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.

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.

Effects of Acoustic Excitation on a Swirling Diffusion Flame

2010 ◽  
Vol 132 (12) ◽  
Author(s):  
Michael E. Loretero ◽  
Rong F. Huang
Keyword(s):  
Flow Field ◽  
Bluff Body ◽  
Excitation Frequency ◽  
Excitation Amplitude ◽  
Mie Scattering ◽  
Flame Length ◽  
Laser Doppler Velocimeter ◽  
Scattering Method ◽  
Acoustic Excitation ◽  
Characteristic Modes

A swirling double concentric jet is commonly used for nonpremixed gas burner application for safety reasons and to improve the combustion performance. Fuel is generally spurted at the central jet while the annular coflowing air is swirled. They are normally separated by a blockage disk where the bluff-body effects further enhance the recirculation of hot gas at the reaction zone. This paper aims to experimentally investigate the behavior of flame and flow in a double concentric jet combustor when the fuel supply is acoustically driven. Laser-light sheet assisted Mie scattering method has been used to visualize the flow, while the flame lengths were measured by a conventional photography technique. The fluctuating velocity at the jet exit was measured by a two-component laser Doppler velocimeter. Flammability and stability at first fuel tube resonant frequency are reported and discussed. The evolution of flame profile with excitation level is presented and discussed, together with the reduction in flame length. The flame in the unforced reacting axisymmetric wake is classified into three characteristic modes, which are weak swirling flame, lifted flame, and transitional reattached flame. These terms reflect their primary features of flame appearances, and when the acoustic excitation is applied, the flame behaviors change with the excitation frequency and amplitude. Four additional characteristic modes are identified; e.g., at low excitation amplitudes, wrinkling flame with a blue annular film is observed because the excitation induces vortices in the central fuel jet and hence gives rise to the wrinkling of flame. The central jet vortices become larger with the increase in excitation amplitude and thus lead to a wider and shorter flame. If the excitation amplitude is increased above a certain value, the central jet vortices change the rotation direction and pacing with the annular jet vortices. These changes in the flow field induce large turbulent intensity and mixing and therefore make the flame looks blue and short. Further increase in the excitation amplitude would lift the flame because the flow field would be dramatically modified.

High Amplitude Sound Propagation at Low Frequencies in a Flow Duct Lined With Resonators: A Time Domain Solution

1988 ◽  
Vol 110 (4) ◽  
pp. 545-551 ◽  
Author(s):  
A. Cummings ◽  
I.-J. Chang
Keyword(s):  
Time Domain ◽  
Internal Combustion Engines ◽  
Sound Propagation ◽  
Sound Field ◽  
High Amplitude ◽  
Combustion Engines ◽  
Low Frequencies ◽  
The Time Domain ◽  
Flow Duct ◽  
Good Agreement

A quasi one-dimensional analysis of sound transmission in a flow duct lined with an array of nonlinear resonators is described. The solution to the equations describing the sound field and the hydrodynamic flow in the neighborhood of the resonator orifices is performed numerically in the time domain, with the object of properly accounting for the nonlinear interaction between the acoustic field and the resonators. Experimental data are compared to numerical computations in the time domain and generally very good agreement is noted. The method described here may readily be extended for use in the design of exhaust mufflers for internal combustion engines.

scholarly journals Sound diffraction on an elastic spheroidal shell

2021 ◽  
Vol 3 (397) ◽  
pp. 97-114
Author(s):  
A. Kleschev ◽  
Keyword(s):  
Group Velocity ◽  
Planar Waveguide ◽  
Sound Propagation ◽  
Spectral Characteristics ◽  
Sound Field ◽  
Signal Propagation ◽  
Elastic Bottom ◽  
Harmonic Signals ◽  
Scattered Fields ◽  
Sound Diffraction

Object and purpose of research. This paper obtains solutions and performs estimations of characteristics of sound reflection and scattering by ideal and elastic bodies of various shapes (analytical and non-analytical) near media interface, or underwater sonic channel, or in a planar waveguide with a solid elastic bottom. Materials and methods. The harmonic signals are investigated with the method of normal waves based on the phase velocity of signal propagation, and impulse signals related to the energy transfer are studied using the method of real and imaginary sources and scatterers based on the group velocity of propagation. Main results. The scattered sound field is calculated for ideal spheroids (elongated and compressed) at fluid – ideal medium interface. The spectrum of a scattered impulse signal is calculated for a body placed in a sonic channel. First reflected impulses are found for an ideal spheroid in a planar waveguide with anisotropic bottom. Conclusion. In the studies of diffraction characteristics of bodies at media interfaces it was found that the main contribution to scattered field is given by interference of scattered fields rather than interaction of scatterers (real or imaginary). It is shown that at long distances the spectral characteristics of the channel itself have a prevalent role. When impulse sound signals in the planar waveguide are used, it is necessary to apply the method of real and imaginary sources and scatterers based on the group velocity of sound propagation.

The local extinction and the nonlinear behaviors of a premixed methane/air flame under low-frequency acoustic excitation

2020 ◽  
Vol 34 (13) ◽  
pp. 2050138 ◽  
Author(s):  
Yongchao Sun ◽  
Mingbo Sun ◽  
Jiajian Zhu ◽  
Yang Xie ◽  
Hongbo Wang ◽  
...  
Keyword(s):  
High Speed ◽  
Excitation Frequency ◽  
Dominant Frequency ◽  
Local Extinction ◽  
Acoustic Excitation ◽  
Flame Extinction ◽  
Buoyant Force ◽  
Hot Gas ◽  
Nature Frequency ◽  
Production Zone

The local extinction and the nonlinear behavior of a premixed methane/air flame under acoustic excitation are investigated experimentally. High-speed photography and high-speed schlieren imaging are used to investigate the oscillation characteristics of the premixed methane/air flame. The flame structure shows a periodic fluctuation when the acoustic excitation is performed to the flame. The local flame extinction can be observed during the flame evolution process. During the local flame extinction process, the flame is found to be cut into two components, then the downstream one extinguishes shortly. The Particle Image Velocimetry (PIV) results suggest that the lower velocity at the separation point is one of the reasons for the flame local extinction. The flame without the acoustic excitation oscillates with a dominant frequency of 18 Hz, which is shown by the schlieren images to be related to the evolution of the hot gas around the flame driven by the buoyant force. When the acoustic excitation frequency is 100 Hz, the structure of the hot gas is destroyed, meanwhile the amplitude of the nature frequency decreases significantly. The hot gas structure appears regularly with the increasing excitation frequency. As a result, the amplitude of the nature frequency also increases gradually. Proper Orthogonal Decomposition (POD) analysis shows that the dominant frequency of the flame without the acoustic excitation is mainly caused by the evolution of the production zone of the flame and the fluctuation of the flame tip. The evolution of the production zone is driven by the buoyant force, which indicates that the result from POD method is consistent with the conclusion obtained from the high-speed schlieren images. Two dominant modes are obtained when the excitation frequencies are 100 and 200 Hz. The two modes are mainly caused by the process of the local flame extinction and the increasing flame length.

Comparison of Sound Propagation Characteristic between Deep and Shallow Water

2014 ◽  
Vol 577 ◽  
pp. 1198-1201
Author(s):  
Zhang Liang ◽  
Chun Xia Meng ◽  
Hai Tao Xiao
Keyword(s):  
Shallow Water ◽  
Deep Water ◽  
Sound Speed ◽  
Sound Propagation ◽  
Spherical Wave ◽  
Sound Field ◽  
Propagation Loss ◽  
Speed Profile ◽  
Sound Speed Profile ◽  
Propagation Law

The physical characteristics are compared between shallow and deep water, in physics and acoustics, respectively. There is a specific sound speed profile in deep water, which is different from which in shallow water, resulting in different sound propagation law between them. In this paper, the sound field distributions are simulated under respective typical sound speed profile. The color figures of sound intensity are obtained, in which the horizontal ordinate is distance, and the vertical ordinate is depth. Then we can get some important characteristics of sound propagation. The results show that the seabed boundary is an important influence on sound propagation in shallow water, and sound propagation loss in deep water convergent zone is visibly less than which in spherical wave spreading. We can realize the remote probing using the acoustic phenomenon.

Flow Field and Vibration Behavior of Straight Transonic Compressor Cascade due to External Acoustic Excitation

2013 ◽  
Author(s):  
Manlu Li ◽  
Anping Hou ◽  
Xiaodong Yang ◽  
Mingming Zhang ◽  
Peng Wang
Keyword(s):  
Flow Field ◽  
Excitation Frequency ◽  
Experimental Studies ◽  
Attack Angle ◽  
Mode Shapes ◽  
Acoustic Excitation ◽  
Fe Model ◽  
Compressor Cascade ◽  
Time Step ◽  
Vibration Behavior

A fluid-structure coupled approach is utilized to study the influence of external acoustic excitation on straight compressor cascade flow field and blade vibration behavior. Interaction between fluid and structure are dealt with in a coupled manner, based on the interface exchange of information between the aerodynamic and structural model. The computation fluid mesh is updated at every time step with an improved algebraic method. The flow field of cascade with/without external acoustic excitation is carried out using a 3D unsteady CFD model based on moving boundary way, as well as some experimental studies based on transonic wind tunnel. Then coupled with blade FE model, mode shapes, frequencies, vibration stress and the structural deformations of blade are identified. The performance of the cascade is obtained by computational and experimental ways, consistency of numerical and test results shows that the numerical model is suitable. The numerical results show that acoustic excitation has a greater impact on negative and designed attack angle in contrast to high positive attack angle. The cascade wake and blade surface pressure frequency characteristic are changed and the main frequency is almost the same as the acoustic excitation frequency. Compared results with no excitation, the vibration characteristics of the blade is changed, also the vibration behavior is sensitive to the excitation amplitude and frequency.

scholarly journals Frequency-dependent variation in the two-dimensional beam pattern of an echolocating dolphin

2011 ◽  
Vol 7 (6) ◽  
pp. 836-839 ◽  
Author(s):  
Josefin Starkhammar ◽  
Patrick W. Moore ◽  
Lois Talmadge ◽  
Dorian S. Houser
Keyword(s):  
Sound Propagation ◽  
Sound Field ◽  
Spatial Separation ◽  
Acoustic Properties ◽  
Maximum Pressure ◽  
Frequency Bandwidth ◽  
Sound Sources ◽  
Pressure Peak ◽  
Prey Localization ◽  
Dominant Peak

Recent recordings of dolphin echolocation using a dense array of hydrophones suggest that the echolocation beam is dynamic and can at times consist of a single dominant peak, while at other times it consists of forward projected primary and secondary peaks with similar energy, partially overlapping in space and frequency bandwidth. The spatial separation of the peaks provides an area in front of the dolphin, where the spectral magnitude slopes drop off quickly for certain frequency bands. This region is potentially used to optimize prey localization by directing the maximum pressure slope of the echolocation beam at the target, rather than the maximum pressure peak. The dolphin was able to steer the beam horizontally to a greater extent than previously described. The complex and dynamic sound field generated by the echolocating dolphin may be due to the use of two sets of phonic lips as sound sources, or an unknown complexity in the sound propagation paths or acoustic properties of the forehead tissues of the dolphin.

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