Spatial correlation in the ambient core noise field of a turbofan engine

An acoustic transfer function relating combustion noise and turbine exit noise in the presence of enclosed ambient core noise is investigated using a dynamic system model and an acoustic system model for the particular turbofan engine studied and for a range of operating conditions. Measurements of cross-spectra magnitude and phase between the combustor and turbine exit and auto-spectra at the turbine exit and combustor are used to show the presence of indirect and direct combustion noise over the frequency range of 0-400 Hz. The procedure used evaluates the ratio of direct to indirect combustion noise. The procedure used also evaluates the post-combustion residence time in the combustor which is a factor in the formation of thermal NO(x) and soot in this region. These measurements are masked by the ambient core noise sound field in this frequency range which is observable since the transducers are situated within an acoustic wavelength of one another. An ambient core noise field model based on one and two dimensional spatial correlation functions is used to replicate the spatially correlated response of the pair of transducers. The spatial correlation function increases measured attenuation due to destructive interference and masks the true attenuation of the turbine.     

Numerical and analytical modelling of entropy noise in a supersonic nozzle with a shock

Analytical and numerical assessments of the indirect noise generated through a nozzle are presented. The configuration corresponds to an experiment achieved at DLR by Bake et al. [The entropy wave generator (EWG): a reference case on entropy noise, Journal of Sound and Vibration 326 (2009) 574-598] where an entropy wave is generated upstream of a nozzle by an electrical heating device. Both 3-D and 2-D axisymmetric simulations are performed to demonstrate that the experiment is mostly driven by linear acoustic phenomena, including pressure wave reflection at the outlet and entropy-to-acoustic conversion in the accelerated regions. Moreover, the spatial inhomogeneity of the upstream entropy fluctuation has no visible effect for the investigated frequency range (0-100 Hz). Similar results are obtained with a purely analytical method based on the compact nozzle approximation of Marble and Candel [Acoustic disturbances from gas nonuniformities convected through a nozzle, Journal of Sound and Vibration 55 (1977) 225-243] demonstrating that the DLR results can be reproduced simply on the basis of a low-frequency compact-elements approximation. Like in the present simulations, the analytical method shows that the acoustic impedance downstream of the nozzle must be accounted for to properly recover the experimental pressure signal. The analytical method can also be used to optimize the experimental parameters and avoid the interaction between transmitted and reflected waves.     

Acoustic characteristics of a rocket combustion chamber: Radial baffle effects

This paper describes methods used for determining the characteristic acoustic modes and frequencies of a liquid-propellant rocket-motor combustion chamber and effects of radial baffles on the chamber’s acoustic field. A multi-point sensing experimental setup, including stationary and moving sensors, was used to measure characteristic frequencies and mode shapes of a combustion chamber. A new technique based on the comparison of signal phase angles from stationary sensors to that of a moving sensor was used to map complex characteristic mode shapes of a combustor. A three-dimensional Helmholtz acoustic solver was also developed using an efficient finite volume approach for complex geometries to simulate the acoustic field inside a combustor. Using this approach the effects of the convergent section of the nozzle and the number of radial baffles on the chamber’s dominant acoustic modes with no mean flow were investigated. We have shown that the classical reduction of characteristic frequency of tangential modes caused by radial baffles is due to longitudinalization of tangential modes and is a function of the blade length and weakly dependent on the number of blades. Also, conjugate spinning modes are decoupled and do not spin in any baffled combustor, independent of the number of blades. On the other hand the converging nozzle section of a combustion chamber modifies pure longitudinal modes in the radial direction and pure tangential modes in the longitudinal direction. Existence of some mixed tangential-longitudinal modes in a combustor is dependent on the ratio of the nozzle throat diameter to the combustor head plate diameter.     

Influence of blade compactness and segmentation strategy on tonal noise prediction of an automotive engine cooling fan

This paper discusses a blade segmentation strategy for tonal noise computation of automotive engine cooling fans. The noise sources are extracted from an Unsteady Reynolds-averaged Naviers Stocks simulation (URANS), and propagated into the far field using dipole noise equations derived from Ffowcs-Williams acoustic analogy. On the aerodynamic side, a mesh independence study is carried out. On the acoustic side, three levels of acoustic meshes are compared to evaluate the assumption of blade compactness and its influence on the final results. The blade is modeled with one dipole at the lowest level of the acoustic mesh: it is assumed acoustically compact regardless of the frequency. At the finest level, the blade is meshed with 26,726 dipoles which are the surface elements of the CFD mesh. An intermediate level of mesh is also studied where the blade is cut into three acoustically compact strips. A comparison is also established between two approaches for summing the contributions of the dipoles in the cases of the intermediate and fine meshes. The first approach is to neglect the axial, tangential and radial phase lags between the dipoles, and assume uncorrelated sources. The second approach takes into account the phase lags between the dipoles due to the blade’s geometry. It is based on the equations derived by Hanson Parzych [4]. The results are additionally compared to values extracted from experiments carried out in an anechoic chamber.     

Turbulence and heat excited noise sources in single and coaxial jets

The generation of noise in subsonic high Reynolds number single and coaxial turbulent jets is analyzed by a hybrid method. The computational approach is based on large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE). The method is used to investigate the acoustic fields of one isothermal single stream jet at a Mach number 0.9 and a Reynolds number 400,000 based on the nozzle diameter and two coaxial jets whose Mach number and Reynolds number based on the secondary jet match the values of the single jet. One coaxial jet configuration possesses a cold primary flow, whereas the other configuration has a hot primary jet. Thus, the configurations allow in a first step the analysis of the relationship of the flow and acoustic fields of a single and a cold coaxial jet and in a second step the investigation of the differences of the fluid mechanics and aeroacoustics of cold and hot coaxial jets. For the isothermal single jet the present hybrid acoustic computation shows convincing agreement with the direct acoustic simulation based on large-eddy simulations. The analysis of the acoustic field of the coaxial jets focuses on two noise sources, the Lamb vector fluctuations and the entropy sources of the APE equations. The power spectral density (PSD) distributions evidence the Lamb vector fluctuations to represent the major acoustic sources of the isothermal jet. Especially the typical downstream and sideline acoustic generations occur on a cone-like surface being wrapped around the end of the potential core. Furthermore, when the coaxial jet possesses a hot primary jet, the acoustic core being characterized by the entropy source terms increases the low frequency acoustics by up to 5 dB, i.e., the sideline acoustics is enhanced by the pronounced temperature gradient.     

半空间球面波函数叠加法重构声源直接辐射声场

提出了基于半空间球面波函数叠加的声场重构方法,以重构含有限声阻抗边界半空间中声源直接辐射的声场.在半空间中多极子声源声压场的解析解的基础上,构造出以边界声阻抗为参量的半空间球面波函数的正交基;通过求逆获得半空间总声压解的基函数系数,同时也获得声源直接辐射声场即自由空间中的基函数系数,进而重构出声源直接辐射的声场.在边界...     

Simulative and experimental investigations on pressure-induced structural vibrations of a rear muffler

The periodically blown out exhaust gas of a combustion engine may excite structural vibrations of the exhaust system. In addition to the noise of the orifice, these vibrations contribute to the overall noise radiation of the exhaust system. In this work, the excitation of structural vibrations of a rear muffler via the acoustic path is investigated both in experiments and simulations. In both cases transfer functions from the acoustic pressure at the inlet to the structural deflection on the surface of the rear muffler are determined and compared to each other. For the simulation an FE-FE (finite element) coupling is applied to account for the fluid-structure interaction. To efficiently predict the fluid-structure coupled behavior, a model reduction technique for the finite element method based on the Craig-Bampton method and the Rubin method is presented. In a last step, the sound radiation is evaluated by solving the exterior acoustic problem with the fast multipole boundary element method. For this purpose, the results of the FE computation are used as boundary datum.     

Jet engine combustion noise: Pressure,entropy and vorticity perturbations produced by unsteady combustion or heat addition

By idealizing combustion or heat addition processes to occur over a short distance in the flow direction it is possible to calculate the amplitude and phase of the disturbances corresponding to small amplitude fluctuations in the heat addition. The fluctuating heat input is assumed to vary sinusoidally with time and with distance along the direction normal to the flow. Pressure waves propagate away from the heat input region upstream and downstream, whilst on the downstream side waves of vorticity and entropy are convected away. Strong resonant peaks in the pressure and vorticity waves are present close to the cut-off condition of the pressure waves in two dimensions. Generally the wave amplitudes tend to be higher when the mean flow velocity into the region is close to sonic and to become smaller as the steady heat input is increased. For a simplified calculation in which the combustion chamber discharges directly into a multi-stage turbine the downstream noise was predominantly due to the interaction of the entropy with the turbine (i.e., “indirect” rather than “direct” combustion noise).     

On the use of a uniformly valid analytical cascade response function for fan broadband noise predictions

The present paper extends an existing analytical model of the aeroacoustic response of a rectilinear cascade of flat-plate blades to three-dimensional incident vortical gusts, to the prediction of the noise generated by a three-dimensional annular blade-row. The extended formulation is meant to be implemented in a fan broadband noise prediction tool. The intended applications include the modern turbofan engines, for which analytical modelling is believed to be a good alternative to more expensive numerical techniques. The prediction noise model resorts to a strip theory approach based on a three-dimensional rectilinear cascade model. The latter is based on the Wiener-Hopf technique, and yields the pressure field in the blade passage and the unsteady blade loading. The analytical pressure solution is derived by making an extensive use of the residue theorem. The obtained unsteady blade loading distribution over the blades is then used as a dipole source distribution in an acoustic analogy applied in the annular rigid duct with uniform mean flow. The new achievements are then tested on three-dimensional annular-benchmark configurations and compared with three-dimensional lifting-surface models and three-dimensional Euler linearized codes available in the literature. The accuracy of the model is shown for high hub-to-tip ratio cases. When used as such in a true rectilinear-cascade configuration, it also reproduces the exact radiated field that can be derived directly. For low hub-to-tip ratio configurations, the model departs from three-dimensional computations, both regarding the blade loading and the acoustic radiation. A correction is proposed to account for the actual annular dispersion relation in the rectilinear-cascade response function. The results suggest that the proposed correction is necessary to get closer to the underlying physics of the annular-space wave equation, but that it is yet not sufficient to fully reproduce three-dimensional results.     

Simulation of unsteady combustion in a solid-propellant rocket motor

Various regimes of combustion in end-burning-grain solid-propellant rocket motors were examined within the framework of the phenomenological theory of unsteady combustion. A system of equations capable of describing the interaction between the process of burning and acoustic waves was derived. A specific feature of the problem is that its formulation involves two characteristic times: the acoustic time and oscillation amplitude variation time. These characteristic times differ by about three orders of magnitude, a circumstance that requires a high accuracy of calculations. Based on the quadratic approximation in oscillation amplitude, a simpler method for solving the problem was proposed, according to which only the effects associated with the oscillation amplitude variation time are taken into account. Numerical results were obtained for the simplest model of propellant burning, which contains the minimum number of parameters and disregards entropy waves in the combustion products. The steady and unsteady regimes of burning were identified. In the latter case, nonlinear effects may generate shock waves in the combustion chamber.     

Active noise control with linear control source and sensor arrays for a noise barrier

A study is conducted on minimizing the sum of the squared acoustic pressures with a linear array of control sources and a perpendicular linear array of error sensors, placed above the top of a noise barrier. Particular angular orientations, with respect to the center of the barrier top, and spacings of the linear arrays of control sources and error sensors result in moderate to significant additional reduction of the acoustic pressure in the shadow zone. Visual inspection of the sound pressure field, with and without active noise control, found that uniform and significant additional insertion loss can be generated near the barrier. Numerical simulations were conducted to test the proposed method. For separations between control sources and error sensors much less than a quarter wavelength of the primary noise disturbance, results show that the angular orientation, of the combined linear control source and sensor arrays, is a weak factor for acoustic pressure reduction in the shadow zone. Weak angle dependence serves as an advantage to the proposed method, which yields uniform performance for any angular orientation. An angular orientation involving the alignment of the furthest error sensor with the first diffracting edge of the barrier and the primary source was observed to perform well for a variety of frequencies, since the spacing between error sensors and between control sources is of the order of a quarter-wavelength. Improved noise control in the shadow zone of a barrier is achieved by the use of two control sources and angular orientation as mentioned above. Further spatial extension of the area of reduced acoustic pressure is possible by utilizing an increased number of control sources.     

非均匀海洋环境噪声中的匹配场处理舰船辐射噪声级估计方法

在实际舰船辐射噪声测量过程中,受非均匀海洋环境噪声的影响,导致常规测量方法的性能急剧下降,基于此,提出了一种非均匀海洋环境噪声背景中的垂直阵舰船辐射噪声测量方法。根据水声信道传播理论,建立了由海面多个空时独立均匀分布噪声源构成的非均匀背景噪声场模型,推导了非均匀海洋环境噪声场中垂直阵舰船辐射噪声估计的理论公式。针对典型的浅海水声信道,进行计算机仿真实验,分析了该方法的测量性能并与常规匹配场测量方法进行对比,结果表明:(1)该方法能有效克服非均匀海洋环境噪声对测量结果的影响,测量误差较小;(2)相同测量条件下,该方法测量性能优于常规匹配场舰船辐射噪声级测量方法;(3)当信噪比满足一定要求时,测量得到的声源级与实际声源级相比,误差小于1 dB。      

一种拖尾噪声环境中的稳健常数模盲均衡新算法

为了改善拖尾噪声环境中盲均衡器的性能,提出了一种稳健常数模盲均衡算法,修改了原常数模算法的误差函数,并进行非线性变换,有效地抑制了拖尾噪声的影响。采用浅海水声信道,在分别叠加高斯噪声与拖尾噪声的情况下,对算法进行了计算机仿真。结果表明:在高斯噪声环境中,新算法具有更快的收敛速度与更低的剩余码间干扰;在拖尾噪声环境中,原算法很不稳定,而新算法则性能稳健,能够稳定地收敛于较低的剩余码间干扰。     

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. “Transient” cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, “stable” cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble–bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both “stable” and “transient” cavitation bubbles which pulsate nonlinearly with the period of ultrasound.     

Row depth effects on turbulence spectra and acoustic vibrations in tube banks

The effect of row depth on Strouhal numbers derived from peaks in the turbulence spectra measured in an in-line tube bank and on the excitation of acoustic standing waves in the duct containing the bank has been investigated. The results indicate significant variations with bank depth and location in the bank although common features are evident. A buffeting type frequency predicted by Owen [1] is clearly shown beyond the fifth row of deeper banks whilst peaks evident in the turbulence spectra at the front of these banks and for less deep banks are assumed to be generated by vortex action and interaction in the wakes of tube rows. For the geometrical configuration tested, acoustic resonance is generated by the coincidence of a vorticity peak frequency with the standing wave frequency for all cases but the two row deep bank in which the excitation source was most probably in the wake of the bank. Finally, a modification of Owen's theory yields an equation which predicts a Strouhal number of the correct order.     

Examination of mode shapes in an unstable model combustor

The coupling between the fluid dynamics, heat addition, and the acoustics of a combustor system determine whether it is prone toward combustion instability. This paper presents results from a benchmark study of the eigenmodes in an unstable experimental combustor. The axisymmetric combustor configuration is representative of a number of practical systems and comprises an injector tube, geometric expansion into a combustion chamber, and a short converging nozzle. Instability limit cycle amplitudes ranged from 5% to nearly 50% of the mean 2.2 MPa pressure. Multiple harmonics were measured for the highly unstable cases. The model combustor was designed to provide a fairly comprehensive set of tested effects: sonic vs subsonic inlets; oxidizer tube lengths that were either quarter-wave, half-wave, or off-resonant acoustic equivalents to the combustion chamber; a significant injector mean flow with Ma∼0.4; and a varied combustion chamber length. The measured mode shape data were analyzed and reduced to provide comparison with results from a linearized one-dimensional Euler model, which included the effects of real boundary conditions, entropy generation, area change, and heat and mass addition, but did not include a model for unsteady heat addition. For low-amplitude instabilities, the measured resonance frequencies agreed with those calculated by the model for the injector tube-combustion chamber system. Resonance frequencies for the high-amplitude oscillation cases corresponded to the first longitudinal frequency of the combustion chamber and its integer multiples. Good quantitative agreement was obtained between computed and measured phase difference profiles, and mode envelopes agreed qualitatively. These results provide a basis for subsequent combustion response studies on the effects of unsteady heat addition.     

Three-dimensional analysis of blade force and sound generation for an annular cascade in distorted flows

An unsteady lifting-surface theory for a rotating subsonic annular cascade has been developed to predict the unsteady blade forces and the acoustic power generation caused by interaction of blades with inlet distortions or wakes. Disturbance pressure and velocity fields induced by the rotor blades with fluctuating blade force are expressed in terms of the blade force distribution and kernel functions. The spanwise distribution of the blade force is given as a sum of blade force modes, and the kernel functions are resolved into the corresponding modal components. The sound pressure and intensity are expressed as a sum of acoustic modes, the modal components of which are given in terms of the blade force mode components.Numerical computations have been conducted .for interaction with the external disturbance flows that are sinusoidal in the circumferential direction, but possess a phase skewing in the radial direction. Correlations among the acoustic modes, the blade force modes and the flow patterns of the external disturbance have been investigated. When the predominant acoustic mode is subresonant, the blade force amplitude is reduced by the three-dimensional effect, which is lessened as the frequency increases. At deeply superresonant states, however, the three-dimensional effect upon the spanwise average of the blade force amplitude is small. The generated sound power is effectively reduced by increasing the radial non-uniformity of the external disturbance.     

Ultrasonics without a source: thermal fluctuation correlations at MHz frequencies

Noise generated in an ultrasonic receiver circuit consisting of transducer and amplifier is usually ignored, or treated as a nuisance. Here it is argued that acoustic thermal fluctuations, with displacement amplitudes of 3 fm, contain substantial ultrasonic information. It is shown that the noise autocorrelation function is the waveform that would be obtained in a direct pulse/echo measurement. That thesis is demonstrated in experiments in which direct measurements are compared to correlation functions. The thermal nature of the elastodynamic noise that generates these correlations is confirmed by an absolute measurement of their strength, essentially a measurement of the sample temperature.     

Experimental and numerical flow visualization in a transonic turbine

This paper focuses on the visualization of both experimental and numerical results and presents research in a highly-loaded cold-flow transonic turbine under continuous and engine-representative conditions. Special focus was placed on blade row interaction at app. 10600 rpm. While the first step was the investigation of a single stage machine (stator-rotor), the second step extended the test rig to a one-and-a-half stage configuration (stator-rotor-stator). Measurements were carried out in the transonic test turbine at Graz University of Technology using Particle-Image-Velocimetry and Laser-Doppler-Anemometry. The main results of these experiments are discussed and compared to numerical simulations.