Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

Copyright: © 2021 by the authors. Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.UK Engineering and Physical Sciences Research Council research grant (Grant No. EP/N018737/1) “Quiet Aerofoils of the Next-Generation”; The third author would like to thank the Israel Science Foundation (Grant No. 207162) for the funding of her time to be spent in this project

Mechanisms of airfoil noise near stall conditions

The focus of this paper is on investigating the noise produced by an airfoil at high angles of attack over a range of Reynolds number Re≈2×10⁵–4×10⁵. The objective is not modeling this source of noise but rather understanding the mechanisms of generation for surface pressure fluctuations, due to a separated boundary layer, that are then scattered by the trailing edge. To this aim, we use simultaneous noise and surface pressure measurement in addition to velocimetric measurements by means of hot wire anemometry and time-resolved particle image velocimetry. Three possible mechanisms for the so-called “separation-stall noise” have been identified in addition to a clear link between far-field noise, surface pressure, and velocity fields in the noise generation

Unsteady aerodynamics loads during flapping flight of birds; case study : starling and sandpiper

Flapping wing is one of the most widespread propulsion methods found in nature. However, the current understanding of the bird aerodynamics is incomplete. The role of unsteady motion in the flow and its contribution to the aerodynamics is still an open question. The current study deals with the estimation of unsteady aerodynamic forces on freely flying birds through analysis of wingbeat kinematics and near wake flow measurements using long duration time-resolved particle image velocimetry. Two bird species have been investigated, the starling and sandpiper. Using long-time sampling data, several wingbeat cycles have been analyzed in order to cover both the downstroke and upstroke phases of flight. Lift and drag were obtained using the momentum equation for viscous flows and were found to share a highly unsteady behavior. The two birds show similar behavior during the downstroke phase of flight, whereas the sandpiper was shown to have a district signature during its upstroke phase. The contribution of the circulatory lift component is shown to be significant when estimating lift (or power) of birds in flapping flight. Moreover, the unsteady drag term was found to have a crucial role in the balance of drag (or thrust), particularly during transition phases. These findings may shed light on the flight efficiency of birds by providing a partial answer to how they minimize drag and maximize lift during flapping flight.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Leading edge serrations for the reduction of aerofoil separation self-noise

This paper presents an experimental investigation into the use of LE serrations for the reduction of trailing edge self-noise, at least for the NACA-65 aerofoil family. It is shown that the leading edge serrations are able to reduce the self-noise in a low frequency range at small and negative angles of attack. The exact mechanism of this reduction is still not completely discovered, but the LE serrations are discovered able to modulate the mean velocity field and turbulent velocity spectrum in that range of frequencies, as well as to dampen the effect of the angle of attack on the pressure field and to reduce its coherence. We emphasise that this paper represents work in progress and further investigations are still necessary in order to completely understand the dynamics behind this reduction

On the manipulation of flow and acoustic fields of a blunt trailing edge aerofoil by serrated leading edges

This paper employs serrated leading edges to inject streamwise vorticity to the downstream boundary layer and wake to manipulate the flow field and noise sources near the blunt trailing edge of an asymmetric aerofoil. The use of a large serration amplitude is found to be effective to suppress the first noise source—bluntness-induced vortex shedding tonal noise—through the destruction of the coherent eigenmodes in the wake. The second noise source is the instability noise, which is produced by the interaction between the boundary layer instability and separation bubble near the blunt edge. The main criterion needed to suppress this noise source is related to a small serration wavelength because, through the generation of more streamwise vortices, it would facilitate a greater level of destructive interaction with the separation bubble. If the leading edge has both a large serration amplitude and wavelength, the interaction between the counter-rotating vortices themselves would trigger a turbulent shear layer through an inviscid mechanism. The turbulent shear layer will produce strong hydrodynamic pressure fluctuations to the trailing edge, which then scatter into broadband noise and transform into a trailing edge noise mechanism. This would become the third noise source that can be identified in several serrated leading edge configurations. Overall, a leading edge with a large serration amplitude and small serration wavelength appears to be the optimum choice to suppress the first and second noise sources and, at the same time, avoid the generation of the third noise source

Leading edge serrations for the reduction of aerofoil self-noise at low angle of attack, pre-stall and post-stall conditions

This paper addresses the usefulness of leading edge serrations for reducing aerofoil self-noise over a wide range of angles of attack. Different serration geometries are studied over a range of Reynolds number (Formula presented.). Design guidelines are proposed that permit noise reductions over most angles of attack. It is shown that serration geometries reduces the noise but adversely effect the aerodynamic performance suggesting that a trade-off should be sought between these two considerations. The self-noise performance of leading edge serrations has been shown to fall into three angle of attack (AoA) regimes: low angles where the flow is mostly attached, moderate angles where the flow is partially to fully separated, and high angles of attack where the flow is fully separated. Leading edge serrations have been demonstrated to be effective in reducing noise at low and high angles of attack but ineffective at moderate angles. The noise reduction mechanisms are explored in each of three angle regimes

Scaling of Lift Coefficient of an Airfoil with Leading Edge Serrations

Israel Science Foundatio

Prediction of broadband trailing-edge noise based on Blake model and Amiet theory

This paper extends previously published TNO-Blake methods to predict airfoil broadband self-noise due to interaction of a turbulent boundary-layer with a sharp trailing edge. The method presented herein combines Blake’s model to predict surface pressure fluctuations and Amiet model to predict trailing edge noise emission. The Blake method is based on the solution of the Poisson equation as an integral of the mean-shear turbulent source interaction source, over the entire boundary layer thickness. A recent advances in description of streamwise turbulent intensity are employed. Surface pressure spectra are measured with remote microphones and compared to the prediction. In the next step, the wavenumber- frequency spectrum as predicted by the TNO model was utilized as an input to the Amiet model to evaluate the far-field trailing edge noise.</p

Evaluation of active flow control applied to wind turbine blade section

A feasibility study for implementing active flow control (AFC) methods to improve the performance of wind turbines was performed. The experimental effort investigated the impact of zero-mass-flux (ZMF) piezofluidic actuators attempting to control boundary layer separation from thick airfoils that are suitable for wind turbine rotor blades. It was demonstrated that the ZMF actuators can replace passive vortex generators that are commonly used for boundary layer separation delay, without the inherent drag penalty that the passive devices impose. It has been shown that ZMF fluidic actuators are suitable for flow control in wind turbine application due to the fact that they are adjustable for wider Reynolds number range, while vortex generators are tuned to perform well in one design point. It was demonstrated that AFC can effectively double the maximum lift of this airfoil at low Reynolds numbers. A possible application is a significant reduction of the turbine start-up velocity. It was also found that even for a contaminated blade, AFC is capable to delay the stall and decrease the drag using low energy expenditure, therefore restoring and even surpassing the clean airfoil performance. The effectiveness of the AFC method was examined using a newly defined aerodynamic figure of merit. Various scaling options for collapsing the effect of the excitation magnitude on the lift alternation due to the activation of zero-mass-flux periodic excitation for boundary layer separation control are proposed and examined using experimental data