摘要 :
A variety of actuation methods have been applied to turbulent jets with the aim of reducing far-field sound. However, a detailed understanding of the mechanisms by which actuation alters the turbulence and far-field sound is lacki...
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A variety of actuation methods have been applied to turbulent jets with the aim of reducing far-field sound. However, a detailed understanding of the mechanisms by which actuation alters the turbulence and far-field sound is lacking. We investigate the effect of periodic acoustic forcing by performing a series of large-eddy simulations of turbulent axisymmetric subsonic and supersonic jets subjected to periodic forcing at several frequencies and amplitudes. To analyze data from the forced jets, we employ cyclostationary analysis, which is an extension of the statistically stationary framework to processes that have periodically varying statistics. Both low- St_f = 0.3 and high-frequency St_f = 1.5 forcing generate an energetic tonal response but have limited effect on the time-averaged mean with a forcing amplitude greater than 1% required to achieve a small change. Similar trends were seen for the turbulent kinetic energy and the energy transfer between the mean and turbulent components. By applying cyclostationary spectral proper orthogonal decomposition (CS-SPOD), we investigate how the dominant coherent structures are modified and modulated by the forcing. For St_f = 0.3, a broadband increase in the energy of the dominant coherent structures was found. The low-frequency coherent structures were found to be strongly phase dependent, with substantial energy coupled to the high-velocity and high-shear regions of the mean flow. In contrast, forcing at St_f = 1.5 resulted in a broadband decrease in the energy of the dominant coherent structures. No phase-dependent modulation of the low-frequency coherent structures was seen due to a large difference in the wavelength and spatial support between the coherent structures and the mean field. A reduced impact of the St_f = 0.3 forcing on the supersonic jet is seen, while the St_f = 1.5 forcing results in a similar impact.
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摘要 :
The statistical analysis of non-axisymmetric turbulent jets using spectral proper orthogonal decomposition (SPOD) is computationally costly; in particular, their non-axisymmetry precludes Fourier decomposition of the three-dimensi...
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The statistical analysis of non-axisymmetric turbulent jets using spectral proper orthogonal decomposition (SPOD) is computationally costly; in particular, their non-axisymmetry precludes Fourier decomposition of the three-dimensional flow field into two-dimensional azimuthal modes. Jets in the dihedral group D_2, including rectangular, elliptic, and twin jets, are invariant under reflection about their major and minor axes. For these jets, we propose an SPOD workflow that exploits their D_2 geometrical symmetry to obtain a reduction in computational effort, accelerate statistical convergence, and improve the interpretability of results. We decompose the three-dimensional snapshots into four symmetry components. For each symmetry component, we independently perform SPOD on one quadrant of the domain. We demonstrate an application of this D_2-symmetric SPOD workflow on a large-eddy simulation of a twin-rectangular supersonic jet. Our analysis indicates that the twin jet exhibits screech at Strouhal number 0.3, and that this screech is dominated by components which are antisymmetric along the minor axis. These observations are consistent with the results from companion experiments at Ohio State University. Furthermore, the same workflow is used to analyze the symmetry-dependence of far-field acoustics at low and high frequencies.
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Large-eddy simulations (LES) of a Mach 1.5, nominally ideally-expanded, twin-rectangular jet are performed, with and without forcing. The numerical implementation of the periodic forcing is based on companion experiments at Ohio S...
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Large-eddy simulations (LES) of a Mach 1.5, nominally ideally-expanded, twin-rectangular jet are performed, with and without forcing. The numerical implementation of the periodic forcing is based on companion experiments at Ohio State University that utilize localized arc filament plasma actuators (LAFPA) to control twin-jet coupling and influence jet screech. In the forced jet LES, twelve independently-controlled energy sources, which model LAFPAs, generate bursts of thermal and pressure perturbations in a pattern that is symmetric about the major and minor axes of the jet, or symmetric-symmetric for short. For rigorous statistical analysis, the natural and forced jet data are decomposed into their D_2-symmetry components, which are either symmetric or antisymmetric about the major and/or minor axes. To identify large-scale structures involved in triadic interactions, bispectral mode decomposition (BMD) is applied to the forced case, leading to a complex mode bispectrum and three-dimensional BMD modes for each D_2 symmetry component (auto-BMD) or set of components (cross-BMD). The BMD analyses reveal dominant triadic interactions between symmetric-symmetric modes, associated with the forcing, and between antisymmetric-symmetric modes, associated with the natural screech tone. Both classes of symmetry-self interactions generate superharmonics and mean flow distortions that are symmetric-symmetric. In contrast, symmetry-cross interactions between the symmetric-symmetric forcing and antisymmetric-symmetric screech form superharmonics and mean flow deformations that are antisymmetric-symmetric. Summed mode spectra, obtained from the average along each diagonal of the BMD bispectra, suggest that under the present forcing, consistent symmetry (i.e., either both symmetric, or both antisymmetric) about the minor axis is a prerequisite for two modes to interact quadratically.
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Response modes computed via linear resolvent analysis have shown promising results for qualitatively modeling both the hydrodynamic and acoustic fields in jets when compared to data-deduced modes from high-fidelity, large-eddy sim...
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Response modes computed via linear resolvent analysis have shown promising results for qualitatively modeling both the hydrodynamic and acoustic fields in jets when compared to data-deduced modes from high-fidelity, large-eddy simulations (LES). For an improved quantitative prediction of the near- and far-field, the role of Reynolds stresses must also be considered. In this study, we propose a methodology to deduce an eddy-viscosity model that optimally captures the nonlinear forcing of resolvent modes. The methodology is based on the maximization of the projection between resolvent analysis and spectral proper orthogonal decomposition (SPOD) modes using a Lagrangian optimization framework. For a Mach 0.4 round, isothermal, turbulent jet, four methods are used to increase the projection coefficients: linear damping, spatially constant eddy-viscosity field, a turbulent kinetic energy derived viscosity field, and an optimized eddy-viscosity field. The resulting projection coefficients for the optimized eddy-viscosity field between SPOD and resolvent can be increased to over 90% for frequencies in the range S_t = 0.35 — 1 with significant improvements to S_t < 0.35. We find that the use of a frequency-independent turbulent kinetic energy turbulent viscosity model produces modes closely inline with optimal results, providing a preliminary eddy-viscosity resolvent model for jets.
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摘要 :
Response modes computed via linear resolvent analysis have shown promising results for qualitatively modeling both the hydrodynamic and acoustic fields in jets when compared to data-deduced modes from high-fidelity, large-eddy sim...
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Response modes computed via linear resolvent analysis have shown promising results for qualitatively modeling both the hydrodynamic and acoustic fields in jets when compared to data-deduced modes from high-fidelity, large-eddy simulations (LES). For an improved quantitative prediction of the near- and far-field, the role of Reynolds stresses must also be considered. In this study, we propose a methodology to deduce an eddy-viscosity model that optimally captures the nonlinear forcing of resolvent modes. The methodology is based on the maximization of the projection between resolvent analysis and spectral proper orthogonal decomposition (SPOD) modes using a Lagrangian optimization framework. For a Mach 0.4 round, isothermal, turbulent jet, four methods are used to increase the projection coefficients: linear damping, spatially constant eddy-viscosity field, a turbulent kinetic energy derived viscosity field, and an optimized eddy-viscosity field. The resulting projection coefficients for the optimized eddy-viscosity field between SPOD and resolvent can be increased to over 90% for frequencies in the range S_t = 0.35 — 1 with significant improvements to S_t < 0.35. We find that the use of a frequency-independent turbulent kinetic energy turbulent viscosity model produces modes closely inline with optimal results, providing a preliminary eddy-viscosity resolvent model for jets.
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The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and that have been previously detected as tones in the near-nozzle region. Using three models (the linearize...
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The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and that have been previously detected as tones in the near-nozzle region. Using three models (the linearized Euler equations, a cylindrical vortex sheet, and a cylindrical duct with pressure release boundary conditions), we show that these waves can be described by linear modes of the jet and correspond to acoustic waves that are trapped within the potential core. At certain frequencies, these trapped waves resonate due to repeated reflection between end conditions provided by the nozzle and the streamwise contraction of the potential core. Our models accurately capture numerous aspects the potential core waves that are extracted from large-eddy-simulation data of a Mach 0.9 isothermal jet. Furthermore, the vortex sheet model indicates that this behavior is possible for only a limited range of Mach numbers that is consistent with previous experimental observations.
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The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and that have been previously detected as tones in the near-nozzle region. Using three models (the linearize...
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The purpose of this paper is to characterize and model waves that are observed within the potential core of subsonic jets and that have been previously detected as tones in the near-nozzle region. Using three models (the linearized Euler equations, a cylindrical vortex sheet, and a cylindrical duct with pressure release boundary conditions), we show that these waves can be described by linear modes of the jet and correspond to acoustic waves that are trapped within the potential core. At certain frequencies, these trapped waves resonate due to repeated reflection between end conditions provided by the nozzle and the streamwise contraction of the potential core. Our models accurately capture numerous aspects the potential core waves that are extracted from large-eddy-simulation data of a Mach 0.9 isothermal jet. Furthermore, the vortex sheet model indicates that this behavior is possible for only a limited range of Mach numbers that is consistent with previous experimental observations.
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Large-eddy simulation of a jet issuing from rectangular nozzles of aspect ratio 2 is performed. The nozzles are operating at their nominal design Mach number of 1.5. This operating condition and the geometry match those of the com...
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Large-eddy simulation of a jet issuing from rectangular nozzles of aspect ratio 2 is performed. The nozzles are operating at their nominal design Mach number of 1.5. This operating condition and the geometry match those of the companion experiment conducted at Ohio State University. The preliminary results show good agreement with near-field and far-field noise measurements in terms of broadband levels and predictions of screech tone frequencies and amplitudes. In particular, the main noise radiation towards the aft angles and the overall sound pressure level directivity are within ldB for most relevant frequencies and angles. For future simulations of active control, a numerical model of a localized arc filament plasma actuator is implemented and tested in a small test domain inside one of the nozzles. A grid resolution study is conducted to investigate the minimum grid resolution required for correct energy transport within the boundary layer.
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Various passive and active control strategies have been applied to turbulent jets and have achieved up to about a 5dB reduction in overall sound pressure level. However, the mechanisms by which forcing alters the turbulence and fa...
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Various passive and active control strategies have been applied to turbulent jets and have achieved up to about a 5dB reduction in overall sound pressure level. However, the mechanisms by which forcing alters the turbulence and far-field sound are poorly understood. We investigate the effect of forcing by performing large-eddy simulations of turbulent axisymmetric jets subjected to periodic forcing at multiple frequencies and amplitudes. Spectral proper orthogonal decomposition is used to study the effect of the forcing on the linear spectrum. Low-frequency periodic forcing, with Stf = 0.3, while producing highly energetic tonal structures and noise, has a limited effect upon the underlying turbulent spectrum of the jet and the most energetic modes. High levels of forcing, 1% of the jet velocity, are required to achieve a small change to the turbulent mean flow and a minor shift in the turbulent spectrum. The changes in the overall spectrum and the shift in the modes are predicted well via the resolvent analysis performed on the new turbulent mean flow. This shows that the turbulent spectrum stems from the turbulent mean flow and not via interactions between phase-locked structures and the natural turbulence. High-frequency periodic forcing, with 57/ = 1.5, is less effective at altering the mean flow field compared to the low-frequency forcing at the same amplitude, but results in a nonlinear interaction, potentially associated with vortex pairing, amplifying the turbulence spectrum at St ≈ 0.75.
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To improve understanding and modeling of jet-noise source mechanisms, extensive experimental and numerical databases are generated for an isothermal Mach 0.9 turbulent jet at Reynolds number Re = 10~6. The large eddy simulations (...
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To improve understanding and modeling of jet-noise source mechanisms, extensive experimental and numerical databases are generated for an isothermal Mach 0.9 turbulent jet at Reynolds number Re = 10~6. The large eddy simulations (LES) feature localized adaptive mesh refinement, synthetic turbulence and wall modeling inside the nozzle to match the fully turbulent nozzle-exit boundary layers in the experiments. Long LES databases are collected for two grids with different mesh resolutions in the jet plume. Comparisons with the experimental measurements show good agreement for the flow and sound predictions, with the far-field noise spectra matching microphone data to within 0.5 dB for most relevant angles and frequencies. Preliminary results on the radiated noise azimuthal decomposition and temporal intermittency are also discussed. The azimuthal analysis shows that the axisymmetric mode is dominant at the peak radiation angles and that the first 3 Fourier azimuthal modes of the LES data recover more than 97% of the total acoustic energy at these angles. The temporal analysis highlights the presence of recurring intermittency in the radiated sound for the low-frequency range and main downstream angles. At these frequencies and angles, temporally-localized bursts of noise can reach levels up to 3 or 4 dB higher (or lower) than the long-time average.
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