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Perrino, MichaelAn Experimental Study into Pylon, Wing, and Flap Installation Effects on Jet Noise Generated by Commercial Aircraft
PhD, University of Cincinnati, 2014, Engineering and Applied Science: Aerospace Engineering
A pylon bottom bifurcation and a wing with variable flaps were designed and built to attach to a scaled model of a coaxial exhaust nozzle system. The presence of the pylon bifurcation, wing, and flaps modify the characteristics of the exhaust flow forc- ing asymmetric flow and acoustics. A parametric study was carried out for assessing and relating the flow field characteristics to the near-field pressure and far-field acous- tic spectra. The flow field was investigated experimentally using both stream-wise and cross-stream PIV techniques where the near-field pressure and far-field acoustic spectra were measured using microphone arrays. Contour mapping of the flow field characteristics (e.g. mean velocity and turbulence kinetic energy levels) and near-field acoustics with and without installation effects were used to explain the changes in the far-field acoustics.

Committee:

Ephraim Gutmark, Ph.D. D.Sc. (Committee Chair); Asif Syed, Ph.D. (Committee Member); Jeffrey Kastner, Ph.D. (Committee Member); Paul Orkwis, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Aeroacoustics;Aircraft Engine;Pylon;Noise;Nozzle

Hahn, Casey BernardDesign and Validation of the New Jet Facility and Anechoic Chamber
Master of Science, The Ohio State University, 2011, Mechanical Engineering

The jet facility and anechoic chamber at the Gas Dynamics and Turbulence Laboratory (GDTL) at The Ohio State University have been redesigned and rebuilt to significantly improve their capabilities. The new jet facility is capable of jets of 2-inch diameter—twice the size of the old jets. The new and much larger anechoic chamber can handle the larger jet and enables the measurements of shock noise generated by the jet of tactical aircraft. Free-field qualification requirements of ISO 3745 standard are met, and the chamber has a cutoff frequency of 160 Hz. A few improvements were incorporated into the new facility including thicker, acoustically-treated walls and an acoustically transparent grating floor above the floor anechoic wedges. Tests showed that very minor variations in the spectra are introduced by the grating floor panels.

Two additional microphones were added to the new facility with three within the upstream region of the acoustic field (a maximum polar angle of 130° compared to the maximum of 90° of the old facility). The radial distances of the microphones were increased, and far-field tests show that the microphones are safely within the far-field of 1-inch and 1.5-inch jets. For a 2-inch jet, some microphones are likely within the transition region of the acoustic field but could be moved farther outward to locate them within the far-field, as there is more room within the chamber. The stagnation chamber diameter was increased from 3.068 inches to 5.047 inches to handle the larger mass flow rate of a 2-inch jet. Initially, spectra suffered from narrowband cavity tones generated by ports upstream. The ports were modified, and a second perforated plate was added to eliminate these tones.

Acoustic data of the new and old jets are compared, and some minor differences in the high frequency content of the spectra are found. Early guesses point to internal rig noise created by flow through the second perforated plate. Work will continue to remove these differences. Finally, PIV results of the old and new jets are compared. The Mach number decay and spreading rates of a new Mach 0.9 jet compare well to an old Mach 0.9 jet. The old Mach 0.9 jets had slightly lower levels of turbulent kinetic energy. A new Mach 1.3 jet compares well with an old Mach 1.3 jet all these statistics.

Committee:

Mo Samimy, PhD (Advisor); Datta Gaitonde, PhD (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Engineering; Experiments; Fluid Dynamics; Mechanical Engineering

Keywords:

jet facility; anechoic chamber; aeroacoustics

Kearney-Fischer, Martin A.The Noise Signature and Production Mechanisms of Excited High Speed Jets
Doctor of Philosophy, The Ohio State University, 2011, Mechanical Engineering

Following on previous works showing that jet noise has significant intermittent aspects, the present work assumes that these intermittent events are the dominant feature of jet noise. A definition and method of detection for intermittent noise events are devised and implemented. Using a large experimental database of acoustically subsonic jets with different acoustic Mach numbers (Ma = 0.5 – 0.9), nozzle exit diameters (D = 2.54, 5.08, & 7.62 cm), and jet exit temperature to ambient temperature ratios (ETR = 0.84 – 2.70), these events are extracted from the noise signals measured in the anechoic chamber of the NASA Glenn AeroAcoustic Propulsion Laboratory. It is shown that a signal containing only these events retains all of the important aspects of the acoustic spectrum for jet noise radiating to shallow angles relative to the jet axis, validating the assumption that intermittent events are the essential feature of the peak noise radiation direction. The characteristics of these noise events are analyzed showing that these events can be statistically described in terms of three parameters (the variance of the original signal, the mean width of the events, and the mean time between events) and two universal statistical distribution curves. The variation of these parameters with radiation direction, nozzle diameter, exit velocity, and temperature are discussed.

A second experimental database from the Ohio State University Gas Dynamics and Turbulence Laboratory of far-field acoustic data from an excited subsonic jet with hydrodynamic Mach number of 0.9 (Mj = 0.9) at various total temperature ratios (TTR = 1.0 - 2.5) is analyzed using the same process. In addition to the experimental acoustic database, conclusions and observations from previous works using Localized Arc Filament Plasma Actuators (LAFPAs) are leveraged to inform discussion of the statistical results and their relationship to the jet flow dynamics. Analysis of the excited jet reveals the existence of a resonance condition. When excited at the resonance condition, large amounts of noise amplification can occur – this is associated with each large-scale structure producing a noise event. Conversely, noise reduction occurs when only one noise event occurs per several large-scale structures. One of the important conclusions from these results is that there seems to be a competition for flow energy among neighboring structures that dictates if and how their dynamics will produce noise that radiates to the far-field.

Utilizing the results from both databases, several models for noise sources addressing different aspects of the results are discussed. A simple model for this kind of noise signal is used to derive a relationship between the characteristics of the noise events and the fluctuations in the integrated noise source volume. Based on the known flow-field dynamics and the acoustic results from the excited jet, a hypothetical model of the competition process is described. These various models speculate on the dynamics relating the noise sources to the signal in the far-field and, as such, the present work cannot provide a definitive description of jet noise sources, but can serve as a guide to future exploration.

Committee:

Mo Samimy, PhD (Advisor); Igor Adamovich, PhD (Committee Member); James Bridges, PhD (Committee Member); Michael Dunn, PhD (Committee Member); Walter Lempert, PhD (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

Jet Noise; Aeroacoustics; Active Flow Control; Turbulence

Durand, ChristopherValidation of a CAA Code for a Case of Vortical Gust-Stator Interaction
Master of Science, University of Toledo, 2016, Mechanical Engineering
In this work, the solution of the NASA Glenn Research Center Broadband Aeroacoustic Stator Simulation (BASS) Computational Aeroacoustics (CAA) code to a benchmark problem has been presented. The test case used for this work is the Category 3 problem from the NASA Second CAA Workshop on Benchmark Problems, which is a 2D gust--cascade problem involving the interaction of an incident vortical gust with a row of infinitely thin stator blades. It has been found that BASS accurately predicts the magnitude of the cut-on acoustic modes that result from the gust impinging on the blades for both the low frequency and the high frequency benchmark case. Spurious reflections from the upstream and downstream boundaries are present in the solution due to limitations of the boundary conditions. Absorbing layers were used to minimize reflections from the boundary, with favorable results achieved by both using absorbing layers and increasing the axial length of the computational domain. Results also have been presented that suggest that dispersion-relation-preserving (DRP) spatial differencing schemes may not accurately resolve the decay rate of decaying acoustic waves with an economical number of grid points per wavelength. This work does not suggest a solution to this issue, but provides a general overview of the problem in order to help form a basis for future research.

Committee:

Ray Hixon, Ph.D (Committee Chair); Sorin Cioc, Ph.D (Committee Member); Mehdi Pourazady, Ph.D (Committee Member)

Subjects:

Acoustics; Aerospace Engineering; Mechanical Engineering

Keywords:

CAA; computational aeroacoustics; gust-cascade; vortical gust; drp; absorbing boundary layer; stator

Sasidharan Nair, UnnikrishnanJet noise source localization and identification
Doctor of Philosophy, The Ohio State University, 2017, Aero/Astro Engineering
The exact mechanism by which turbulent fluctuations in jets are converted into acoustic energy remains unexplained. The current work aims to improve our understanding of this problem by localizing acoustic sources and identifying its causal dynamics. We use Large-Eddy Simulations of a Mach 1.3 turbulent cold jet for this purpose. The localization question is resolved by using a novel technique, termed Synchronous Large-Eddy Simulations (SLES), that tracks the non-linear evolution of small perturbations from any region (window) in a time-varying base flow. This provides superior insights into the generation of intermittency and directivity compared to traditional approaches that use linear stability analyses based on steady basic states or backward correlations between different regions of the flow. In SLES, two simulations are performed in a lock-step manner. At each step, native fluctuations from a desired spatial window in the first (or baseline simulation) are scaled to small values and then injected into the second (or twin simulation) to provide a forcing in the targeted region. At subsequent times, the difference between the two simulations provides a snapshot of the evolution of the perturbation field associated with the continuous forcing in the chosen spatial window. The perturbation field, which is equivalent to the solution of the forced Navier-Stokes equations linearized about the time-evolving base flow, is then statistically analyzed to identify its modulation by the turbulent region of the jet. Results are distilled by examining forcing at lipline and centerline locations in detail. The end of the potential core is found to be a sensitive zone where perturbations are amplified and lead to secondary sources. Perturbations within the shear layer on the other hand, are initially channeled toward the core and undergo higher amplification compared to those originating from the centerline, before propagating outward. Statistical analyses quantify intermittent events which have a major role in creating the nearfield sound signature and yield polar variation of the most significant frequency band. The flowfield is decomposed into its acoustic, hydrodynamic and thermal modes (which are referred to as the Fluid-Thermodynamic (FT) modes) using the Momentum Potential Theory to identify the acoustic sources and extract the propagated field of the jet. The hydrodynamic mode highlights the shear layer roll up and turbulent mixing, while the acoustic and thermal modes exhibit a wavepacket nature in the core. The acoustic wavepacket develops into the nearfield radiation pattern of the jet, with spatio-temporal amplifications in the core due to the presence of vorticity resulting in nearfield intermittent events. The acoustic and hydrodynamic modes closely follow the theoretical decay rates and the former possess the features of experimentally observed model sound spectra along the downstream and sideline polar angles. Inter-modal energy transfers in the non-linear flow are analyzed using the transport equation for the universal acoustic variable, Total Fluctuating Enthalpy (TFE). Production terms of TFE identify intruding vortices in the potential core as the principal physical mechanism by which intermittent acoustic sources are generated in the jet. The acoustic wavepacket and its nearfield fluctuations play a central role in transporting TFE outward from the core, resulting in the near and farfield sound signature of the jet. The present work provides unique insights into time-accurate linear response of a turbulent jet, and quantifies the relative prominence of various core locations in generating the nearfield acoustic signature. The evolution of the linear perturbation field highlights the modulation of random turbulent fluctuations into spatio-temporally persistent intermittent events. While the intermediate frequencies in the jet propagate in the upstream direction, the highest and lowest frequency-bands prevail in the sideline and downstream directions respectively. The FT mode decomposition elucidates the energy transfer mechanisms in the jet. It provides a phenomenological model to explain the amplification of the acoustic mode, leading to intermittent sound events in the nearfield as well as generation of acoustic sources. The FT modes also quantify the net energy flux scattered and transmitted out of the jet, delineating the stochastic turbulence from a relatively orderly acoustic transport.

Committee:

Datta Gaitonde, Dr. (Advisor); Jen-Ping Chen, Dr. (Committee Member); Brenda Henderson, Dr. (Committee Member); Sandip Mazumder, Dr. (Committee Member); Mo Samimy, Dr. (Committee Member)

Subjects:

Aerospace Engineering; Mechanical Engineering

Keywords:

Jet noise, Aeroacoustics, Large-eddy simulation, Acoustic wavepacket

Ingraham, DanielVerification of a Computational Aeroacoustics Code Using External Verification Analysis (EVA)
Master of Science in Mechanical Engineering, University of Toledo, 2010, College of Engineering
As Computational Aeroacoustics (CAA) codes become more complex andwidely used, robust Verification of such codes becomes more and more important. Recently, Hixon et al. proposed a variation of the Method of Manufactured Solutions of Roache especially suited for Verifying unsteady CFD and CAA codes that does not require the generation of source terms or any modification of the code being Verified. This work will present the development of the External Verification Analysis (EVA) method and the results of its application to some popular model equations of CFD/CAA and a high-order nonlinear CAA code.

Committee:

Ray Hixon, PhD (Committee Chair); Douglas Oliver, PhD (Committee Member); Chunhua Sheng, PhD (Committee Member)

Subjects:

Acoustics; Mechanical Engineering

Keywords:

Computational Aeroacoustics; Computational Fluid Dynamics; Code Verification; External Verification Analysis; Method of Manufactured Solutions; CAA; CFD; EVA; MMS

CALLENDER, WILLIAM BRYANAN INVESTIGATION OF INNOVATIVE TECHNOLOGIES FOR REDUCTION OF JET NOISE IN MEDIUM AND HIGH BYPASS RATIO TURBOFAN ENGINES
PhD, University of Cincinnati, 2004, Engineering : Aerospace Engineering
This research project has developed a new, large-scale, nozzle acoustic test rig capable of simulating the exhaust flows of separate flow exhaust systems in medium and high bypass turbofan engines. This rig has subsequently been used to advance the understanding of two state-of-the-art jet noise reduction technologies. The first technology investigated is an emerging jet noise reduction technology known as chevron nozzles. The fundamental goal of this investigation was to advance the understanding of the fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles. Additionally, this study sought to establish the relationship between these physical mechanisms and the chevron geometric parameters. A comprehensive set of data was collected, including far-field and near-field acoustic data as well as flow field measurements. In addition to illustrating the ability of the chevron nozzles to provide acoustic benefits in important aircraft certification metrics such as effective perceived noise level (EPNL), this investigation successfully identified two of the fundamental physical mechanisms responsible for this reduction. The flow field measurements showed the chevron to redistribute energy between the core and fan streams to effectively reduce low frequency noise by reducing the length of the jet potential core. However, this redistribution of energy produced increases in turbulent kinetic energy of up to 45% leading to a degradation of the chevron benefit at higher frequencies. Trends observed with respect to the chevron geometry showed that the chevron penetration could be matched to the exhaust flow conditions to optimally balance the trade between low frequency reduction and high frequency increase to maximize reductions in EPNL. Secondly, a completely new technology, known as fluidic injection, was investigated. This technology consists of applying continuous air injection, from a number of small injection jets, at the nozzle exit plane to reduce jet noise. The principal advantage of such an approach is that it is an active technology that can be activated as needed and, as such, may be more acceptable in aircraft engines from a performance standpoint than passive technologies. This study successfully demonstrated the feasibility of this technology by showing that effective jet noise reduction can be provided in a broad range of flow conditions using less than 1% of the mean jet mass flow. An investigation of injection geometric parameters identified the injection pitch angle as the most influential parameter with respect to jet noise reduction. Furthermore, an investigation of scaling effects showed a momentum ratio of approximately 1.5% to provide reductions in sound pressure level between 1 and 2 dB across a wide range of frequencies for a wide range of flow conditions and scales including both single stream and dual stream flows. PIV flow field measurements identified the fundamental physical mechanism of the noise reduction to be a near uniform reduction in shear layer turbulence.

Committee:

Dr. Ephraim Gutmark (Advisor)

Keywords:

Aeroacoustics; Jet Noise; Separate Flow Exhaust Systems; Separate Flow Exhaust Nozzles; Chevron Nozzles; Chevrons; Fluidic Injection; Fluidics

Harris, Christopher A.Acoustics and Fluid Dynamics Studies of High Speed Jet Noise Reduction Devices
MS, University of Cincinnati, 2009, Engineering : Aerospace Engineering
Jet noise reduction was investigated on a scale model turbofan exhaust simulator rig at a Reynold's Number O(10e6) through mean and time-resolved flow and aeroacoustic measurements. Various stream-wise vorticity production devices, including conventional and modified chevron nozzles and CVG's (Coupled Vortex Generators), were installed to increase turbulent shear layer mixing and ultimately reduce far-field radiated noise. Simplified flow simulations using a steady RANS k-epsilon turbulence model aid to elucidate the initial vortex development for several geometries. CVG's were installed in axisymmetric arrangements on both the core and fan streams of the exhaust simulator, and in the various boundary layers. Measurements of the nozzle boundary layer characteristics were performed using a total pressure probe on the baseline hardware to determine appropriate mean spatial scales, and to evaluate the boundary layer momentum thickness influence on noise for a coaxial, turbulent jet. LDV of two velocity components determined the turbulence properties in the jet at various locations in the initial mixing region and past the potential core. Acoustic far-field measurements showed that high levels of peak noise reduction were possible with added high-frequency energy. One purpose was to offer an explanation of this 'self' noise component and mixing mechanisms, in comparison with delta tabs which also incur high-frequency noise. With properly scaled geometry design, and installation configurations, the CVG's can achieve SPL peak noise reductions for essentially all directivity angles, with the addition of a high-frequency source that appears consistent with a self-noise induced dipole.

Committee:

Dr. Ephraim J. Gutmark (Committee Chair); Dr. Paul Orkwis (Committee Member); Dr. John Wojno (Committee Member); Dr. Steve Martens (Committee Member)

Subjects:

Acoustics; Aerospace Materials; Engineering; Experiments; Fluid Dynamics

Keywords:

jet; aeroacoustics; noise, reduction; suppression; vortex generators; vg; experimental

Rejent, AndrewExperimental Study of the Flow and Acoustic Characteristics of a High-Bypass Coaxial Nozzle with Pylon Bifurcations
MS, University of Cincinnati, 2009, Engineering : Aerospace Engineering

The thrust of this thesis is to initiate an investigation into the acoustic effects related to the presence of a pylon installed on a high bypass ratio turbofan engine. It is well known that the presence of a pylon bifurcation generates an asymmetric sound field and modifies the characteristics of the exhaust flow. This study was designed to gain an understanding between these two results of the pylon’s presence. To accomplish this, a pylon was designed and built to modify the existing bypass ratio 5 nozzle in the Aeroacoustic Test Facility at the University of Cincinnati’s Gas Dynamics and Propulsions Laboratory. This pylon and bottom bifurcation modifies the baseline nozzle in a manner geometrically similar to that of a real engine configuration.

Experiments were carried out to measure the acoustic properties of the pylon configuration and understand their connection to the observed flow field. Both near and far field recordings were made of the baseline nozzle and the pylon nozzle at several azimuthal positions. Velocimetry measurements were also taken for these configurations.

It was seen that the classic pylon effects were present on the tested configuration; the core flow was turned towards the pylon, the fan stream was directed away from the pylon. The resulting far field and near field signatures were asymmetric. In the far-field, the presence of the pylon at the highest bypass cycle condition exhibited a maximum increase in noise production of 2.2 EPNL dB, at the sideline angle, and a minimum increase of 1.1 EPNL dB directly under the pylon. Increasing the shear velocity lowered the increase in sound production due to the pylon, but the azimuthal variation was largely unaffected.

A chevron nozzle, an existing noise reduction technology, was tested on the pylon nozzle configuration to study how the pylon affects the acoustic benefits of this technology across a range of cycle conditions. Also, a new technology known as an internal chevron nozzle was designed and tested with the baseline and pylon configurations. This internal chevron nozzle was designed as an alternative to the existing chevron technology; intended to reduce the sensitivity to shear velocities exhibited by traditional chevron nozzles.

The 8LP core chevron reduced the EPNL of the baseline nozzle by up to 1.6 dB, and the internal chevron nozzle provided up to a 0.8 EPNL dB reduction. However, the presence of the pylon modified the effectiveness of these nozzles. The chevron nozzle increased sound production at high shear velocity, but reduced noise up to 2.0dB for lower shear cases. The effectiveness of the internal chevron nozzle grew at both the medium and low shear conditions for all azimuthal positions, up to a 1.3 EPNL dB reduction. However, reductions seen at high shear velocity were reduced by the presence of the pylon. The noise reduction of the internal chevron nozzle was less than the chevron nozzle, but its design was successful in being less dependent on the cycle condition.

Committee:

Dr. Ephraim Gutmark (Committee Chair); Dr. Shaaban Abdallah (Committee Member); Dr. Mark Turner (Committee Member); Dr. John Wojno (Committee Member)

Subjects:

Acoustics; Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

jet noise; acoustics; aeroacoustics; pylon; far-field; near-field; piv; jet-pylon interaction; EPNL

Heeb, Nicholas S.Azimuthally Varying Noise Reduction Techniques Applied to Supersonic Jets
PhD, University of Cincinnati, 2015, Engineering and Applied Science: Aerospace Engineering
An experimental investigation into the effect of azimuthal variance of chevrons and fluidically enhanced chevrons applied to supersonic jets is presented. Flow field measurements of streamwise and cross-stream particle imaging velocimetry were employed to determine the causes of noise reduction, which was demonstrated through acoustic measurements. Results were obtained in the over- and under- expanded regimes, and at the design condition, though emphasis was placed on the overexpanded regime due to practical application. Surveys of chevron geometry, number, and arrangement were undertaken in an effort to reduce noise and/or incurred performance penalties. Penetration was found to be positively correlated with noise reduction in the overexpanded regime, and negatively correlated in underexpanded operation due to increased effective penetration and high frequency penalty, respectively. The effect of arrangement indicated the beveled configuration achieved optimal abatement in the ideally and underexpanded regimes due to superior BSAN reduction. The symmetric configuration achieved optimal overexpanded noise reduction due to LSS suppression from improved vortex persistence. Increases in chevron number generally improved reduction of all noise components for lower penetration configurations. Higher penetration configurations reached levels of saturation in the four chevron range, with the potential to introduce secondary shock structures and generate additional noise with higher number. Alternation of penetration generated limited benefit, with slight reduction of the high frequency penalty caused by increased shock spacing. The combination of alternating penetration with beveled and clustered configurations achieved comparable noise reduction to the standard counterparts. Analysis of the entire data set indicated initial improvements with projected area that saturated after a given level and either plateaued or degraded with additional increases. Optimal reductions were 3-7dB depending on operating condition and observation angle. The fluidic enhancement of the low penetration chevrons indicated significant improvement in the overexpanded regime, with detrimental effect at higher conditions. Improvements were generally due to shock noise and turbulent mixing noise reductions caused by decreased shock strength and LSS growth inhibition. Investigation of azimuthal configurations indicated further improvements were achieved by the clustered configuration due to additional BSAN reductions caused by drastic modification of the shock cell structure due to elliptification of the jet cross section.

Committee:

Ephraim Gutmark, Ph.D. D.Sc. (Committee Chair); James Bridges, Ph.D. (Committee Member); Kailas Kailasanmath, Ph.D. (Committee Member); Steve Martens, Ph.D. (Committee Member); Shaaban Abdallah, Ph.D. (Committee Member); Paul Orkwis, Ph.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Supersonic Jet;Aeroacoustics;Jet Noise;Chevrons;Fluidic Injection;Compressible Flow

Ingraham, DanielExternal Verification Analysis: A Code-Independent Approach to Verifying Unsteady Partial Differential Equation Solvers
Doctor of Philosophy, University of Toledo, 2015, Mechanical Engineering
External Verification Analysis (EVA), a new approach to verifying unsteady partial differential equation codes, is presented. After a review of the relevant code verification literature, the mathematical foundation and solution method of the EVA tool is discussed in detail. The implementation of the EVA tool itself is verified through an independent Python program. A procedure for code verification with the EVA tool is described and then applied to the three-dimensional form of a high-order non-linear computational aeroacoustics code.

Committee:

Ray Hixon (Advisor); Sorin Cioc (Committee Member); James DeBonis (Committee Member); Mehdi Pourazady (Committee Member); Chunhua Sheng (Committee Member)

Subjects:

Aerospace Engineering; Fluid Dynamics; Mechanical Engineering

Keywords:

code verification; computational aeroacoustics; computational fluid dynamics; numerical partial differential equations

Crawley, Michael BUnderstanding the Aeroacoustic Radiation Sources and Mechanisms in High-Speed Jets
Doctor of Philosophy, The Ohio State University, 2015, Mechanical Engineering
It has been well-known within the aeroacoustic community that the dominant noise sources in high-speed turbulent jets are related to the large-scale structures which are generated in the initial shear layer by instabilities and rapidly grow, interact, and disintegrate as they convect downstream. However, the exact dynamics of these large-scale structures which are relevant to the noise generation process are less clear. This work aims to study the dynamics of, and noise generated by, the large-scale structures in high-fidelity in a Mach 0.9 turbulent jet using simultaneous pressure and velocity data acquisition systems alongside plasma-based excitation to produce either individual or periodic coherent ring vortices in the shear layer. In the first phase, the irrotational near-field pressure is decomposed into its constitutive acoustic and hydrodynamic components, and two-point cross-correlations are used between the acoustic near-field and far-field in order to identify the dominant noise source region. Building upon the work of previous researchers, the decomposition is performed using a spatio-temporal wavelet transform, which was developed during the current work and found to be more robust than previous techniques. Results indicated that for both individual as well as periodic large-scale structures, the dominant noise reaching the far-field at low angles to the jet axis is being generated in the upstream region of the jet, ending just before the end of the potential core (in a time-averaged sense) in the unexcited jet. This is not to say that no noise is generated outside of this region, just that the most energetic and coherent acoustic radiation is emitted here. The large-scale structure interactions were then investigated by stochastically-estimating the time-resolved velocity fields from time-resolved near-field pressure traces and non-time-resolved planar velocity snapshots. For computational efficiency, the ensemble velocity snapshots were first decomposed into orthogonal modes, and a mapping from the near-field pressure to the expansion coefficients was then produced using a feedforward neural network using backpropagation for learning. The coherent structures generated by the excitation were then identified and tracked using standard vortex identification routines. When exciting the jet at very low frequencies, an individual structure quickly rolled up into a coherent structure within two jet diameters and then advected until roughly four jet diameters downstream, at which point it underwent a rapid disintegration. For the periodically-excited jet, multiple smaller-scale structures are initially apparent just downstream of the nozzle exit. These structures quickly undergo multiple mergings to produce a single large-scale structure with a separation distance that matches the excitation wavelength. Similar to the impulsively-excited structures, these now large-scale structures advect downstream and undergo a rapid disintegration near the end of the potential core. Finally, from Ribner's dilatation-based acoustic analogy the aeroacoustic source terms were computed using the time-resolved velocity field produced by the stochastic estimation. Interpretation of the results is challenging however, due to the number of assumptions and simplifications necessary for the computations given the limitations of the current experimental capabilities. Analysis of the computed source fields found that the coherent structures produced a convected wavepacket-like event, centered on the jet lipline though reaching into the potential core. For the individual vortex rings, a clear modulation of the spatial extent and amplitude was observed as the vortex began to break down just upstream of the end of the potential core. This behavior is also present for the periodic train of vortices observed at higher excitation frequencies, however it is obscured by an amplification of the source in the upstream region where the multiple smaller-scale structures merge. As the excitation frequency was increased, and multiple vortex mergings occurred before the end of the potential core, the aeroacoustic source associated with the merging amplified such that it was distinct from the vortex disintegration source. The results from this work indicate that the disintegration of the coherent ring vortices are the dominant aeroacoustic source mechanism for the Mach 0.9, high Reynolds number jet studied here. However, the merging of vortices in the initial shear layer was also identified as a non-trivial noise source mechanism in high-speed, turbulent jets. Future work will focus on improving the source localization by utilizing acoustic beamforming techniques to identify the source region from the acoustic near-field, in place of the two-point correlations used in this work. Additionally, the structure dynamics and noise generation process will be explored in high-order azimuthal modes.

Committee:

Mo Samimy (Advisor); Datta Gaitonde (Committee Member); James Gregory (Committee Member); Mei Zhuang (Committee Member)

Subjects:

Mechanical Engineering

Keywords:

aeroacoustics; high-speed jets; flow control

Wukie, Nathan AAn analysis of booster tone noise using a time-linearized Navier-Stokes solver
MS, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
This thesis details a computational investigation of tone noise generated from a booster(low-pressure compressor) in a fan test rig. The computational study consisted of sets of time-linearized Navier-Stokes simulations in the booster region to investigate the blade-wake interactions that act as the primary noise-generating mechanism for the booster blade-passing frequency and harmonics. An acoustic test database existed with data at several operating points for the fan test rig that was used to compare against the predicted noise data from the computational study. It is shown that the computational methodology is able to capture trends in sound power for the 1st and 2nd booster tones along the operating line for the rig. It is also shown that the computational study underpredicts one of the tones at low power and is not able to capture a peak in the data at the Cutback condition. Further investigation of this type is warranted to quantify the source of discrepancies between the computational and experimental data as the reflected transmisison of sound off the fan through the bypass duct was not accounted for in this study.

Committee:

Paul Orkwis, Ph.D. (Committee Chair); Shaaban Abdallah, Ph.D. (Committee Member); Ephraim Gutmark, Ph.D. D.Sc. (Committee Member); Mark Turner, Sc.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Computational Aeroacoustics;Turbomachinery;Computational Fluid Dynamics;Acoustics;Frequency-Domain Solvers

Cuppoletti, Daniel RSupersonic Jet Noise Reduction with Novel Fluidic Injection Techniques
PhD, University of Cincinnati, 2013, Engineering and Applied Science: Aerospace Engineering
Supersonic jets provide unique challenges in the aeroacoustic field due to very high jet velocities, shock associated noise components, flow dependence on jet expansion, and stringent performance requirements. Current noise suppression technology for commercial and military jet engines revolves around using chevrons or mechanical vortex generators to increase mixing near the nozzle exit, subsequently reducing peak turbulence levels in the mixing region. Passive noise control methods such as mechanical chevrons cause thrust loss throughout the flight envelope and performance can vary with the engine operating condition. Development of active noise control methods have the potential of improved performance throughout the flight envelope and the benefit of being deactivated when noise control is unnecessary. Fluidic injection of air into a supersonic jet is studied as an active control method with an emphasis on understanding the physics of the problem and identifying the controlling parameters. An experimental investigation with computational collaboration was conducted to understand the effect of nozzle design on supersonic jet noise and to develop various fluidic injection techniques to control noise from a supersonic jet with a design Mach number of 1.56. The jet was studied at overexpanded, ideally expanded, and underexpanded conditions to evaluate the effects throughout the operational envelope. As a passive noise control method, the internal contour of a realistic nozzle was modified to investigate the effect on acoustics and performance. Thrust was improved up to 10% with no acoustic penalties through nozzle design, however it was found that the shock noise components were highly sensitive to the shock structure in the jet. Steady fluidic injection was used to generate vorticity at the trailing edge of the nozzle showing that noise reduction is achieved through vorticity generation, modification of the shock structure, and interference with the screech feedback mechanism by decoupling the phase relationship between jet turbulence and shock spacing. Reduction of shock noise was found to be optimum at an intermediate injection pressure due to shock weakening from the fluidic injectors and injector interactions with the jet shock-expansion structure. Large-scale mixing noise reduction was shown to depend on the vorticity strength and circulation. Unprecedented reduction of OASPL up to -8.5 dB were achieved at the peak noise direction through strong jet mixing and rapid collapse of the potential core. Pulsed fluidic injection was investigated to understand the acoustic benefits and drawbacks of unsteady injection. Valve frequency response up to 500 Hz was achieved but noise reduction dropped off above 100 Hz due to poor flow response as verified by hot-wire and dynamic pressure measurements. At low pulse frequencies it was found that moderate noise reduction could be achieved with less flow than steady injection, but in general the mixing noise reduction scaled with the time integrated mass flow injection. It was discovered that the different components of supersonic jet noise had different characteristic response times to unsteady injection. Analysis of high speed shadowgraph images and acoustic spectra was used to identify time response of the jet during the unsteady injection cycle.

Committee:

Ephraim Gutmark, Ph.D., D.Sc. (Committee Chair); Steve Martens, Ph.D. (Committee Member); Awatef Hamed, Ph.D. (Committee Member); Jeffrey Kastner, Ph.D. (Committee Member); David Munday, Ph.D. (Committee Member); Mark Turner, Sc.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Jet Noise;Aeroacoustics;Fluid Dynamics;Particle Image Velocimetry;Acoustics;Supersonic

Kilburn, KoreyA Laplace Transform/Potential-Theoretic Method for Transient Acoustic Propagation in Three-Dimensional Subsonic Flows
Doctor of Philosophy, University of Akron, 2010, Engineering-Applied Mathematics
This dissertation presents the development of a semi-analytic technique developed for the determination of far field acoustic radiation in the time domain. This method solves linear, time dependent wave propagation in an unbounded medium using a numerical Laplace transform and potential theory. The end result is a robust procedure that is accurate and computationally efficient. The Transform Potential Theoretic (TPT) method is meshless and can handle arbitrary geometries. The procedure assumes the linearity of the sound field away from a bounded region surrounding the object. The TPT method depends on the sound pressure on the boundary of this region (referred to as the Kirchhoff surface). The Euler equations are linearized about a uniform mean flow. First, the problem is transformed via the Laplace transform (with appropriate initial conditions) into a reduced wave equation. By application of a dependent variable transformation, the anisotropic terms are removed and a Helmholtz-like equation with complex wave number is obtained where both single and double layer potential theory applies. This allows the calculation of the far-field acoustic pressure in the Laplace domain. Then, an inversion of the dependent variable transform is applied. Upon application of numerical inverse Laplace transform techniques, far-field acoustic pressure is then successfully obtained as a function of space and time. Using transient monopole radiation in a uniform freestream, accuracy is analyzed with excellent results. This method shows many advantages over direct simulation, including vast savings in computational time. The freestream Mach number is only a parameter in the TPT method and has no bearing on the run time, unlike direct methods.

Committee:

Scott Sawyer, PhD (Advisor)

Subjects:

Acoustics; Engineering; Physics

Keywords:

Aeroacoustics; Potential Theory; TPT; Transient Acoustic Propagation; Three Dimensional; Subsonic Flows; Exterior Problem

Kastner, Jeffrey F.Far-field radiated noise mechanisms in high reynolds number and high-speed jets
Doctor of Philosophy, The Ohio State University, 2007, Mechanical Engineering
The present research examines the relationship between the large-scale structure dynamics of a jet and the far-field sound. This was achieved by exploring the flowfield and the far field of an axisymmetric Mach 0.9 jet with a Reynolds number of approximately 0.76 million. The jet is controlled by eight plasma actuators, which operate over a large frequency range and have independent phase control allowing excitation of azimuthal modes (m) 0, 1, 2, and 3. The jet’s far field is probed with a microphone array positioned at 30 degrees with respect to the downstream jet axis. The array is used to estimate the origin of peak sound events in space, and find the sound pressure level (SPL) and overall sound pressure level (OASPL). The lower forcing Strouhal numbers (StDF’s) increase the OASPL and move noise sources upstream while higher StDF’s decrease the OASPL and have noise source distributions similar to the baseline jet. The flowfield was investigated using particle image velocimetry (PIV). A Reynolds decomposition of the PIV data emphasized the importance of the streamwise velocity fluctuations for the symmetric azimuthal modes (m = 0 and 2) and the cross-stream velocity fluctuations for the asymmetric azimuthal modes (m = 1 and 3). A proper orthogonal decomposition of the PIV data was performed to extract information about how forcing affects the large-scale flow features and conditionally average the PIV data. When forcing at StD’s other than the preferred mode, the conditional-averaged images show large-scale flow features that grow, saturate, and decay closer to the nozzle exit. When exciting a symmetric azimuthal mode, m = 0, near the preferred StDF, the streamwise phase-averaged velocity grows quickly and saturates over a relatively long spatial range. When exciting an asymmetric azimuthal mode, m = 1, near the preferred StDF, the cross-stream phase-averaged velocity grows slowly, saturates, and then decays relatively quickly. The noise source distribution occurred in the decay region for both m = 0 and m = 1, and the distribution changed in accordance with changes in the decay rate.

Committee:

Mo Samimy (Advisor)

Subjects:

Engineering, Mechanical

Keywords:

Fluid Dynamics; Gas Dynamics; Aeroacoustics; Optical Diagnostics