Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F06: Aeroacoustics |
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Chair: Yuji Hattori, Tohoku University Room: Georgia World Congress Center B208 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F06.00001: Resolvent analysis of a Mach 1.5 jet for noise source identification Ethan M Pickering, Oliver T Schmidt, Georgios Rigas, Tim E Colonius Acoustic sources for an isothermal Mach 1.5 round jet are determined through resolvent analysis and spectral proper orthogonal decomposition (SPOD). Both physics-based resolvent analysis and data-driven SPOD (using a high-fidelity, experimentally-verified, large-eddy simulation (LES) database) provide a basis for predicting the perturbation field. Singular value decomposition of the resolvent operator constructed using an LES baseflow identifies optimal volumetric forcing modes and their corresponding linear responses. In order to determine physically relevant modes, projections of the highest gain responses and highest energy SPOD modes from LES realizations are computed. To determine the occurrence of the associated resolvent forcings, projections are then computed upon the full LES nonlinear terms and the distribution of these values leveraged for jet noise modeling. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F06.00002: Continued Development of a Coupled LES/Stochastic Approach to Jet Noise Prediction Joshua D Blake, Adrian Sescu, David Thompson, Yuji Hattori Turbulence modeling for jet noise prediction remains a challenge due to the wide range of temporal and spatial scales present in the jet. Smaller turbulent scales corresponding to higher-frequency jet noise content often reside in a critical range for human hearing. Resolving this content with LES or DNS requires a large amount of computational resources. With the goal of extending the high frequency content of LES jet noise predictions at a reduced cost, we employ a coupled LES/stochastic method, wherein large flow structures are solved via very large eddy simulations (VLES), while the smaller-scale flow structures are modeled via synthetic turbulence. We show the feasibility of such a coupled approach with a Fourier-based stochastic method, and we present progress toward developing a synthetic-eddy-based method for the supplemented small-scale turbulence. Comparisons between the current approach and pure LES show that predicting additional higher frequency content in a more cost-effective approach is more efficient than simply increasing the grid resolution in the LES framework. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F06.00003: Data-informed dynamic acoustic source modeling in high-speed jets Armin Zare, Jinah Jeun, Joseph W Nichols, Mihailo R Jovanovic We build on work by Zare, Jovanovic, and Georgiou (JFM, vol. 812, 2017) to develop dynamic noise models that account for the far-field acoustics of a Mach 0.9 subsonic turbulent jet. Given far-field time-averaged correlations of pressure we formulate an inverse problem to determine the forcing statistics to the linearized model that provide consistency with high-fidelity large-eddy simulation. Fluctuations in the near-field of the jet are projected to far-field sound via the Ffowcs Williams-Hawkings method. To reduce computational effort, we utilize the method of snapshots to obtain a reduced-order model of the input-output behavior of the jet, and design filters to provide an explicit dynamical representation of acoustic sources. Our data-driven approach reconciles linearized models of near-field fluctuations to acoustics at various observer angles by introducing dynamical modifications to the linearized operator. We conduct a frequency response analysis to examine the predictive capability of our model in capturing near-field coherent flow structures and far-field acoustic radiation. |
Monday, November 19, 2018 8:39AM - 8:52AM |
F06.00004: Delineation and evolution of aerodynamic and acoustic components of pressure in turbulence Unnikrishnan Sasidharan Nair, Datta V Gaitonde To aid in jet noise source identification, several studies have attempted to isolate the propagating component of pressure from that which convects with structures. While the former contributes to farfield sound of the jet (acoustic pressure), the latter decays rapidly in the nearfield (aerodynamic or hydrodynamic pressure). Fourier- and wavelet-analyses usually used to split pressure in this manner rely on various pre-defined parameters such as wave-speed and statistics of coherent events, which introduce ambiguity in the analysis. The availability of accurate spatio-temporal data from high-fidelity simulations offers an opportunity to perform this separation in a more rigorous manner. We propose a first-principle-based approach predicated on the availability of pure acoustic and hydrodynamic components of fluctuations in any turbulent medium. Specifically, the momentum potential theory is extended to derive Poisson equations which separate the acoustic and hydrodynamic components of pressure from the momentum equations. The splitting is successfully applied to a turbulent jet and yields insights into the spatio-temporal segregation that prevails among the constituent components of pressure fluctuations. |
Monday, November 19, 2018 8:52AM - 9:05AM |
F06.00005: Nonlinear dynamics of sound production by a mixing layer: an investigation using Lyapunov covariant vectors Zhong-Nan Wang, Qiqi Wang, Paul Tucker Current design process of noise control is empirical and depends on expensive rig test data. We aim to use eddy resolving simulation along with Lyapunov analysis to understand the nonlinearity in sound source compared with linear wavepackets and provide physical insights into chevron design. In this research, a mixing layer with Re_{δ}^{*}=300 is designed to produce the basic ingredients of jet mixing noise. 152 largest Lyapunov exponents are obtained and neutral modes are reached around the 100^{th} Lyapunov exponent. Vorticity and pressure components of the covariant Lyapunov vectors are investigated and compared with Spectral POD (SPOD) modes (linear wavepackets). It is found that the similarity between covariant Lyapunov vectors and SPOD modes exist at the early stage of shear layer development. This indicates that the nonlinearity becomes more important in the late stage, which might be linked with the missing intermittency dynamics in wavepackets model. This database will be used for computation of sensitivity information of serration change and far-field sound to near-field source using both tangent and adjoint method in the future. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F06.00006: Navier-Stokes Equations based Acoustic Analogy with Shock-Noise Prediction Example Steven A E Miller, Trushant Patel We propose a new acoustic analogy that retains the Navier-Stokes equations as both the propagator and source terms. This approach relies on a new decomposition involving the base flow, aerodynamic fluctuations, and acoustic fluctuations. The aerodynamic fluctuating quantities are further decomposed into large-scale highly spatially coherent turbulence and fine-scale spatially incoherent turbulence. The resultant sources of sound are written as two-point cross-correlations involving each term of the Navier-Stokes equations. The linearized Navier-Stokes equations are solved for the vector Green’s function for the purpose of predicting the acoustic propagation. A closed-form equation for the spectral density of the fluctuating acoustic quantities is derived by a convolution integral involving the vector Green’s function and the two-point cross-correlation source term from Navier-Stokes equations. The closed-form equation is valid for acoustic fluctuations. We model the source terms using a combination of experimental data, source term analysis, and numerical simulation. We show a sample prediction for shock-associated noise using the dominant term in the source model for an off-design supersonic jet flow. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F06.00007: An analytical and experimental study on the optimal serration geometry for leading-edge noise reduction Benshuai Lyu, Lorna J. Ayton, Paruchuri Chaitanya Leading-edge noise refers to the sound generated due to the scattering of unsteady flow by the leading-edge of an airfoil. It is a common noise source in many multi-row rotor and stator systems, such as jet engines and contra-rotating open rotors. Using serrated leading edges has been shown to be capable of reducing leading-edge noise significantly, but the optimal serration geometry has not been known. In this study, by performing an asymptotic analysis, we show that in order to achieve greater noise reduction at high frequencies, the serration profile cannot have stationary points. Therefore, piecewise smooth profiles free of stationary points are more desirable. Moreover, we show that greater noise can be achieved in the high-frequency regime by using serrations that are sharper only around the non-smooth points. Based on these findings, a new type of serration profile is proposed. Both numerical evaluations and experimental tests confirm its improved acoustic performance. In particular, the trends predicted by the analytical study are well supported by the experimental results. It is expected that these findings can serve as an essential guide for designing serrations. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F06.00008: Prediction of broadband trailing edge noise from a NACA0012 airfoil using DNS and WMLES Mohammad Mehrabadi, Daniel Joseph Bodony Motivated by the need to predict the broadband noise generated by the main propulsive fan of modern gas turbine engines, we study the broadband noise due to the turbulent flow on a NACA0012 airfoil at zero degree angle-of-attack, a chord-based Reynolds number of 408,000 and a Mach number of 0.1 using direct numerical simulation (DNS) and wall-modeled large-eddy simulation (WMLES). We investigate the flow hydrodynamics and sound generation from the WMLES and examine its predictability compared with our DNS results, as well as available datasets for the same flow conditions. Verification of WMLES for such a canonical problem is crucial since it provides useful insight about the WMLES approach before using it for broadband fan noise prediction. We discuss the resolution requirements necessary for the accurate prediction of wall surface pressures using WMLES. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F06.00009: Reduction of Aeroacoustic Noise Generated from a Flow past a Cylinder by Porous Materials Yasunori Sato, Yuji Hattori It is important to reduce the aerodynamic noise generated from high-speed trains in order to achieve their further speed-up in the near future. In recent years, a new method for reducing aerodynamic noise by using porous materials has been proposed, and confirmed to be effective experimentally. In this study, by modeling the porous material by a large number of small cylinders, the flow around the cylinder covered by the porous materials is simulated by direct numerical simulation based on the modified volume penalization (VP) method. It is confirmed that the aerodynamic noise from the cylinder can be reduced by using porous materials. In particular, the aerodynamic sound reduction effect is significant when installing only one row of the small cylinders in the radical direction. In this case, depending on the number of the cylinders, the reduction rate of acoustic power becomes about of the case of a bare cylinder. In contrast to the case of a bare cylinder, the Karman vortex shedding is delayed downstream by installing the small cylinders. In other words, vortex shedding in the wake is suppressed by using the porous material. The essential mechanism of the reduction of aerodynamic sound is that placing small cylinders makes the wake behind the cylinder nearly steady. |
Monday, November 19, 2018 9:57AM - 10:10AM |
F06.00010: Global response of cavity-flow shear layer to low and high amplitude acoustic forcing Claire Bourquard, Abel Faure Beaulieu, Nicolas Noiray Aeroacoustic instabilities in deep cavities subject to grazing flows are governed by the nonlinear response of the shear layer to transverse acoustic velocity forcing. We characterise experimentally the shear layer dynamics with and without acoustic forcing using time-resolved particle image velocimetry (PIV) and acoustic measurements. Considering the Navier-Stokes equations linarized around the mean flow from the PIV, we predict the harmonic response of the flow for a range of acoustic forcing amplitudes and frequencies. A global stability analysis is also carried out, showing that the modes, which are marginally stable, define the hydrodynamic feedback responsible for the aeroacoustic instabilities. These analysis are performed for low Mach and highly turbulent conditions, for different cavity opening geometries, with sharp and round corners. These results allow us to explain the self-sustained aeroacoustic oscillations for a range of grazing flow velocities and predict the associated limit cycle amplitudes. |
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