Understanding the impact of jet noise and other high-amplitude sound sources can be improved by quantifying the nonlinearity in a signal with a single-microphone measurement. An ensemble-averaged, frequency-domain version of the generalized Burgers equation has been used to derive a quantitative expression for the change in spectral levels (in decibels) over distance due to geometric spreading, thermoviscous absorption, and nonlinearity, respectively. The nonlinearity indicator, called νN , is based on the quadspectral Morfey-Howell indicator, which has been used in the past to characterize nonlinearity in noise waveforms. Unlike the Morfey-Howell indicator, the νN indicator has direct physical significance, giving a change in decibels per meter of the sound pressure level spectrum specifically due to nonlinearity. However, a detailed characterization of the expected behavior and potential issues for the nonlinearity indicator has been lacking. The quadspectral nonlinearity indicator is first calculated for well-known solutions to several basic acoustical scenarios to determine its expected behavior in both the near field and far field. Next, the accuracy of νN is examined as a function of measurement parameters such as sampling frequency, signal bandwidth, scattering, and noise. Recommendations for conducting experiments are given based on the findings. Finally, the indicator is calculated for model-scale and military jet noise waveforms. These tests reveal the utility and accuracy of the νN indicator for characterizing broadband noise; the indicator gives frequency-dependent information about the waveform from a single-point measurement.

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 scientific community has employed multiple methods to analyze and describe the jet noise emanating from the turbulent exhaust flow from modern military aircraft engines, with the goal that better characterization of the sound radiation will improve noise reduction efforts. This thesis utilizes three different approaches to characterize the noise source region from a static F-22A Raptor. First, the energy flow field along planes near the aircraft and along an arc is measured using a multidimensional vector intensity probe. The resulting vector intensity maps give a clear indication of the directionality of the noise as a function of frequency at different engine conditions. A straightforward ray-tracing method show the utility of vector intensity measurements in source characterization by estimating the region from which the loudest portions of sound are emanating. Second, intensity reconstructions from near-field acoustical holography (NAH) provide an estimate of the three-dimensional radiated energy flow field. The sound field is shown to be dominated by mutually incoherent radiation lobes, which can be partially isolated by a partial decomposition method. Lastly, a wavepacket source model is optimized in light of amplitude-based NAH reconstructions near the jet axis. The wavepacket model successfully fits the NAH-reconstructed partial fields, especially at frequencies above 50 Hz, indicating that the source may be modeled by multiple wave packets at each frequency.

The noise emitted from tactical supersonic aircraft presents a dangerous risk of noise-induced hearing loss for personnel who work near these jets. Although jet noise has many interacting features, large-scale turbulent structures are believed to dominate the noise produced by heated supersonic jets. To characterize the unsteady behavior of these large-scale turbulent structures, which can be correlated over several jet diameters, a velocimetry technique resolving a large region of the flow spatially and temporally is desired. This work details the development of time-resolved Doppler global velocimetry (TRDGV) for the study of large-scale turbulence in high-speed flows. The technique has been used to demonstrate three-component velocity measurements acquired at 250 kHz, and an analysis is presented to explore the implications of scaling the technique for studying large-scale turbulent behavior. The work suggests that the observation of low-wavenumber structures will not be affected by the large-scale measurement. Finally, a spatiotemporally-resolved measurement of a heated supersonic jet is achieved using large-scale TRDGV. By measuring a region spanning several jet diameters, the lifetime of turbulent features can be observed. The work presented in this dissertation suggests that TRDGV can be an invaluable tool for the discussion of turbulence with respect to aeroacoustics, providing a path for linking the flow to far-field noise. / Doctor of Philosophy / During takeoff, the intense noise emitted from tactical supersonic aircraft exposes personnel to dangerous risks of noise-induced hearing loss. In order to develop noise-reduction techniques which can be applied to these aircraft, a better understanding of the links between the jet flow and sound is needed. Laser-based diagnostics present an opportunity for studying the flow-field through time and space; however, achieving both temporal and spatial resolution is a technically challenging task. The research presented herein seeks to develop a diagnostic technique which is optimized for the study of turbulent structures which dominate jet noise production. The technique, Doppler global velocimetry (DGV), uses the Doppler shift principle to measure the velocity of the flow. First, the ability of DGV to measure the three orthogonal components of velocity is demonstrated, acquiring data at 250 kHz. Since turbulent structures in heated jets can be correlated over long distances, the effects on measurement error due to a large field-of-view measurement are investigated. The work suggests that DGV can be an invaluable tool for the discussion of turbulence and aeroacoustics, particularly for the consideration of full-scale measurements. Finally, a large-scale velocity measurement resolved in time and space is demonstrated on a heated supersonic jet and used to make observations about the turbulence structure of the flow field.

Continued growth of the aviation industry and increasingly strict noise requirements set by international bodies and airport authorities alike means that novel methods of reducing aircraft noise must be found. Engine noise represents a majority contribution to total aircraft noise during take-off and turbulent mixing of the exhaust gases is the dominant noise source of the engine at take-off. While bypass ratio has been the historical, and rather convenient means, of reducing jet noise, an upper limit to bypass ratio is now being approached and additional means of reducing jet noise must be found. One method that has shown potential for reducing aeroacoustic jet noise is the application of small, high pressure jets to the circumference of the jet nozzle. These jets, termed microjets, have the advantage over static devices that the microjets can be activated only when the noise benefit is required and deactivated when emitted noise is not an issue, such as in cruise, thereby reducing the thrust penalty associated with the devices over the majority of the flight. Large eddy simulations have been performed to investigate the impact that the addition of microjets has on the aerodynamic flowfield and radiated far-field noise of a high Reynolds number, Mach 0.9, propulsive, laboratory scale jet. Far-field noise was predicted through a new implementation of the permeable Ffowcs Williams Hawkings surface method in the solver. In addition to single-point flowfield statistics and far-field noise, spatio-temporal second- and fourth-order correlations are investigated. Two pairs of simulations were conducted, a coarse mesh containing 100 million elements and a fine mesh with 200 million elements. The coarse mesh included an azimuthal clustering of the cells in the near-microjet region. The non-uniformity of the azimuthal cell size was shown to adversely affect the development of the initial shear layer, yielding a delay in transition to a fully turbulent state and larger coherent structures in regions with larger cells. Radial velocity and turbulent kinetic energy profiles show good agreement with experimental results. A previously unidentified periodic interaction between the main jet and microjets was found. The dynamic interaction gives rise to velocity and pressure fluctuations in the near microjet region that match a tonal frequency found in the microjet far-field spectra that is absent from the clean jet case. Second- and fourth-order correlation distributions show large periodic regions of high correlation amplitude in the near microjet region. The evidence demonstrates that the main-microjet interaction is a clear high-frequency noise source. Despite the high-frequency noise associated with the main-microjet interaction, the addition of microjets yields a 1-2 dB reduction in overall sound pressure level. Additionally, over a significant portion of the length of the potential core the microjets reduce the amplitude of the majority of the six main correlation amplitudes that can be used in far-field noise prediction. Finally, the generation of the counter-rotating vortex pair downstream of the microjets was investigated. It is commonly presumed that this vortex pair is similar in origin to the counter-rotating vortex pair present in a jet in a crossflow. Vortex identification methods, velocity vectors and streamlines in the near microjet region demonstrate that the horseshoe-like vortex is the source of the counter rotating vortex pair that is present downstream of the microjets. The horseshoe-like vortex in the microjet case has the same sense as the vortices in the microjet shear layer and appears to be generated by the development of a recirculation region of microjet fluid during the main-microjet interaction.

The evaluation of the noise produced by flows has reached a high level of importance in the past years. The physics surrounding flow-induced noise is quite complex and sensitive to various flow conditions like temperature, shape. Empirical models were built in the past for some special geometries but they cannot be used in a general case for a shape optimization for instance. Experimental aeroacoustic facilities represent the main tool for acoustic analyses of flow fields, but are quite expensive because extreme care must be exercised not to introduce acoustic perturbations in the flow (silent facilities). These tools allow a good analysis of the physical phenomena responsible for noise generation in the flow by a comparison of the noise sources and the flow characteristics (pressure, turbulence,...). Nevertheless, the identification and location of noise sources to compare with flow structures requires quite complex methods. The numerical approach complements the experimental one in the sense that the flow characteristics are deeply analyzed where experiments suggest noise production. For the numerical approach, the turbulence modeling is quite important. In the past, some models were appreciated for their good prediction of some aerodynamic parameters as lift and drag for instance. The challenge is now to tune these models for a correct prediction of the noise sources. In the low subsonic range, the flow field is completely decoupled from acoustics, and noise sources can be computed from a purely hydrodynamic simulation before this information is transferred to an acoustical solver which will compute the acoustic field at the listener position. This post processing of the aerodynamic results is not obvious since it can introduce non-physical noise into the solution. This project considers the aspect of noise generation in turbulent jets and especially the noise generated by vortex pairing, as it occurs for instance in jet flows. The axisymmetric version of the flow solver SFELES has been part of this PhD research, and numerical results obtained on the jet are similar to the experimental values. Analyses performed on the numerical results are interesting to go to complete turbulence modeling for aeroacoustics since vortex pairing is one of the basic acoustical processes in vortex dynamics. Currently, a standard static Smagorinski model is used for turbulence modeling. However, this model has well known limitations, and its influence on the noise sources extracted from the flow field is not very clear. For this reason, it is planned to adopt a dynamic procedure in which the subgrid scale model automatically adapts to the flow. We planned also to perform simulations with the variational multiscale approach to better simulate the different interactions between large and unresolved scales. The commercial software ACTRAN distributed by Free Field Technologies is used for the computation of sound propagation inside the acoustic domain.

In recent years, jet noise has been an active area of research due to an increase in the use of aircraft in both commercial and military applications. To meet the noise standards laid out by government agencies, novel nozzle design concepts are being developed with an aim to attenuate the noise levels. To reduce the high costs incurred by experiments, simulation techniques such as large eddy simulation (LES) in combination with a surface integral acoustic method have received much attention for investigating various nozzle concepts. LES is utilized to predict the unsteady flow in the nearfield, whereas the surface integral acoustic method is used for the computation of noise in the farfield. However, Reynolds numbers at which nozzles operate in the real world are very high making wall-resolved LES simulations prohibitively expensive. To make LES simulations affordable, wall-models are being used to model the flow in the near wall region. Using a highly scalable, sixth-order finite-difference-based, in-house LES code, both wall-resolved and wall-modeled simulations of jets through the baseline short metal chevron (SMC000) nozzle were carried out earlier using an implicit LES (ILES) approach. However, differences exist in noise levels between the two simulations. Understanding the cause and reducing the differences between the two methodologies, while at the same time improving the fidelity of the wall-modeled LES is the main aim of the present work. Three new wall-models are implemented in the in-house LES code. A generalized equilibrium wall-model (GEWM) is implemented along with two wall-models that can account for non-equilibrium effects. First, a series of preliminary SMC000 wall-modeled LES simulations were performed and analyzed using the GEWM. The effect of turbulent length scales and velocity fluctuations specified at the inflow, wall-model formulation, and wall-normal grid refinement are analyzed. The adjustment of the fluctuations levels at the inflow proves to be useful in producing flowfields similar to that of the wall-resolved simulation. The newly implemented wall-models are validated for non-canonical problems such as an accelerating boundary layer developing over a flat plate and flow through a converging-diverging channel. It is noticed that the Reynolds number should be high enough for the non-equilibrium wall-models to be effective. At low Reynolds numbers, both equilibrium and non-equilibrium models produce similar wall shear-stresses. However, the wall shear stress boundary conditions supplied by the wall-models do not affect the mean velocity, turbulent kinetic energy, and Reynolds shear stress. Since all the wall-models produce similar results, and the GEWM is the most economical among the implemented wall-models, it is used in performing two wall-modeled LES SMC000 nozzle simulations for noise predictions. The inflow velocity and density fluctuations are varied between the simulations. The first SMC000 simulation uses similar inflow conditions as the previous wall-resolved SMC000 simulation. The second wall-modeled simulation was carried out by reducing the density and velocity fluctuations added to the mean flow at the inlet by 65%. The flowfield and acoustics agree reasonably well in comparison with the wall-resolved LES and similar experiments. Lowering of the velocity and density fluctuations in the wall-model LES improves the agreement of the far-field noise predictions with the wall-resolved LES at most observer locations. However, the preliminary SMC000 simulations performed using a higher Reynolds number and Mach number than that of the previous case show that the approach of adjusting the velocity and density fluctuations added to the mean flow have minimal impact on the developing flowfield which in turn affects the farfield noise. Thus, unless a more effective wall-modeling method is developed, possibly employing an explicit SGS model, the postdictive process of using a wall-model while adjusting the velocity and density fluctuations, seems to be an affordable tool for testing various nozzle designs, subject to the Reynolds number and Mach number being used.

Au sein d’un échangeur à air, les trompes à air permettent de créer l’écoulement d’air froid nécessaireà son bon fonctionnement. Ces dispositifs, qui peuvent ^etre assimilés à des jets subsoniques confinésen conduit, peuvent contribuer au bruit rayonné par les avions lors des phases au sol. Nous proposonsdans cette thèse de développer un outil numérique prédictif de l’acoustique rayonnée par ces dispositifsafin de pouvoir proposer des solutions de réduction de bruit appropriées. Cet outil est adapté au contexteindustriel de Liebherr Aerospace. Une méthode stochastique permet, à partir d’un calcul stationnaireRANS, de générer un champ de vitesse turbulente qui autorise la formation d’un terme de forçage dansles équations d’Euler linéarisées qui sont alors utilisées comme un opérateur de propagation. Un nouveaumodèle stochastique basé sur l’hypothèse de sweeping est développé. Ce dernier permet de produiredes champs instationnaires respectant certaines propriétés aérodynamiques statistiques dans le cadre dejets libres subsoniques. Cette méthode est couplée avec le solveur Euler de l’Onera sAbrinA_v0 et l’outilrésultant est appliqué sur le cas d’un jet libre subsonique à M=0.72. Moyennant une calibration duterme source, la méthodologie permet de reproduire les spectres acoustiques en champ lointain, exceptépour les angles faibles. L’outil numérique est ensuite couplé avec un solveur FW-H pour étudier le casconcret de la trompe à air. Les résultats aérodynamiques et acoustiques sont validés par comparaison àune base de données aérodynamique et acoustique constituée au préalable à partir d’une campagne d’essaiscomprenant des mesures par anémométrie laser Doppler à l’intérieur du conduit et des microphonesacoustiques en champ lointain. / In air exchangers, the cool air flow can be produced by jet pumps. These devices, which can be consideredas subsonic jets confined in ducts, could contribute directly to ramp noise. A predictive numerical toolof the acoustic radiated by jet pumps is therefore developped in order to be able to propose appropriatenoise reduction solutions. This tool is adapted to Liebherr Aerospace’s industrial context. A stochasticmethod allows, starting from a steady RANS computation, to synthetise a turbulent velocity fields andto enforce source terms in the linearized Euler equations therefore used as a wave propagator. A newstochastic model relying on the sweeping hypothesis is developped. Unsteady fields reproducing someaerodynamics features of a free subsonic jet flow can be generated. This method is then coupled withOnera’s Euler solver sAbrinA_v0 and the resulting tool is applied on a free subsonic jet configuration atMach 0.72. Assuming a cabration of the source terms, this methodology models properly the far fieldacoustic spectra except for small angles. The numerical tool is then coupled with a FW-H solver to studya realistic jet pump. Aerodynamic and acoustic results are validated by comparison with a data baseobtained from an experimental campaign including laser Doppler anemometry measures inside the ductand pressure recording in the far-field.

Cette étude s’inscrit dans le domaine de la réduction du bruit des avions et plus précisément du bruit de jet représentant la première source de bruit au décollage. Les travaux de thèse consistent en l’identification et l’analyse des mécanismes de génération du bruit de jet à partir de résultats expérimentaux et de simulations numériques. La démarche a porté dans un premier temps sur l’analyse de la structure du champ de pression proche d’un jet chaud subsonique turbulent à partir de données expérimentales acquises au moyen d’une antenne azimutale de microphones. Dans un second temps, une simulation numérique par l’approche LES, avec déclenchement de la turbulence, reproduisant la configuration expérimentale a été mise en place et validée dans le but de poursuivre l’analyse des données expérimentales. L’analyse des données issues de la simulation a permis de relier, au moyen de calculs de corrélation, les comportements identifiés dans le champ proche à des ondes de pression se développant dans la couche de cisaillement et se propageant vers la fin du cône potentiel. Cette analyse a également mis en avant le caractère intermittent du rayonnement acoustique dans la direction aval, direction pour laquelle l’énergie acoustique est maximale, ce caractère intermittent provenant des structures cohérentes se développant dans la couche de cisaillement. L’analyse réalisée à partir des données de la simulation serait utilement complétée par des calculs de cohérences entre le champ aérodynamique et le champ acoustique à partir de signaux expérimentaux provenant de mesures synchronisées. / This study falls within the field of aircraft noise reduction and more precisely jet noise as it represents the main noise source during take-off. The present work consists in the identification and the analysis of the jet noise source mecanisms using experimental results and numerical simulation. First, an analysis of the near field pressure of a hot subsonic turbulent jet has been done from experimental data acquired with an azimuthal array of microphones. Secondly, in order to continue the analysis, a numerical simulation using the LES approach with turbulence triggering reproducing the experimental configuration has been set up and validated. The data obtained from the simulation enable to link the highlighted behaviours in the near field to pressure waves developping in the shear layer and propagating toward the potential core end, through correlation calculations. The intermittency of the downstream acoustic radiation, i.e. the main direction of radiation, has been evidenced and related to the coherent structures developping in the shear layer. To deepen the analysis, it would be interesting for instance to calculate spectral coherence between the aerodynamic and the acoustic fields from synchronised measurements.

The sound produced by military aircraft is dominated by noise generated by the turbulent mixing of the jetted exhaust with the ambient air. This jet noise has the potential to annoy the community and pose a hearing loss risk for military personnel. The goal of this dissertation is to characterize spatiospectral features in the field produced by full-scale military aircraft that are not traditionally seen at the laboratory scale and identify potential noise mechanisms for these features. Measurements of two military aircraft jet noise fields are found to be best described as a superposition of spatiospectral lobes, whose relative amplitudes dictate the overall directivity at each engine power. Near-field acoustical holography techniques are applied to one of the military aircraft measurements to characterize the behavior of the lobes as a function of engine power. The simulated jet noise of a highly heated laboratory-scale jet is then analyzed to compare with the military aircraft measurement and is found to only partially contain the spatiospectral lobe phenomenon. Application of near to far field coherence tracing and near-field acoustical holography to the simulations provides validation of the methods used on the military aircraft and illuminate potential source mechanisms that may explain the presence of the spatiospectral lobes.