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No description available.

The combination of traditional passive acoustic liners with Herschel-Quincke (HQ) waveguides is proposed in this work as a device for Turbofan Engine Noise Control. The approach consists of installing circumferential arrays of HQ tubes on the lined sector of the inlet of a turbofan engine. A theoretical model is developed to predict the performance of this system assuming that the engine inlet is a circular lined duct with uniform mean flow. The tube-duct interfaces are modeled as finite piston sources that couple the sound field inside the duct with the dynamics of the HQ tubes. The finite piston source radiation is modeled in terms of a new closed form Green's function, which is found as the solution of the non-homogeneous convected acoustic wave equation with soft wall boundary conditions. The Green's function is extended from a point source to a finite piston by using the Divergence Theorem in the appropriate form. The dynamics of the HQ tube are both modeled as plane waves inside a straight tube and experimentally determined. The experimental determination of the HQ-dynamics is undertaken using impedance tubes with a 4-microphone technique. The newly developed theoretical model was used to predict the performance of a combined HQ-Liner system, which was tested on a scale simulated turbofan rig. The model is validated for broadband noise with the experimental data obtained from this test rig. The analytical predictions are shown to correlate well with experimental data. The results of the application of a HQ-Liner on a turbofan engine show a great potential in order to improve the performance of traditional passive acoustic liners. / Ph. D.

For decades noise-induced hearing loss has been a concern of the Department of Defense (DoD). My research investigates noise generation and dispersion in supersonic jets and focuses on the fluid-dynamic regime typical of high-performance turbojet and turbofan engines. The goal of my research is to understand how dispersion and propagation of wavepackets can be modified by noise reduction strategies based on secondary injections of fluid with a different temperature from the main jet. The research is organized into three studies that focus on instability, large eddy simulations, and resolvent modes. The first study is a computational investigation of the role of thermal non-uniformity on the development of instability modes in the shear-layer of a supersonic $M= 1.5$, $Re=850,000$ jet. Cold fluid is injected at the axis of a heated jet to introduce radial non-uniformity and control the spatial development of the shear layer. The mean flow is analyzed with an efficient 2D and 3D Reynolds-averaged Navier-Stokes (RANS) approach using the SU2 code platform for 3 different cases -baseline, centered, and offset injection. Different turbulence models are tested and compared with the experiments. The coherent perturbation is analyzed using linear parallel and parabolized stability equations (PSEs). The second study investigates novel formulations of large eddy simulation models using an arbitrary high order discontinuous Galerkin scheme. The LES analysis focuses on both numerical issues (such as convergence against the polynomial order of the mesh), modeling issues (such as the choice of subgrid model), and underlying physics (such as vortex stretching and noise generation). Wall models are used to capture the viscous sublayer at the nozzle. The Ffowcs Williams-Hawkings (FW-H) method is used for far-field noise predictions for all cases. Three-dimensionality is studied to investigate how injection in the shear layer acts to create a rotational inviscid core and affects the mixing of the cold fluid and noise dispersion. The third study extends the (first) instability study by considering (global) resolvent modes. Such optimally forced modes of the turbulent mean flow field will identify the turbulent coherent structures (wavepackets) for different turbulence models at $M=1.5$. The LES simulations performed in the second study will be used to extract the mean flow and the dynamic modes for comparison. My research plan is to perform the resolvent analysis of the axisymmetric mean flow fields for the thermally activated case (i.e., the centered injection) and compare it to the baseline jet case. Different turbulence models will be investigated to determine the correct alignment of dynamic and resolvent modes. Finally, I will consider the three-dimensional, non-axisymmetric mean flow created by offset injection described in the second study, which requires evaluating the convolution products of resolvent modes and base flow. Such three-dimensional resolvent compressible modes have never been identified in the context of supersonic jets. / Doctor of Philosophy / For decades noise-induced hearing loss has been a concern of the Department of Defense (DoD). Research in this area is critical to US national security and valued by both the aircraft industry and government. The noise generated during take-off and landing is hazardous to the crew personnel who work around this vicinity. A reduction of noise can significantly decrease medical expenditure and allow the aircraft industry to meet the stringent community noise requirements. Among the various techniques of noise reduction analyzed over the years, thermal non-uniformity stands out for its simple implementation and cost-effectiveness, especially in after-burner turbojets. Thermal non-uniformity with a cold secondary stream introduces low-velocity fluid in a supersonic jet by locally increasing the density while matching the mass flow rate. Changes to the velocity profile are localized; different regions of the jet emit sound at different frequencies and radiation angles, thus the link between injection location and noise control is not well understood. Using different computational tools this research investigates the link connecting thermal non-uniformity, turbulent production, and sound generation. Injection at different radial locations affects the two mechanisms of sound radiation in different ways. The first mechanism, the Kelvin Helmholtz instability, can be studied as an eigenvalue problem that represents the spatial growth of normal modes. De-coherence of these modal fluctuations can be obtained by injecting secondary fluid directly into the shear layer. This injection mode is called offset injection. The present research shows that the thickening of the shear layer due to low-velocity fluid delays the formation of Kelvin-Helmholtz modes in the offset case. Thus, the outskirts of the jet produce pressure fluctuations with a lower spectral energy density. The second mechanism, the Orr instability, can be analyzed as non-modal growth of acoustic perturbation forced by the breakdown of the core of the jet. LES and stability analysis shows that centered injection is highly effective in reducing the Orr radiation. Resolvent modes explain that the rationale is the delay and reduction of a secondary resonant peak between spatial eddies and forcing caused by changes in the mean profile responsive to secondary injection. Our analysis also explains why the offset injection is more effective at a low polar angle, while centered injection reduces acoustic radiation towards high polar angles. Parametric studies of different injection strategies, i.e., location and number of injection ports are performed to demonstrate the best strategy for noise level reductions.

Commercial aviation transportation has experienced an overwhelming growth over the years. However, this expansion has encountered an important barrier: noise. Several studies have shown that residents in these areas experience problems such as stress and sleep disturbance. These problems have translated into demands for a better quality of life from airport residents which in turn have translated into more stringent aircraft noise regulations. As a result, large amounts of resources have been diverted towards the improvement of existing noise attenuation technologies and the development of more effective ones. In terms of turbofan generated noise, the most widely used technology is that of absorbent materials or liners. In recent investigations Alonso et al. have combined Herschel/Quincke (HQ) tubes with liners. This combination has the potential of effectively controlling pure tones and broadband noise in inlet sections of modern turbofan engines. Since a comprehensive approach for engine noise reduction will involve both inlet and aft HQ-Liner systems, additional research efforts were needed to evaluate their performance at reducing aft fan radiation In the present work, a combination of traditional liners and Herschel/Quincke waveguide resonators for aft fan radiation control is proposed. A theoretical model is developed in order to predict noise reduction due to such systems. The newly developed tool was then utilized to design an HQ-liner that was installed and tested in the aft section of the NASA Active Noise Control Fan (ANCF) rig. This experimental data was utilized to prove the potential of these systems and to validate the mathematical model. Analytical predictions correlate well with experiments. The NASA ANCF rig is not representative of a real turbofan engine. In order to assess the behavior of HQ-Liners in a more realistic environment a new system was specifically designed for a generic turbofan engine and its performance analyzed. The sound field inside HQ tubes has been described assuming plane waves only. This assumption limits the model to frequencies below the tube first resonance. In order to overcome this limitation a new model accounting for higher order modes inside the tubes has been developed. / Ph. D.

A new technique to de-Dopplerize microphone signals from moving sources of sound is derived. Currently available time domain de-Dopplerization techniques require oversampling and interpolation of the microphone time data. In contrast, the technique presented in this dissertation performs the de-Dopplerization entirely in the frequency domain eliminating the need for oversampling and interpolation of the microphone data. As a consequence, the new de-Dopplerization technique is computationally more efficient. The new de-Dopplerization technique is then implemented into a frequency domain beamforming algorithm to locate moving sources of sound. The mathematical formulation for the implementation of the new de-Dopplerization technique is presented for sources moving along a linear trajectory and for sources moving along a circular trajectory, i.e. rotating sources. The resulting frequency domain beamforming method to locate moving sound sources is then validated using numerical simulations for various source configurations (e.g. emission angle, emission frequency, and source velocity), and different processing parameters (e.g. time window length). Numerical datasets for sources with linear motion as well as for rotating sources were simulated. For comparison purposes, selected datasets were also processed using traditional time domain beamforming. The results from the numerical simulations show that the frequency domain beamforming method is at least 10 times faster than the traditional time domain beamforming method with the same performance. Furthermore, the results show that as the number of microphones and/or grid points increase, the processing time for the traditional time domain beamforming method increases at a rate 20 times larger than the rate of increase in processing time of the new frequency domain beamforming method. / Ph. D.

The research effort presented in this dissertation consists of employing active trailing edge blowing control to reduce the unsteady stator-rotor interaction in a turbofan simulator. Two active flow control systems with different wake sensing approaches are successfully implemented on the engine simulator. The first flow control system utilizes Pitot probes as flow sensors. Use of Pitot probes as sensors is appropriate as a first step toward a more in depth investigation of active trailing edge blowing control. An upper performance limit in terms of wake-filling can be obtained and serves as the baseline in evaluating other control systems with indirect wake sensors. The ability of the system to achieve effective wake filling when subjected to a change in inlet flow conditions demonstrates the feasibility and advantage of active flow control. Significant tonal noise reductions in the far field are also obtained. The second control system involves using microphones as indirect wake sensors. The significance of these acoustic sensing approaches is to provide a practical TEB approach for realistic engines implementations. Microphones are flush mounted on the inlet case to sense the tonal noise at the blade passing frequency. The first sensing approach only uses the tone magnitude while the second novel sensing approach utilizes both the tone magnitude and phase as error information. The convergence rate of the second sensing approach is comparable with that of the Pitot-probe based experiments. The acoustic results obtained from both sensing approaches agree well with those obtained using Pitot probes as sensors. In addition to the experimental part of this research, analytical studies are also conducted on the trailing edge blowing modeling using an aeroacoustic code. An analytical model for trailing edge blowing is first proposed. This model is then introduced into the two-dimensional aeroacoustic code to investigate effect of various trailing edge blowing managements in the tonal sound generation. / Ph. D.

An innovative implementation of the Herschel-Quincke tubes concept for the reduction of noise from turbofan engines is proposed here. The approach consists of installing circumferential arrays of Herschel-Quincke (HQ) tubes or waveguides in the inlet of the turbofan engine. An analytical technique was developed to predict the effects of HQ tubes applied to circular inlets. The modeling technique involves modeling the tubes-inlet interfaces as finite piston sources that couple the acoustic field inside the inlet with the acoustic field within the HQ tubes. An optimization technique based on genetic algorithms was also developed to be able to design and optimize the system parameters. The accuracy of the model was validated with experimental data obtained from two types of turbofan engines. Analytical predictions are shown to correlate well with experimental data. The analytical model is then used to provide insight into the noise control mechanisms involved in the system. It is shown that the energy in an incident mode is in part reflected back to the fan and that some energy is also scattered into other higher-order modes. Thus, the suppression of a particular mode is due to the combination of the scattered contributions from the various incident modes. The effects of the system parameters were analyzed and parametric studies were conducted. Different configurations for the arrays of HQ tubes such as helical patterns or tubes at an angle with respect to the inlet axis were also investigated. The results show the great potential of the HQ tubes system to reduce noise from turbofan engines. / Master of Science

Noise from fluid flow over rough surfaces is an important consideration in the design and performance of certain vehicles with high surface-area-to-perimeter ratios. A new method of noise control based on the anatomy of owls is developed and consists of fabric or fibrous canopies suspended above the surface. The method is tested experimentally and is found to reduce the total far-field noise emitted by the surface. The treatment also is found to reduce the magnitude of pressure fluctuations felt by the underlying surface by up to three orders of magnitude. Experimental investigations into the effects of geometric parameters of the canopies lead to an optimized design which maximizes noise reduction. The results obtained during the canopy experiment inspired a separate new device for the reduction of trailing edge noise. This type of noise is generated by flow past the wing of an aircraft or the blades of a wind turbine, and is a source of annoyance for those in surrounding communities. The newly developed treatment consists of small fins, or "finlets," placed near the trailing edge of an airfoil. The treatment is tested experimentally at near-full-scale conditions and is found to reduce the magnitude of far-field noise by up to 10 dB. Geometric parameters of the finlets are tested to determine the optimal size and spacing of the finlets to maximize noise reduction. Follow-up computational and experimental studies reveal the fluid mechanics behind the noise reduction by showing that the finlets produce a velocity deficit in the flow near the trailing edge and limit the magnitude and spanwise correlation lengthscale of turbulence near the trailing edge, factors which determine the magnitude of far-field noise. In a final experiment, the finlets are applied to a marine propeller and are found to reduce not only trailing edge noise, but also noise caused by the bluntness of the trailing edge. The results of this experiment show the potential usefulness of finlets to reduce noise from rotating systems, such as fans or propellers, as well as from structures which feature blunt trailing edges. / Ph. D.

Uma nova ferramenta de aeroacústica computacional baseada em simulações RANS (Reynolds Averaged Navier-Stokes) foi desenvolvida para a predição do ruído gerado pelo escoamento tri-dimensional de jatos complexos. O método é denominado de LRT o qual surgiu da combinação da analogia acústica de Lighthill com o método de acústica geométrica Ray-Tracing. A grande vantagem da utilização do método LRT para predições de ruído é que este determina não apenas as fontes sonoras presentes no escoamento, mas também modela os efeitos da interação fluidoacústica e, sua influência no ruído em um campo distante. Esta característica tornou-se extremamente importante para a indústria de motores aeronáuticos já que investigações em bocais assimétricos estão atualmente em andamento devido à necessidade de redução de ruído. O método LRT é uma ferramenta relativamente rápida de predição de ruído de jatos baseado na Analogia Acústica de Lighthill e que usa como dados de entrada os resultados obtidos à partir de uma simulação RANS do escoamento. A interação fluidoacústica é calculada através da utilização da Teoria de Traçamento de Raios. O método LRT foi formulado como um método tri-dimensional e, portanto, não possui limitações de aplicabilidade para a predição sonora em relação ao tipo de escoamento ou à geometria do bocal. Diversas simulações numéricas foram conduzidas com sucesso para uma grande variedade de escoamento de jatos (jatos simples, coaxiais e assimétricos) utilizando o LRT como uma ferramenta de engenharia. O resultado deste trabalho é uma ferramenta numérica que permite a realização de predições de ruído para casos de escoamento de jatos complexos, assim como possibilita sua aplicação para a investigação de efeitos de interação do escoamento do jato com superfícies hiper-sustentadoras no campo acústico. Adicionalmente, o método LRT pode ser aplicado para complementar análises experimentais possibilitando, portanto, um melhor entendimento sobre os mecanismos fluidodinâmicos e acústicos presentes em escoamentos de jatos complexos. / A novel computational aeroacoustics tool based on RANS (Reynolds Averaged Navier-Stokes method) is developed for predicting the noise generated by complex three-dimensional jet flows. The new method is called LRT which arises from the combination of Lighthills acoustic analogy with Ray-Tracing acoustics. The powerful advantage of applying the LRT method for noise predictions is that it calculates not only the noise sources but it also models and takes into account sound-flow interaction effects without any geometric simplification, such as flow symmetries of the problem. This is now a strong requirement from aero-engines manufactures since investigations on asymmetric nozzles, as a means of noise reductions are in progress. The LRT method is a relatively fast jet noise prediction tool based on Lighthills Acoustic Analogy and it uses a Reynolds-Average Navier-Stokes (RANS) computational fluid dynamics (CFD) simulation as input information. The sound-flow interaction is computed by solving the propagation using Ray-Tracing equations. The LRT method has been formulated as a general three-dimensional method and it has no restrictions on the type of the flow field or nozzle geometry for noise prediction. Successful numerical noise predictions have been carried out for a variety of jet flows (single, coaxial and asymmetric jets) using the LRT as an engineering tool. The outcome from this thesis is a numerical tool that allows noise predictions of complex exhaust systems and the variations in sound field due to modifications of the flow field generated by the interaction of the jet flow with high-lift surfaces. In addition, the LRT method can be applied to complement experimental analysis providing a better understanding about the flow and acoustics mechanisms for complex jets.