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

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

Optimization of leading-edge undulation of a NACA 65(12)-10 aerofoil for noise reduction and aerodynamic enhancement

Leading-edge undulations or tubercles of humpback whale flippers have been known as one of biomimetic technologies adaptable to flow control of aerofoils, particularly at post stall conditions. These leading-edge undulations are also known to reduce noise resulting from an interaction with on-coming turbulence. We have recently carried out a parametric study of a NACA 65(12)-10 aerofoil with a view to optimise the amplitude and the wavelength of leading-edge undulations for noise reduction and aerodynamic enhancement. A 3x3 test matrix composing of three amplitudes (h = 3%, 6% and 12% chord) and three wavelengths (? = 10%, 20% and 30% chord) was used in the investigation, where lift, drag and noise were measured at the Reynolds number of 105. In this test, a turbulence-generating grid was installed at the inlet of the test section to increase the turbulence level in the freestream and to promote transition to turbulence near the leading edge of aerofoils without a need for a trip device. Within the test matrix considered, we found that the best improvement in CL,max is given with the greatest wavelength and amplitude, whereas the maximum noise reduction is obtained with the small wavelength and the large amplitude

Aerodynamic and Aeroacoustic Optimization of Leading-Edge Undulation of a NACA 65(12)-10 Airfoil

Experimental studies of a NACA 65(12)-10 airfoil with a sinusoidal leading-edge undulation (LEU) were carried out to simultaneously optimize its aerodynamic and aeroacoustic performances by considering the attached as well as the separated flow at the effective Reynolds number of 106, where the maximum lift was increased without sacrificing drag or overall noise at near- and poststall angles. Further aerodynamic and aeroacoustic tests indicated that a combination of LEU wavelength λ/c=30% and amplitude h/c=6% gave an optimum LEU by considering the aerodynamic performance as well as the noise reduction. Particle image velocimetry measurements of the flow over the optimized airfoil showed biperiodic velocity fluctuations downstream of the LEU peaks that were associated with unsteady stall cell structure near the trailing edge

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

Acoustic radiation from a shell-encapsulated baffled cylindrical cap

An exact study of radiation of an acoustic field due to radial/axial vibrations of a baffled cylindrical piston, eccentrically positioned within a fluid-filled thin cylindrical elastic shell, into an external fluid medium is presented. This configuration, which is a realistic idealization of a liquid-filled cylindrical acoustic lens with a focal point inside the lens when used as a sound projector, is of practical importance with a multitude of possible applications in underwater acoustics and ocean engineering. The formulation utilizes the appropriate wave field expansions along with the translational addition theorems for cylindrical wave functions to develop a closed-form solution in the form of an infinite series. Numerical results reveal the key effects of excitation frequency, cap angle, radiator position (eccentricity), dynamics of the elastic shell, and cap surface velocity distribution on sound radiation

Optimization of leading-edge undulation of a NACA 65(12)-10 aerofoil for noise reduction and aerodynamic enhancement

Copyright © 2019 The Author(s). Leading-edge undulations or tubercles of humpback whale flippers have been known as one of biomimetic technologies adaptable to flow control of aerofoils, particularly at post stall conditions. These leading-edge undulations are also known to reduce noise resulting from an interaction with on-coming turbulence. We have recently carried out a parametric study of a NACA 65(12)-10 aerofoil with a view to optimise the amplitude and the wavelength of leading-edge undulations for noise reduction and aerodynamic enhancement. A 3x3 test matrix composing of three amplitudes (h = 3%, 6% and 12% chord) and three wavelengths (? = 10%, 20% and 30% chord) was used in the investigation, where lift, drag and noise were measured at the Reynolds number of 105. In this test, a turbulence-generating grid was installed at the inlet of the test section to increase the turbulence level in the freestream and to promote transition to turbulence near the leading edge of aerofoils without a need for a trip device. Within the test matrix considered, we found that the best improvement in CL,max is given with the greatest wavelength and amplitude, whereas the maximum noise reduction is obtained with the small wavelength and the large amplitude.EPSRC (Grant Number EP/N018486/1) in the UK