Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session A34: Aeroacoustics |
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Chair: Benshuai Lyu, Peking University Room: 255 F |
Sunday, November 24, 2024 8:00AM - 8:13AM |
A34.00001: Fluid mechanics of a recorder: coherent structures and sound generation Miguel Fosas de Pando, Ammara Tassawar, Rodolfo Ostilla Monico The recorder is perhaps the simplest woodwind musical instrument, and despite its 700-year history, a complete description of the interplay between the hydrodynamic and sound generation processes in the labium is yet lacking. The aim of this work is to gain insight into these processes using simplified simulations and hydrodynamic stability theory. To this end, we consider a computational model of a recorder parametrized by its main geometric and flow features, and explore the parameter space using two-dimensional flow simulations based on immersed boundaries to determine the onset of coherent structures and their main statistical properties. Then, these structures and their sensitivity to changes in the geometry of the labium and flow parameters are analyzed, as well as their interaction with resonant acoustic modes in the chimney. Finally, the implication of these results to the design of these instruments and their playability will be discussed. |
Sunday, November 24, 2024 8:13AM - 8:26AM |
A34.00002: Near field jet noise by acoustic wave radiation Simon Görtz, Lara De Broeck, Martin Oberlack, Olivia Grace Martin, Sanjiva K Lele We consider the problem of noise emission in supersonic jets by radiation of acoustic waves into the far field. The initial part of the jet close to the nozzle with a constant core flow is examined, which contributes significantly to noise emission. The plane parallel jet is modelled as a composition of two opposite parallel hyperbolic tangent shear flows allowing us to derive exact solutions as expansions about singularities for the underlying linearized Euler equations for compressible fluids. Using this we analyze two effects of acoustic wave radiation into the far field: (i) we derive acoustic waves directly radiated by the shear layers which seem to be linked to a class of unstable jet modes described by Tam & Hu (1989); (ii) the opposition arrangement of the shear layer leads to an interaction of the wave phenomena. Waves are both partially reflected, leading to trapped waves, and partially transmitted to the outside of the jet, thus contributing to radiation. For both radiation effects, we study the influence of Mach number on characteristic quantities of the radiated waves, such as propagation angle. The key observations were confirmed by DNS with the analytical eigenfunctions used as initial DNS values. |
Sunday, November 24, 2024 8:26AM - 8:39AM |
A34.00003: Direct Numerical Simulation of the Trailing Edge Noise on a Flat Plate and the Effect of Finlets Natsumi Hirao, Makoto Hirota, Yuji Hattori There are increasing needs for reduction of trailing-edge (TE) noise from airplane wings. Finlets are known to be effective in reducing TE noise. The generation mechanism of the TE noise from a flow past a flat plate with/without finlets is studied by direct numerical simulations (DNS). First, the flow field around an untreated flat plate was conducted at Mach number M=0.2. The corrected volume penalization (VP) method, one of the immersed boundary methods, was used to represent the flat plate. In this method, the rigid bodies are regarded as porous materials with small permeability. The appropriate value of the permeability was determined carefully by checking convergence of the surface pressure distribution on the flat plate. The distance between the trailing edge and the observation points of the sound pressure is comparable to the length of the flat plate. Thus, the properties of the pressure at the observation points were checked to see whether we can detect the sound pressure correctly. The propagation velocity of the pressure fluctuations determined by the correlation between different points coincided with the sound speed. This confirms that sound generation is captured, although there exist components other than the acoustic waves. The reduction effects of finlets will be also discussed. |
Sunday, November 24, 2024 8:39AM - 8:52AM |
A34.00004: Role of separation bubbles in the presence of airfoil secondary tones for NACA profiles Jean Ribeiro, William R Wolf We perform numerical simulations to investigate the presence of secondary tones in airfoil trailing-edge noise for various NACA profiles at a moderate Reynolds number. The study focuses on two-dimensional airfoils with symmetric NACA profiles 0006 to 0018, at angles of attack ranging from 1 to 5 degrees. The free-stream Mach number is set as 0.2 and the chord-based Reynolds number is 50,000. Our results indicate that the secondary tones are intrinsically linked to the formation of separation bubbles on either side of the airfoils. The noise spectra become increasingly chaotic as the wake dynamics on both sides strengthen with increasing airfoil thickness and incidence angle. At lower angles of attack, thinner airfoils are dominated by a single tone, while thicker airfoils exhibit broadband spectra due to large separation bubbles near the trailing edge on the pressure side. As the angle of attack increases, secondary tones emerge for thinner airfoils. At higher angles of attack, separation on the pressure side is suppressed for all airfoils, resulting in noise spectra dominated by suction side wake dynamics. This study provides physical insights into the effects of adverse pressure gradients on airfoil tonal noise. |
Sunday, November 24, 2024 8:52AM - 9:05AM |
A34.00005: Sources of sound in the near-nozzle region of a subsonic Mach 0.9 jet Chandan Vempati, Santosh Hemchandra, Arnab Samanta We investigate a M = 0.9 turbulent round jet issued from a straight, cylindrical nozzle with the aim of understanding the behavior of near- nozzle noise sources, and their contribution to the radiated far-field spectra. The numerical database for this study is obtained via a Large- Eddy Simulation (LES) of a Mach 0.9 round jet with a diameter-based Reynolds number of 106, where a geometric trip forces the transition of the upstream boundary-layer inside the nozzle. The far-field spectra is computed by solving the permeable Ffowcs Williams-Hawkings (FW-H) equation in the frequency domain. Next, we perform a Spectral Proper Orthogonal Decomposition (SPOD) on the noise sources identified by the FW-H equation. Preliminary results reveal the presence of strong tonal peaks at high-frequencies (St >~ 2) in the SPOD spectra whose corresponding mode shapes show the presence of strong acoustic signatures inside the nozzle. In fact, once the far-field spectra is recalculated with only the modal structures present outside the nozzle, these high-frequency tones disappear. Further, some of the SPOD modes also seem to be scattered predominantly in the upstream direction with such scattering originating at the nozzle lip, as expected. The nature of these latter SPOD modes are different to the ones that do not scatter which we analyze in this work. |
Sunday, November 24, 2024 9:05AM - 9:18AM |
A34.00006: Aeroacoustic source diagnosis for turbulent subsonic jets using Canonical Correlation Decomposition Benshuai Lyu Common flow decompositions methods such as POD are widely used to diagnose the main aeroacoustic sources of turbulent jets. But the most energic modes extracted from POD are not necessarily the most acoustically-dominant flow structures. In this work, we use the recently-proposed Canonical Correlation Decomposition (CCD) method to probe the flow structures that are most correlated with acoustic pressure fluctuations of a turbulent subsonic jet. Both far-field and near-field pressure fluctuations are used as observables. In the near-field case, it is found that CCD is able to extract the well-known large-scale structures connected with K-H instabilities, which agrees current understanding on the jet’s near-field dynamics. The resulting CCD spectrum shows a significant low-rank behavior, and the near-field pressure fluctuations can be efficiently reconstructed using only the first two modes. In the far-field case, CCD spectrum also exhibit a much quicker decay. The leading-order modes are of large scale and low frequencies. As the mode number increases, CCD modes are of increasingly short scales and high frequencies. It is shown that some of the resulting CCD modes are directly connected with the spectral features of the far-field sound. It is shown that CCD can be a useful tool for observable diagnosis such as aeroacoustic source diagnosis for turbulent flows. |
Sunday, November 24, 2024 9:18AM - 9:31AM |
A34.00007: An extension of Hanson's theory of harmonic noise of rotors Niranjan Nanjappa, Matthew A Clarke The recent advent of EVTOLs has brought rotorcraft noise back into focus within aeroacoustics. Unlike traditional rotorcraft, EVTOLs feature multiple propellers operating simultaneously, complicating the noise they produce and necessitating higher fidelity aeroacoustic models. The present work builds on Hanson's theory for harmonic noise of propellers in the frequency domain, utilizing more sophisticated aerodynamic models. Hanson's theory addresses point, line, and planar acoustic compactness of loading source distributions on the blade. |
Sunday, November 24, 2024 9:31AM - 9:44AM |
A34.00008: Reynolds number dependence of wall-pressure fluctuations in weakly compressible turbulent channel flow Yi Liu, Meng Wang Compressible direct numerical simulations are employed to examine the Reynolds number dependence of the spatiotemporal characteristics of wall-pressure fluctuations in weakly compressible turbulent channel flow. Simulations are conducted at a bulk Mach number of 0.4 and friction Reynolds numbers of 180, 500, and 1000. With increasing Reynolds number, the results show a significant growth in the spectral level in the subconvective wavenumber range, as well as an elevation and broadening of the convective ridge, particularly at higher frequencies. At the low friction Reynolds number of 180, acoustic peaks are barely visible in the one-dimensional streamwise wavenumber-frequency spectrum. However, at higher Reynolds numbers, acoustic peaks associated with longitudinal and oblique waves emerge in the subconvective wavenumber range, accompanied by a broadband rise in spectral levels. The Reynolds number effect on acoustic ridges is most prominently displayed in the two-dimensional wavenumber-frequency spectra at the zeroth spanwise wavenumber, and the use of inner scaling for normalization is found to largely account for this effect in both convective and subconvective wavenumber ranges. |
Sunday, November 24, 2024 9:44AM - 9:57AM |
A34.00009: A Mathematical Model for the Interaction of Anisotropic Turbulence with Porous Surfaces Alistair Hales, Lorna J Ayton, Angus Wills, Chaoyang Jiang, Charitha M De Silva, Danielle Moreau, Con Doolan Leading-edge noise is a complex phenomenon that occurs when a turbulent fluid encounters a solid object and is a notable concern in various engineering applications. |
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