Numerical Investigation of Tonal Trailing-Edge Noise Radiated by Low Reynolds Number Airfoils (original) (raw)
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Recent Advances in Acoustics of Transitional Airfoils with Feedback-Loop Interactions: A Review
Applied Sciences, 2021
We discuss herein recent experimental and numerical studies examining resonant flow-acoustic feedback–loop interactions in transitional airfoils (i.e., possessing a notable area of laminar-to-turbulent boundary-layer transition) characteristic of low-to-medium Reynolds number flow regimes. Such interactions are commonly attributed to the viscous dynamics of the convected boundary-layer structures scattering into acoustic waves at the trailing edge which propagate upstream and re-excite the convected vortical structures. While it has been long suspected that the acoustic feedback mechanism is responsible for the highly pronounced, often multi-tonal response, the exact reason of how the boundary-layer instability structures could reach a sufficient degree of amplification to sustain the feedback-loop process and exhibit specific tonal signature remained unclear. This review thus pays particular attention to the critical role of the separation bubble in the feedback process and emphasi...
Acoustic Feedback-Loop Interactions in Transitional Airfoils
19th AIAA/CEAS Aeroacoustics Conference, 2013
Our recent numerical and experimental efforts are reviewed examining flow-acoustic resonant interactions in transitional airfoil boundary layers and means of control of the resulting prominent, tonal trailing-edge noise sources. Experimentally recorded unsteady responses of loaded, transitional NACA0012 airfoil reveal operational regimes characterized by the presence of the shifted ladder-type tonal structures with dual velocity dependence observed in the surface pressure and the acoustic signals. High-fidelity numerical efforts employ a 6 th-order Navier-Stokes solver implementing a low-pass filtering of poorly resolved high-frequency solution content to retain numerical accuracy and stability over a range of transitional flow regimes. 2D and 3D (ILES) numerical experiments investigate the behavior of the boundary-layer statistical moments during the transitional flow regimes characterized by the presence of separation regions and the resulting formation of the highly-amplified instability waves scattered into noise at the airfoil trailing edge, thus triggering and sustaining the acoustic feedback-loop process. The current paper particularly focuses on the sensitivity of the airfoil flow-acoustic interactions (and the resulting acoustic signature) to the upstream flow conditions.
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Journal of Fluid Mechanics, 2007
This paper presents the first numerical investigation via direct numerical simulation of the tone noise phenomenon occurring in the flow past laminar airfoils. This phenomenon corresponds to the radiation of discrete acoustic tones in some specific flow conditions, and has received much attention since the 1970s, and several experimental studies have been carried out to identify and understand the underlying physical mechanisms. However, several points remain to be clarified in order to provide a complete explanation of its origin. The flow around a two-dimensional NACA0012 airfoil is considered in order to have a deeper understanding of the tone noise phenomenon. Consistently with previous experimental studies, it is shown that depending on the Reynolds number and angle of attack, two different types of acoustic spectrum are observed: one which exhibits a broadband contribution with a dominant frequency together with a sequence of regularly spaced discrete frequencies, while the ot...
Switch of tonal noise generation mechanisms in airfoil transitional flows
Physical Review Fluids
Large eddy simulations are performed to study tonal noise generation by a NACA0012 airfoil at an angle of attack α = 3 deg. and freestream Mach number of M ∞ = 0.3. Different Reynolds numbers are analyzed spanning 0.5 × 10 5 ≤ Re ≤ 4 × 10 5. Results show that the flow patterns responsible for noise generation appear from different laminar separation bubbles, including one observed over the airfoil suction side and another near the trailing edge, on the pressure side. For lower Reynolds numbers, intermittent vortex dynamics on the suction side results in either coherent structures or turbulent packets advected towards the trailing edge. Such flow dynamics also affects the separation bubbles on the pressure side, which become intermittent. Despite the irregular occurrence of laminar-turbulent transition, the noise spectrum depicts a main tone with multiple equidistant secondary tones. Increasing the Reynolds number leads to a permanent turbulent regime on the suction side that reduces the coherence level causing only small scale turbulent eddies to be observed. Furthermore, the laminar separation bubble on the suction side almost vanishes while that on the pressure side becomes more pronounced and permanent. As a consequence, the dominant noise generation mechanism becomes the vortex shedding along the wake.
Acoustics, 2020
An experimental and analytical study of the tonal trailing-edge noise of a symmetric NACA-0012 aerofoil and of a cambered SD7003 aerofoil has been achieved. It provides a complete experimental database for both aerofoils and improves the understanding of the underlying mechanisms. The analysis stresses the high sensitivity of the tonal noise phenomenon to the flow velocity and the angle of attack. Several regimes of the noise emission are observed depending on the aforementioned parameters. The contributions of the pressure and the suction sides are found to vary with the flow parameters too. A special attention has been paid to the role of the separation bubble in the tonal noise generation. Hot-wire measurements and flow visualization prove that the separation bubble is a necessary condition for the tonal noise production. Moreover, the bubble must be located close enough to the trailing edge. Several tests with small-scale upstream turbulence confirm the existence of the feedback loop. Analytical predictions with a classical trailing-edge noise model show a good agreement with the experimental data; they confirm the cause-to-effect relationship between the wall-pressure fluctuations and the radiated sound. Finally, previously reported works on fans and propellers are shortly re-addressed to show that the tonal noise associated with laminar-boundary-layer instabilities can take place in rotating blade technology.
On flow-acoustic resonant interactions in transitional airfoils
International Journal of Aeroacoustics, 2014
We discuss our recent experimental and preliminary numerical efforts examining resonant feedback mechanism of flow-acoustic interactions in airfoil transitional boundary layers. Experimentally recorded unsteady responses of loaded, transitional NACA0012 airfoil with tripped suction or pressure sides confirm the presence of shifted ladder-type tonal structures with dual velocity dependence in the acoustic signal previously reported in the literature. The installation of upstream grid generating low-intensity turbulence appears to eliminate the feedback mechanism leaving just a single velocity dependence for the dominant spectral components. Complementary numerical efforts employ a high-order Navier-Stokes solver implementing low-pass filtering of poorly resolved high-frequency solution content to retain numerical accuracy and stability over the range of transitional flow regimes. Within the scope of 2D analysis, the conducted numerical experiments particularly investigate the behavior of the boundary-layer statistical moments during the transitional flow regimes characterized by the presence of the acoustic feedback, and address sensitivity of the latter to the flow Reynolds number.
Investigation of Airfoil Trailing Edge Noise with Advanced Experimental and Numerical Methods
2014
The investigation of the noise emitted from the trailing edge (TE) of a Somers S834 airfoil section with advanced experimental and numerical methods is presented. The airfoil section is placed in a low noise, low turbulence small aeroacoustic wind tunnel. To mimic a relatively large target Reynolds number the boundary layer on the airfoil has to be tripped. An unsteady large-eddy simulation (LES) provides the input data for airfoil self noise prediction models. Standard microphone correlation as well as microphone array based techniques were applied to distinguish trailing edge noise (TEN) from ambient sound and for quantification. The carefully applied boundary layer tripping produced the targeted boundary layer parameters in the TE region of the airfoil. The TE of the airfoil was found to be the most dominant sound radiation region on the airfoil section. The microphone array measurements revealed dominant contributions from the TE from 160 to at least 2500 Hz. The peak level of T...
Journal of Fluid Mechanics, 2022
A large eddy simulation is performed to study secondary tones generated by a NACA0012 airfoil at angle of attack of alpha=3circ\alpha = 3^{\circ}alpha=3circ with freestream Mach number of Minfty=0.3M_{\infty} = 0.3Minfty=0.3 and Reynolds number of Re=5times104Re = 5 \times 10^4Re=5times104 . Laminar separation bubbles are observed over the suction side and near the trailing edge, on the pressure side. Vortex shedding occurs aft of the suction side separation bubble, and vortex interaction results in merging or bursting such that coherent structures or turbulent packets are advected towards the trailing edge. This mechanism modulates the amplitude of the incident pressure signal, leading to different levels of noise emission. Despite the intermittent occurrence of laminar–turbulent transition, the noise spectrum depicts a main tone with multiple equidistant secondary tones. To understand the role of flow instabilities on the tones, the linearised Navier–Stokes equations are examined in their operator form through biglobal stability and resolv...
A global analysis of tonal noise in flows around aerofoils
Journal of Fluid Mechanics, 2014
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 mean-flow dynamics reveal that the linearized operator is stable. However, the flow response to incoming disturbances exhibits important transient growth effects that culminate in the onset of aeroacoustic feedback loops, involving instability processes on the suction-and 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 displays the coupled dynamics of hydrodynamic instabilities on the suction surface and in the near wake. Finally, different families of global modes will be identified and the dynamics that they represent will be discussed.
International Journal of Aeroacoustics, 2018
A direct numerical simulation of the flow field around a controlled-diffusion airfoil within an anechoic wind-tunnel at [Formula: see text] incidence and a high Reynolds number of [Formula: see text] is performed for the first time using a lattice Boltzmann method. The simulation compares favorably with experimental measurements of wall-pressure, wake statistics, and far-field sound. The simulation noticeably captures experimentally observed high-amplitude acoustic tones that rise above a broadband hump. Both noise components are related to a breathing of a recirculation bubble formed around 65–70% of the chord, and to Kelvin-Helmholtz instabilities in the separated shear layer that yield rollers that break down into turbulent vortices whose diffraction at the trailing edge produces a strong dipole acoustic field. A wavelet analysis of the wall-pressure signals combined with some flow visualization has shown that the flow statistics are dominated by intense events caused by intermit...