto the leading edge, can substantially reduce broadband leading-edge noise. All of this work has focused on sinusoidal (single-wavelength) leading-edge serration profiles. In this paper, a new leading-edge serration geometry is proposed which provides significantly greater noise reductions compared to the maximum noise reductions achievable by single-wavelength serrations of the same amplitude. This is achieved through destructive interference between different parts of the aerofoil leading edge, and therefore involves a fundamentally different noise reduction mechanism from conventional single-wavelength serrations. The new leading-edge serration profiles simply comprise the superposition of two single-wavelength components of different wavelength, amplitude and phase with the objective of forming two roots that are sufficiently close together and separated in the streamwise direction. Compact sources located at these root locations then interfere, leading to less efficient radiation than single-wavelength geometries. A detailed parametric study is performed experimentally to investigate the sensitivity of the noise reductions to the profile geometry. A simple model is proposed to explain the noise reduction mechanism for these double-wavelength serration profiles and shown to be in close agreement with the measured noise reduction spectra. The study is primarily performed on flat plates in an idealized turbulent flow. The paper concludes by introducing the double-wavelength serration on a 10 % thick aerofoil, where near-identical noise reductions are obtained compared to the flat plate.

Aerodynamic robustness in owl-inspired leading-edge serrations: a computational wind-gust model

Owls are a master to achieve silent flight in gliding and flapping flights under natural turbulent environments owing to their unique wing morphologies. While the leading-edge serrations are recently revealed, as a passive flow control micro-device, to play a crucial role in aerodynamic force production and sound suppression (Rao et al 2017 Bioinspiration Biomim. 12 1–13), the characteristics of wind-gust rejection associated with leading-edge serrations remain unclear. Here we address a large-eddy simulation-based study of aerodynamic robustness in owl-inspired leading-edge serrations, which is conducted with clean and serrated wing models through mimicking wind-gusts under a longitudinal fluctuation in free-stream inflow and a lateral fluctuation in pitch angle over a broad range of angles of attack (AoAs) over 0° ⩽ Φ ⩽ 20°. Our results show that the leading-edge serration-based passive flow control mechanisms associated with laminar-turbulent transition work effectively under fluctuated inflow and wing pitch, indicating that the leading-edge serrations are of potential gust fluctuation rejection or robustness in aerodynamic performance. Moreover, it is revealed that the tradeoff between turbulent flow control (i.e. aero-acoustic suppression) and force production in the serrated model holds independently to the wind-gust environments: poor at lower AoAs but capable of achieving equivalent aerodynamic performance at higher AoAs >15° compared to the clean model. Our results reveal that the owl-inspired leading-edge serrations can be a robust micro-device for aero-acoustic control coping with unsteady and complex wind environments in biomimetic rotor designs for various fluid machineries.

Morphology Effects of Leading-edge Serrations on Aerodynamic Force Production: An Integrated Study Using PIV and Force Measurements

While the leading-edge serration in owls’ wing is known to be responsible for low noise gliding and flapping flights, the findings on its aero-acoustic role have been diverse or even controversial. Here we present an experimental study of the morphological effects of leading-edge serrations on aerodynamic force production by utilizing owl-inspired, single-feather, clean and serrated wing models with different serration lengths and spacing, and by combining Particle Image Velocimetry (PIV) and force measurements. Force measurements show that an increase in the length and density of the leading-edge serrations leads to a reduction in the lift coefficient and lift-to-drag ratio at Angles of Attack (AoAs) 15°, which owl wings often reach in flight. Furthermore PIV visualization of the flow fluctuations demonstrates that the leading-edge serration-based mechanism is consistent in all serrated wing models in terms of passive control of the laminar-turbulent transition while at AoAs > 15° similar suction flow is present at leading edge resulting in a comparable aerodynamic performance to that of the clean wing. Our results indicate the robustness and usefulness of leading-edge serration-inspired devices for aero-acoustic control in biomimetic rotor designs.

Slitted leading edge profiles for the reduction of turbulence-aerofoil interaction noise.

Aerofoils operating in a turbulent flow are an efficient source of noise radiation by scattering vorticity into sound at the leading edge (LE). Much work has now been undertaken demonstrating the effectiveness of serrations, or undulations, introduced onto the LE can substantially reduce broadband LE interaction noise. However, all of this work is focused on sinusoidal LE serration profiles. In this paper, an alternative profile is proposed that is capable of providing significantly greater noise reductions than the maximum noise reductions that can be achieved by single-wavelength serrations of the same peak-to-root amplitude. In its most general realization, the LE serration profiles simply comprise of a sawtooth or single wavelength serration for which every root has a single narrow slit. This simple geometry, upon interaction with a turbulent flow, produces compact source regions at either end of the slit, which then destructively interfere, leading to considerably less efficient noise radiation than conventional single-wavelength geometries. The paper will demonstrate experimentally that even slits by themselves can provide greater levels of noise reduction than conventional profiles of the same peak-to-root amplitude.

On the leading edge noise and aerodynamics of thin aerofoil subjected to the straight and curved serrations

This paper presents the results of an experimental study into the effect of add-on type leading edge serrations on the aeroacoustic and aerodynamic performances of a symmetrical NACA0008 aerofoil. The aeroacoustic part of this paper studies the reduction of interaction broadband noise in the presence of elevated freestream turbulence by employing serrated leading edges. For non-dimensional frequencies f′ < 1, the resulting sound pressure level reduction (ΔSPL) was found to be a linear function of f′ and the ΔSPL depends only on the serration amplitude, serration wavelength and freestream velocity. Leading edge with a large serration amplitude was found to be very effective in the reduction of broadband noise where up to ΔSPL ≈ 8 dB is achievable. It is generally more beneficial to choose a leading edge with a smaller serration wavelength, although the most effective configuration actually combines the largest serration amplitude and the largest serration wavelength. Interestingly, for a curved-serration, the most optimised configuration (with small serration wavelength, large serration amplitude, small inclination angle and large curvature radius) was found to outperform its straight-serration counterpart by a further 5 dB reduction of broadband noise at the same frequency. Concerning the aerodynamic part of the study, to effectively suppress boundary layer stall without incurring severe drag penalties for low freestream turbulence intensity, the most effective leading edge serration should possess a large serration wavelength and small serration amplitude. Hence, the serration geometry that works very well for a low noise aerofoil is usually inferior in the aerodynamic performance, and vice versa. The best compromise for the serration geometry that can still harness good performances in both the aeroacoustic and aerodynamic should possess the largest serration amplitude and the largest serration wavelength. This paper demonstrates that, when optimised properly, the add-on type leading edge serration can be very effective in both the reduction of the interaction broadband noise, and the suppression of the boundary layer separation at high angle of attack.

An analytically-based method for predicting the noise generated by the interaction between turbulence and a serrated leading edge

This paper considers the interaction of turbulence with a serrated leading edge. We investigate the noise produced by an aerofoil moving through a turbulent perturbation to uniform flow by considering the scattered pressure from the leading edge. We model the aerofoil as an infinite half plane with a leading edge serration, and develop an analytical model using a Green's function based upon the work of Howe. This allows us to consider both deterministic eddies and synthetic turbulence interacting with the leading edge. We show that it is possible to reduce the noise by using a serrated leading edge compared with a straight edge, but the optimal noise-reducing choice of serration is hard to predict due to the complex interaction. We also consider the effect of angle of attack, and find that in general the serrations are less effective at higher angles of attack.

Leading-edge serrations for performance improvement on a vertical-axis wind turbine at low tip-speed-ratios

The performance of vertical-axis wind turbines (VAWTs) are substantially affected by the phenomenon of dynamic stall which is induced by the variations of angle of attack of rotating blades, especially at low tip-speed-ratios (TSRs). Large and sudden torque fluctuations are observed to take place when the dynamic stall vortices, formed near the blade leading-edge, are transported downstream. At low TSRs (λTSR<4) and relatively low Reynolds number (Re<105), dynamic stall occurs periodically during the rotation of turbine blades. This results in a sharp drop in lift coefficient and therefore rotor torque and power output are essentially reduced. The purpose of the present study is to investigate the concepts for improving the power performance of a conventional H-type VAWT model by implementing sinusoidal serrations on the leading-edge of turbine blades to control the dynamic flow separation at low TSRs. A thorough numerical study has been carried out to obtain the detailed flow fields for analysis and visualization. The power output results show that the improved turbine design with the sinusoidal serration profile of the wave amplitude h=0.025c and the wavelength λs=0.33c not only increases the power generation at low TSRs, but also enhances the capability of wind energy extraction at the optimal TSR in comparison to the baseline model. The flow separation is significantly controlled in the azimuth angle ranges from 75° to 160°, where the positive torque generation is also found to be considerably increased in the improved turbine model. Counter-rotating vortex pairs are generated due to the existence of serrations, which suppress the flow separation, especially in the regions near the peak-serration sections. Additionally, the lift coefficients illustrate a delay of occurrence of dynamic stall and a notable improvement of maximum lift in the improved wind turbine model in comparison to the baseline model. The effects of Reynolds number variation also reveal that the improved model would gain more benefits in power generation at low Reynolds number compared with that at high Reynolds number. The simulated results demonstrate that the leading-edge serration strategy could be an effective solution to control the dynamic stall in the operation of VAWTs.

On the noise prediction for serrated leading edges

An analytical model is developed for the prediction of noise radiated by an aerofoil with leading-edge serration in a subsonic turbulent stream. The model makes use of Fourier expansion and Schwarzschild techniques in order to solve a set of coupled differential equations iteratively and express the far-field sound power spectral density in terms of the statistics of incoming turbulent upwash velocity. The model has shown that the primary noise-reduction mechanism is due to the destructive interference of the scattered pressure induced by the leading-edge serrations. It has also shown that in order to achieve significant sound reduction, the serration must satisfy two geometrical criteria related to the serration sharpness and hydrodynamic properties of the turbulence. A parametric study has been carried out and it is shown that serrations can reduce the overall sound pressure level at most radiation angles, particularly at small aft angles. The sound directivity results have also shown that the use of leading-edge serration does not significantly change the dipolar pattern of the far-field noise at low frequencies, but it changes the cardioid directivity pattern associated with radiation from straight-edge scattering at high frequencies to a tilted dipolar pattern.

Owl-inspired leading-edge serrations play a crucial role in aerodynamic force production and sound suppression

Owls are widely known for silent flight, achieving remarkably low noise gliding and flapping flights owing to their unique wing morphologies, which are normally characterized by leading-edge serrations, trailing-edge fringes and velvet-like surfaces. How these morphological features affect aerodynamic force production and sound suppression or noise reduction, however, is still not well known. Here we address an integrated study of owl-inspired single feather wing models with and without leading-edge serrations by combining large-eddy simulations (LES) with particle-image velocimetry (PIV) and force measurements in a low-speed wind tunnel. With velocity and pressure spectra analysis, we demonstrate that leading-edge serrations can passively control the laminar-turbulent transition over the upper wing surface, i.e. the suction surface at all angles of attack (0° < AoA < 20°), and hence play a crucial role in aerodynamic force and sound production. We find that there exists a tradeoff between force production and sound suppression: serrated leading-edges reduce aerodynamic performance at lower AoAs < 15° compared to clean leading-edges but are capable of achieving both noise reduction and aerodynamic performance at higher AoAs > 15° where owl wings often reach in flight. Our results indicate that the owl-inspired leading-edge serrations may be a useful device for aero-acoustic control in biomimetic rotor designs for wind turbines, aircrafts, multi-rotor drones as well as other fluid machinery.

Statistical–Empirical Modeling of Airfoil Noise Subjected to Leading-Edge Serrations

With the objective of reducing the broadband noise from the interaction of highly turbulent flow and airfoil leading edge, sinusoidal leading-edge serrations were investigated as an effective passive treatment. An extensive aeroacoustic study was performed to determine the main influences and interdependencies of factors, such as the Reynolds number, turbulence intensity, serration amplitude, and wavelength as well as the angle of attack on the noise-reduction capability. A statistical–empirical model was developed to predict the overall sound pressure level and noise reduction of a NACA 65(12)-10 airfoil with and without leading-edge serrations in the range of chord-based Reynolds numbers of . The observed main influencing factors on the noise radiation were quantified in a systematic order for the first time. Moreover, significant interdependencies of the turbulence intensity and the serration wavelength, as well as the serration wavelength and the angle of attack, were observed, validated, and quantified. The statistical–empirical model was validated against an external set of experimental data, which is shown to be accurate and reliable.

Performance and mechanism of sinusoidal leading edge serrations for the reduction of turbulence–aerofoil interaction noise

This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto three-dimensional aerofoils. The influence on the noise reduction of the turbulence integral length scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length scale is approximately one-fourth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number $St_{h}=fh/U$, where $f$, $h$ and $U$ are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce aerofoil self-noise. The mechanism for this phenomenon is explored through particle image velocimetry measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.