Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Q12: Acoustics: Aero |
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Chair: Hirotaka Sakaue, University of Notre Dame Room: 139 |
Monday, November 21, 2022 1:25PM - 1:38PM Not Participating |
Q12.00001: Microfiber coating as a noise reducing device for a propeller blade Vince DiFilippo, Mitsugu Hasegawa, Hirotaka Sakaue A microfiber coating has been studied as a noise reducing device for a propeller blade. It is a hair-like structure attached to the blade via an adhesive. To determine the optimal microfiber locations for a blade-noise reduction, the microfiber coating is placed symmetrically at different spanwise and chordwise locations on each blade of the 16-inch diameter rotor, and rotated at a range of wingtip speeds. Sound data is captured by a microphone placed 3 feet from the rotor and processed resulting in an average sound level in decibels and a spectral density plot over the relevant frequency range for each rotor configuration and wingtip speed. When the coating is placed along the leading edge of the inner portion of the rotor and the trailing edge of the outer portion of the rotor, the microfiber coating leads to noise reductions of up to 2.3 dBA. The low fiber density corresponded with larger noise reduction than the high fiber density for a given location. At the highest wingtip speeds of 72 to 83 meters per second, the high density fiber resulted in increased noise of up to 5.2 dBA relative to the baseline. Current status of this study as well as our future studies will be presented. |
Monday, November 21, 2022 1:38PM - 1:51PM |
Q12.00002: Aeroacoustics Analysis of Rifle Silencers Using Unsteady Overset CFD Methods Bryan Lewis, Eric R Anderson, Robert Kunz, Jacob Cox, Michael Quintana, Hunter Troutman Rifle silencers are used in both military and recreational applications to prevent hearing damage for firearm operators. The acoustic signature of a rifle firing is composed of both a spherical shock wake produced by the propellent combustion and an oblique shock wave form the projectile. Silencers reduce the peak sound pressure and intensity of the combustion shock by increasing the time over which the combustion gases are released from the barrel. Many silencer designs have been employed over the years, which varying degrees of success. While silencer manufactures conduct performance evaluations for new products, few studies have been published on the gas dynamic flow through a silencer. |
Monday, November 21, 2022 1:51PM - 2:04PM |
Q12.00003: Aliasing Effect and Mitigation Strategy in Combined CFD-CAA Simulations Ang Li, Jun Chen, Jordan A Awan, David Eddins, Shaheen N Awan Many aeroacoustics problems are solved by combined CFD-CAA simulations in which the time step typically cannot be too small due to the limited computational resource. This situation results in an identified aliasing error in spectral analysis, so an anti-aliasing operation prior to sampling is necessary. In this study, an anti-aliasing filter in a time-domain is designed based on the compact finite difference scheme. This filter is applied on the Navier-Stokes solver directly, prior to sampling for CAA analysis. A pipe flow with oscillating inlet velocity validates this mitigation strategy. The result shows the fake peak induced by the aliasing error is removed while not affecting other signature peaks. The filter is applied to more complicated cases with signature peaks in predicting acoustic fields of a vortex whistle. The acoustic field around the vortex whistle with constant and variable inlet flow rates is simulated and the aliasing peak is successfully removed. Although the peak magnitudes are decreased slightly by the filter, the signature frequencies are not changed. Therefore, the simulation with anti-aliasing filter can predict the acoustic feature without the aliasing error, even if the time step is not sufficiently small, and thus reduce the simulation time significantly. |
Monday, November 21, 2022 2:04PM - 2:17PM |
Q12.00004: Generation of Screech Tones at the 2nd Harmonic for a Rectangular Jet Olivia G Martin, Gao Jun Wu, Sanjiva K Lele Supersonic jet screech is a form of aero-acoustic resonance observed in non-perfectly expanded jets. Downstream-traveling hydrodynamic waves and upstream-traveling disturbances in the jet form a constructive feedback loop which produces high-intensity tones. The upstream-traveling waves comprise of both external acoustic waves and a guided jet mode. The fundamental screech tone is believed to result from interaction between the Kelvin-Helmholtz instability waves and shock cells. It has also been observed that the guided jet mode plays an important role in closing the feedback loop at the fundamental (Wu et al., 2021). The mechanisms responsible for producing higher harmonic screech tones, however, are less well understood. We perform high-fidelity LES simulations of jet screech from a 4:1 rectangular nozzle. SPOD is used to analyze the role of the upstream-traveling guided mode in closing the feedback loop at the second harmonic frequency. Analysis of the spatial wavenumbers also suggests that self-interaction of the guided jet mode at the fundamental frequency could produce the second harmonic guided mode. The performance of phased acoustic source models based on this theory are assessed for their abilities to reproduce the directivity patterns of the second harmonic. |
Monday, November 21, 2022 2:17PM - 2:30PM |
Q12.00005: Tonal noise generation mechanisms in airfoil transitional flows up to $Re 4\times10^5$ Tulio R Ricciardi, William R Wolf Large eddy simulations are performed to study tonal noise generation by a NACA0012 airfoil at an angle of attack $\alpha = 3 deg.$ and freestream Mach number of $M_{\infty} = 0.3$. Different Reynolds numbers are analyzed in the range $0.5 \times 10^5 \le Re \le 4 \times 10^5$. The flow patterns responsible for noise generation originate 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. As a consequence, the separation bubbles on the pressure side also 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 boundary layer on the suction side that reduces the coherence level, causing only small scale turbulent eddies to be observed at the trailing edge. 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 and the noise spectrum presents a single tonal peak. |
Monday, November 21, 2022 2:30PM - 2:43PM |
Q12.00006: Suppression of acoustic instabilities in boundary layer flows by heating / cooling of the wall and wall impedance Lara De Broeck, Simon Goertz, Tim J Flint, Carlos A Gonzalez, Martin Oberlack, Sanjiva K Lele We address the problem of acoustic instabilities in compressible boundary layers with the aim of suppressing the instabilities and delaying the transition by means of cooled walls of impedance type. |
Monday, November 21, 2022 2:43PM - 2:56PM |
Q12.00007: Whistling of a deep axisymmetric cavity: interactions between an azimuthal aeroacoustic mode and the mean flow. Abel Faure Beaulieu, Tiemo Pedergnana, Yuan Xiong, Nicolas Noiray Our experimental, numerical and theoretical study deals with the whistling of a deep axisymmetric cavity in a cylindrical duct subject to a low Mach turbulent flow. The aeroacoustic instabilities result from the constructive feedback between one of the trapped acoustic modes of the cavity and the dynamics of the shear layer at the cavity opening. Acoustic measurements reveal that the predominant instabilities involve the first azimuthal acoustic mode. For the first time in such configuration, time-resolved stereoscopic particle image velocimetry (PIV) is performed to visualize the three components of the velocity field in a two-dimensional cut of the cavity. The velocity fluctuations are then phase averaged to unravel the structure of the different hydrodynamic modes that contribute to the instabilities. The experiments allow us to discover a spontaneous symmetry breaking of the mean flow when a self-sustained high-amplitude aeroacoustic wave steadily spin around the cavity. It manifest itself by a mean swirling motion which predominantly rotates against the spinning direction of the wave. With a second order perturbation model on the Navier-Stokes equations, we show that the emergence of this mean swirl is caused by the Reynolds stress tensors arising from the hydrodynamic part of the aeroacoustic wave. |
Monday, November 21, 2022 2:56PM - 3:09PM |
Q12.00008: Aeroacoustic investigation of a helicopter tail rotor. Prateek Jaiswal, Stephane Moreau, Marlene Sanjose The present study focuses on the aeroacoustic aspects of a full-scale helicopter tail rotor. To this end, the newly conceived, Electronically Distributed Anti Torque (EDAT), tail rotor is investigated both experimentally and numerically. Acoustic directivity in the far-field was measured in the anechoic wind tunnel at Université de Sherbrooke. These acoustic measurements confirm the presence of tones at the blade passing frequency, furthermore, quasi-tonal and broadband acoustic components were also obtained. In order to quantify the installation effects on the measurements and source generation mechanisms, high-fidelity Very Large Eddy Simulations (VLES) with the Lattice Boltzmann method were performed at a single operating condition. This is achieved by simulating the complete experimental setup, e.g. the anechoic room and the tail rotor geometry. A good agreement in the overall mean performances, such as torque and mass flow, is achieved between the VLES simulations and the measurements. Furthermore, direct far-field acoustic spectra computations show a good agreement with the experimental measurements. Cross correlation between the near-field and far-field pressure was performed in order to reveal possible locations of noise generation and propagation on the tail rotor. |
Monday, November 21, 2022 3:09PM - 3:22PM |
Q12.00009: Wall pressure spectra modelling for trailing edge noise in non-equilibrium adverse pressure gradient boundary layers. Saurabh Pargal, Junlin Yuan, Stephane Moreau Based on existing direct numerical simulation (DNS) datasets of non-equilibrium boundary layers over flat plate or airfoils with and without separation, widely used semi-empirical wall-pressure-spectra models such as Goody’s, Rozenberg’s, and Lee’s models are tested. All tested models display large errors near region of strong adverse pressure gradients (APG) and in the region of separated flow, mainly due to the models’ direct dependences on local friction velocity as a parameter. |
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