Trailing edge noise is a common noise source in aerodynamic applications, such as wind turbines, duct fan blades, and propellers. As sound is a nuisance for people near this machinery, methods of reducing trailing edge noise are being investigated. A proven method of trailing edge noise reduction is using a serrated trailing edge. Many prior experiments have shown that a trailing edge with sawtooth serrations can reduce trailing edge noise compared to a straight trailing edge, but the mechanism by which sawtooth serrations reduce noise is not fully understood. Previous theoretical models have assumed that the turbulence field convecting past a serrated trailing edge is unchanged by the presence of the serrations, but experiments have shown that this is not the case in reality. This work attempts to further explore the mechanisms behind why trailing edge serrations reduce trailing edge noise. Additionally, it evaluates the usefulness of a wall jet wind tunnel for use in the study of serrated trailing edges. Experiments were conducted in an anechoic wall jet wind tunnel using a straight trailing edge configuration and a serrated trailing edge configuration. It was found that there may be differences in the unsteady surface pressure over serrated edges in one-sided flows as compared to two-sided flows, like on that of an airfoil, most notably in relation to the magnitude of the unsteady pressure on the serrations. In the wall jet and in agreement with other studies, it was found that the unsteady pressure fluctuations increase towards the tip of the serration in one-sided flows. This is counter to what is found in some studies of two-sided flows. Good agreement was found between some models of the wavenumber-frequency wall pressure spectrum of a turbulent boundary layer and the measured wall pressure spectrum on the straight trailing edge, and these models also produced good predictions of the noise produced by this trailing edge using Amiet's equation. A surface pressure microphone array was used to estimate the zero spanwise wavenumber surface pressure spectrum. This spectrum was used in Amiet's method to predict the measured trailing edge noise. Predictions using the wavenumber-filtered measurement tended to overpredict the measured far field noise most likely due to the inclusion of broader wavenumber content through the array's side lobe response and the breadth of the main lobe. The serrated trailing edge was found to increase coherence between two points on the same serration while reducing coherence between two points on different serrations. It was concluded that the presence of the serrations decorrelates small-scale turbulent eddies. Additionally, it was found that while the serrated trailing edge did reduce the noise produced, its destructive effect on the geometry-based resonance of the straight trailing edge configuration may have amplified the magnitude of the reduction. Finally, it was concluded that the serrations do indeed affect the hydrodynamic field near the trailing edge, and the theoretical models which make the assumption otherwise must be refined. / Master of Science / Trailing edge noise is a common noise source in aerodynamic applications, such as wind turbines, duct fan blades, and propellers. As sound is a nuisance for people near this machinery, methods of reducing trailing edge noise are being investigated. A proven method of trailing edge noise reduction is using a serrated trailing edge. Many prior experiments have shown that a trailing edge with sawtooth serrations can reduce trailing edge noise compared to a straight trailing edge, but the mechanism by which sawtooth serrations reduce noise is not fully understood. This work attempts to further explore the mechanisms behind why trailing edge serrations reduce noise. Experiments were conducted in an anechoic wind tunnel facility. It was found that a one-sided flow over a serrated trailing edge may be significantly different from that over a two-sided flow. Good agreement was found between prediction models and measurements of trailing edge noise. The serrated trailing edge was effective at reducing the coherence of turbulent eddies across the roots of the sawtooth serrations. It was concluded that the serrated trailing edge is effective at reducing noise, and that one means of doing so is decreasing the correlation of small-scale turbulent eddies, and that current models of the flow over serrations may need to be refined.

Crackle is a perceptual feature of supersonic jet noise that is related to the presence of acoustic shocks. The skewness of the time-derivative of the pressure waveform, or derivative skewness, is used as a metric indicative of crackle perception. The three main objectives of this work are: 1) Determine the potential spatial origin of crackle-related events in the near field of a high-performance military aircraft via an event-based beamforming method. 2) Investigate the potential for nonlinear, irregular shock reflections occurring along the near-field ground array and their implications on derivative skewness. 3) Relate the near-field, crackle-related events to far-field crackle perception by comparing nonlinearly propagated waveforms with measured far-field data. The event-based beamforming method used to determine source and far-field relationship of shock-like events utilizes the cross correlation between adjacent microphone waveform segments to determine the angle of propagation for an ensemble of crackle-related events within the waveform. The angle of propagation is traced towards the source for each event to find its apparent origin along the jet lipline. Beamforming results indicate that crackle-related events appear to originate anywhere from 2 to 14.5 m downstream along the jet lipline, with distributions that shift downstream and broaden with increasing engine power. The shock reflection classification method builds on the event-based beamforming method to calculate angle of incidence relative to the ground for an ensemble of shock events. The combination of angles of incidence and the measured shock strengths of the events reveal that irregular reflections are likely to occur over the majority of the array, which likely elevates the derivative skewness values due to steeper shocks with greater peak-to-peak pressures relative to off-ground measurements. Near-field, crackle-related events are extrapolated to the far field using a nonlinear propagation model to determine their prevalence in the far field. Cross-correlation coefficients of waveform segments centered about the propagated events indicates that for farther aft angles, near-field events are more related to far-field measurements. Waveform observations show that shock-like events in the near field that are more spiked in nature tend not propagate into the far field. However, near-field, large-derivative events with broader, high-pressure peaks nonlinearly steepen and form shocks in the far field that are likely contribute to crackle perception.

Aircraft noise mitigation has been an ongoing challenge for the aeronautics research community. In response to this challenge, aircraft concepts have been developed in which the propulsion system is integrated with the airframe to shield the noise from the observer. These concepts exhibit situations where the jet exhaust interacts with an airframe surface. Jet flows interacting with nearby surfaces exhibit a complex behavior in which acoustic and aerodynamic characteristics are altered. The physical understanding and accurate modeling of these characteristics are essential to designing future low-noise aircraft. In this thesis, an alternative approach is created for predicting jet mixing noise that utilizes an acoustic analogy and the solution of the steady Reynolds-Averaged Navier-Stokes (RANS) equations using a two equation turbulence model. A tailored Green's function is used in conjunction with the acoustic analogy to account for the propagation effects of mixing noise due to a nearby airframe surface. The tailored Green's function is found numerically using a newly developed ray tracing method. The variation of the aerodynamics, acoustic source, and far- field acoustic intensity are examined as a large flat plate is moved relative to the nozzle exit. Steady RANS solutions are used to study the aerodynamic changes in the field-variables and turbulence statistics. To quantify the propulsion airframe aeroacoustic (PAA) installation effects on the aerodynamic source, a non-dimensional number is formed that can be used as a basic guide to determine if the aerodynamic source is affected by the airframe and if additional noise produced by the airframe surface is present. The aerodynamic and noise prediction models are validated by comparing results with Particle Image Velocimetry (PIV) and far-field acoustic data respectively. The developed jet noise scattering methodology is then used to demonstrate the shielding effects of the Hybrid Wing Body (HWB) aircraft. The validation assessment shows that the acoustic analogy and tailored Green's function provided by the ray tracing method are capable of capturing jet shielding characteristics for multiple configurations and jet exit conditions. / Master of Science

The entropy noise in modern engines is mainly originating from two types of mechanisms.First, chemical reactions in the combustion chamber lead to unsteady heat releasewhich is responsible of the direct combustion noise. Second, hot and cold blobsof air coming from the combustion chamber are advected and accelerated throughturbine stages, giving rise to the so-called entropy noise (or indirect combustionnoise). In the present work, numerical characterization of indirect combustion noiseof a Nozzle Guide Vane passage was assessed using three-dimensional Large EddySimulations. The study was conducted on a simplified topology of a real turbinestator passage, for which experimental data were available in transonic operatingconditions. First, a baseline case was reproduced to validate a numerical finite volumesolver against the experimental measurements. Then, the same solver is used toreproduce the effects of incoming entropy waves from the combustion chamber andto characterize the additional generated acoustic power. Periodic temperature fluctuationsare imposed at the inlet, permitting to simulate hot and cold packets of aircoming from the unsteady combustion. The incoming waves are characterized bytheir characteristic wavelength; therefore, a parametric study has been conductedvarying the inlet temperature of the passage, generating entropy waves of greaterwavelengths. The study proves that the generated indirect combustion noise canbe significant. Moreover, the generated indirect combustion noise increases as thewavelength of the incoming disturbances increases. Finally, the present work suggeststhat, in transonic conditions, there might be flow features which enhance theindirect combustion noise generation mechanism.

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Sound generation and radiation properties are studied of full-scale tactical jet engine noise. This is motivated by the high sound exposure levels from jet noise, particularly for tactical engines. Acoustic source reconstruction methods are implemented computationally on existing jet noise data. A comparative study is performed using numerical simulations to understand the capabilities of more advanced beamforming methods to successfully estimate the source properties of a distributed, partially correlated source distribution. The properties and limitations of each beamforming method are described. Having validated the methods, beamforming with regularization—via the Hybrid Method—is implemented on linear array measurements near an installed tactical engine. A detailed analysis of the correlation and coherence properties associated with the phased array measurements guides the implementation of the beamforming. When the measurements are used as inputs to the beamforming, they produce partially correlated, distributed sources in a full-order model representation. A processing technique is also implemented that increases the usable bandwidth of the array measurements to almost an order of magnitude above the array design frequency. To more appropriately study the equivalent sources, a decomposition technique is designed and implemented to create a reduced-order wavepacket model of the jet noise. The wavepacket model is modular and scalable to allow for the efficient characterization of similar jet noise measurements. It is also appropriate for its physical significance, as wavepackets are attributed to the turbulent flow as well as the hydrodynamic and acoustic properties of the radiation. The reduced order model can estimate the levels and coherence properties of the acoustic radiation and represents a significant step towards a complete jet noise prediction model.

Sound generation and radiation properties are studied of full-scale tactical jet engine noise. This is motivated by the high sound exposure levels from jet noise, particularly for tactical engines. Acoustic source reconstruction methods are implemented computationally on existing jet noise data. A comparative study is performed using numerical simulations to understand the capabilities of more advanced beamforming methods to successfully estimate the source properties of a distributed, partially correlated source distribution. The properties and limitations of each beamforming method are described. Having validated the methods, beamforming with regularization”via the Hybrid Method”is implemented on linear array measurements near an installed tactical engine. A detailed analysis of the correlation and coherence properties associated with the phased array measurements guides the implementation of the beamforming. When the measurements are used as inputs to the beamforming, they produce partially correlated, distributed sources in a full-order model representation. A processing technique is also implemented that increases the usable bandwidth of the array measurements to almost an order of magnitude above the array design frequency. To more appropriately study the equivalent sources, a decomposition technique is designed and implemented to create a reduced-order wavepacket model of the jet noise. The wavepacket model is modular and scalable to allow for the efficient characterization of similar jet noise measurements. It is also appropriate for its physical significance, as wavepackets are attributed to the turbulent flow as well as the hydrodynamic and acoustic properties of the radiation. The reduced order model can estimate the levels and coherence properties of the acoustic radiation and represents a significant step towards a complete jet noise prediction model.

The design of aircraft propulsion configurations must digress from the typical configurations that are utilized on the majority of aircraft in order to consider the effects of environmental issues as well as the noise that is generated from the engines. One unconventional approach under consideration involves rectangular jets near flat surfaces that are parallel to the jet axis. This type of configuration makes an attempt to muffle the noise that propagates to the ground, but previous experimental work showed that the noise generated by this configuration was actually increased due to the effect that the plate trailing edge exerts on the flow. In this thesis, a large eddy simulation study is conducted to determine whether wall deformations at the plate trailing edge could reduce the jet noise. A high aspect ratio rectangular nozzle is placed over a flat surface featuring sinusoidal deformations at the trailing edge. A range of amplitudes and wavenumbers, characterizing the deformations at the trailing edge, is considered to determine the parameter range that corresponds to noise reduction.

This thesis details the design and construction of an anechoic chamber for acoustic testing and measurements of large UAS propellers. Three propellers are considered, as they are common propeller designs used for large UAS today. The knowledge and practices involved with acoustic testing and measurements in anechoic chambers, as well as the results of noise studies related to large UAS, are not widely available due in large part to the limited availability and use of large UAS in the public domain. Using established principles related to fundamental acoustic theory and propeller noise, the aeroacoustic noise from large UAS propellers was measured to study and evaluate the reduction in total aerodynamic noise. This data and research provides the ability to evaluate propeller noise in relation to the overall detectability of large unmanned aircraft systems.

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