Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session KN: Acoustics II |
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Chair: Lawrence Ukeiley, University of Florida Room: Tampa Marriott Waterside Hotel and Marina Meeting Room 9 |
Monday, November 20, 2006 5:15PM - 5:28PM |
KN.00001: Acoustically coupled jet resonance in a finite-length duct Victor Topalian, Arnab Samanta, Jonathan Freund Remarkably high amplitude fluctuations have been observed in jet engine test cells, which most simplistically are a jet confined in a finite-length outer duct. The dominant frequencies correspond well with the acoustic modes of the outer duct, which suggests that the resonance involves strong acoustic coupling, but the mechanism's details remain unclear, which hampers mitigation efforts. We investigate the coupling through a specially designed direct numerical simulation code that uses a high-order staggered finite difference formulation. The current focus is on a parametric study in two space dimensions. The acoustic modes of the finite duct are excited by the jet but not to the degree anticipated unless the configuration is carefully tuned to match the natural instability frequencies supported by the confined jet. The basic mechanism of receptivity at the nozzle lip is reasonably well understood, but the acoustic reaction of the duct to an outgoing hydrodynamic instability is not. For a jet engine test cell, the instability wavelength is comparable to the duct diameter, so a constant pressure boundary condition is inappropriate. This part of the feedback mechanism is investigated analytically, in conjunction with the simulation, using a vortex sheet model. [Preview Abstract] |
Monday, November 20, 2006 5:28PM - 5:41PM |
KN.00002: Adjoint-based analysis of turbulent free shear flow noise Randall Kleinman, Jonathan Freund An adjoint-based control methodology has been developed that circumvents the complexities of the noise generation mechanisms in a free shear flow and allows for direct study of noise control. The numerical solution of the adjoint of the linearized and perturbed compressible flow equations provides the gradient information needed to improve controls in a conjugate-gradient optimization procedure. For generic forcing, the method has been implemented within a direct numerical simulation (DNS) framework and demonstrated successfully on a two-dimensional mixing layer. Currently, a three-dimensional turbulent mixing layer has been simulated and controlled via the adjoint method. The inflow of this calculation comes from an auxillary simulation of a streamwise-periodic mixing layer. Preliminary results show a corresponding 28\% reduction, with simulations continuing. A sensitivity analysis has also been carried out using the adjoint system to assess the turbulence length scales affected by the control, as well as to study their contributions to different components of the sound field. [Preview Abstract] |
Monday, November 20, 2006 5:41PM - 5:54PM |
KN.00003: The pressure field of co-axial subsonic jets with and without serrations Peter Jordan, Charles Tinney Data from measurements performed in the near pressure fields of subsonic co-axial jets (serrations, bypass-ratio, temperature ratio) are analyzed, where the pressure field is considered to constitute a reasonable footprint of the large-scale coherent structures (or low-order dynamic) of the flows. A comparative study shows how serrations produce considerable reductions in the nearfield pressure levels, however structural changes in the flows are more subtle. A transformation from (x-t) to (k-omega) is used to study the pressure fields in wavenumber-frequency space and permits a clearer separation of the measured pressure field into (1) purely hydrodynamic components, with subsonic phase velocity (redundant in the production of sound) and (2) hydrodynamic-acoustic mixed components with sonic and supersonic phase velocities (active in the production of radiating sound). A filtering operation, based on the dispersion inequality (omega$>$ka), leads to a reconstruction of the sound-producing dynamic of the low-order flow structure and the statistics of this field are compared to those of the unfiltered flow dynamic. POD of the filtered pressure field identifies the subtleties of the structural changes induced by the serrations. [Preview Abstract] |
Monday, November 20, 2006 5:54PM - 6:07PM |
KN.00004: Sound radiation from instability waves in subsonic jets: entropy sound and superdirectivity Lutz Lesshafft, Patrick Huerre, Pierre Sagaut The acoustic field radiated by instability wave packets in subsonic round jets is investigated by direct numerical simulation. The directly computed acoustic far field is compared to solutions of the Lighthill equation. Wave packets that emerge in response to forcing in convectively unstable isothermal jets emit a quadrupole sound field. If the jet is sufficiently heated with respect to the ambient medium, the occurrence of absolute instability gives rise to self-sustained global oscillations (global modes). The acoustic field emitted by such global modes displays the directivity of a dipole. An investigation of the Lighthill equation reveals that this dipole field in hot jets is caused by entropy-related source terms, whereas the dominant acoustic sources in isothermal jets are related to Reynolds stresses. Conditions for the occurrence of superdirective sound fields and for their numerical observation are explored. [Preview Abstract] |
Monday, November 20, 2006 6:07PM - 6:20PM |
KN.00005: Correlation of Large-scale Structures and Far-field Radiated Noise in High-speed and High Reynolds Number Jets Jeff Kastner, Jin-Hwa Kim, Mo Samimy The importance of dynamic processes involving large coherent structures in entrainment, mixing, and noise generation in free jets and shear layers is well established. The present research attempts to further the understanding of the relationship between the large coherent structure dynamics and the far-field sound of high-speed and high Reynolds number jets. This was achieved by exploring the flow field and the far field of two axisymmetric jets with a Mach number of 0.9 (Re$_{D}$ of 7.6 x 10$^{5})$ and 1.3 (Re$_{D}$ of 1.1 x 10$^{6})$. The jets were controlled by eight localized arc filament plasma actuators distributed around the nozzle exit. The actuators operate over a frequency range of 0 to 200 kHz, and the phase between the eight actuators can be varied to allow excitation of azimuthal modes 0, 1, 2, 3, $\pm $1, $\pm $2 and $\pm $4. The far field of the jets was probed with a three-dimensional microphone array at 30\r{ } to the downstream jet axis. The array estimates the origin of sound waves in space and time and also provides the sound pressure level. The flow field of the jets was investigated using flow visualization and particle image velocimetry. Both techniques are non-intrusive planar measurements that provide mean and fluctuating quantities of the flow field. Detailed results from the far-field and flow field diagnostics and their correlation will be presented and discussed. [Preview Abstract] |
Monday, November 20, 2006 6:20PM - 6:33PM |
KN.00006: Identification and Control of Noise Source Mechanisms in a Transonic Axisymmetric Jet Andre Hall, Jeremy Pinier, Mark Glauser An experimental examination aimed at characterizing the aeroacoustic effect linked to the turbulent mixing of the exhausted jet plume with the ambient air in high-speed jets is comprised of a 50.8mm nozzle at Mach 0.85, operated under both heated (260oC) and room temperature (0oC) conditions. Both the hydrodynamic near- field and acoustic far-field pressure regions are examined. The near- field using an azimuthal array of fifteen (15) dynamic response pressure transducers positioned near the jet’s lip, and the far- field using a boom array of six (6) acoustic microphones (6.35mm in diameter). Instantaneous 3 component velocity measurements are acquired, simultaneously, in the r, theta plane at several streamwise positions between z/D=3:8 (the region where the sound producing events are found to be dominant) using a stereo PIV system. This data set is utilized in conjunction with multi-point low-dimensional techniques to characterize a low-dimensional description of the velocity field, with minimal effect on far-field acoustics. The low- dimensional description of the velocity field is examined to identify the dominant noise source mechanism in both jets. Calculation of a modified Lighthill source term, and azimuthal modal forcing, are used as a measure of source intensity and a gauge for noise reduction schemes, respectively. Where control of noise sources is concerned, a modal analysis of the near-field region has shown that modal forcing may prove a promising method. We greatly acknowledge the support of the AFOSR and the CNY-PR AGEP Alliance. [Preview Abstract] |
Monday, November 20, 2006 6:33PM - 6:46PM |
KN.00007: Aeroacoustic Noise Predictions using Nonlinear PSE Based Methods Lawrence Cheung, Sanjiva Lele High-fidelity simulations of aeroacoustic radiation from free shear flows typically involve computationally expensive DNS or LES computations to resolve the dynamically important temporal and spatial scales. We present a hybrid method to predict the far-field sound of shear layers, using a combination of the Nonlinear Parabolized Stability Equations (NPSE) and an appropriate acoustic analogy. The NPSE provide an efficient and accurate way to capture the large-scale hydrodynamics of the flow, and are used to calculate the source terms for an acoustic analogy method to predict the far-field sound. Four cases are considered in this study: an isothermal supersonic, a heated supersonic, an isothermal subsonic, and a heated supersonic shear layer. Comparisons between the NPSE simulations and their DNS counterparts are shown. In both of the supersonic shear layers, we find that the NPSE solution is accurate in both the near field and far field regions. For the subsonic shear layers, the NPSE only captures the near field hydrodynamics and underpredicts the acoustic radiation. However, when combined with Lilley's acoustic analogy, reasonable agreement is found with DNS calculations. Comparisons with other acoustic analogies, such as Lighthill's method, will also be shown. [Preview Abstract] |
Monday, November 20, 2006 6:46PM - 6:59PM |
KN.00008: Noise prediction of a low speed airfoil Yaser Khalighi, Meng Wang, Daniel Bodony, Parviz Moin The goal of this work is accurate and efficient prediction of flow generated noise in the presence of solid objects. Flow around a low speed airfoil is considered at chord Reynolds number of Re = 150 000. This flow configuration represents an important aeroacoustics problem with complex physical effects including solid boundaries, boundary layers with pressure gradient, transition and turbulent wake. In the framework of Lighthill's acoustics analogy the procedure of sound prediction is divided into two steps: calculation of sound sources generated by flow and propagation of sound into far field. To calculate flow induced noise sources we performed a high resolution LES as well as a more affordable LES on fewer grid points. Sound propagation and diffraction by the airfoil are accurately are accounted for using numerically computed Green's functions tailored to the airfoil geometry. We will compare the noise spectra using this Green's function with previous calculations using an approximate Green's function. [Preview Abstract] |
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