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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