An Acoustic Excitation System for the Generation of Turbomachinery Specific Sound Fields: Part I — Design and Methodology

2016 ◽  
Author(s):  
Akif Mumcu ◽  
Christian Keller ◽  
C. Mandanna Hurfar ◽  
Joerg R. Seume
Keyword(s):  
Sound Propagation ◽  
Microphone Array ◽  
Sound Field ◽  
Radial Mode ◽  
Mode Analysis ◽  
Acoustic Excitation ◽  
Axial Distribution ◽  
Excitation System ◽  
Sound Fields ◽  
Duct Wall

A strong focus in the development of modern aircraft engines is the reduction of the engine tonal core noise. For the development of efficient noise reduction techniques, a detailed understanding of the sound transmission throughout all turbomachinery components of the engine is mandatory. In this paper an excitation system is developed to generate turbomachinery-specific sound fields by controlling their circumferential and radial mode order. The excitation system consists of two rows of eight loudspeakers distributed circumferentially around the outer duct wall. This paper gives a detailed description of the analytically- and numerically-supported design methodology of an optimized excitation system, as well as an optimized microphone array mounted flush with the outer duct wall. A sensitivity analysis of the loudspeaker array and of the microphone array with respect to distance and frequency is then carried out numerically. To analyze the microphone signals and to deconstruct the propagating sound field into its modal components, a Radial Mode Analysis (RMA) is carried out. To ensure high-quality RMA results, the axial distribution of the microphones is optimized with respect to the condition number of the array’s transfer matrix. The procedure explained in this paper shall help guide the development of acoustic excitation and microphone array systems for experiments to better understand sound propagation in turbomachinery and flow ducts.

Synthetic Sound Source Generation for Acoustical Measurements in Turbomachines

2013 ◽  
Author(s):  
Michael Bartelt ◽  
Juan D. Laguna ◽  
Joerg R. Seume
Keyword(s):  
Wind Tunnel ◽  
Sound Source ◽  
Sound Propagation ◽  
Microphone Array ◽  
Sound Field ◽  
Acoustic Mode ◽  
Acoustic Excitation ◽  
Noise Emission ◽  
Noise Sources ◽  
Sound Fields

One of the greatest challenges in modern aircraft propulsion design is the reduction of the engine noise emission in order to develop quieter aircrafts. In the course of a current research project, the sound transport in low pressure turbines is investigated. For the corresponding experimental measurements, a specific acoustic excitation system is developed which can be implemented into the inlet of a turbine test rig and into an aeroacoustic wind tunnel. This allows for an acoustic mode generation and a synthesis of various sound source patterns to simulate typical turbomachinery noise sources such as rotor-stator interaction, etc. The paper presents the acoustical and technical design methodology in detail and addresses the experimental options of the system. Particular attention is paid to the design and the numerical optimization of the acoustic excitation units. To validate the sound generator during operation, measurements are performed in an aeroacoustic wind tunnel. For this purpose, an in-duct microphone array with a specific beamforming algorithm for hard-walled ducts is developed and applied to identify the source locations. The synthetically excited sound fields and the propagating acoustic modes are measured and analyzed by means of modal decomposition techniques. The measurement principles and the results are discussed in detail and it is shown that the intended sound source is produced and the intended sound field is excited. This paper shall contribute to help guide the development of excitation systems for aeroacoustic experiments to better understanding the physics of sound propagation within turbomachines.

Impact of Swirl on the Sensitivity of the Radial Mode Analysis in Turbomachinery

2013 ◽  
Author(s):  
Juan D. Laguna ◽  
Michael Bartelt ◽  
Joerg R. Seume
Keyword(s):  
Sensitivity Analysis ◽  
Measurement Errors ◽  
Sound Propagation ◽  
Swirling Flow ◽  
Signal To Noise Ratio ◽  
Sound Field ◽  
Radial Mode ◽  
Mode Analysis ◽  
Mean Flow ◽  
Sound Structure

Sound measurements in turbomachinery are a prerequisite for the study and consequent understanding of sound propagation mechanisms. For analyzing these measurements, the Radial Mode Analysis (RMA) is applied. This method decomposes the transmitted sound field in dominant acoustical modes at specific frequencies. Before an experimental campaign is carried out, measurement parameters are selected such that the uncertainty in the results from the application of the RMA is minimized. In order to minimize uncertainties, a sensitivity analysis of the parameters which influence the overall error of the RMA is performed. This analysis focuses mainly on the output of a measurable quantity, namely on the propagating mode amplitudes. Using a numerical simulation, modal structures are generated based upon real turbine operating data with swirling flow and a characteristic operating temperature. The swirling flow is generated by adding an axial vortex to a constant flow-velocity profile. The results show that the sound field varies under consideration of swirling mean flow compared to uniform flow conditions. In the present case, higher-order modes dominate the propagating sound structure. The parameters studied for assessing the sensitivity are the signal-to-noise ratio of the measurement sensors, the number of triggered revolutions, the azimuthal spacing of the sensors, and a triggering delay. The sensitivity analysis gives a detailed insight into the measurement parameters influencing the output of the RMA, e.g. that small triggering delays cause appreciable measurement errors. This knowledge is used to define the requirements for high fidelity measurements.

An Acoustic Excitation System for the Generation of Turbomachinery Specific Sound Fields: Part II — Experimental Verification

2016 ◽  
Author(s):  
C. Mandanna Hurfar ◽  
Christian Keller ◽  
Akif Mumcu ◽  
Joerg R. Seume
Keyword(s):  
Sound Propagation ◽  
Excitation Frequency ◽  
Sound Field ◽  
Wide Frequency Range ◽  
Acoustic Excitation ◽  
Excitation System ◽  
Stable Mode ◽  
Resonance Effects ◽  
Mode Order ◽  
Sound Generator

A detailed understanding of the sound propagation and transmission within the engine and adjacent ducts is mandatory for the development of efficient noise reduction techniques for the tonal sound field produced by the turbomachinery components of aircraft engines. For this purpose, experimental acoustic investigations are needed. In the first part of this paper, an acoustic excitation system for the generation of acoustic spinning modes with circumferential mode order one and varying radial mode order, as well as a microphone array optimized for a radial decomposition of the sound field have been systematically designed. To verify the excitation method and the design of the excitation system, corresponding experimental measurements are carried out in an acoustic wind tunnel. Amongst others, the sound power of the specific excited acoustic modes of order (1,0) and (1,1) are compared with the respective powers achieved with a non-specific sound field excitation. To test the range of flexible use of the sound generator, measurements are carried out over a wide frequency range. It is shown that the intended modes can be controlled at the design frequency of the sound generator as well as off-design frequencies. However, the dominance of the excited modes strongly depends on the number of cut-on modes and the excitation frequency as non-linear resonance effects may interfere. Furthermore, the benefit of an increased number of loudspeaker rows for stable mode excitation is discussed. The experimental results are supported by numerical simulations.

THE METHOD OF SEQUENTIAL TRANSFORMATION OF THE SOUND FIELDS

Akustika ◽  
2021 ◽  
pp. 141
Author(s):  
Nickolay Ivanov ◽  
Gennady Kurtsev ◽  
Aleksandr Shashurin
Keyword(s):  
Noise Level ◽  
Sound Absorption ◽  
Sound Propagation ◽  
Sound Field ◽  
Structural Elements ◽  
Noise And Vibration ◽  
Main Assumption ◽  
Efficiency Calculation ◽  
Sound Fields ◽  
Sequential Transformation

A rule for describing the sequential transformation of the sound fields when properties of the surfaces or structural elements change due to such basic processes as sound absorption, reflection, diffraction, or sound divergence is proposed. The main assumption is that sound fields are non-coherent, i.e., resonant phenomena and sound interference are not considered. The examples show solutions to such problems: - sound propagation in space if there are artificial structures; - sound propagation in the rooms; - efficiency calculation of the noise protection structures; - calculation of the expected noise level of the machinery and separation of the contribution of noise and vibration sources to sound fields (for example, an external sound field, a sound field in the office, etc.)

Reconstruction of Interior Sound Fields of Vibrating Shells with an Open Spherical Microphone Array

2016 ◽  
Vol 24 (04) ◽  
pp. 1650013 ◽  
Author(s):  
Minzong Li ◽  
Huancai Lu
Keyword(s):  
Cross Validation ◽  
Microphone Array ◽  
Sound Field ◽  
Reconstruction Error ◽  
Wave Number ◽  
Tikhonov Regularization Method ◽  
Sound Fields ◽  
Noise Regularization ◽  
Enclosed Space ◽  
The Impact

Spherical acoustic holography was utilized to reconstruct the interior sound field of an enclosed space with vibrating boundaries using an open spherical microphone array. The interior sound fields of vibrating shells, including a pulsating shell, a [Formula: see text]-axis oriented oscillating shell, a partially vibrating shell and a point-excited vibrating shell, were reconstructed, and numerical simulations were carried out to examine the impact of reconstruction parameters, the radius of the microphone array, the number of microphones, the distribution of microphones on the array surface, the wave number, the number of basis functions used, and the radius of the reconstruction surface on the accuracy of reconstruction. In order to minimize the error of reconstruction caused by a variety of factors and uncertainties, such as the measurement noise, regularization treatments were introduced into the process of reconstructing, to suppress the divergent trends of the reconstruction error along with the increase of the wave number and the increase of the radius of the reconstruction surface. Results showed that a Tikhonov regularization method with generalized cross validation (GCV) could yield the least error of reconstruction among the investigated regularization methods.

scholarly journals Numerical and Experimental Studies on Inclined Incidence Parametric Sound Propagation

2019 ◽  
Vol 2019 ◽  
pp. 1-10 ◽  
Author(s):  
Haisen Li ◽  
Jingxin Ma ◽  
Jianjun Zhu ◽  
Baowei Chen
Keyword(s):  
Sound Propagation ◽  
Experimental Studies ◽  
Sound Field ◽  
Source Model ◽  
Calculation Model ◽  
Theoretical Research ◽  
Field Calculation ◽  
Sound Beam ◽  
Sound Fields ◽  
Kzk Equation

The Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation has been widely used in the simulation and calculation of nonlinear sound fields. However, the accuracy of KZK equation reduced due to the deflection of the direction of the sound beam when the sound beam is inclined incidence. In this paper, an equivalent sound source model is proposed to make the calculation direction of KZK calculation model consistent with the sound propagation direction after acoustic refraction, so as to improve the accuracy of sound field calculation under the inclined incident conditions. The theoretical research and pool experiment verify the feasibility and effectiveness of the proposed method.

On the Acoustics of Auditoria

1994 ◽  
Vol 1 (1) ◽  
pp. 27-48 ◽  
Author(s):  
H. Kuttruff
Keyword(s):  
Sound Propagation ◽  
Sound Field ◽  
Great Benefit ◽  
Room Acoustics ◽  
Short Introduction ◽  
New Developments ◽  
Sound Fields ◽  
Field Simulation ◽  
Basic Facts

The paper presents a short introduction into auditorium acoustics and reports on a few new developments in this field, which are believed to be of great benefit both for the acoustical design of auditoria and for research in practical room acoustics. The first part describes in a rather elementary way the basic facts of sound propagation in enclosures, including the effects of reflections and the role of reverberation. Furthermore, some of the numerous objective parameters are discussed which have been introduced in order to characterize particular aspects of sound fields. In the second part, recently developed methods of sound field simulation are described by which such parameters can be predicted. Methods of “auralization” are briefly discussed by which aural impressions from non-existing halls can be created on the basis of digital sound field simulation.

Technical Note: Prediction of the Characteristics of Complicated Sound Fields in Long Enclosures by a Beam-Tracing Method

2002 ◽  
Vol 9 (2) ◽  
pp. 139-150 ◽  
Author(s):  
Xiangyang Zeng ◽  
Jincai Sun ◽  
Ke'an Chen
Keyword(s):  
Sound Pressure ◽  
Sound Propagation ◽  
Sound Field ◽  
Technical Note ◽  
Pressure Level ◽  
Reverberation Time ◽  
Complex Sound ◽  
Sound Fields ◽  
Sound Receiver ◽  
The Subject

The subject of this paper is the characterisation of the sound field in long enclosures. A beam-tracing computer model has been developed especially for the simulation of sound propagation throughout long enclosures. Surface diffusing reflection and air absorption are included in the model, which can predict the impulse response and acoustic indexes at arbitrary positions in the enclosure. This paper describes how the algorithm models the sound source, sound propagation and sound receiver. The algorithm was then tested in both common rooms and long enclosures by comparison of the measurement, theoretical calculation and prediction results. The characteristics of more complex sound fields in long enclosures, the prediction of reverberation time, early decay time and sound pressure level, etc, at individual points are discussed in terms of the algorithm. The results indicate that the primary characteristics of complicated sound fields in non-rectangular long enclosures are similar to those in rectangular ones.

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