A Numerical Study on Noise Generation Caused by Slat Tracks

The effect of nonlinear interaction of instability eigenmodes on jet flow transition and its near acoustic field for a high-subsonic round jet at a Reynolds number of Re=4.5×105 and a Mach number of Ma=0.9 is investigated using large-eddy simulations. At the inflow, helical perturbations of azimuthal wavenumbers |n|=4,…,8 determined from linear stability theory are superimposed on a laminar base flow in order to trigger transition to turbulence. The disturbance amplitude is varied parametrically in the range from 1.5% to 4.5% of the jet exit velocity Uj. Thereby we aim to characterize sources of noise generation and, in particular, underlying mode interactions. With increasing forcing amplitude, the transitional behavior of the jet changes which affects the mean flow and also the acoustic near-field, which are both analyzed in detail. As the forcing amplitude is increased, the axial root-mean-square peak levels along the jet centerline are reduced by approximately 7%. Simultaneously, pronounced dual-peak distributions are generated along the jet lip line which are related to the localization of vortex pairings of the jet column mode. For low-amplitude excitation the azimuthal turbulent kinetic energy spectra show that the unexcited, naturally least stable axisymmetric mode n=0 and the helical mode n=1 dominate the early nonlinear regimes between z≈6r0 and 9r0 where r0 is the jet radius. An analysis of the Fourier mode amplitude clarifies that this energy rise is linked to the helical mode n=1. For higher forcing amplitudes, in addition to the varicose mode n=0 interactions between the excited even mode n=4 and higher azimuthal harmonics thereof dominate the azimuthal energy spectra. These differences in the early nonlinear development of the eigenmodes are found to alter the acoustic near-field. At small angles from the downstream jet axis, the peak acoustic frequency occurs at a Strouhal number based on the angular frequency ω and the jet diameter Dj of St=ωDj/(2πUj)≈0.4. For low-amplitude forcing sound pressure levels are slightly enhanced which can be linked to the dominant low azimuthal wavenumbers identified in the transitional region. In the sideline direction, regardless of the excitation level, broadbanded spectra with maxima in the band 0.7≤St≤0.8 are found which is maintained at intermediate observer angles. For high forcing amplitude, however, a tonal component outside the initially excited frequency range is observed. This peak at St≈0.88 can be explained by weakly nonlinear interactions of initially forced eigenmodes n=4 and n=8 together with the jet column mode.

The present paper reviews and discusses the physical mechanisms of noise generation and reduction in turbulent flows with their applications towards aircraft noise reduction at takeoff and on the approach. This work began in 1948 when Lilley undertook an experimental investigation into the source of jet noise as a necessary precursor to finding methods for the reduction of high speed jet engine noise on civil jet airliners. Westley and Lilley completed this experimental programme in 1951, which included the design of a range of devices for high speed jet noise reduction. It was about this time that similar studies on jet noise were being started elsewhere and in particular by Lassiter and Hubbard in USA. The major contribution to the subject of turbulence as a source of noise came from Sir James Lighthill’s remarkable theory in 1952. In spite of the difficulties attached to theoretical and experimental studies on noise from turbulence, it is shown that with the accumulated knowledge on aerodynamic noise over the past 50 years, together with an optimisation of aircraft operations including flight trajectories, we are today on the threshold of approaching the design of commercial aircraft with turbofan propulsion engines that will not be heard above the background noise of the airport at takeoff and landing beyond 1-2km, from the airport boundary fence.It is evident that in the application of this work, which centres on the physical mechanisms relating to the generation of noise from turbulence and turbulent shear flows, to jet noise, there is not one unique mechanism of jet noise generation for all jet Mach numbers. This author in this publication has concentrated on what appears to be the dominant mechanism of noise generation from turbulence, where the mean convection speeds of the turbulence are subsonic. The noise generated at transonic and supersonic jet speeds invariably involves extra mechanisms, which are only briefly referred to here.

Sources of noise in axial flow fans

Effects of slat track on the flow and acoustic field of high-lift devices

Flow noise in planar sonar applications

Turbulent flow through tandem orifices in a circular duct is numerically modeled through dynamic delayed detached-eddy simulations to clarify the unsteady flow behaviors and noise generation mechanism. The characteristics of four configurations with different separation distances L/D = 0 (single orifice), 1, 2, and 4 are compared at a Reynolds number of 10 000. The acoustic sources and their noise-propagation behaviors are analyzed using Lighthill's acoustic analogy. The coherent flow structures (wavepackets) are determined through spectral proper orthogonal decomposition to clarify the resulting flow noise mechanism. The dominant noise sources are acoustic dipoles that are alternately energetic on the orifice's leading and trailing faces subjected to intermittent interaction with the unsteady flow. The total sound pressure level (SPL) for a single orifice is alternately dominated by the shedding and flapping behaviors of the large-scale vortical structures in the low-frequency range and Kelvin–Helmholtz (K–H) type wavepackets at high frequencies. For the tandem orifice configuration with L/D = 1, the total SPL is dominated by the contribution of the trailing face, attributable to the interactions between the K–H-type double-wavepacket structures. Both the upstream and downstream wavepackets start to split and generate four-wavepacket structures in the high-frequency range. In the cases of L/D = 2 and 4, the total SPL is dominated by the dipole sources at the downstream leading face that subjected to the intermittent interaction with the upstream separated shear layers. The dominant flow structures are independent wavepackets through each orifice at low frequencies, while at high frequencies the four-wavepacket structures break into two independent double-wavepacket structures close to each orifice.

Turbulent flow through tandem orifices in a circular duct is numerically modeled through dynamic delayed detached-eddy simulations to clarify the unsteady flow behaviors and noise generation mechanism. The characteristics of four configurations with different separation distances L/D = 0 (single orifice), 1, 2, and 4 are compared at a Reynolds number of 10 000. The acoustic sources and their noise-propagation behaviors are analyzed using Lighthill's acoustic analogy. The coherent flow structures (wavepackets) are determined through spectral proper orthogonal decomposition to clarify the resulting flow noise mechanism. The dominant noise sources are acoustic dipoles that are alternately energetic on the orifice's leading and trailing faces subjected to intermittent interaction with the unsteady flow. The total sound pressure level (SPL) for a single orifice is alternately dominated by the shedding and flapping behaviors of the large-scale vortical structures in the low-frequency range and Kelvin–Helmholtz (K–H) type wavepackets at high frequencies. For the tandem orifice configuration with L/D = 1, the total SPL is dominated by the contribution of the trailing face, attributable to the interactions between the K–H-type double-wavepacket structures. Both the upstream and downstream wavepackets start to split and generate four-wavepacket structures in the high-frequency range. In the cases of L/D = 2 and 4, the total SPL is dominated by the dipole sources at the downstream leading face that subjected to the intermittent interaction with the upstream separated shear layers. The dominant flow structures are independent wavepackets through each orifice at low frequencies, while at high frequencies the four-wavepacket structures break into two independent double-wavepacket structures close to each orifice.

In recent years, the Lattice Boltzmann (LB) method has become a powerful numerical \ntool for simulating complex fluid flows and modeling physics and chemistry in \nfluids. Derived from the continuum Boltzmann equation used in statistical \nmechanics, the LB method has the advantage of describing non-equilibrium dynamics, \nespecially in fluid-flow applications involving interfacial dynamics and complex \nboundaries, without simplifying the physics. In addition, the parallel structure \ninherent in the LB method makes it extremely suitable for parallel computing. \nBecause of these features, the LB method affords the most comprehensive pore-scale \napproach to systematically investigate fundamental issues involving flow and \nreactive transport in porous media. In this paper, the state of the art of this \nmethod is discussed. Specifically, a multi-component LB model for simulating \nreactive transport in porous media at the pore scale is presented. In the model, a \nset of distribution functions is introduced to simulate fluid flow and solute \ntransport. The LB equation for flow recovers the correct pore-scale continuity and \nNavier-Stokes equations. The LB equations for solute transport are modified to \nrecover advection-diffusion equations for total concentrations at the pore scale. \nThe model takes into account advection, diffusion, homogeneous reactions among \nmultiple aqueous species, heterogeneous reactions between the aqueous solution and \nminerals, as well as changes in solid and pore geometry. Homogeneous reactions are \ndescribed through local equilibrium mass action relations. Mineral reactions are \ntreated kinetically through boundary conditions at the mineral surface. Simulation \nexamples presented include injection of carbon dioxide saturated brine into a \nlimestone rock with pore geometry derived from a thin section, crystal formation \nfrom a supersaturated solution without flow, and crystal formation during carbon \ndioxide sequestration in oceanic sediments.

On-chip noise generation and coupling is an important issue in deep-submicron technologies. Advanced IC technology faces new challenges to ensure function and performance integrity. Selecting adequate test techniques depends on the circuit, its implementation, and the possible physical failures and parasitic coupling models. This new demand for test technology practices precipitated the investigation of dl/dt and dV/dt noise generation and propagation mechanisms.

Excitation of the walls of a vehicle by turbulent boundary layers constitutes a major source of interior noise in the field of transport industry. A turbulent boundary layer over a wall can provide a direct contribution to the noise and also an indirect one due to the excitation of the structure below. In the present study, simulations of turbulent boundary layers at Mach=0.5 are carried out using direct noise computation by solving the compressible NavierStokes equations. This method provides the direct contribution (despite its very low intensity) and the wall pressure fluctuations which are responsible for the indirect contribution to the aerodynamic noise. The low efficiency of the noise radiated inthe freestream can be explained by the fact that the sources are turbulence quadrupoles. In order to determine the effects of pressure gradients on both noise contributions, adverse and favorable cases are investigated. The results are compared to a reference case with zero pressure gradient computed by Gloerfelt 1,2 . Results show that an adverse pressure gradient leads to higher levels of the direct acoustic emission whereas lower levels are obtained in presence of a favorable pressure gradient in comparison to the zero gradient case. The same hierarchy of levels is visible in the wavenumber pressure spectra of wall pressure fluctuations. In the field of transport industry, boundary layer constitutes a fundamental source of aerodynamic noise (Blake 3 ). A turbulent boundary layer over a vehicle has two mechanisms of noise generation: an indirect contribution corresponding to structural excitation and a direct contribution consisting of propagation of pressure waves. Due to very low amplitudes of the direct contribution in subsonic flows compared to the overall pressure levels, it is difficult to evaluate the direct acoustic contribution experimentally and numerically. The noise radiated by a zero pressure gradient (ZPG) turbulent boundary layer has been investigated for subsonic flows (Mach 0.5, 0.7 and 0.9) by Gloerfelt 1,2,4 . In these studies, the author especially confirmed the U 8 power law dependence of the acoustic intensities and showed that the acoustic domain and the convective ridge merge at these relatively high speeds. The amplitude of the radiated pressure waves which propagate in the opposite direction of the flow is less than 1% of the aerodynamic pressure for the Mach 0.5 case. In this work, a numerical study of a Mach 0.5 turbulent boundary layer is investigated in presence of adverse and favorable pressure gradients (APG and FPG) using Large Eddy Simulation (LES). Pressure gradients are generated by means of curved geometries. An adverse pressure gradient leads to a deceleration � = − � @� + @� +

The major purpose of the present research is developing the Lattice Boltzmann (LB) method as an extremely efficient computational technique in the pragmatic field of fluid-structure interactions. Hence, the LB method is employed in simulating the incompressible flow past a square unit of four equal diameter circular cylinders. The simulation is conducted at Re of 200 and the spacing ratio (L/D) increases from 1.5 to 5 in 0.5 steps to cover all closely, moderately and widely spaced cases. Accordingly, the effects of the spacing ratio on the shear layer’s behaviour and vortex shedding pattern are studied in detail. Furthermore, its effects on the main force characteristics, i.e., lift and drag coefficients and also the Strouhal number are fully investigated. The observations and results obtained by the LB method employed in the current work are in strict accordance with experimental evidence of other researchers and other existent numerical data in the literature.

Identification of the two-sources at the aircraft slat/fuselage juncture based on phased array

The growth that aviation has seen in the last decades has drawn the attention on the environmental impact of aircraft. An important part of this environmental impact is the noise emitted by air vehicles, which is considered rather significant for community annoyance. The generation and propagation of air vehicle noise are two different areas of interest, which require accurate prediction in order to control the emitted noise levels. The present thesis employs numerical methods in order to investigate various air vehicle noise propagation effects. It is divided in two parts: the far field and the near field study. Each of these studies is concentrated on the sound propagation mechanisms that are dominant for each case and uses a numerical method that is best suited for it in terms of mechanisms incorporated and cost effectiveness. The far-field study of this thesis focuses on the nonlinear propagation of helicopter rotor noise using the Burgers equation, a well known one direction propagation method. The Burgers equation incorporates geometrical spreading, atmospheric absorption and nonlinear distortion effects. Towards this study, the HELISHAPE descent case experimental database is used. Blade Vortex Interaction (BVI) noise, the dominant noise contributor during descent, is mainly examined. It is shown that advancing side BVI noise is affected by nonlinear distortion, while retreating side BVI noise is not. For some frequency bands the difference between linear and nonlinear calculations can be as high as 7 dB. Based on signal characteristics at source, two quantities are derived. The first quantity (termed polarity) is based on the pressure gradient of the source signal and can be used to determine whether a BVI signal will evolve as an advancing or a retreating side signal. The second quantity (termed weighted rise time) is a measure of the impulsiveness of the BVI signal and can be used to determine at which frequency nonlinear effects start to appear. Finally, polarity and weighted rise time are shown to be applicable in cases of BVI noise generated from different blade tips, as well as, in cases of non-BVI noise. However, employment of the Burgers equation can be time consuming to be included in routine calculations. It also requires knowledge of the initial pressure time signal. The power spectrum alone, which is usually known, is not sufficient. In order to ii overcome these difficulties, three prediction methods are presented that are based on the Burgers equation. These are: i) a numerically generated database, ii) correlation equations and iii) the phase assignment method. Near field propagation of air vehicle noise requires different treatment than far field. The effects which are mainly affecting the propagation are geometrical spreading, convection and refraction effects due to the flow field, as well as reflections and diffraction on the air vehicle surfaces. Towards these objectives, a new low-order flow/acoustics interaction method for the prediction of sound propagation and diffraction in unsteady compressible flow using adaptive 3-D hybrid grids is investigated. The total field is decomposed into the flow field described by the Euler equations, and the acoustics field described by the Nonlinear Perturbation equations. The method is shown capable of predicting monopole sound propagation, while employment of acoustics-guided adapted grid refinement improves the accuracy of capturing the acoustic field. Interaction of sound with solid boundaries is also examined in terms of reflection and diffraction. Sound propagation through an unsteady flow field is examined using static and dynamic flow/acoustics coupling demonstrating the importance of the latter. Proof of concept for the new method is provided by its application to the case of a conventional jet transport airplane, examining the effect of flow field and wing shielding on the near field noise levels. During the aforementioned noise investigation and analysis, results on Blade Wake Interaction (BWI) noise were also reached. Presently, the mechanism of BWI noise generation, as well as the corresponding prediction model, are still under consideration. Helicopter rotor BWI noise is known to be significant during take-off and level flight, while less attention has been given to descent flight conditions, where BVI noise is dominant. Through signal analysis of the HELISHAPE descent case acoustic database, the rotor azimuthal region responsible for BWI noise is localized and the dominance of BVI noise in the BWI frequency region is shown. Coherence analysis of the blade pressure data indicate significant chordwise coherence in the 3 to 4 Struhal number range and absence of acoustic dipoles in the BWI frequency range. The findings of this study support BWI prediction models based on Amiet’s theory and suggest that BWI noise can be ignored for predictions of rotor noise in descent flight conditions.

<div class="section abstract"><div class="htmlview paragraph">Tire noise is caused due to the complex interactions between the rotating tire and the road surface at the tire/road interface. It is usually caused due to a combination of individual noise generation mechanisms, which can either be structural or air-borne. The influence of each of these noise generation mechanism may vary, depending on various conditions such as tire design, road surface and operating conditions. Due to the many variables that affect the noise generation mechanisms in tires, it is usually a very complex task to isolate and categorize those that are present in the overall tire/road noise spectrum. Various approaches are used to categorize noise generation mechanisms in tires. In this paper, a statistical model based on the assumption that the tire noise acoustic pressure at a specific frequency band is related to the vehicle speed, is used, in order to study tire noise at different speeds. Consequently, the variation of the speed coefficients with frequency bands is studied for a tire with two transverse slots cut into it, and an effort is presented here, to isolate the different noise generation mechanisms as structural or air-borne. While the goal of the statistical model is to indirectly isolate different noise generation mechanisms, the experimental studies presented here, directly attempt to isolate air-borne and structure-borne noise using a foam. Overall, the current study, tries to establish a trend for the variation of tire noise with tire speed, and also serves to reinforce the notion that statistical models can be used to isolate the different tire noise generation mechanisms.</div></div>

Vibro-acoustic characterization of railway wheels