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Acoustic Perturbation
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Wolfgang Schroder – One of the best experts on this subject based on the ideXlab platform.

Noise sources of an unconfined and a confined swirl burner
Journal of Sound and Vibration, 2020CoAuthors: Konrad Pausch, Sohel Herff, Wolfgang SchroderAbstract:Abstract The sound sources of an unconfined and a confined swirl burner are investigated by a twostep approach. The Reynolds number based on the injector diameter downstream of the swirler is ReD = 8800 and the swirl number is S = 0.73. First, the conservation equations of a compressible fluid are solved to determine the flow field under reacting and nonreacting conditions. The solution determines the mean flow field and the source terms of the Acoustic Perturbation equations (APE). The contributions of the various sound sources are analyzed by solving the Acoustic Perturbation equations (APE) in a computational aeroAcoustics (CAA) simulation for each of the source terms. For the unconfined swirl burner, the precessing vortex core (PVC) of the swirl flow is a dominant sound source in nonreacting flow, whereas in reacting flow the unsteady heat release dominates the sound field. This changes when the burner is confined. A selfexcited instability occurs at the quarterwave mode of the burner and further sources, i.e., the momentum source due to fluctuations of the pressure at the flame front and the energy source due to velocity fluctuations at the flame front, become important to accurately predict the sound pressure amplitude of the limit cycle.

Noise Sources of Lean Premixed Flames
Flow Turbulence and Combustion, 2019CoAuthors: Konrad Pausch, Sohel Herff, Feichi Zhang, Henning Bockhorn, Wolfgang SchroderAbstract:The thermoAcoustic sound generation mechanisms of lean premixed laminar and turbulent flames are investigated by a twostep approach. First, the conservation equations of a reacting compressible fluid are solved. This solution is used to determine the Acoustic source terms of the Acoustic Perturbation equations (APE). Second, the contributions of the different source terms to the amplitude and phase of the Acoustic pressure signal are analyzed by solving the APE in a computational aeroAcoustics (CAA) simulation. The results show that it is not sufficient to only consider the unsteady heat release rate fluctuations which occur in the substantial pressuredensity relation. The acceleration of density gradients occurring at the flame front is a significant contributor to the overall sound emission. For the investigated laminar and turbulent flames the amplitude and phase of the Acoustic pressure signal can only be predicted accurately if both source terms are included in the Acoustic analysis.

disturbance evolution in the leading edge region of a blunt flat plate in supersonic flow
Fluid Dynamics Research, 2008CoAuthors: Stefan Mahlmann, Wolfgang SchroderAbstract:Abstract The spatiotemporal dynamics of small disturbances in viscous supersonic flow over a blunt flat plate at freestream Mach number M ∞ = 2.5 is numerically simulated using a spectral approximation to the Navier–Stokes equations. The unsteady solutions are computed by imposing weak Acoustic waves onto the steady base flow. In addition, the unsteady response of the flow to velocity Perturbations introduced by local suction and blowing through a slot in the body surface is investigated. The results indicate distinct disturbance/shockwave interactions in the subsonic region around the leading edge for both types of forcing. While the disturbance amplitudes on the wall retain a constant level for the Acoustic Perturbation, those generated by local suction and blowing experience a strong decay downstream of the slot. Furthermore, the results prove the importance of the shock in the distribution of Perturbations, which have their origin in the leadingedge region. These disturbance waves may enter the boundary layer further downstream to excite instability modes.
R Ewert – One of the best experts on this subject based on the ideXlab platform.

Prediction of Porous Trailing Edge Noise Reduction via Acoustic Perturbation Equations and Volume Averaging
, 2015CoAuthors: Benjamin Faßmann, R Ewert, Christof Rautmann, Jan Werner DelfsAbstract:Edge noise is generated if turbulence interacts with solid edges. Reduction of trailing edge noise of airfoils can be achieved by replacing the solid material at the trailing edge by inlays of porous permeable material. The Acoustic benefit of approximately 6 dB of such treatment is known from experiments. Enroute to numerically optimized porous properties, this paper presents a first principle based Computational AeroAcoustics (CAA) method for predicting the Acoustic effect of a porous NACA0012 trailing edge. In a hybrid twostep CFD/CAA procedure the turbulence statistics from a solution of the Volume Averaged NavierStokes (VANS) equations is used as a basis for the prediction of turbulentboundarylayer trailingedge noise (TBLTEN). For the Acoustic part of the calculation, the Acoustic Perturbation Equations (APE) are solved in the flow field. Inside the porous regions, a different set of governing equations, referred to as Linear Perturbation Equations (LPE) will be solved. The LPE represent a modified form of the Linearized Euler Equations (LEE) with the APE vorticity source term shifted to the righthand side. The new set of equations is derived by volume averaging the NavierStokes equations and decomposing the flow variables into a timeaveraged mean part and a fluctuating part and isolating the vorticity source term to the righthand side of the momentum equation. The LPE are verified by an analytical solution. The simulation results of a NACA0012 airfoil geometry with and without porous trailing edge treatment are compared to wind tunnel measurements. The noise reduction effect of such a trailing edge treatment is successfully demonstrated.

Application of a Discontinuous Galerkin Method to Discretize Acoustic Perturbation Equations
AIAA Journal, 2011CoAuthors: Marcus Bauer, Jiirgen Dierke, R EwertAbstract:Airframe noise of complex geometries, like e.g. highlift airfoil configurations, may conveniently be computed on unstructured grids. A Discontinuous Galerkin Method (DGM) provides a robust, highorder accurate discretization of systems of partial differential equations like for example the Acoustic Perturbation Equations (APE) even on this type of grid. In particular, a DGM based on Lagrange polynomials may be used, since it enables simple and cheap truncation of flux terms. The goal of the work reported herein is to verify such a DGM. Therefore, the sound field of a monopole situated in a laminar bounboundary layer is computed. Computations are stable, free from spurious numerical oscillations, and show very good agreement with other computations and with theoretical results.

the simulation of airframe noise applying euler Perturbation and Acoustic analogy approaches
International Journal of Aeroacoustics, 2005CoAuthors: R Ewert, Jan Werner Delfs, Markus LummerAbstract:The capability of three different Perturbation approaches to tackle airframe noise problems is studied. The three approaches represent different levels of complexity and are applied to trailing edge noise problems. In the EulerPerturbation approach the linearized Euler equations without sources are used as governing Acoustic equations. The sound generation and propagation is studied for several trailing edge shapes (blunt, sharp, and round trailing edges) by injecting upstream of the trailing edge test vortices into the meanflow field. The efficiency to generate noise is determined for the trailing edge shapes by comparing the different generated sound intensities due to an initial standard vortex. Mach number scaling laws are determined varying the meanflow Mach number. In the second simulation approach an extended Acoustic analogy based on Acoustic Perturbation equations (APEs) is applied to simulate trailing edge noise of a flat plate. The Acoustic source terms are computed from a synthetic turbulen…
Abhishek Saha – One of the best experts on this subject based on the ideXlab platform.

synchronization framework for modeling transition to thermoAcoustic instability in laminar combustors
Nonlinear Dynamics, 2020CoAuthors: Yue Weng, Vishnu R Unni, R I Sujith, Abhishek SahaAbstract:We, herein, present a new model based on the framework of synchronization to describe a thermoAcoustic system and capture the multiple bifurcations that such a system undergoes. Instead of applying flame describing function to depict the unsteady heat release rate as the flame’s response to Acoustic Perturbation, the new model considers the Acoustic field and the unsteady heat release rate as a pair of nonlinearly coupled damped oscillators. By varying the coupling strength, multiple dynamical behaviors, including limit cycle oscillation, quasiperiodic oscillation, strange nonchaos, and chaos, can be captured. Furthermore, the model was able to qualitatively replicate the different behaviors of a laminar thermoAcoustic system observed in experiments by Kabiraj et al. (Chaos (Woodbury, N Y) 22:023129, 2012). By analyzing the temporal variation of phase difference between heat release rate oscillations and pressure oscioscillations under different dynamical states, we show that the characteristics of the dynamical states depend on the nature of synchronization between the two signals, which is consistent with previous experimental findings.

synchronization framework for modeling transition to thermoAcoustic instability in laminar combustors
arXiv: Adaptation and SelfOrganizing Systems, 2020CoAuthors: Yue Weng, Vishnu R Unni, R I Sujith, Abhishek SahaAbstract:We, herein, present a new model based on the framework of synchronization to describe a thermoAcoustic system and capture the multiple bifurcations that such a system undergoes. Instead of applying flame describing function to depict the unsteady heat release rate as the flame’s response to Acoustic Perturbation, the new model considers the Acoustic field and the unsteady heat release rate as a pair of nonlinearly coupled damped oscillators. By varying the coupling strength, multiple dynamical behaviors, including limit cycle oscillation, quasiperiodic oscillation, strange nonchaos, and chaos can be captured. Furthermore, the model was able to qualitatively replicate the different behaviors of a laminar thermoAcoustic system observed in experiments by Kabiraj et al.~[Chaos 22, 023129 (2012)]. By analyzing the temporal variation of the phase difference between heat release rate oscillations and pressure oscioscillations under different dynamical states, we show that the characteristics of the dynamical states depend on the nature of synchronization between the two signals, which is consistent with previous experimental findings.

Acoustic Perturbation Effects on the Fluid Dynamics and Swirling Flame Response in a NonPremixed CoFlow Burner
Volume 2: Combustion Fuels and Emissions Parts A and B, 2010CoAuthors: Uyi Idahosa, Abhishek Saha, Navid Khatami, Saptarshi BasuAbstract:An investigation into the response of nonpremixed swirling flames to Acoustic Perturbations at various frequencies (fp = 0–315 Hz) and swirl intensities (S = 0.09 and 0.34) is carried out. Perturbations are generated using a loudspeaker at the base of an atmospheric coflow burner with resulting velocity oscillation amplitudes u′ /Uavg  in the 0.03–0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. Flame heat release is quantitatively measured and simultaneously imaged using a photomultiplier (PMT) and a phaselocked CCD camera. Both of which are fitted with 430nm bandpass filters for observing CH*chemiluminescence. The flame response is observed to exhibit a lowpass filter characteristic with minimal flame response beyond pulsing frequencies of 200Hz. Flames at lower fuel flow rates are observed to remain attached to the central fuel pipe at all Acoustic pulsing frequencies. PIV imaging of the associated isothermal fields show the amplification in flame aspect ratio is caused by the narrowing of the inner recirculation zone (IRZ). The Rayleigh criterion (R) is used to assess the potential for instability of specific Perturbation configurations and is found to be a good predictor of unstable modes. Phase conditioned analysis of the flame dynamics yield additional criteria in highly responsive modes to include the effective amplitude of velocity oscillations induced by the Acoustic pulsing. Highly amplified responses were observed in pulsed flame configurations with Strouhal numbers (St = fp Uavg /dm ) in the 1–3.5 range. Heat release to velocity Perturbation time delays on the order of the Acoustic pulsing period also characterized the highly responsive flames. Finally, wavelet analyses of heat release Perturbations indicate sustained low frequency oscillations that become more prominent for low Acoustic pulsing frequencies in lean flame configurations.Copyright © 2010 by ASME
Michael M. Yartsev – One of the best experts on this subject based on the ideXlab platform.

Longterm and persistent vocal plasticity in adult bats
Nature Communications, 2019CoAuthors: Daria Genzel, Janki Desai, Elana Paras, Michael M. YartsevAbstract:Bats exhibit a diverse and complex vocabulary of social communication calls some of which are believed to be learned during development. This ability to produce learned, speciesspecific vocalizations – a rare trait in the animal kingdom – requires a highdegree of vocal plasticity. Bats live extremely long lives in highly complex and dynamic social environments, which suggests that they might also retain a high degree of vocal plasticity in adulthood, much as humans do. Here, we report persistent vocal plasticity in adult bats (Rousettus aegyptiacus) following exposure to broadband, Acoustic Perturbation. Our results show that adult bats can not only modify distinct parameters of their vocalizations, but that these changes persist even after noise cessation – in some cases lasting several weeks or months. Combined, these findings underscore the potential importance of bats as a model organism for studies of vocal plasticity, including in adulthood.Bats are longlived animals that can produce a complex vocabulary of social communication calls. Here, the authors show that even in adulthood, bats retain the ability to adaptively introduce longterm modifications to their vocalizations, showing persistent vocal plasticity.
Matthias Meinke – One of the best experts on this subject based on the ideXlab platform.

Acoustic Refraction Effects in Turbulent Reacting Flows
15th AIAA CEAS Aeroacoustics Conference (30th AIAA Aeroacoustics Conference), 2009CoAuthors: Wolfgang Schr, Matthias MeinkeAbstract:A weak nonisotropic component of the Acoustic field has been stated by classical theories and observed in experiments. In several investigations the deviations from the isotropic character of the Acoustic field were suspected to be related to refraction effects due to the varying mean speed of sound. In this investigation this hypothesis is substantiated. Using the Acoustic Perturbation equations for reacting flows (APERF) the Acoustic impact of spatially varying mean speed of sound within a turbulent reacting flow field is numerically investigated. The resultsshowthis effect to be frequencydependent and the meanflowfield, especially the mean speed of sound, to be taken into account to accurately predict combustion generated noise of open turbulent flames.

Numerical Applicability of Different Sound Source Formulations to Compute Combustion Noise Using Acoustic Perturbation Equations for Reacting Flows
14th AIAA CEAS Aeroacoustics Conference (29th AIAA Aeroacoustics Conference), 2008CoAuthors: Thanh Phong Bui, Matthias Meinke, Matthias Ihme, Wolfgang Schroeder, Heinz PitschAbstract:Combustion noise analyses in a hybrid computational aeroAcoustics (CAA) context are presented. Acoustic Perturbation equations for reacting flows (APERF) describe the wave propagation, while unsteady results from a variable density incompressible reactive largeeddy simulation (LES) are used to evaluate the Acoustic source terms. To simulate combustion generated noise via such a hybrid approach, an appropriate source description has to be taken, which preferably matches two requirements, i.e., on the one hand, to efficiently and accurately predict the generated Acoustic field, and on the other hand, to easily determine the source term from the LES. In this study, three source formulations for the APERF system are examined on two Acoustic meshes with different resolutions in the source region to predict the Acoustic field of an open turbulent nonpremixed flame. Using the source term, which is expressed via the scaled partial time derivative of the density, the Acoustic field can be reproduced best up to a maximum Strouhal number of StD = 2 on the fine mesh. However, for this source formulation spurious noise can be observed depending on the CAA resolution. It will be shown in this study that this observation can be related to the “artificial interpolation induced acceleration” effect. A compromise between efficiency and accuracy represents the source formulation expressed via the scaled material derivative of the density.

Numerical analysis of the Acoustic field of reacting flows via Acoustic Perturbation equations
Computers & Fluids, 2008CoAuthors: T. Ph. Bui, Wolfgang Schroder, Matthias MeinkeAbstract:Abstract The Acoustic radiation of a turbulent nonpremixed flame using a hybrid method is numerically simulated. The twostep method consists of an incompressible largeeddy simulation (LES) and Acoustic Perturbation equations (APE), which are reformulated to account for reacting flow effects (APERF). In reacting flows, hybrid methods to compute Acoustic radiation have some advantages compared to the direct simulation of the Acoustic field using a compressible LES. Considering the different characteristic length scales optimized schemes can be applied to each subproblem, i.e., to the hydrodynamic and the Acoustic problem. This is of interest since the fluid mechanics is governed by the combustion process and to compute the highly intricate chemistry tailormade schemes with reasonable computational costs combustion models can be implemented to resolve this phenomenon by, for instance, preprocessed databases for the chemical reactions, like in the steady flamelet approach. The APERF system possesses several source terms on the righthand side (RHS), which are thoroughly discussed to their relation to various sound mechanisms. The Acoustic sources describe the impact of unsteady heat release, nonisomolar combustion, species diffusion, heat diffusion, viscous effects, nonuniform mean flow and nonconstant combustion pressure effects, and the influence of acceleration of density inhomogeneities. Moreover, an additional source term within the APERF pressure–density relation can be identified to describe the local Acoustic wave amplification due to Acoustic–flame interaction. It is evidenced that the wellknown Rayleigh criterion can be directly given by this source. The unsteady heat release is shown to occur in the total time derivative of the density that is directly provided from an LES solution. By analyzing via the twostep method the Acoustic field of an open turbulent nonpremixed flame being generated just by the total temporal derivative of the density, and by comparing the numerical data with experimental findings the total substantial derivative is shown to describe for a wide frequency range the essential sound propagation caused by reacting flows. Nevertheless, it is also discussed that to simulate all the details over the complete frequency range additional source mechanisms occurring on the RHS of the APERF system are to be considered in the investigation.