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Aircraft Auxiliary Power Unit (APU) is one of the major aircraft systems and is reported to be a key driver of unscheduled maintenance. So far, the research has been focused on the implementation of the APU thermodynamic state data to isolate and diagnose faults. To advance the available diagnostic techniques, research work has been initiated to explore the potential of employing far-field microphone data for the identification and isolation of APU faults. This paper aims to address the first step required in the overall effort and proposes a novel methodology for the development of a noise model that can be used for evaluating noise as a source of fault diagnostics. The methodology integrates experimentally acquired full-scale aircraft state and noise data, a physics-based APU thermodynamic model, and semi-empirical noise models to estimate the noise produced by an aircraft APU based on a limited parameter-set. The methodology leads to a model which works by estimating the unknown thermodynamic parameters from the limited dataset and then passes on the relevant parameters to noise estimation models (combustion/jet noise models). An inherent part of the model is the effect of multipath propagation and ground reflections for which a relationship has been analytically derived that considers all the necessary parameters. The developed model has been validated against experimental noise and thermodynamic data acquired from a Boeing 737–400 aircraft APU under several different operating conditions. The acquired noise estimates suggest that the proposed approach provides an accurate estimation of the far-field noise under a wide range of APU operating conditions, both at the sub-system and APU level. The model would act as an enabler to simulate APU noise data under degraded functional states and subsequently developing fault diagnostic schemes based on the far-field noise data.

The directivity of noise from three large-eddy simulations of turbulent jets at Mach 0.7, 0.9, and 1.5 is investigated using spectral proper orthogonal decomposition (SPOD). The most energetic patt...

Imperfectly expanded jet flows are known to have additional noise sources known as Screech and broadband shock-associated noise. They are generated by the interaction between the instability waves that propagate from the lip of the nozzle and the shock cell structures. In this study, thorough experimental investigations were carried out on chevron nozzles to assess the importance of chevron parameters such as the chevron count and chevron penetration angle on the pressure field emitted by the jet. Data were acquired in the state-of-the-art aeroacoustic facility at the University of Bristol. Acoustic measurements such as pressure spectra, directivity and overall sound pressure levels along with near-field measurements were acquired for jet Mach numbers ranging from M = 1.1–1.4. Fourier-based and Wavelet-based analyses were used to highlight the different features of the various tested nozzles. Wavelet decomposition results highlight that the presence of the chevrons reduce the acoustic noise especially at a higher axial distance with increased levels of noise reduction achieved by chevron nozzle with deep penetration angle.

We investigate the mechanisms leading to acoustic whistling for a jet passing through a circular hole in a thick plate connecting two domains. Two generic situations are considered. In the first one, the upstream domain is a closed cavity while the downstream domain is open, leading to a class of conditionally unstable modes. In this case, the instability source lies in the recirculation region within the thickness of the plate, but coupling with a conveniently tuned resonator is needed to select the conditional instability range. In the second situation, the two regions, upstream and downstream of the hole, are considered as open, leading to a class of hydrodynamic modes where instability of the recirculation region is sufficient to generate self-oscillations without the need of any resonator. A matched asymptotic model, valid in the low Mach limit, is used to derive a global impedance of the system, combining the impedance of the hole and the modelled impedances of the upstream and downstream domains. It is shown that the knowledge of this global impedance along the real $\omega$ -axis provides an instability criterion and a prediction of the eigenvalues of the full system. Validations against the solutions of the eigenvalue problem obtained from the linearized fully compressible formulation confirm the accuracy of the approach. Then, it is subsequently used to characterise the range of existence of instabilities as a function of the Reynolds number, the Mach number, the aspect ratio of the hole and (for the cavity configuration) the dimensionless volume of the cavity.

Near-field hydrodynamic and acoustic characteristics of an unheated subsonic jet with chevron nozzles were investigated experimentally with and without jet installation effects. Near-field pressure fluctuations were acquired for Mach numbers ranging from 0.3 to 0.9 for four different types of chevron nozzles in an anechoic jet facility at the University of Bristol. The measurements were carried out on an axial microphone array traversed in the radial direction and placed close to the jet. The tests were performed for several plate heights from the nozzle to investigate the effect of plate distance from the jet. The results are presented for sound pressure level and overall sound pressure level for several radial locations. Further analyses were carried out by decomposing the near-fieldpressure into hydrodynamic and acoustic components. The investigation clearly shows the effect ofjet installation on the hydrodynamic and acoustic fields. Auto-correlation and cross-correlation analysis was also carried out for the near-field pressure fluctuations. The convective velocity of the flow isreduced by the use of chevrons. The use of chevron nozzles changes the acoustic pressure in the nearfield region thereby shifting the dominant noise source towards the nozzle exit for both the installedand isolated configurations.

The effects of scarfed nozzles on the jet-installation noise were experimentally investigated using unheated subsonic jets in an anechoic jet noise facility. The experiments were carried out for four different types of scarfed nozzles with increasing nozzle lip angles. Installation effects were investigated at a plate distance of one jet diameter away from the jet for Mach numbers ranging from 0.3 to 0.8. The use of scarfed nozzles has been known to deflect the flow away from the center axis and induce asymmetry in the jet shears leading to azimuthal variation in the spectra and thus noise reduction. In this study, the possibility of jet-installation noise reduction with the use of scarfed nozzleshas been explored at subsonic flow conditions. The characteristics of the jet hydrodynamic pressure fluctuations were investigated in the axial direction using far-field measurements. The near-field flow features were studied using surface pressure transducers installed on the flat plate for the installedconfigurations. Detailed spectral, coherence, and correlation analysis were carried out to determine noise reduction mechanisms involved with scarfed nozzles at the proximity of a flat plate.

Jet installation beneath a wing significantly enhances jet noise at low frequencies, and itsphysical mechanism must be comprehended to develop efficient noise reduction solutions. Anumerical investigation on the jet-installation noise is performed usingWall Modelled LargeEddy Simulation (WMLES) performed using the high-resolutionCABARET method acceleratedon Graphics Processing Units. To simulate jet installation, a flat plate is put outside of the jet’splume, causing a rise in noise levels due to the scattering of near-field hydrodynamic wavesat the trailing edge of the plate. The configuration adopted in this work replicates a seriesof experiments performed at the University of Bristol, against which the numerical resultsare validated. The numerical simulation is performed for Mach numbers of 0.5 and 0.9, andthe influence of the selected noise reduction technique, i.e., the usage of chevron nozzles incomparison with the baseline round nozzle, on the jet-installation is studied by modellingSMC006 chevron nozzle. The properties of jet-hydrodynamic pressure variations and theireffect on nozzle type and Mach number are investigated. Far-field noise spectra from theisolated and installed jet cases, obtained through the Ffowcs-Williams Hawkings method, arecompared at different polar angles. In addition, a hybrid semi-analytical hydrodynamic-edgescattering prediction model is implemented following the model of Lyu and Dowling [1] toanalyse jet-installation noise, using inputs obtained directly from the LES calculation. Theimplemented model is found to capture the correct physics at peak jet installation frequenciesand can be used as a robust prediction tool for jet-installation noise optimisation in the future.

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computatio...

Using the example of an integrated airframe, we performed a computational and experimental study of the possibility of using geometric diffraction theory (GDT) to calculate the sound diffraction when the airframe is replaced with simulating flat polygonal screen as pertains to the effectiveness of noise screening of aircraft power plants. We compared (1) impulse responses experimentally measured with maximum length sequences and the reciprocity theorem for both a three-dimensional model of the airframe and its plane model with (2) the responses calculated using GDT; the results showed good qualitative agreement for observation points in the geometric shadow zone and in the illuminated area, as well as quantitative coincidence in the frequency range characteristic of jet noise and the first harmonics of the blade passing frequency of a propeller or fan.

International audience; Acoustic measurements of unheated supersonic underexpanded jets with ideally-expanded Mach numbers of 1.14, 1.38 and 1.50 are presented. Of the three components of supersonic jet noise, the focus is on the broadband shock-associated noise (BBSAN) component. Motivated by the modelling of BBSAN using the wavepacket framework, a traversable microphone ring is used to decompose the acoustic pressure into azimuthal Fourier modes. Unlike noise radiated downstream, BBSAN is dominated by azimuthal modes 1 through 3 which are approximately 3-4dB/St stronger than the axisymmetric component. Crucially, the relative contribution of successive modes to BBSAN is sensitive to observer angle and jet operating condition. Four azimuthal modes are necessary to reconstruct the total BBSAN signal to within 1dB/St accuracy for the conditions presented here. The analysis suggests, however, that the number of modes required to maintain this accuracy increases as the peak frequency shifts upward. The results demonstrate the need to carefully consider the azimuthal content of BBSAN when comparing acoustic measurements to predictions made by jet noise models built on instability theory.

The paper presents the results of a computational–experimental study of jet noise reduction using the screening effect. According to correlation theory, the noise sources of a single circular jet are represented by a set of uncorrelated quadrupoles. Using the basic relations of geometric diffraction theory (GDT), expressions are obtained for the sound field emitted by a point quadrupole source located near an acoustically rigid infinite half-plane. Using the expressions obtained in the framework of the GDT, the correlation model of the jet noise sources was adapted to the calculation of the sound field in the presence of a flat rectangular screen. A comparison of the calculated and experimentally measured spectra of sound pressure levels, performed at a given jet velocity and various screen positions, showed good qualitative, and for certain observation angles, quantitative agreement.

Measurements of full-scale high-performance military aircraft reveal phenomena that are not widely seen at laboratory scales. However, recent modifications to large eddy-simulation (LES) methods al...

The noise sources of the tip-jet driven rotor are mainly classified as rotor blade noise and jet noise. The rotor blade noise includes thickness noise, loading noise, and nonlinear quadrupole noise, while the jet noise includes nozzle momentum noise and jet radiation noise. Each noise component has different noise source mechanisms related to flow characteristics and the rotating motion. Especially, the nozzle momentum noise has two noise source mechanisms based on the rotating steady aerodynamic load and the nozzle mass flow rate. The details of the time signal and spectrum are discussed to study the unique spectral characteristics. The tip-jet driven rotor noise has both tonal and broadband spectral characteristics. The periodicity of each instance of jet radiation noise is discussed, while the total jet radiation noise also has a tonal noise component. The noise source mechanisms and spectral characteristics are important considerations for an acoustically efficient tip-jet driven rotor.

This paper deals with the topic of upper surface blowing noise. Using a model-scale rectangular nozzle of an aspect ratio of 10 and a sharp trailing edge, detailed noise contours were acquired with and without a subsonic jet blowing over a flat surface to determine the noise source location as a function of frequency. Additionally, velocity scaling of the upper surface blowing noise was carried out. It was found that the upper surface blowing increases the noise significantly. This is a result of both the trailing edge noise and turbulence downstream of the trailing edge, referred to as wake noise in the paper. It was found that low-frequency noise with a peak Strouhal number of 0.02 originates from the trailing edge whereas the high-frequency noise with the peak in the vicinity of Strouhal number of 0.2 originates near the nozzle exit. Low frequency (low Strouhal number) follows a velocity scaling corresponding to a dipole source where as the high Strouhal numbers as quadrupole sources. The culmination of these two effects is a cardioid-shaped directivity pattern. On the shielded side, the most dominant noise sources were at the trailing edge and in the near wake. The trailing edge mounting geometry also created anomalous acoustic diffraction indicating that not only is the geometry of the edge itself important, but also all geometry near the trailing edge.

Aeroacoustic analyses of jet flows have benefited greatly from a decomposition of turbulent pressure fluctuations into hydrodynamic and acoustic components. This is typically accomplished using signal processing techniques based on phase speeds, coherence properties or spectral analyses. We present an approach, building on the Momentum Potential Theory (MPT) approach of (Doak, 1989), to split pressure fluctuations into their hydrodynamic, acoustic and entropic, collectively designated fluid-thermodynamic (FT), components. Key advantages are that the approach is applicable everywhere in the jet i.e, not restricted to the near-acoustic field, and does not need user-defined thresholds. The effectiveness of the technique is demonstrated by analyzing the flowfields of three simulated jets, to encompass moderate-compressible to supersonic conditions. The statistical properties and wavepacket dynamics of each pressure component, and their relationships with the unsplit pressure are elaborated. The acoustic pressure field has the form of a wavepacket that attenuates downstream and whose modal analysis reveals low-rank behavior. At each Mach number examined, the acoustic pressure also identifies the relative prominence of each of three components: i) waves with upstream propagating energy content (negative group velocity), ii) supersonically traveling radiating downstream waves, and iii) subsonically convected evanescent waves, which follow the convection pattern of hydrodynamic eddies in the turbulent region. With increasing Mach number, the radiating and convected bands of energy move closer to each other. The hydrodynamic pressure also displays a wavepacket structure, but its features are different: it displays large-scale subsonically convected structures even past the core collapse region. Thus, in the turbulent region of the jet, the acoustic pressure displays smaller integral time scales of fluctuations than the hydrodynamic component. The acoustic pressure field, which includes a zero-crossing in its radial profiles, displays larger wavelengths than the hydrodynamic pressure field, correlates better with the near-field pressure signal and captures the radiated component of noise, especially at shallow angles. These properties make it a suitable field for informing pressure-based wavepacket models for jet noise.

The current state of the use of large-eddy simulations (LESs) in jet aeroacoustics is reviewed. Various computational techniques are discussed, as well as methods to extend the LES results to the a...

Surveys to locate and monitor bubble gas seeps in Lake Baikal have been conducted mainly using the active echolocation methods since the 2000s. This paper compares the active acoustic method with a newly developed optical and passive acoustic methods using the example of the shallow-water gas seep Istok in the Selenga shoal. The optical method is based on processing of underwater video records of ascending bubbles while the passive acoustic uses spectral analysis of gas jet noise recorded using a hydrophone. Optical and passive acoustic methods enables estimation of the bubble size distribution function, which is necessary for the calculation of the bubble gas flux. The latter can be evaluated from active echolocation or optical data. By applying these methods, their possibilities and limitations are revealed as well as benefits of their combined use and ways to improve their accuracy in future.

In many areas of data science, deep neural networks (DNNs) have shown a remarkable ability to learn complex, nonlinear relationships between sets of variables. In this paper, this network architect...

This paper presents an analysis of turbulence characteristic scales and eddy convection velocity of jet flows computed using joint statistical moments, digital filters, and a modified version of the empirical mode decomposition (EMD). The ongoing aim of this study is to develop semi-empirical space-time cross-correlation models based on stationary statistics and jet physical lengths. Multivariate statistics are used to correlate jet properties to one-dimensional time series. The data available to this study were recorded from single-point and two-point hot-wire anemometry experiments carried out for a range of jet Mach numbers (0.2≤M≤0.8). Firstly, the jet eddy convection velocity, turbulence length, and time scales are computed using space-time cross-correlation functions. Isotropic flow and frozen turbulence hypothesis are then used to estimate the joint moments from single-point statistics in the fully developed turbulence region. An EMD-based decomposition method is presented and assessed in both the Gaussian and non-Gaussian signal regions. It is demonstrated that the artificially filtered signal reconstructs the physical properties of single and multi-point jet statistics. The relationship between central moments and joint moments presented here focuses on the region of high turbulence levels, which generates the vast majority of jet mixing noise produced by turbofan engines. Further analysis is required to extend this investigation to intermittent zones and other jet noise sources, such as jet-surface installation noise.

The use of surrogate models in recent times plays a key role in the design process and, overthe past few years, machine learning algorithms are adapting to the use of active metamodellingtechniques. In the present study, Artificial Neural Networks formulation is used as a data–driven nonlinear model aimed at describing the dynamics of an experimental test campaignrelated to the noise emitted by a single–stream jet in under–expanded conditions. The architectureof the neural network is selected employing a deterministic optimization algorithm,coupled with data informed tuning of the input parameters. The data set explored here wasacquired in the state–of–the–art aeroacoustic facility at the University of Bristol. Both the nearandfar–field acoustic measurements were carried out for a cold under–expanded jet for Machnumbers ranging from 1.1 to 1.4. The predictions by the metamodel are in good agreementwith the experimental data, and the results demonstrate the capability of metamodels as areliable tool to estimate jet noise in under-expanded flow conditions for a wide range of Machnumbers and near–field locations.

В работе экспериментально исследуется возможность снижения шума дозвуковой турбулентной струи за счет использования гофрированной формы сечения сопла. Рассматривались гофрированные сопла с числом гофров от 6 до 12 различной высоты. Эксперименты проводились в двух заглушенных камерах: заглушенной камере с аэродинамическими источниками шума Лаборатории механизмов генерации шума и модального анализа ПНИПУ и заглушенной камере с потоком АК-2 ЦАГИ. Звуковое поле дозвуковой струи, истекающей из гофрированного сопла сравнивалось со звуковым полем эквивалентной круглой струи. Экспериментально подтвержден результат, демонстрирующий, что при увеличении амплитуды гофров при сохранении площади выходного сечения сопла и скорости истечения усиливается эффект снижения шума турбулентной струи. Также получено, что при увеличении числа гофров увеличивается частотный диапазон снижения шума. Найдены конфигурации с числом гофров, при котором снижение шума наблюдается во всем диапазоне частот., The paper considers the possibility of reducing the noise of a subsonic turbulent jet by using the lobed nozzle. The nozzles with the lobes from 6 to 12 of different heights were used. The experiments were carried out in two anechoic chambers: anechoic chamber with aerodynamic noise sources at PNRPU and anechoic chamber with flow AC-2 at TsAGI. The sound field of a subsonic jet flowing out from lobed nozzles was compared with the sound field of an equivalent circular jet. The experiments have confirmed that with a constant outlet crosssection of the nozzle area and outflow velocity, an increase in the amplitude of the lobe leads to a decrease in the noise of the turbulent jet. It was also found that with an increase in the number of lobes, the frequency range of noise reduction increases. Configurations with the number of lobes are found, at which noise reduction is observed over the total frequency range.

На основе результатов измерений и численного моделирования анализируется взаимодействие турбулентных струй с окружающей средой. Показано, что динамическое воздействие струи создает условия для индуцированного подтекания окружающего воздуха к струе.Наблюдается взаимосвязь между движением внешней среды и областями пониженного давления. Эти области движутся и вызывают перемещение линий тока втекания в струю, которые совершают периодические перемещения вблизи границ струи.Создаваемые их движением возмущения во внешней среде приводят к образованию звуковых волн. Период этого движения в каждом сечении струи соответствует определённой по опытным данным зависимости частоты звука, излучаемого отдельным участком струи, от продольной координаты., Based on the results of measurements and numerical modeling, the interaction of turbulent jets with the environment is analyzed. It is shown that the dynamic effect of the jet creates conditions for the induced inflow of ambient air to the jet.The relationship between the movement of the external environment and regions of low pressure was established. These regions move downstream and cause displacement of the streamlines of inflow into the jet, which make periodic disturbances near the jet boundaries.This disturbances in the external environment lead to the formation of sound waves. The period of this movement in each section of the jet corresponds to the dependence of the frequency of sound emitted by a separate section of the jet on the longitudinal coordinate, determined from the experimental data.

The connections between the vortical and near-acoustic fields of three-stream, high-speed jets are investigated for the ultimate purpose of developing linear surface-based models for the noise source. Those models would be informed by low-cost, Reynolds-averaged Navier-Stokes (RANS) computations of the flow field. The study uses two triple-stream jets, one is coaxial and the other has eccentric tertiary flow that yields noise suppression in preferred directions. Large eddy simulations (LES) validate the RANS-based models for the convective velocity Uc of the noise-generating turbulent eddies. In addition, the LES results help define a “radiator surface” on which the jet noise source model would be prescribed. The radiator surface is located near the boundary between the rotational and irrotational fields and defined as the surface on which the Uc distribution, obtained from the space-time correlations of the pressure, matches that inferred from the RANS model. The edge of the mean vorticity field is nearly coincident with the radiator surface, which suggests a RANS-based criterion for locating this surface. The two-dimensional space-time correlations show how the asymmetry of the tertiary stream and the resulting thicker low-speed flow weakens the generation of acoustic disturbances from the vortical field.

An experimental study is conducted on unsteady pressure fluctuations occurring near the nozzle exit and just outside the shear layer of compressible jets. These fluctuations are related to ‘trapped waves’ within the jet's potential core, as investigated and reported recently by other researchers. Round nozzles of three different diameters and rectangular nozzles of various aspect ratios are studied. The fluctuations manifest as a series of peaks in the spectra of the fluctuating pressure. Usually the first peak at the lowest frequency (fundamental) has the highest amplitude and the amplitude decreases progressively for successive peaks at higher frequencies. These ‘trapped wave spectral peaks’ are found to occur with all jets at high subsonic conditions and persist into the supersonic regime. Their characteristics and variations with axial and radial distances, jet Mach number and aspect ratio of the nozzle are documented. For round nozzles, the frequency of the fundamental is found to be independent of the jet's exit boundary layer characteristics and scales with the nozzle diameter. On a Strouhal number (based on diameter) versus jet Mach number plot it is represented by a unique curve. Relative to the fundamental the frequencies of the successive peaks are found to bear the ratios of 5/3, 7/3, 9/3 and so on, at a given Mach number. For rectangular nozzles, the number of peaks observed on the major axis is found to be greater than that observed on the minor axis by a factor approximately equal to the nozzle's aspect ratio; the fundamental is the same on either edge. For all nozzles the onset of screech tones appears as a continuation of the evolution of these peaks; it is as if one of these peaks abruptly increases in amplitude and turns into a screech tone as the jet Mach number is increased.

In an attempt to reduce jet noise, blended wing concept vehicle utilizes rectangular jet exhaust ports exiting from above the wing ahead of the trailing edge. In this study, we take another look at the rectangular exhaust port configuration with some notional modifications to the geometry of the trailing edge to determine if the emitted noise levels due to jet interactions can be reduced with respect to a baseline configuration. We consider various horizontal and vertical offsets of the jet exit with respect to a flat plate standing in for the aft wing surface. We then introduce a series of sinusoidal deformations to the trailing edge of the plate of varying amplitude and wave number. Our results show that the emitted sound levels due to the jet--surface interactions can be significantly altered by the proposed geometry modifications. While sound levels remained fairly consistent over many configurations, there were some that showed both increased and decreased sound levels in specific directions. We present results here for the simulated configurations which showed the greatest decrease in overall sound levels with respect to the baseline. These results provide strong indications that such geometry modifications can potentially be tailored to optimize for further reductions in sound levels., 23 pages