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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

Munday, DavidFlow and Acoustics of Jets from Practical Nozzles for High-Performance Military Aircraft
PhD, University of Cincinnati, 2010, Engineering and Applied Science: Aerospace Engineering

This research project examines supersonic jets from nozzles representative of the practical variable-geometry convergent-divergent nozzles used on high-performance military aircraft. The nozzles employed have conical convergent sections, sharp throats and conical divergent sections. Nozzles with design Mach numbers of 1.3, 1.5, 1.56 and 1.65 are tested and the flow and acoustics examined. Such nozzles are found to produce a double-diamond shock structure consisting of two overlapping sets of shock cells, one cast from the nozzle lip and one cast from the nozzle throat. These nozzles are found to produce no shock-free condition at or near the design condition. As a result they produce shock-associated noise at all supersonic conditions. The shock cell spacing, broad-band shock-associated noise peak frequency and screech frequency all match those of more traditional nearly isentropic convergent-divergent nozzles.

A correlation is proposed which improves upon the Prandtl-Pack relation for shock cell spacing in that it accounts for differences in nozzle design Mach number which the Prandtl-Pack relation does not. This proposed relation reverts to the Prandtl-Pack equation for the case of a design Mach number of 1.0.

Chevrons are applied to the nozzles with design Mach numbers of 1.5 and 1.56. The effective penetration of the chevrons is found to be a function of the jet Mach number. Increasing jet Mach number increases effective penetration of the chevrons and increases the magnitude of all chevron effects. Chevrons on supersonic jets are found to reduce shock cell length, increase mixing and spreading, decrease turbulent kinetic energy at the end of the potential core and increase it near the nozzle. Chevrons corrugate the shear layer but not the shock structures inside the jet which remain axisymmetric. Chevrons thicken the shear layer, reducing the sonic diameter and reducing the diameter of the shock cells. By reducing their diameter they also reduce the shock cell spacing. Chevrons reduce low-frequency mixing noise near the end of the potential core, increase high-frequency noise near the nozzle exit. They eliminate screech and reduce broad-band shock-associated noise and shift it to higher frequencies.

Fluidic injection is applied to the nozzle with design Mach number of 1.5. Fluidic injection corrugates the shear layer, increases mixing and spreading, reduces low frequency mixing noise, increases high frequency noise, reduces broad-band shock-associated noise and shifts its peak to higher frequency.

Committee:

Ephraim Gutmark, PhD, DSc (Committee Chair); Shaaban Abdallah, PhD (Committee Member); Paul Orkwis, PhD (Committee Member); James Bridges, PhD (Committee Member); Kailas Kailasanmath, PhD (Committee Member)

Subjects:

Aerospace Materials

Keywords:

jet noise;chevrons;microjets;fluidics;Prantdl-Pack;supersonic jets

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

Hileman, James IsaacLarge-scale structures and noise generation in high-speed jets
Doctor of Philosophy, The Ohio State University, 2004, Mechanical Engineering
This work examines the relationship between the dynamics of large-scale turbulence structures and the acoustic far-field of high Reynolds number, high-speed jets. Three Mach numbers were examined: 0.9, 1.3 and 2.0. The Mach 1.3 jet was also modified with delta tabs. A novel microphone array / algorithm was developed, tested and then used to locate sources of individual sound waves in space and time. Noise source distributions were compared to and correlated with flow visualization images that were examined with Proper Orthogonal Decomposition (POD). Time and frequency-domain analyses showed the acoustics of Mach 0.9 and 1.3 jets differed from the Mach 2.0 jet due to Mach wave emission in the latter case. Differences associated with turbulence structure scale were observed within the acoustic measurements. The addition of delta tabs led to streamwise vorticity production and the regulation and augmentation of spanwise vorticity. These modifications led to an upstream shift in the noise production regions of the jet and a shift away from the delta tab location. The regions of noise generation coincided with the location where the sides of the mixing layer merge (Mach 0.9, 1.3, 2.0, single-tab, quad-tab jets) or were dramatically altered (bifurcating region of the dual-tab jet). The streamwise vortices were not a strong, direct acoustic source for frequencies on the order of the peak jet radiation at the angle of maximum sound emission. The Mach 1.3 jet was analyzed for periods of noise generation (NG) and relative quiet (RQ) using simultaneously acquired flow and noise source localization data. POD modes were used to reconstruct cross-stream images and a series of crudely phase-locked streamwise images for the two cases. Both image planes showed the lower order POD modes that possess larger scale structures are important to the RQ while the higher order modes with relatively smaller scales dominate the NG. Within the phase-locked NG streamwise images, a series of robust structures form approximately one convective time scale before noise emission and then rapidly disintegrate as fluid is entrained to the jet’s core. The observed NG process bares many similarities to the breakdown of an instability wave.

Committee:

Mo Samimy (Advisor)

Subjects:

Engineering, Aerospace

Keywords:

Jet noise; Jet exhaust; Large-scale structure; Large structure; Coherent structure; Turbulence mixing noise; Mach wave radiation; Mach wave emission; Delta tab; Streamwise vorticity; Proper Orthogonal Decomposition; Mexican hat wavelet; Microphone array

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

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

Rask, Olaf HallerThe Reduction of Mixing Noise and Shock Associated Noise using Chevrons and other Mixing Enhancement Devices
PhD, University of Cincinnati, 2009, Engineering : Aerospace Engineering

This experimental research project varied nozzle discharge geometries in an effort to reduce jet noise. Chevrons and other mixing enhancement devices introduced streamwise vortices into the initial shear layer of the jet.

Single Flow Noise Reduction: Air was injected into the center of streamwise vortices. They were energized and increased mixing across the shear layer, which reduced low frequency noise. The vortices broke down with lower circumferential velocities, which reduced high frequency noise. More robust streamwise vortices created additional mixing. This reduced low frequency noise but created additional high frequency noise. The size of the streamwise vortices was reduced. This did not effect low frequency noise, but reduced high frequency noise.

Coaxial Flow Noise Reduction: The noise reduction when using fan and core chevrons together was shown to be additive. Reductions attributed to core chevrons plus reductions attributed to fan chevrons were roughly equal to reductions when both chevron nozzles were installed. Low frequency noise was predominantly generated in the mixed region, which can be modified by either nozzle. The effect was compounded when both chevron nozzles were installed. High frequency noise was predominantly generated in the shear layers. Under the current test conditions, the shear layers were close enough that acoustic measurements could only resolve one noise source. Either chevron nozzle modified noise in the shear layer(s), the effect was compounded when both were installed.

Underexpanded Flow Noise Reduction: Baseline and chevron nozzles were used on an underexpanded core stream. Chevrons increased shock associated noise for lower fan flow velocities but reduced shock associated noise for the highest fan flow velocity. Shock associated noise is created as turbulence interacts with shock cells. Both the values of turbulence and the magnitude of pressure fluctuations are important. For the lower fan flow velocities, chevrons increased turbulence but did not effect the pressure fluctuations. This increased shock noise. For the highest fan flow velocity, chevrons did not change turbulence levels but significantly reduced pressure fluctuations. This reduced shock associated noise.

Committee:

Ephraim Gutmark, PhD (Committee Chair); Richard Cedar, PhD (Committee Member); Philip Gliebe (Committee Member); Jay Kim, PhD (Committee Member); Prem Khosla, PhD (Committee Member); James Bridges, PhD (Committee Member)

Subjects:

Acoustics; Engineering

Keywords:

jet noise; shock noise; shock associated noise; chevrons; fluidic injection; mixing noise; vortex generators;

Malla, BhupatindraStudy of High-speed Subsonic Jets using Proper Orthogonal Decomposition
MS, University of Cincinnati, 2012, Engineering and Applied Science: Aerospace Engineering

The primary objective of this thesis is to advance Proper Orthogonal Decomposition (POD) methods to quantify similarities and differences between the turbulent structures in the mixing layer of transonic jets issued through baseline axisymmetric conical nozzle and ones with chevrons. The analysis is done using flow velocity data obtained through Particle Image Velocimetry (PIV). Two chevron nozzles with penetration levels at 2% and 4% are used. The mean velocity and TKE results show that chevrons reduce the potential core length, increase the jet spread, increase TKE levels immediately after the nozzle exit, and reduce TKE levels downstream.

POD is used to further quantify these results. First 35 converged POD modes are investigated since these modes comprise of the majority of the large scale structures. The similarities and differences between the POD modes from different data sets vary along the jet.

The study of the POD modes along the jet showed the growth of the structures in both size and strength in the downstream direction. The growth rates of the modes are suppressed by the chevrons and high penetration chevrons are more effective. This trend is also noticed in the energy distribution study among the modes. Despite having both the axial and radial modes energized by the chevrons near the nozzle, the stabilization of the modes resulted in lower energy contents in the downstream regions. Projection of the PIV images from the chevron configurations onto the baseline POD modes showed that both near the nozzle and downstream the highest energy containing baseline modes are attenuated by the chevrons, but the radial modes in the near-nozzle region. A metric used in order to quantify the similarities among the mode shapes between different flow data sets showed that in the near nozzle region the correlation between the modes corresponding to the chevron nozzles and baseline nozzle is very low due to the increased mixing. Correlation increases as the flow moves downstream indicating the regaining of the baseline flow features. The correlation levels seem to be reduced in the transitional region of the shear layer profile; however increases again afterwards until the end of the PIV domain, again indicating the reorganization of the flow. This metric also identifies the changes in the energy distributions among the modes due to the flow and chevron interactions.

The secondary objective of this research is to use POD to further enhance validation methods when comparing computational and experimental data. The experimental (PIV) data consists of the 2D visualization of the velocity field corresponding to the baseline nozzle. The computational results are obtained through the Large Eddy Simulation(LES). A significant match in the characteristics of the mode shapes has been observed, especially in the lower modes, however, there is noticeable difference in the energy distributions between the flows. The LES predicts higher energy content at the lowest modes and the energy drop off rate is also higher. Getting the energy distribution matching between the experimental and computational results is necessary to ensure complete agreement between the turbulence levels.

Committee:

Ephraim Gutmark, PhD DSc (Committee Chair); Kelly Cohen, PhD (Committee Member); Jeffrey Kastner, PhD (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Proper Orthogonal Decomposition; Jet Shear Layer; Turbulent Kinetic Energy; Coherent Structures; Transonic Jet; Jet Noise

Mustafa, MansoorInvestigation into Offset Streams for Jet Noise Reduction
Master of Science, The Ohio State University, 2015, Aero/Astro Engineering
This effort investigates the near field behavior of two ideally-expanded subsonic dual-stream jets. One case implements a traditional symmetric, concentric dual-stream nozzle configuration while the other imposes an asymmetric, eccentric layout to model the behavior of an offset stream. The essence of an offset stream is to force an uneven azimuthal distribution of the secondary coflow and create an outside stream that varies in thickness. Past studies have shown a benefit in acoustic propagation in the direction of the thickest coflow and the present work further analyzes this phenomenon. A LES (Large Eddy Simulation) approach is implemented to run the simulations for both cases and a number of qualitative and quantitative analyses tools are used for post-processing. A reduction in the noise levels for the lower, thicker side of the eccentric nozzle is observed in comparison to the baseline concentric case. Examination of the mean flow behavior shows a shorter, thinner primary potential core for the offset case and a faster axial velocity decay rate. The asymmetric distribution of the coflow causes varying velocity profiles in the radial direction for the top and bottom regions and consequently produces unique flow features on either side. Lower levels of shear stress and slower decay rates lead to less turbulence production on the lower side of the eccentric nozzle. An investigation into the flow structures reveals lower vorticity and weaker convective structures on the bottom which influences propagation in that direction. Two-point correlation analysis reveals the presence of smaller turbulence scales in the lower, thicker portion of the eccentric case. This is further confirmed by an Empirical Mode Decomposition (EMD) study that shows lower frequency ranges dominate the concentric near field in comparison to the eccentric. The combination of these unique features demonstrate the principles behind the acoustic benefit of implementing offset stream flows in dual-stream nozzle configurations.

Committee:

Datta Gaitonde (Advisor); Mei Zhuang (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

offset; noise reduction; jets; aerospace; aerodynamics; LES; computational simulation; jet noise; eccentric; concentric; dual-stream; nozzle

Brown, Clifford A.Simulation of the Localized Arc Filament Plasma Actuators for Jet Excitation
Doctor of Philosophy, University of Akron, 2010, Mechanical Engineering
The concept of jet control by external forcing is not new. The first published demonstration of a jet responding to outside forces occurred in the mid-1800's. It was not, however, until the 1950's, with the advent of commercial jet aircraft, that scientific study of the subject greatly increased as researchers used external forcing to study the structure and noise sources present in a jet plume. Interest in active jet control continues today, particularly with the additional possibilities afforded by significant advances in measurement and simulation technology, even though it remains limited by the available jet actuators to relatively small, low Reynolds number jets that are of little similarity to the large, highly turbulent jets common in real world applications. However, the recently developed Localized Arc Filament Plasma Actuators (LAFPA) have the potential to expand active jet control research to include these larger, higher Reynolds number jets. The LAFPA have been used successfully to control a small-diameter, high-speed turbulent jet to achieve some plume mixing enhancement and limited noise mitigation. The system, however, must still be extended to a larger class of jets common in the real world and optimized for an application. This work addresses both of these issues First, experiments are conducted to determine the impact of the LAFPA on a large-scale jet. A model of the LAFPA is then developed for use in the future CFD based studies of excited jets. The model performance is investigated using multiple CFD methodologies to determine the optimal combination of actuator model and CFD scheme for excited jet simulations. Ultimately, the model developed will be used by researchers in future simulations to optimize the actuator system for noise reduction, IR signature reduction, or to system scalability for deployment on larger jets in real-world applications.

Committee:

Scott Sawyer, Dr. (Advisor); Braun Minel, PhD (Committee Member); Quinn Dane, PhD (Committee Member); Mugler Dale, PhD (Committee Member); Young Jerry, PhD (Committee Member); Hariharan Subramaniya, PhD (Committee Member); Georgiadis Nicholas, PhD (Committee Member)

Subjects:

Fluid Dynamics

Keywords:

jet excitation; jet noise; CFD; LES; RANS; actuators; plasma

Mora Sánchez, Pablo AInvestigation of the Noise Radiation from Heated Supersonic Jets
PhD, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering
This work focuses in the investigation of crackle and Mach wave radiation in heated supersonic jets. The skewness and kurtosis of the acoustic pressure signal and its time derivative were adopted as metrics for identifying crackling jets and quantifying levels of crackle. Cold and heated jets from supersonic nozzles with different geometric parameters and scales are analyzed to draw conclusions on noise sources and propagation. In order to complement the investigation, results are also presented for the mixing noise, broadband shock-associated noise and screech. Chapter 4 focuses on the impact of jet operating condition on the skewness and kurtosis levels of a jet issuing from a converging-diverging conical nozzle, with a 1.5 design Mach number. An increase in convective Mach number, achieved by increasing jet temperature, proved to be related to elevated values of OASPL, skewness, and kurtosis, in both the near and far fields. Intense levels of the dP/dt high-order statistics appear to be generated at different locations in the shear layer of the jet and strengthen away from the jet by non-linear propagation effects. Chapter 5 studies how adding chevrons to a converging-diverging nozzle impacts Mach wave radiation and crackle. The chevrons decreased OASPL in the downstream angles but increased the broadband shock-associated noise. Pressure skewness, dP/dt skewness and kurtosis were all reduced by the chevrons in the near field and far field, and thus they effectively mitigated crackle and Mach wave radiation; however, chevrons showed no evidence of changing the convective Mach number. The evolution of noise signals was analyzed in the near-field to the far-field to identify the strengthening of skewness through nonlinear propagation effects. Chapter 6 investigates a jet exhausting over a plate at different stand-off distances, to simulate jets exhausting over airframe surfaces and jet-ground interaction during take-off and landing operations. Far-field acoustics were measured at the reflected direction, sideline, and shielded azimuthal directions. At the sideline, the plate attached to the nozzle lip diminished broadband shock-associated noise, and mitigated screech for the cold case. When the plate was moved away from the nozzle, screech tones were intensified at the under-expanded condition. Crackle levels were significantly intensified in the sideline, within a range of stand-off positions. Chapter 7 analyzes the impact of nozzle scale and nozzle internal contours on the levels of crackle. Three scaled converging-diverging nozzles, with jet exit diameters of 0.542 in, 0.813 in, and 1.085 in were investigated. Far-field arrays were setup at a constant non-dimensionalized radial distance of 40 nozzle exit diameters. The pressure skewness and kurtosis plots collapsed for all three scaled nozzles when the pressure signals were not filtered. The dP/dt statistics collapsed when the signals were downsampled proportional to the nozzle exit diameters. Baseline nozzle results were also compared to a smooth contoured nozzle designed by the Method of Characteristics. This nozzle almost had no broadband shock-associated noise, but contained the same skewness and kurtosis levels, concluding that crackle is not linked to the shock-cell structures in the jet.

Committee:

Ephraim Gutmark, Ph.D. D.Sc. (Committee Chair); Kailas Kailasanath, Ph.D. (Committee Member); Jeffrey Kastner, Ph.D. (Committee Member); Mark Turner, Sc.D. (Committee Member)

Subjects:

Aerospace Materials

Keywords:

Supersonic Jet Noise;Crackle;Mach Wave Radiation;Supersonic Heated Jets

Gonzalez, David RDevelopment of a Semi-Lagrangian Methodology for Jet Aeroacoustics Analysis
Doctor of Philosophy, The Ohio State University, 2016, Aero/Astro Engineering
A novel analysis technique is developed for the time-accurate analysis of noise sources in compressible jet flows. By adopting a Lagrangian point of view, the finite-time Lyapunov exponent (FTLE) method provides a means of linking discrete events in the vicinity of a high-subsonic jet shear layer to aft- and sideline-radiated noise. The FTLE is first validated in the compressible flow regime, as it was originally developed for incompressible flows, where it has shown substantial success in analyzing the dynamics of Lagrangian coherent structures. It is demonstrated by theoretical and numerical experiments that judicious choice of temporal intergration parameters highlights distinct features in the flow field. Due to the non-solenoidal nature of compressible flows, Lagrangian FTLE coefficients computed in forward and backward time can extract wave dynamics from the velocity field in addition to the flow-organizing convective features. Integration time, thus, acts as a pseudo-filter, smoothly separating coherent (convective) structures from propagating waves. Results are confirmed by first examining forward and backward FTLE coefficients for several simple, well-known acoustic fields and in the propagation of a two-dimensional acoustic pulse. Each coefficient is shown to capture a distinct portion of the traveling acoustic waves, with the forward FTLE (sf) focusing on the positive stroke and the negative portion extracted by the backward- integrated (sb) coefficient. Analysis of the mono-chromatic acoustic flow fields shows the peak FTLE magnitudes directly scale with the acoustic frequency and a complete reconstruction of the underlying dilatational field can be achieved by conducting the Lagrangian analysis over a single time step for each time instant. An increase in integration time subsequently leads to both damping and phase-shifting of the resolved acoustic waves as a consequence of the contribution from multiple temporal snapshots. Having established the theoretical foundation of the Lagrangian dynamics in compressible flows, the technique is then applied to identify events associated with intermittency in jet noise pressure probe data. Although intermittent events are known to be dominant causes of jet noise, their direct source in turbulent jet flows has remained unexplained. To this end, Large-Eddy Simulation (LES) data of several Mach 0.9 jets are subjected to FTLE to simultaneously examine, and thus expose, the causal relationship between coherent structures and the corresponding acoustic waves. The analysis illustrates that intermittent events are associated with entrainment in the initial roll up region and emissive events downstream of the potential-core collapse. Instantaneous acoustic disturbances are observed to be primarily induced near the collapse of the potential core and continue propagating towards the far-field at the experimentally-observed, approximately thirty-degree angle relative to the jet axis. Analysis of a fully-turbulent jet LES, where inflow turbulence was accounted for with a Reynolds- averaged-Navier-Stokes-initiated digital filter, further demonstrates that repelling Lagrangian structures given by the forward-integrated FTLE play a dominant role in establishing the near-acoustic field. By examining the evolution of the FTLE coefficients and the Lagrangian reconstruction of dilatation along the direction of maximum noise directivity, it is shown that sf contributes the bulk of the acoustic energy in the LES 'far-field'; and is also better correlated with the Eulerian dilatation at these locations than the backward-integrated field, despite the fact that the latter field is associated with the large-scale coherent structures. A two- point correlation analysis between the farfield dilatation and the Lagrangian coefficients along the time-mean potential core and shear layer boundaries further demonstrates that the farfield signals are intimately linked to the dynamics of coherent structures in these regions. Vortex merging events in the turbulent jet are also shown to be significant contributors to the genesis of farfield-radiating disturbances. In particular, they play a key role in establishing and modulating wavepackets within the potential core, which are known to be an important component of aft-radiated noise. The modulation of the wavepackets and the entrainment and emmission of acoustic energy to and from the ambient is highly influenced by the presence of strong repelling structures in the turbulent shear layer.

Committee:

Datta Gaitonde (Advisor); Jen-Ping Chen (Committee Member); Mark Lewis (Committee Member); Mohammad Samimy (Committee Member); Mei Zhuang (Committee Member)

Subjects:

Aerospace Engineering

Keywords:

Jet noise, finite-time Lypaunov exponents, Lagrangian dynamics, computational fluid dynamics

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

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