Aeracoustic Measurements

by Mueller, Thomas J.; Blake, William K. (CON); Dougherty, Robert P. (CON); Lynch, Denis (CON); Soderman, Paul T. (CON)

• ISBN13: 9783540417576
• ISBN10: 3540417575
• Format: Hardcover
• Copyright: 2002-09-01
• Publisher: Springer Verlag

Summary

The goal of Aeroacoustic Measurements is to provide a basis for assessing mechanisms of noise generation, and to develop methods of reducing noise to more acceptable levels. However, the measurements themselves are complex, and require a deep understanding of the experimental facility utilized (such as a wind tunnel), measurement instrumentation, and data analysis techniques. In this volume recent advances in the measurement and understanding of aerodynamically generated sound are presented by leaders in the development of new techniques in this field. Both basic and applied problems are covered in detail.

Table of Contents

Preface	p. Vü
Microphone Measurements In and Out of Airstream	p. 1
Introduction	p. 1
Research Objectives (and What to Measure)	p. 2
Wind Tunnel Background Noise Including Flow-Induced Noise	p. 3sw
Drive Fan	p. 4
Self-Noise from Strut-Mounted Microphones	p. 6
Air Ducts	p. 11
Voice Communication in Wind	p. 13
Strut Noise	p. 13
Wall-Mounted Microphones	p. 18
Background Noise from Wall Boundary Layer	p. 22
Open-Jet Background Noise	p. 23
Microphone Placement	p. 24
Directivity	p. 24
Near-Field Effects	p. 25
Background Noise and Microphone Placement	p. 25
Reverberant Field	p. 26
Reflections in a Semi-Anechoic Environment	p. 26
Tonal Sound Interference	p. 27
Random Sound Interference	p. 29
Source Identification by Signal Processing	p. 30
Convection Effects and Doppler Shift	p. 31
Sound Propagation Through Shear Layers in Open-Jet Wind Tunnels	p. 34
Change in Acoustic Propagation Direction by Refraction	p. 34
Change in Sound Pressure Level Caused by Refraction	p. 38
Wave Absorption And Scattering by Shear Layer Turbulence	p. 40
Procedure for Applying Shear Layer Corrections	p. 41
Microphone Corrections at High Frequency	p. 42
Free-Field Response	p. 42
Pressure Response	p. 43
Free-Field Correction	p. 45
Microphone Directional Response	p. 47
Aerodynamic Microphone Forebody Frequency Response and Directivity	p. 47
Scaling, Extrapolation and Flight Simulation	p. 51
Removal of Test Day Effects	p. 51
Scaling From Small Scale to Full Scale	p. 52
Flyover Simulation	p. 53
References	p. 58
Beamforming in Acoustic Testing	p. 62
Nomenclature	p. 62
Introduction	p. 64
Analysis of Wind Tunnel Acoustic Propagation by Geometrical Optics	p. 66
Uniform Flow	p. 66
Amplitude Approximations for Nonuniform Flow	p. 69
Ray Tracing for Travel Time	p. 69
Array Source-Receiver Model	p. 72
Temporal Considerations	p. 72
Distributed Sources	p. 73
Beamforming	p. 75
Microphone Weight Vectors	p. 75
Beamforming Expressions	p. 76
Performance Analysis	p. 76
The Point Spread Function and Sidelobes	p. 77
Effect of Wind Tunnel Walls	p. 79
Reflected Images	p. 80
Sidelobes from Reflected Images	p. 81
Resolution Requirement	p. 81
Removal of Flow Noise	p. 83
Isolation of the Diagonal Elements of the Cross Spectral Matrix	p. 83
Diagonal Elements Are Not Helpful	p. 83
Diagonal Elements Are Harmful	p. 84
Beamforming without the Diagonal	p. 85
Determination of Quantitative Source Spectra by Integrating the Beamform Map	p. 86
Normalizing the Integral to Account for the Width of the Point Spread Function	p. 86
Rejecting Sidelobes with a Threshold	p. 87
he Sidelobes Are Controlled by the Loudest Source	p. 88
The Threshold Excludes Some Real Noise	p. 88
Array Performance May Be less than Optimal	p. 88
Successful Integration	p. 88
Eigenvalue Classification for Quantitative Source Spectra	p. 89
Relationship between Beamforming Weight Vectors and Cross Spectral Matrix Eigenvectors	p. 89
The Array-Centric Definition of Sources	p. 90
Classification	p. 91
Benefits	p. 91
Coherent Sources and Virtual Microphones	p. 92
Array Calibration Using a Speaker	p. 93
Setup	p. 93
The Diagonal Calibration Matrix	p. 94
The Effect of Speaker Calibration for Microphone Position Errors	p. 94
Array Level Calibration	p. 96
Conclusions	p. 96
References	p. 97
3	p. 98
Introduction	p. 98
Justifying the Cost of Aeroacoustic Phased Array Testing	p. 99
An Overview of Aeroacoustic Phased Array Deployment	p. 100
Array Design	p. 100
Array Mount Design and Build	p. 100
Instrumentation Plan	p. 101
Data Acquisition System Configuration	p. 101
Installation	p. 101
Array Calibration	p. 101
Testing	p. 102
Tear Down	p. 102
Non-intrusive Aeroacoustic Array Measurement	p. 102
Broadband Array Design	p. 103
Background	p. 103
Beamforming	p. 104
Evaluating Array Performance	p. 108
Array Resolution	p. 109
Spatial Aliasing	p. 112
Array Design Strategy	p. 114
Random Array Theory	p. 117
Aperiodic Array Design and The Coarray	p. 117
Spiral Arrays	p. 119
Other Array Design Strategies	p. 128
Designing Arrays for Existing Facilities	p. 128
Array Design Process	p. 129
What to Do in Difficult Situations	p. 131
Array Construction and Installation	p. 141
Panel Strength Requirements	p. 142
Simultaneous Measurement Considerations	p. 142
SensorMounting	p. 144
Calibration Requirements	p. 144
Traverse Requirements	p. 145
Nested Arrays	p. 149
Cable Strain Relief	p. 149
Heating to Avoid Potential Condensation Problems	p. 151
Geometric Survey Considerations	p. 154
Recessed Array Considerations	p. 154
Laminar Flow Control	p. 155
Array Cover	p. 155
Sensor Location Identification	p. 155
Hole Location Accuracy	p. 156
Hole Plugs	p. 158
Instrumentation	p. 158
Sensors	p. 159
Signal Conditioning	p. 175
Instrumentation Setup and Checkout	p. 178
Phased Array Data Acquisition	p. 179
Data Acquisition System Requirements	p. 179
Data Acquisition System Architecture	p. 186
Acquiring the Data	p. 193
Data Management	p. 196
Array Calibration	p. 199
Calibration Enclosure	p. 200
Array Calibration Source Requirements	p. 201
Calibration Source Evaluation	p. 201
Locating the Calibration Source in the Wind Tunnel - Geometric Survey Techniques	p. 202
Calibration Data Acquisition	p. 204
Evaluating Array Calibration Goodness	p. 205
Known Sources	p. 205
Multiple Array Calibration	p. 206
Phased Array Data Reduction	p. 208
The cross-spectral matrix	p. 208
Use of Parallel Processing	p. 209
Special Considerations for Pressurized Wind Tunnel Testing	p. 211
Pressurization of Instrumentation	p. 211
Special Instrumentation Configuration Requirements	p. 211
Reference Microphone Electrostatic Response Calibration Under Pressure	p. 212
Tunnel Operation	p. 213
Acoustic Phased Array Testing in Conjunction with Traditional Aerodynamic Test Techniques	p. 213
Beyond Aeroacoustic Phased Array Measurements in Low Speed Wind Tunnels	p. 214
References	p. 215
4	p. 218
Nomenclature	p. 218
Introduction	p. 219
Mathematical Definitions	p. 220
Cross Spectral Analysis of Linear Systems	p. 222
Application to Aeroacoustic Applications: The Correlation Volume, Correlation Area, and Correlation Time	p. 225
Examples ofCorrelations	p. 228
Correlation of Fluid Motion Variables	p. 228
Correlation of Acoustic Pressure	p. 229
References	p. 256
An Anechoic Facility for Basic Aeroacoustic Research	p. 258
Nomenclature	p. 258
Introduction	p. 259
The Design of an Anechoic Wind Tunnel Facility	p. 260
The Anechoic Room	p. 260
Wind Tunnel Design Criteria	p. 261
Inlet Section	p. 262
Diffuser Components	p. 264
Diffuser Materials	p. 266
Wind Tunnel Drive System	p. 268
Analysis of the Anechoic Wind Tunnel Facility Performance	p. 268
Acoustic Calibration	p. 269
Aerodynamic Calibration	p. 270
Design Summary	p. 274
Propeller Response to Inflow Distortions	p. 275
Background	p. 275
Initial Measurements and Analysis	p. 276
Development of Unsteady Surface Pressure Sensors for Model Propellers	p. 278
Unsteady Pressure Measurements Using a Single, Thin Airfoil	p. 279
Experimental Characterization of Grid-Generated Turbulence, Including the Aeroacoustic Response of a Downstream Propeller	p. 280
Unsteady Pressure Measurements of a Four-Bladed Propeller Ingesting Turbulence	p. 281
Summary	p. 306
References	p. 306
Authors	p. 309
Biographical Sketch of Authors	p. 311
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