Investigation of Curle's Dipolar Sources on a Porous Airfoil Interacting with Incoming Turbulence (original) (raw)
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The effect of leading edge porosity on airfoil turbulence interaction noise
The Journal of the Acoustical Society of America
Airfoil turbulence interaction noise and the flow field up to and over the porous leading edge is experimentally studied. The porous leading edges were of the same base triply periodic minimal surface structure with varying porosity to enable us to understand how the porosity, permeability, and pore size affect the generated turbulence interaction noise. The turbulent flow was generated by means of a passive turbulence grid that does not affect the normal background noise of the wind tunnel. Far-field noise results were obtained from a polar microphone array to assess the directivity of the sound as well as the narrowband frequency contributions. Far-field noise results demonstrate that increasing porosity reduces the turbulence interaction noise over low-to-mid frequencies, with a penalty of a high frequency noise increase. Flow measurement results indicate hydrodynamic penetration of the flow into the porous structure at the leading edge. Furthermore, the two-point correlation ana...
This paper presents the results of an experimental investigation to study the effect of a porous airfoil Trailing-Edge (TE) with different open-area ratios at Reynolds number by 2.9 x 10^5 ≤ Re_{c} ≤ 4.06 x 10^5 (based on the chord length) and the zero angle of attack (alpha = 0) on the flow-induced noise generation mechanism. Acoustic pressure measurements were taken in an Anechoic Wind Tunnel (AWT) using a spiral array of 31 microphones. Five different TE configurations were considered; these are airfoil with a porous TE modelled by 15, 36, 45 and 72 slits and a reference non-porous plate. A comparison of the Power Spectral Density (PSD) graphs of a porous TE with that obtained due to reference non-porous plate indicates up to 5 dB reduction (on an average) in the noise levels between the frequency range given by 2500 Hz to 8000 Hz due to the use of porous TE. The results obtained from the cross-spectral Conventional Beamforming (CB) algorithm indicate that the noise generation source is closer to the TE at these frequencies. However, in the mid-frequency range between 2000 Hz to 2500 Hz, the noise from the Leading-Edge (LE) is comparable to that observed at the TE. This is hypothesised to be due to the interaction between the LE and the boundary-layer exiting the contraction-outlet.
EXPERIMENTAL ANALYSIS OF THE AERODYNAMICS OF POROUS FLAP SIDE-EDGE FOR NOISE REDUCTION
Experimental testes were carried out to investigate the flow near a flap side-edge with attached porous plate as a potential noise reduction device. The experiments were performed over a wing to simplify the installation. The objective was to perform a first approximation to the flow field produced by the addition of porosity of different sizes. Specifically, the changes in tip vortex were studied. Baseline case and plates of 10mm, 20mm, 30mm, 50mm and 70mm placed in the bottom side of the wing tip were tested. Also another configuration was tested, with a plate of 25mm roughly placed in the attachment line of the vortex. Aerodynamic experiments were performed to evaluate the changes in the lift, drag and pitching moment coefficients. Hot wire anemometry measurements were performed in perpendicular planes to the direction of the undisturbed flow. Problems in the calibration were detected, that corrupts all the values. Nevertheless, first results about the behavior of the flow field were achieved. The addition of the porous plate transforms the structure of the vortex by dramatically decreasing the axial velocity in the core center transforming the tip vortex from a jet vortex to a wake vortex. Vorticity was also changed with a noticeable decrease. From 30mm porous strip, this decrease becomes asymptotic. There are some structures that are not completely explained. The attachment line plate and the 10mm plate cases present a complex structure, which seems due to various vortices. The 50mm and 70mm plates show a merging between the wake and the vortex. The addition of the porous plate weakens the mechanisms of noise production. It is judged that this will lead to a noise reduction.
WIND TUNNEL TESTING OF POROUS DEVICES FOR THE REDUCTION OF FLAP SIDE-EDGE NOISE
This work investigates experimentally several flap side-edge treatments on a MD 30P30N wing model for noise reduction. The application of porous edges of various extensions, splitter plate were among the investigated treatments. The experiments were carried out in a solid wall low speed wind tunnel with turbulence level of 0.2% and capability of aeroacoustic testing with an overall SPL at 37m/s from 500 Hz to 20 kHz of 80 dB , with a peak 1/24 octave-band level of 75 dB at 500Hz that decreases to 67 dB at 2 kHz. The baseline configuration and those with side-edge treatments were tested and equivalent noise levels were obtained for low frequencies.
The generation of discrete acoustic tones in the compressible flow around an aerofoil is addressed in this work by means of nonlinear numerical simulations and global stability analyses. The nonlinear simulations confirm the appearance of discrete tones in the acoustic spectrum, and for the chosen flow case, the global stability analyses of the meanflow dynamics reveal that the linearized operator is stable. However, the flow response to incoming disturbances exhibits important transient growth effects that culminate into the onset of aeroacoustic feedback loops, involving instability processes on the suctionand pressure-surface boundary-layers together with their cross interaction by acoustic radiation at the trailing edge. The features of the aeroacoustic feedback loops and the appearance of discrete tones are then related to the features of the least stable modes in the global spectrum: the spatial structure of the direct modes display the coupled dynamics of hydrodynamic instabilities on the suction surface and the near wake. Finally, different families of global modes will be identified and the dynamics that they represent will be discussed. despite four decades of research efforts and studies, an encompassing and widely accepted description of the physical mechanisms underlying the rise of discrete tones is still wanting. Based on the above-mentioned investigations, we shall give a brief résumé of the principal features and our current understanding of tonal-noise on aerofoils; for a chronological presentation we refer the reader to .
Acoustics of turbulent flows: a report on Euromech 142
Journal of Fluid …, 1982
The European Mechanics Colloquium, Euromech 142, ww held at the Ecole Centrale de Lyon from 23 to 26 September 1981 and WM attended by 70 participants, from 9 countries, active in the fields of (i) sound production by turbulent flows and (ii) the effects of flow and turbulence on the propagation of acoustic waves. For topic (i), attention was mostly paid to shear-layer and jet instabilities and to the flow-surface interaction of flexible boundaries, vibrating blades and rigid thick airfoils. Applications were concerned in particular with propeller noise and airframe self-noise of large aircraft. Impinging shear layers were also considered for single and multiple cavities in which self-sustained oscillations can occur. Another subject discussed at the meeting w a the noise from inhomogeneities, with applications to flames. For topic (ii), theoretical formulations were presented for the far field of moving multipole sources in the presence of flow and for sound propagation in ducts of variable cross section, with flow, for frequencies around the cutoff. The effect of turbulence was investigated in terms of the space-time coherence of the transmitted pressure fields. Broadband active sound control in the presence of flow was also considered, with emphasis on the progress made possible by use of digital filters. Finally, new experimental techniques, such = acoustic intensity measurements, were presented and large anechoic wind tunnels and other acoustic facilities were described.
On the attenuating effect of permeability on the low frequency sound of an airfoil
Journal of Sound and Vibration, 2016
The effect of structure permeability on the far-field radiation of a thin airfoil is studied. Assuming low-Mach and high-Reynolds number flow, the near-and far-field descriptions are investigated at flapping-flight and unsteady flow conditions. Analysis is carried out using thin-airfoil theory and compact-body-based calculations for the hydrodynamic and acoustic fields, respectively. Airfoil porosity is modeled via Darcy's law, governed by prescribed distribution of surface intrinsic permeability. Discrete vortex model is applied to describe airfoil wake evolution. To assess the impact of penetrability, results are compared to counterpart predictions for the sound of an impermeable airfoil. Considering the finite-chord airfoil as "acoustically transparent", the leading-order contribution of surface porosity is obtained in terms of an acoustic dipole. It is shown that, at all flow conditions considered, porosity causes attenuation in outcome sound level. This is accompanied by a time-delay in the pressure signal, reflecting the mediating effect of permeability on the interaction of fluid flow with airfoil edge points. To the extent that thin-airfoil theory holds (requiring small normal-to-airfoil flow velocities), the results indicate on a decrease of $ 10 percent and more in the total energy radiated by a permeable versus an impermeable airfoil. This amounts to a reduction in system sound pressure level of 3 dB and above at pitching flight conditions, where the sound-reducing effect of the seepage dipole pressure becomes dominant. The applicability of Darcy's law to model the effect of material porosity is discussed in light of existing literature.
Experimental investigation of aerofoil tonal noise generation
Journal of Fluid Mechanics, 2014
The present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence ($0^{\circ },1^{\circ },2^{\circ }$ and 4circ4^{\circ }4circ angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant to...
Airfoil Trailing Edge Noise Generation and Its Surface Pressure Fluctuation
In the present work, Large Eddy Simulation (LES) of turbulent flows over a NACA 0015 airfoil is performed. The purpose of such numerical study is to relate the aerodynamic surface pressure with the noise generation. The results from LES are validated against detailed surface pressure measurements where the time history pressure data are recorded by the surface pressure microphones. After the flow-field is stabilized, the generated noise from the airfoil Trailing Edge (TE) is predicted using the acoustic analogy solver, where the results from LES are the input. It is found that there is a strong relation between TE noise and the aerodynamic pressure. The results of power spectrum density show that the fluctuation of aerodynamic pressure is responsible for noise generation.