Quantitative imaging of acoustically coupled flows over symmetrically located side branches

Fully turbulent inflow past symmetrically located side branches mounted in a duct can give rise to pronounced flow oscillations due to coupling between separated shear layers and standing acoustic waves. Experimental investigation of acoustically coupled shear layers was conducted using digital particle image velocimetry in conjunction with unsteady pressure measurements. Global instantaneous and time-averaged flow images, as well as turbulence statistics, were evaluated to provide insight into the flow physics during flow tone generation. The emphasis was on the acoustic response of the resonator during the first and second hydrodynamic modes of the shear layer oscillation. Onset of the locked-on resonant states was characterized in terms of the acoustic pressure amplitude and the quality factor of the corresponding spectral peak. In addition, visco-thermal acoustic damping and patterns of generated acoustic power were calculated using a semi-empirical approach.     

Experimental investigation of coaxial side branch resonators

Digital particle image velocimetry is employed to investigate acoustically-coupled flow past a coaxial deep cavity (side branch) resonator mounted in a duct. The emphasis is on the effect of the separation between the coaxial side branches on the interaction between separated shear layers that form across the side branch openings. Various resonator geometries are characterized in terms of patterns of instantaneous and time-averaged flow velocity, vorticity, and streamline topology at several phases of the acoustic cycle. In addition, phase-averaged images of the flow in conjunction with unsteady pressure measurements are evaluated in order to provide insight into the mechanisms of acoustic power generation. Generally speaking, the acoustic source undergoes a significant transformation as the distance between the coaxial side branches changes. As the distance between the side branches decreases, interaction between the associated oscillating shear layers results in substantial changes in the spatial structure of the acoustic source region.     

Generation of locked-on flow tones: Effect of damping

The overall objective of this investigation is to determine the effect of variable damping on the pressure response of a deep cavity. The pressure fluctuations arise from coupling between the unsteady shear layer along the cavity opening and a resonant mode of the cavity. The damping of the cavity is tuned to desired values without changes of geometry or other parameters.The amplitude of the cavity pressure fluctuation as a function of flow velocity is characterized for the first, second and third acoustic modes of the cavity. For each mode, variation of the value of damping over a relatively wide range yields corresponding attenuation of the pressure amplitude. For higher acoustic modes and sufficiently large damping, abrupt decreases of the pressure amplitude occur at threshold values of flow velocity.The variable damping of the deep cavity does not significantly alter the eigenfrequencies of the system. The peak response amplitude of the pressure fluctuation, however, occurs at a value of Strouhal number that increases with increasing values of damping. Moreover, this peak response amplitude, when normalized by the free stream dynamic head, generally shows a linear variation with the value of damping, for three acoustic modes of the cavity.The strength of lock-on of the pressure oscillation, as a function of the degree of damping, is evaluated in terms of the coherent and broadband pressure amplitudes. Both amplitudes are attenuated for increased damping; the difference between them, however, remains relatively large (40 dB minimum), thereby indicating well-defined lock-on, even when the amplitude of the spectral peak of the coherent component is substantially attenuated.     

Pressure pulsations in piping system excited by a centrifugal turbomachinery taking the damping characteristics into consideration

Pressure pulsations excited by a centrifugal turbomachinery such as compressor, fan or pump at the blade passing frequency may cause severe noise and vibrations in piping system. Therefore, the practical evaluation method of pressure pulsations is strongly recommended. In particular, the maximum pressure amplitude under the resonant conditions should be appropriately evaluated. In this study, a one-dimensional excitation source model for a compressor or pump is introduced based on the equation of motion, so as to incorporate the non-linear damping proportional to velocity squared in the total piping system including the compressor or pump. The damping characteristics of the compressor or pump are investigated by using the semi-empirical model. It is shown that the resistance coefficient of the compressor or pump depends on the Reynolds number that is defined using the equivalent velocity of the pulsating flow. The frequency response of the pressure amplitude and the pressure distribution in the piping system can be evaluated by introducing the equivalent resistance of the compressor or pump and that of piping system. In particular, the relation of the maximum pressure amplitude in piping system to the location of the excitation source under resonant conditions can be evaluated. Finally, the reduction of the pressure pulsations by use of an orifice plate is discussed in terms of the pulsation energy loss.     

A finite element study on the large amplitude flexural vibration characteristics of FGM plates under aerodynamic load

The large amplitude flexural vibration characteristics of functionally graded material (FGM) plates are investigated here using a shear flexible finite element approach. Material properties of the plate are assumed to be graded in the thickness direction according to a simple power-law distribution in terms of volume fractions of the constituents. The effective material properties are then evaluated based on the rule of mixture. The FGM plate is modeled using the first-order shear deformation theory based on exact neutral surface position and von Kármán’s assumptions for large displacement. The third-order piston theory is employed to evaluate the aerodynamic pressure. The governing equations of motion are solved by harmonic balance method to study the vibration amplitude of FGM plates under supersonic air flow. Thereafter, the non-linear equations of motion are solved using Newmark’s time integration technique to understand the flexural vibration behavior of FGM plates in time domain (simple harmonic or periodic or quasi-periodic). This work is new in the sense that it deals with the non-linear flutter characteristics of FGM plates under high supersonic airflow accounting for both the geometric and aerodynamic non-linearities. Some parametric study is conducted to understand the influence of these non-linearities on the flutter characteristics of FGM plates.     

某调距导管桨系泊工况水力性能和流激直发声预报

采用经验证的计算流体力学方法,对某艏辅推调距导管桨设计螺距和系泊工况螺距的水动力性能进行了有效预报,并对系泊工况装船桨流激噪声进行了分析。系泊工况下,由于导管桨的抽吸作用在导管外壁近壁面区域存在与导管内部流动方向相反的逆向流动,且导管桨尾流场速度梯度分布不均匀、流动紊乱,此时桨叶与导管的推力之比约为1.2∶1。系泊工况船+桨的瞬态流场脉动信息表明,导管桨各部件噪声源强度均表现出从1倍到4倍叶频依次下降的规律,最强幅值集中在桨叶导边和导管内壁;在远场声源级频谱曲线中轴向测点线谱较高峰值位置体现出导管桨进流流场的流动特性。对比分析该艏辅推整体和各部件宽带声源级指向性,可知旋转部件(桨叶、桨榖)对总噪声级的贡献较大,静止部件是径向测点噪声的主要贡献源。     

Application of an acoustic analogy to PIV data from rectangular cavity flows

The present paper describes a method to derive information about the acoustic emission of a flow using particle image velocimetry (PIV) data. The advantage of the method is that it allows studying sound sources, the related flow phenomena and their acoustic radiation into the far field, simultaneously. In a first step the time history of two-dimensional instantaneous pressure fields is derived from planar PIV data. In a successive step the Curle’s acoustic analogy is applied to the pressure data to obtain the acoustic radiation of the flow. The test cases studied here are two rectangular cavity flows at very low Mach number with different aspect ratios L/H. The main sound source is located at the cavity trailing edge and it is due to the impingement of vortices shed in the shear layer. It is shown that the flow emits sound with a main directivity in the upstream direction for the smaller aspect ratio and the directivity is more uniform for the larger aspect ratio. In the latter case the acoustic pressure spectra has a broader character due to the impact of the downstream recirculation zone onto the shear layer instabilities, destroying their regular pattern and alternating the main sound source.     

Extension de la DRBEM à la propagation guidée en écoulement cisaillé

The present study aims to extend the Dual Reciprocity Boundary Element Method in order to solve acoustic wave propagation equations in the frequency domain for a parallel shear flow. The Linearized Euler Equations are written as a coupled pair of equations, which are second-order in terms of acoustic pressure and first-order in terms of normal acoustic velocity. Good agreement between numerical results and analytical solutions for a low Mach number shear flow (M<0.1) shows the interest of the method.     

Flow tones in a pipeline-cavity system: effect of pipe asymmetry

Flow tones in a pipeline-cavity system are characterized in terms of unsteady pressure within the cavity and along the pipe. The reference case corresponds to equal lengths of pipe connected to the inlet and outlet ends of the cavity. Varying degrees of asymmetry of this pipe arrangement are investigated. The asymmetry is achieved by an extension of variable length, which is added to the pipe at the cavity outlet. An extension length as small as a few percent of the acoustic wavelength of the resonant mode can yield a substantial reduction in the pressure amplitude of the flow tone. This amplitude decrease occurs in a similar fashion within both the cavity and the pipe resonator, which indicates that it is a global phenomenon. Furthermore, the decrease of pressure amplitude is closely correlated with a decrease of the Q (quality)-factor of the predominant spectral component of pressure. At a sufficiently large value of extension length, however, the overall form of the pressure spectrum recovers to the form that exists at zero length of the extension.Further insight is provided by variation of the inflow velocity at selected values of extension length. Irrespective of its value, both the magnitude and frequency of the peak pressure exhibit a sequence of resonant-like states. Moreover, the maximum attainable magnitude of the peak pressure decreases with increasing extension length.     

Generation of tones due to flow past a deep cavity: Effect of streamwise length

Shear flow past a deep cavity can generate self-sustained oscillations, including locked-on flow tones, due to coupling between the inherent instability of the separated shear layer and an acoustic mode of the cavity resonator. This investigation focuses on the dimensionless pressure amplitude response within a deep cavity, as a function of the streamwise length of the cavity opening; for each length, the pressure response is characterized over a wide range of dimensionless inflow velocity. Criteria for locked-on flow tones are assessed. They include a measure of the strength of lock-on, SoL and the quality factor Q. All self-excited oscillations are assessed using both of these criteria, in order to interpret dimensionless forms of the fluctuation pressure amplitude. The dimensionless pressure amplitude response of the cavity involves several successive regimes, due to variations of streamwise length L of the cavity opening. These regimes are defined in relation to L/θ, where θ is the momentum thickness of the inflow boundary layer. Below a minimum value of L/θ, flow tones cannot be generated. Furthermore, these regimes are defined in terms of the possible hydrodynamic modes (stages) of the unsteady shear layer and the acoustic modes of the deep cavity.     

Attenuation of cavity internal pressure oscillations by shear layer forcing with pulsed micro-jets

Experimental results concerning the pressure oscillations induced by a grazing flow over a deep cavity like a Helmholtz resonator are presented. The study deals with the forcing of the neck shear layer instability in an opened-loop control scheme by means of pulsed micro-jets. The effects of the frequency and amplitude are investigated. It is found that efficient attenuation of the pressure oscillations can be reached when the forcing frequency is larger than the cavity resonance frequency. Then the shear layer is locked with the forcing and resonance with the cavity is lost, inducing a significant decrease of the acoustic pressure level in the cavity. Effects of the jet amplitude are weak, a very small amplitude being capable of forcing the shear layer. By contrast, when the forcing frequency is lower than the cavity resonance frequency (the forcing wave length is greater than twice the neck length) the forcing is ineffective.     

Experimental and numerical investigation on the flow field within a compact inlet duct

A combined experimental and numerical investigation of the flow field in a short, rectangular, diffusing S-shape inlet duct was conducted. The inlet duct had a length-to-hydraulic diameter ratio of 1.5 and an inflow Mach number of 0.44. The flow field was diagnosed utilizing stereoscopic particle image velocimetry, surface static pressure measurements, and high frequency total pressure measurements both on the lower surface and at the duct’s aerodynamic interface plane. To complement the experimental investigation and to aid in understanding the flow field associated with this complex geometry, a numerical flow simulation was undertaken. The flow field exhibited massive flow separations and shear layer formations at both turns of the compact inlet. Moreover, secondary flow structures along the duct’s lower surface and along the duct’s side walls were identified. It was shown that the two counter-rotating flow structures along the duct’s lower surface resulted in high levels of total pressure loss at the aerodynamic interface plane. A high fidelity spectral analysis of the pressure signals at the aerodynamic interface plane and along the lower surface was conducted and demonstrated that a high frequency surface static pressure sensor could identify flow separation in a non-intrusive fashion, allowing for future use in a closed-loop control scheme for active flow control. This work was part of a more comprehensive study which was to utilize active flow control to improve performance metrics of such compact inlets.     

Oscillation of shallow flow past a cavity: Resonant coupling with a gravity wave

Self-excited oscillations of flow past a cavity are generated in a shallow free-surface system. The shear layer past the cavity opening has two basic forms: a separated free-shear flow; and a shear flow along a slotted plate. Instabilities of these classes of shear flows can couple with the fundamental gravity-wave mode of the adjacent cavity. The dimensionless frequencies of both types of oscillations scale on the length of the cavity opening, rather than the gap distance between the slats, i.e., a large-scale instability is always prevalent. A technique of high-image-density particle image velocimetry allows acquisition and interpretation of global, instantaneous images of the flow pattern, including patterns of vorticity and Reynolds stress correlation. Use of a cinema approach provides representations of the timewise evolution of the global, instantaneous flow structure, and thereby definition of the amplitude peaks and phase angles of the coupled fluctuations via auto- and cross-spectral techniques. These methods, along with global, averaged representations of the fluctuating flow field, provide insight into the onset of fully coupled (phase-locked) oscillations of the shear flow past the resonator cavity. The common, as well as the distinctive, features of the resonant-coupled instability of the shear flow past the slotted plate are characterized, relative to the corresponding coupled instability of the free-shear layer. Varying degrees of resonant coupling between the unstable shear layer and the adjacent resonator are attained by variations of the inflow velocity, which yield changes of the predominant oscillation frequency, relative to the resonant frequency of the adjacent cavity. Well-defined, coherent oscillations are indeed attainable for the case of the shear flow along the slotted plate, though their amplitude is significantly mitigated relative to the case of a free-shear layer. The degree of organization of the self-excited, resonant-coupled oscillation and the manner in which it varies with open area ratio and geometry of the plate are interpreted in terms of the flow structure on either side of, and within, the slotted plate; these features are compared with the corresponding structure of the free-shear layer oscillations.     

Prediction of vortex shedding from bluff bodies in the presence of a sound field

The separated flow around a rectangular cylinder, in the presence of a transverse duct resonant acoustic mode, is modelled using a vortex method. The instantaneous transfer of power between the mean flow and the acoustic field is predicted using Howe's theory of aerodynamic sound. Whether the net acoustic energy per cycle generated is positive or negative depends on the phase of the acoustic cycle at which vortex clouds arrive at the trailing edge of the cylinder.     

The effect of laminations on the vibrational modes of circular annular plates

The purpose of this article is to present an experimental study of the effect of laminations on the vibrations of circular annular plates. To obtain a basis for comparison with experimental data, the natural frequencies and mode shapes of a series of solid circular annular plates were calculated using the finite element method. An extensive range of experiments were performed on both a series of solid models and a series of laminated models under a range of normal clamping pressures. Based on the analytical and experimental results, it was found that the vibrational behavior of the laminated plates was dominated by that of the individual plate of which they were composed and that the effects of the laminations on vibrations were mode type dependent. The effects on the transverse vibrational modes were dependent on both the normal clamping pressure and the number of plates. The amplitude of the frequency response function for these modes reduced quickly, and the resonant frequency of such modes shifted higher as the clamping pressure or the number of plates increased. For the in-plane vibrational modes, the amplitude of the frequency response function reduced slightly as the number of plates increased; the resonant frequency of such modes could be considered to be a constant and independent of both the clamping pressure and the number of plates.     

Imaging of the self-excited oscillation of flow past a cavity during generation of a flow tone

Flow through a pipeline-cavity system can give rise to pronounced flow tones, even when the inflow boundary layer is fully turbulent. Such tones arise from the coupling between the inherent instability of the shear flow past the cavity and a resonant acoustic mode of the system. A technique of high-image-density particle image velocimetry is employed in conjunction with a special test section, which allows effective laser illumination and digital acquisition of patterns of particle images. This approach leads to patterns of velocity, vorticity, streamline topology and hydrodynamic contributions to the acoustic power integral. Comparison of global, instantaneous images with time- and phase-averaged representations provides insight into the small-scale and large-scale concentrations of vorticity, and their consequences on the topological features of streamline patterns, as well as the streamwise and transverse projections of the hydrodynamic contribution to the acoustic power integral. Furthermore, these global approaches allow the definition of effective wavelengths and phase speeds of the vortical structures, which can lead to guidance for physical models of the dimensionless frequency of oscillation.     

Measurements of the dynamic lift force acting on a circular cylinder in cross-flow and exposed to acoustic resonance

A piezoelectric transducer is developed to perform direct measurements of the dynamic lift force acting on a circular cylinder in cross-flow, in the presence and absence of acoustic resonance. Details of the force transducer design are presented in the paper. The dynamic lift force is measured for a single cylinder with two different diameters, D=12.7 and 15.8 mm. During the tests, the first transverse acoustic mode of the duct housing the cylinder is self-excited. The fluctuating pressure on the top wall of the duct is measured simultaneously with the dynamic lift force. In the absence of acoustic resonance, the measured dynamic lift coefficients agree favorably with those reported in the literature. However, when the acoustic resonance is initiated, the dynamic lift experiences a drastic increase in amplitude associated with abrupt changes in the phase between the lift force and the acoustic pressure. A methodology to extract the hydrodynamic lift component from the total lift measured during acoustic resonance is also proposed. The hydrodynamic lift force is then decomposed into in-phase and out-of-phase components, with respect to the resonant sound pressure. This decomposition procedure provides new insights into the nature of the aeroacoustic sources in the cylinder wake. The proposed methodology, together with the test results provide a general design approach to assess the increase in the dynamic fluid loading on bluff bodies in cross-flow due to the excitation of acoustic resonance.     

Thresholds for rectified diffusion and acoustic microstreaming by bubbles in biological tissue

Results are presented of the calculation of the thresholds (in terms of peak acoustic pressure as a function of frequency of the incident ultrasonic wave), at which rectified diffusion may begin in environments represented by the liquid/cell structure of biological tissue, exposed to ultrasonic frequencies, 1–4 MHz, typical for clinical devices. Computations based on excitation by peak pressure amplitude values typical for continuous and pulse-echo diagnostic devices, suggests that rectified diffusion is unlikely to occur for the latter only. Acoustically induced shear stresses, caused by bubble pulsation produced microstreaming and affecting the integrity of cellular membranes, are evaluated and are found to lie above levels at which biological effects have been observed.     

Numerical investigation of a laminar pulsating flow in a rectangular duct

A pulsating laminar flow of a viscous, incompressible liquid in a rectangular duct has been studied. The motion is induced under an imposed pulsating pressure difference. The problem is solved numerically. Different flow regimes are characterized by a non‐dimensional parameter based on the frequency (ω) of the imposed pressure gradient oscillations and the width of the duct (h). This, in fact, is the Reynolds number of the problem at hand. The induced velocity has a phase lag (shift) with respect to the imposed pressure oscillations, which varies from zero at very slow oscillations, to 90° at fast oscillations. The influence of the aspect ratio of the rectangular duct and the pulsating pressure gradient frequency on the phase lag, the amplitude of the induced oscillating velocity, and the wall shear were analyzed. Copyright © 1999 John Wiley & Sons, Ltd.     

Erratum to: Vibration Analysis of Perforated Plates Using Time-Average Digital Speckle Pattern Interferometry

Digital Speckle Interferometry is a non invasive full-field coherent optical technique used in mechanical vibration measurement. In this research, it is used for tuning resonant frequencies of vibrating plates in order to investigate the dynamical behavior of perforated plates. The plate was excited to resonant vibration by a sinusoidal acoustic source. Fringe pattern produced during the time-average recording of the vibrating plate, for several resonant frequencies were registered. Results of plates fixed at one edge having internal holes and attached masses are presented. Experimental natural frequencies and modal shapes are compared to those obtained by an analytical approximate solution based on the Rayleigh–Ritz method with the use of orthogonal polynomials as coordinate function. A high degree of correlation between computational analysis and experimental results was observed, proving the potentiality of the optical technique as experimental validation of the numerical simulations.