WHISTLES WITH A GENERIC SIDEBRANCH: PRODUCTION AND SUPPRESSION

The present study investigates experimentally the production and suppression of whistle noise resulting from the shear layer instabilities coupled with the acoustic resonances at the interface of two ducts, a main duct and connecting sidebranch. A generic sidebranch adapter is built to allow for mounting downstream of the throttle body in the induction system of a production engine, and the adjustment of sidebranch length. The adapter has also the provision to investigate a number of suppression methods such as: (1) ramps mounted in the main duct right upstream of the sidebranch opening; (2) a spacer to increase the distance between the throttle plate and sidebranch opening; and (3) the rotation of the throttle body from its original position. Experiments with the same hardware are conducted in both a flow laboratory and an engine dynamometer facility. The effectiveness of these suppression techniques is examined experimentally along with the correlation between the two facilities.     

矢量拖线阵不同形状水听器流噪声响应特性

拖线阵声呐流噪声大小与水听器形状密切相关,为了揭示矢量拖线阵流噪声响应特性,研究了不同形状矢量水听器滤波特性及其噪声抑制特点。基于波数-频率谱分析方法,导出了圆柱形、球形、胶囊形矢量水听器的波数响应函数以及噪声功率谱解析表达式;并根据湍流边界层压力起伏Carpenter模型数值计算了矢量水听器波数响应函数、流噪声响应特性与水听器形状和几何尺寸之间的变化规律,得到了不同形状矢量水听器抑制流噪声特点。研究结果表明,细长胶囊形矢量水听器具有较好的流噪声抑制性能。      

Shadowing of directional noise sources by finite noise barriers

The present study investigates the shadowing effect of barriers of infinite or finite length in the presence of directional noise sources. The diffraction model termed [Directive Line Source Model (DLSM) J. Acoust. Soc. Am. 107 (2000) 2973-2986] is employed. DLSM is appropriately modified and extended to include the effects of ground reflection, diffraction by the side edges of a finite length barrier, and diffraction by directional noise sources. Results obtained are shown to be in good agreement with available experimental data and known analytical solutions. An application of the enhanced DLSM is illustrated using helicopter type noise, which is highly directive. The noise source is modeled as a directional point source with far field directivity data and the enhanced DLSM is employed to compare the noise field with and without the barrier present for three different directivity patterns, various source locations and orientations, as well as, for various barrier lengths.     

Features of Laser Through-Cutting of Silicate Glass by a Fiber Ytterbium Laser

Some principal aspects of silicate glass cutting by controlled laser through thermal cleavage are considered. In particular, it is shown that the cutting speed in the case of ytterbium fiber laser radiation with a wavelength of 1.065 μm lying in fact in the glass transmission range (more precisely, in the low absorption region) depends linearly on the laser power. It is shown that the glass end face takes various geometrical shapes under various conditions of bulk heating and cooling. Therefore, to obtain a homogeneous end face, it is required to stabilize both the laser radiation power and the laser beam speed at a corresponding laser beam geometry in the cut region. Methods for obtaining various cross section shapes of the glass end face and methods for obtaining blunt edges of end faces are presented.     

Study of shear transfer in Al/epoxy joint by digital photoelasticity

An improved six-step phase-shifting method is proposed for calculating full-field shear stress based on a four-step color phase-shifting method in digital photoelasticity. The method was verified using a disk under diametral compression and then applied to an aluminum alloy/epoxy joint for studying the shear transfer behavior. Experimental results revealed that the isochromatic fringe order and shear stress at the bonding interface are distributed continuously and increased with compression. In particular, an antisymmetric thermal residual shear stress appears at the bonding interface, because of the difference in the thermal expansion coefficients of Al and the resin. This indicates that the thermal residual shear stress at the bonding interface is self-balanced and reaches a peak at the edges of the bonding interface. The load transfer is realized by the shear band from the bonding interface to the bottom support. Basically, the bonding interfacial shear stress is balanced with the load.     

An experimental study of transmission,reflection and scattering of sound in a free jet flight simulation facility and comparison with theory

When a free jet (or open jet) is used as a wind tunnel to simulate the effects of flight on model noise sources, it is necessary to calibrate out the effects of the free jet shear layer on the transmitted sound, since the shear layer is absent in the real flight case. In this paper, a theoretical calibration procedure for this purpose is first summarized; following this, the results of an experimental program, designed to test the validity of the various components of the calibration procedure, are described. The experiments are conducted by using a point sound source located at various axial positions within the free jet potential core. By using broadband excitation and cross-correlation methods, the angle changes associated with ray paths across the shear layer are first established. Measurements are then made simultaneously inside and outside the free jet along the proper ray paths to determine the amplitude changes across the shear layer. It is shown that both the angle and amplitude changes can be predicted accurately by theory. It is also found that internal reflection at the shear layer is significant only for large ray angles in the forward quadrant where total internal reflection occurs. Finally, the effects of sound absorption and scattering by the shear layer turbulence are also examined experimentally.     

微尺度驻停气泡柔性气-液界面抗流体剪切能力

在微流控系统中,稳定、可控的柔性气-液界面可实现声流体颗粒富集、微纳操作、快速气-液反应等各种实际物理和生化应用.微流道内气-液界面的抗流体剪切能力对于增强微尺度下气-液界面的可控性具有十分重要的意义.为此,文章研究了具有高稳定性、高可控性、可阵列化的微尺度驻停气泡现象.利用嵌入局部裂隙的微流道以及与之平行的气体流道,可对驻停气泡的生成和形态进行有效调节,并利用其可控的气-液界面实现多种功能化应用.在此基础上,文章进一步研究柔性可控气-液界面的抗流体剪切能力,对形态变化中的气-液界面受力进行分析,利用仿真和实验手段研究不同状态下气-液界面的形状特征,研究不同的液体驱动压力、裂隙尺寸以及裂隙形状对气-液界面抗剪切能力的影响,并将界面的曲率半径作为气泡驻留与否的判定依据.文章对驻停气泡柔性气-液界面抗流体剪切能力的研究有助于优化其控制方法,增强其控制稳定性并拓展其潜在应用场合.      

An effective noise removal technique for recovering the shapes of diamond abrasive grains in SLM images degraded by clustered spike noise

A new noise-removal technique is applied to scanning laser microscopic (SLM) images to remove clustered spike noise in the images and to recover the shapes of diamond abrasive grains degraded by the noise. For achievement of this purpose, noise points in the SLM image are accurately detected by taking advantage of their properties in the space and spatial-frequency regions. The noise points are removed by a method of smoothing that is based on linear interpolation; that is, their pixel values are replaced by the interpolated values of their non-noise neighboring points. Noise-point information in the space region is acquired from image segmentation based on pixel classification, while noise-point information in the frequency region is derived from redundant wavelet decomposition for the SLM image. Fisher's linear discriminant method is used to yield the two noise-point images. The degraded grain shapes in the SLM images at different noise levels are satisfactorily recovered with a single iteration of smoothing without losses in sharp edges although a single smoothing needed four interpolations. Thus, the present noise-removal technique is shown to be effective for recovering the original shapes of the grains in every SLM image.     

An experimental study of airfoil instability tonal noise with trailing edge serrations

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien–Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback models. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.     

Investigation of higher spanwise Helmholtz resonance modes in slender covered cavities

Cavity aeroacoustic noise is relevant for aerospace and automotive industries and widely investigated since the 1950s. Most investigations so far consider cavities where opening length and width are of similar scale. The present investigation focuses on a less investigated setup, namely cavities that resemble the door gaps of automobiles. These cavities are both slender (width much greater than length or depth) and partially covered. Furthermore they are under influence of a low Mach number flow with a relatively thick boundary layer. Under certain conditions, these gaps can produce tonal noise. The present investigation attempts to reveal the aeroacoustic mechanism of this tonal noise for higher resonance modes. Experiments have been conducted on a simplified geometry, where unsteady internal pressures have been measured at different spanwise locations. With increasing velocity, several resonance modes occur. In order to obtain higher mode shapes, the cavity acoustic response is simulated and compared with experiment. Using the frequency-filtered simulation pressure field, the higher modes shapes are retrieved. The mode shapes can be interpreted as the slender cavity self-organizing into separate Helmholtz resonators that interact with each other. Based on this, an analytical model is derived that shows good agreement with the simulations and experimental results.     

A Quadratic Spline based Interface (QUASI) reconstruction algorithm for accurate tracking of two-phase flows

A new Quadratic Spline based Interface (QUASI) reconstruction algorithm is presented which provides an accurate and continuous representation of the interface in a multiphase domain and facilitates the direct estimation of local interfacial curvature. The fluid interface in each of the mixed cells is represented by piecewise parabolic curves and an initial discontinuous PLIC approximation of the interface is progressively converted into a smooth quadratic spline made of these parabolic curves. The conversion is achieved by a sequence of predictor–corrector operations enforcing function (C⁰) and derivative (C¹) continuity at the cell boundaries using simple analytical expressions for the continuity requirements. The efficacy and accuracy of the current algorithm has been demonstrated using standard test cases involving reconstruction of known static interface shapes and dynamically evolving interfaces in prescribed flow situations. These benchmark studies illustrate that the present algorithm performs excellently as compared to the other interface reconstruction methods available in literature. Quadratic rate of error reduction with respect to grid size has been observed in all the cases with curved interface shapes; only in situations where the interface geometry is primarily flat, the rate of convergence becomes linear with the mesh size. The flow algorithm implemented in the current work is designed to accurately balance the pressure gradients with the surface tension force at any location. As a consequence, it is able to minimize spurious flow currents arising from imperfect normal stress balance at the interface. This has been demonstrated through the standard test problem of an inviscid droplet placed in a quiescent medium. Finally, the direct curvature estimation ability of the current algorithm is illustrated through the coupled multiphase flow problem of a deformable air bubble rising through a column of water.     

Finite element prediction of debonding onset in short fiber reinforced polymers incorporating a hybrid interphase region

A robust finite element procedure for investigating damage evolution in short fiber reinforced polymeric composites under external loads is developed. This procedure is based on an axisymmetric unit cell composed of a fiber, surrounding interphase and bulk matrix. The hybrid interphase concept involves a degraded material phase, the extent of which is material and property dependent. One of the most significant features of the model relies on establishment of variable adhesion conditions between the primary material phases. The unit cell is discretized into linearly elastic elements for the fiber and the matrix and interface elements which allow debonding in the fiber–matrix interface. The interface elements fail according to critical stress and critical energy release rate criteria. The tension and shear aspects of failure are uncoupled, although the resulting nonlinear problem is solved implicitly utilizing quasi-static incremental loading conditions. Final failure resulting from saturation and breakage is modeled by the vanishing interface element technique. Details of the propagation of interface cracks and the initiation of debonds are also observed and discussed for various shapes of fiber end. Numerical results reveal an intense effect of the fiber-end geometry on the initial fiber–matrix de-cohesion. The present finite element procedures can generate meaningful results in the analysis of fiber-reinforced composites.     

Free vibration analysis of Mindlin plates with linearly varying thickness

A method based on the variational principles in conjunction with the finite difference technique is applied to examine the free vibration characteristics of isotropic rectangular plates of linearly varying thickness by including the effects of transverse shear deformation and rotary inertia. The validity of the present approach is demonstrated by comparing the results with other solutions proposed for plates with uniform and linearly varying thickness. Natural frequencies and mode shapes of Mindlin plates with simply supported and clamped edges are determined for various values of relative thickness ratio and the taper thickness constant.     

High amplitude acoustic transmission through duct terminations: Theory

Recent experimental measurements have demonstrated that net acoustic energy dissipation can occur when sound waves interact with free shear layers, which are produced either by boundary layer separation in mean fluid flow at sharp edges, or by separation of the boundary layer in the acoustic flow at an edge in the absence of mean flow. This paper presents theoretical results which are offered in an attempt to explain these observations quantitatively. Comparison is made between the predicted and measured net energy loss which occurs upon transmission of high amplitude impulsive acoustic waves through various duct terminations, and also between calculated and measured reflection coefficients in the duct. The agreement is generally at least qualitatively good, and would appear to justify the physical assumptions on which the theoretical arguments are based.     

Numerical study of the single fibre push-out test: Part III. Singularity of interface stresses

The singular behaviour at the free edges of the fibre-matrix interface is analysed for the fibre push-out test geometry based on the boundary element method. The fibre push-out test has been extensively used to measure the fibre-matrix interfacial properties in polymer, ceramic and metal matrix composites. There are two free edges in the fibre push-out specimen: one is at the loaded fibre end and the other at the supported fibre end. The singular stresses can be expressed as a function of singular exponent and singular stress intensity. It is shown that the singular exponents obtained at both fibre ends are characteristic of composite constituent properties, such as Young's moduli of fibre and matrix, and does not vary with specimen dimensions. The singular exponents are real and identical for the shear and radial stress components at fibre ends where the wedge angles are the same. The singular stress intensities are also implicit in material properties, and vary with specimen dimensions, such as fibre to matrix radius ratio, fibre aspect ratio and support hole size. An interfacial failure criterion is proposed here based on the average stress concept to determine the critical singular stress intensities in mode I and mode II loads.     

襟翼侧缘噪声机理及修型降噪设计

襟翼侧缘噪声是飞机起降阶段机体噪声的重要噪声源。采用极大涡模拟对襟翼侧缘非定常流场进行数值模拟,分析其噪声产生机理.基于此,提出了两种襟翼侧缘修型方式,应用虚拟渗透面的Ffowcs Williams and Hawkings(FW-H)声比拟方法将修型构型的远场噪声频谱特性和指向性与基准构型对比分析,研究其降噪效果。通过流场和声场的数值模拟表明,襟翼侧缘噪声属于宽频噪声。不同的襟翼侧缘形状改变了流场形态、侧缘涡结构以及涡系的发展过程,进而对声源分布和远场噪声特性产生影响。结果表明:在给定的5°计算迎角下,两种襟翼侧缘修型方式在保证增升装置的原有升阻气动特性的前提下,能达到减小全场总声压级1~2 dB的降噪效果。      

The rôle of surface shear stress fluctuations in the generation of boundary layer noise

The influence of unsteady wall shear stress on boundary layer noise and wall pressure fluctuations is discussed. It is argued that in the acoustic analogy theory of boundary layer noise the surface shear stress “dipole” characterizes acoustic propagation and not generation. Analytical results are presented in support of this view which, in addition, indicate that the effect of the surface dipole is to dininish rather than enhance boundary layer radiation at low Mach numbers.     

Sharp interface tracking using the phase-field equation

A general interface tracking method based on the phase-field equation is presented. The zero phase-field contour is used to implicitly track the sharp interface on a fixed grid. The phase-field propagation equation is derived from an interface advection equation by expressing the interface normal and curvature in terms of a hyperbolic tangent phase-field profile across the interface. In addition to normal interface motion driven by a given interface speed or by interface curvature, interface advection by an arbitrary external velocity field is also considered. In the absence of curvature-driven interface motion, a previously developed counter term is used in the phase-field equation to cancel out such motion. Various modifications of the phase-field equation, including nonlinear preconditioning, are also investigated. The accuracy of the present method is demonstrated in several numerical examples for a variety of interface motions and shapes that include singularities, such as sharp corners and topology changes. Good convergence with respect to the grid spacing is obtained. Mass conservation is achieved without the use of separate re-initialization schemes or Lagrangian marker particles. Similarities with and differences to other interface tracking approaches are emphasized.     

Anisotropic surface melting in lyotropic cubic crystals

From experiments with ice or metal crystals, in the vicinity of their crystal/liquid/vapor triple points, it is known that melting of crystals starts on their surfaces and is anisotropic. It is shown here by direct observations under an optical microscope that this anisotropic surface melting phenomenon occurs also in lyotropic systems. In the case of C12EO2/water mixture, it takes place in the vicinity of the peritectic Pn3m/L3/L1 triple point. Above the peritectic triple point, where the Pn3m and L1 phases coexist in the bulk, the surface of a Pn3m-in-L1 crystal is composed of (111)-type facets surrounded by rough surfaces. The angular junction suggests that rough surfaces are wet by a L3-like layer while facets stay “dry”. This is analogous to the pre-melting at rough surfaces in solid crystals. Upon cooling below the peritectic triple point, where L3 and L1 phases coexist in the bulk, a thick layer of the L3 phase grows from the pre-melted, rough Pn3m/L1 interface. Simultaneously, facets stay dry and their radius decreases. In this tri-phasic configuration, stable in a narrow temperature range, the L3/L1 and L3/Pn3m interfaces have shapes of constant mean curvature surfaces having common borders: edges of facets.     

Investigation of the SiO<Subscript>2</Subscript>(Co)/GaAs heterostructures using the surface scattering of synchrotron radiation

The giant injection magnetoresistive effect has been observed in a granulated Co/SiO₂ film on a semiconductor GaAs substrate in a narrow temperature range near T = 300 K. According to the existing theory, the nature of the effect is due to the structure and physical problems of the interface layer. The spatial distribution of cobalt nanoparticles in the bulk of the Co/SiO₂ granular film and at the granular film/semiconductor substrate (GF/SS) interface has been investigated by the reflectometry and small-angle scattering of synchrotron radiation in the grazing geometry. It has been shown that the characteristic average distance between the cobalt granules in the bulk of the film is 7.3 nm. At the same time, the average distance between the granules with a vertical size of about 7.5 nm at the GF/SS interface is 32 nm. The experimental data indicate the low concentration of cobalt at the interface and the point character of the contact of the main bulk of the Co/SiO₂ film with the GaAs substrate through a relatively diluted layer of ferromagnetic cobalt granules.