ON THE MODELING AND DESIGN OF ZERO-NET MASS FLUX ACTUATORS
By
QUENTIN GALLAS
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2005
Copyright 2005
by
Quentin Gallas
Pour ma famille et mes amis, d’ici et de là-bas… (To my family and friends, from here and over there…)
ACKNOWLEDGMENTS
Financial support for the research project was provided by a NASA-Langley
Research Center Grant and an AFOSR grant. First, I would like to thank my advisor, Dr.
Louis N. Cattafesta. His continual guidance and support gave me the motivation and encouragement that made this work possible. I would also like to express my gratitude especially to Dr. Mark Sheplak, and to the other members of my committee (Dr. Bruce
Carroll, Dr. Bhavani Sankar, and Dr. Toshikazu Nishida) for advising and guiding me with various aspects of this project. I thank the members of the Interdisciplinary
Microsystems group and of the Mechanical and Aerospace Engineering department
(particularly fellow student Ryan Holman) for their help with my research and their friendship. I thank everyone who contributed in a small but significant way to this work.
I also thank Dr. Rajat Mittal (George Washington University) and his student Reni Raju, who greatly helped me with the computational part of this work.
Finally, special thanks go to my family and friends, from the States and from
France, for always encouraging me to pursue my interests and for making that pursuit possible.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...... iv
LIST OF TABLES...... ix
LIST OF FIGURES ...... xi
LIST OF SYMBOLS AND ABBREVIATIONS ...... xix
ABSTRACT...... xxvi
CHAPTER
1 INTRODUCTION ...... 1
Motivation...... 1 Overview of a Zero-Net Mass Flux Actuator...... 3 Literature Review ...... 7 Isolated Zero-Net Mass Flux Devices ...... 7 Applications ...... 8 Modeling approaches ...... 11 Zero-Net Mass Flux Devices with the Addition of Crossflow...... 15 Fluid dynamic applications ...... 16 Aeroacoustics applications...... 18 Modeling approaches ...... 19 Unresolved Technical Issues ...... 25 Objectives ...... 27 Approach and Outline of Thesis...... 28
2 DYNAMICS OF ISOLATED ZERO-NET MASS FLUX ACTUATORS ...... 30
Characterization and Parameter Definitions...... 31 Lumped Element Modeling ...... 34 Summary of Previous Work ...... 34 Limitations and Extensions of Existing Model ...... 38 Dimensional Analysis...... 44 Definition and Discussion ...... 44 Dimensionless Linear Transfer Function for a Generic Driver...... 46
v
Modeling Issues...... 51 Cavity Effect...... 51 Orifice Effect...... 52 Lumped element modeling in the time domain...... 52 Loss mechanism ...... 61 Driving-Transducer Effect...... 63 Test Matrix...... 69
3 EXPERIMENTAL SETUP...... 72
Experimental Setup...... 72 Cavity Pressure...... 75 Diaphragm Deflection ...... 76 Velocity Measurement...... 79 Data-Acquisition System...... 82 Data Processing ...... 85 Fourier Series Decomposition ...... 92 Flow Visualization...... 97
4 RESULTS: ORIFICE FLOW PHYSICS...... 99
Local Flow Field...... 100 Velocity Profile through the Orifice: Numerical Results...... 100 Exit Velocity Profile: Experimental Results ...... 109 Jet Formation...... 116 Influence of Governing Parameters...... 118 Empirical Nonlinear Threshold ...... 119 Strouhal, Reynolds, and Stokes Numbers versus Pressure Loss...... 121 Nonlinear Mechanisms in a ZNMF Actuator ...... 128
5 RESULTS: CAVITY INVESTIGATION...... 137
Cavity Pressure Field...... 137 Experimental Results...... 138 Numerical Simulation Results...... 141 Computational fluid dynamics ...... 142 Femlab...... 147 Compressibility of the Cavity...... 150 LEM-Based Analysis...... 151 Experimental Results...... 156 Driver, Cavity, and Orifice Volume Velocities...... 162
6 REDUCED-ORDER MODEL OF ISOLATED ZNMF ACTUATOR...... 171
Orifice Pressure Drop ...... 171 Control Volume Analysis...... 172 Validation through Numerical Results ...... 175
vi Discussion: Orifice Flow Physics...... 181 Development of Approximate Scaling Laws ...... 188 Experimental results...... 188 Nonlinear pressure loss correlation ...... 194 Refined Lumped Element Model...... 198 Implementation...... 198 Comparison with Experimental Data ...... 202
7 ZERO-NET MASS FLUX ACTUATOR INTERACTING WITH AN EXTERNAL BOUNDARY LAYER ...... 211
On the Influence of Grazing Flow...... 211 Dimensional Analysis...... 218 Reduced-Order Models...... 223 Lumped Element Modeling-Based Semi-Empirical Model of the External Boundary Layer ...... 224 Definition ...... 224 Boundary layer impedance implementation in Helmholtz resonators ...... 229 Boundary layer impedance implementation in ZNMF actuator...... 238 Velocity Profile Scaling Laws...... 241 Scaling law based on the jet exit velocity profile...... 244 Scaling law based on the jet exit integral parameters ...... 261 Validation and Application ...... 270
8 CONCLUSIONS AND FUTURE WORK...... 273
Conclusions...... 273 Recommendations for Future Research...... 276 Need in Extracting Specific Quantities ...... 276 Proper Orthogonal Decomposition...... 277 Boundary Layer Impedance Characterization...... 279 MEMS Scale Implementation ...... 280 Design Synthesis Problem...... 282
APPENDIX
A EXAMPLES OF GRAZING FLOW MODELS PAST HELMHOLTZ RESONATORS ...... 283
B ON THE NATURAL FREQUENCY OF A HELMHOLTZ RESONATOR ...... 291
C DERIVATION OF THE ORIFICE IMPEDANCE OF AN OSCILLATING PRESSURE DRIVEN CHANNEL FLOW...... 295
D NON-DIMENSIONALIZATION OF A ZNMF ACTUATOR ...... 303
E NON-DIMENSIONALIZATION OF A PIEZOELECTRIC-DRIVEN ZNMF ACTUATOR WITHOUT CROSSFLOW...... 312
vii F NUMERICAL METHODOLOGY ...... 326
G EXPERIMENTAL RESULTS: POWER ANALYSIS ...... 331
LIST OF REFERENCES...... 348
BIOGRAPHICAL SKETCH ...... 359
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LIST OF TABLES
Table page
2-1 Correspondence between synthetic jet parameter definitions...... 34
2-2 Dimensional parameters for circular and rectangular orifices...... 49
2-3 Test matrix for ZNMF actuator in quiescent medium ...... 69
3-1 ZNMF device characteristic dimensions used in Test 1 ...... 75
3-2 LDV measurement details...... 82
3-3 Repeatability in the experimental results...... 92
4-1 Ratio of the diffusive to convective time scales ...... 109
5-1 Cavity volume effect on the device frequency response for Case 1 (Gallas et al.) from the LEM prediction...... 153
5-2 Cavity volume effect on the device frequency response for Case 1 (CFDVal) from the LEM prediction...... 154
5-3 ZNMF device characteristic dimensions used in Test 2 ...... 156
5-4 Effect of the cavity volume decrease on the ZNMF actuator frequency response for Cases A, B, C, and D...... 157
7-1 List of configurations used for impedance tube simulations used in Choudhari et al...... 216
7-2 Experimental operating conditions from Hersh and Walker...... 230
7-3 Experimental operating conditions from Jing et al...... 236
7-4 Tests cases from numerical simulations used in the development of the velocity profiles scaling laws...... 242
7-5 Coefficients of the nonlinear least square fits on the decomposed jet velocity profile...... 254
ix 7-6 Results from the nonlinear regression analysis for the velocity profile based scaling law ...... 259
7-7 Results for the parameters a, b and c from the nonlinear system ...... 265
7-8 Integral parameters results ...... 266
7-9 Results from the nonlinear regression analysis for the integral parameters based velocity profile...... 267
A-1 Experimental database for grazing flow impedance models ...... 290
B-1 Calculation of Helmholtz resonator frequency...... 293
D-1 Dimensional matrix of parameter variables for the isolated actuator case...... 304
D-2 Dimensional matrix of parameter variables for the general case...... 308
E-1 Dimensional matrix of parameter variables...... 314
G-1 Power in the experimental time data...... 332
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LIST OF FIGURES
Figure page
1-1 Schematic of typical zero-net mass flux devices interacting with a boundary layer, showing three different types of excitation mechanisms...... 4
1-2 Orifice geometry...... 5
1-3 Helmholtz resonators arrays...... 6
2-1 Equivalent circuit model of a piezoelectric-driven synthetic jet actuator...... 35
2-2 Comparison between the lumped element model and experimental frequency response measured using phase-locked LDV for two prototypical synthetic jets. ...37
2-3 Comparison between the lumped element model (—) and experimental frequency response measured using phase-locked LDV (¡) for four prototypical synthetic jets...... 41
2-4 Variation in velocity profile vs. S = 1, 12, 20, and 50 for oscillatory pipe flow in a circular duct...... 42
2-5 Ratio of spatial average velocity to centerline velocity vs. Stokes number for oscillatory pipe flow in a circular duct...... 43
2-6 Schematic representation of a generic-driver ZNMF actuator...... 47
2-7 Bode diagram of the second order system given by Eq. 2-20, for different damping ratio...... 48
2-8 Coordinate system and sign convention definition in a ZNMF actuator...... 53
2-9 Geometry of the piezoelectric-driven ZNMF actuator from Case 1 (CFDVal)...... 55
2-10 Geometry of the piston-driven ZNMF actuator from Case 2 (CFDVal)...... 55
2-11 Time signals of the jet orifice velocity, pressure across the orifice, and driver displacement during one cycle for Case 1...... 57
2-12 Time signals of the jet orifice velocity, pressure across the orifice and driver displacement during one cycle for Case 2...... 58
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2-13 Numerical results of the time signals for A) pressure drop and B) velocity perturbation at selected locations along the resonator orifice...... 59
2-14 Schematic of the different flow regions inside a ZNMF actuator orifice...... 62
2-15 Equivalent two-port circuit representation of piezoelectric transduction...... 64
2-16 Speaker-driven ZNMF actuator...... 66
2-17 Schematic of a shaker-driven ZNMF actuator, showing the vent channel between the two sealed cavities...... 67
2-18 Circuit representation of a shaker-driven ZNMF actuator...... 68
3-1 Schematic of the experimental setup for phase-locked cavity pressure, diaphragm deflection and off-axis, two-component LDV measurements...... 73
3-2 Exploded view of the modular piezoelectric-driven ZNMF actuator used in the experimental test...... 73
3-3 Schematic (to scale) of the location of the two 1/8” microphones inside the ZNMF actuator cavity...... 76
3-4 Laser displacement sensor apparatus to measure the diaphragm deflection with sign convention...... 77
3-5 Diaphragm mode shape comparison between linear model and experimental data at three test conditions...... 79
3-6 LDV 3-beam optical configuration...... 80
3-7 Flow chart of measurement setup...... 83
3-8 Phase-locked signals acquired from the DSA card, showing the normalized trigger signal, displacement signal, pressure signals and excitation signal...... 84
3-9 Percentage error in Error! Objects cannot be created from editing field codes. from simulated LDV data at different signal to noise ratio, using 8192 samples.....87
3-10 Phase-locked velocity profiles and corresponding volume flow rate acquired with LDV for Case 14...... 89
3-11 Noise floor in the microphone measurements compared with Case 52...... 91
3-12 Normalized quantities vs. phase angle...... 93
3-13 Power spectrum of the two pressure recorded and the diaphragm displacement. ...95
3-14 Schematic of the flow visualization setup...... 97
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4-1 Numerical results of the orifice flow pattern showing axial and longitudinal velocities, azimuthal vorticity contours, and instantaneous streamlines at the time of maximum expulsion...... 101
4-2 Velocity profile at different locations inside the orifice for Case 1...... 103
4-3 Velocity profile at different locations inside the orifice for Case 2...... 105
4-4 Velocity profile at different locations inside the orifice for Case 3...... 106
4-5 Vertical velocity contours inside the orifice during the time of maximum expulsion...... 107
4-6 Experimental vertical velocity profiles across the orifice for a ZNMF actuator in quiescent medium at different instant in time for Case 71...... 110
4-7 Experimental vertical velocity profiles across the orifice for a ZNMF actuator in quiescent medium at different instant in time for Case 43...... 111
4-8 Experimental vertical velocity profiles across the orifice for a ZNMF actuator in quiescent medium at different instant in time for Case 69...... 113
4-9 Experimental vertical velocity profiles across the orifice for a ZNMF actuator in quiescent medium at different instant in time for Case 55...... 114
4-10 Experimental results of the ratio between the time- and spatial-averaged velocity and time-averaged centerline velocity...... 116
4-11 Experimental results on the jet formation criterion...... 118
4-12 Averaged jet velocity vs. pressure fluctuation for different Stokes number...... 120
4-13 Pressure fluctuation normalized by the dynamic pressure based on averaged velocity vs. St⋅ h d ...... 122
4-14 Pressure fluctuation normalized by the dynamic pressure based on averaged velocity vs. Strouhal number...... 123
4-15 Vorticity contours during the maximum expulsion portion of the cycle from numerical simulations...... 124
4-16 Pressure fluctuation normalized by the dynamic pressure based on ingestion time averaged velocity vs. St⋅ h d ...... 125
4-17 Vorticity contours during the maximum ingestion portion of the cycle from numerical simulations...... 126
4-18 Comparison between Case 1 vertical velocity profiles at the orifice ends...... 127
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4-19 Comparison between Case 2 vertical velocity profiles at the orifice ends...... 128
4-20 Comparison between Case 3 vertical velocity profiles at the orifice ends...... 128
4-21 Determination of the validity of the small-signal assumption in a closed cavity. ..131
4-22 Log-log plot of the cavity pressure total harmonic distortion in the experimental time signals...... 132
4-23 Log-log plot of the total harmonic distortion in the experimental time signals vs. Strouhal number as a function of Stokes number...... 134
5-1 Coherent power spectrum of the pressure signal for Cases 9 to 20...... 138
5-2 Phase plot of the normalized pressures taken by microphone 1 versus microphone 2...... 139
5-3 Pressure signals experimentally recorded by microphone 1 and microphone 2 as a function of phase in Case 59...... 140
5-4 Ratio of microphone amplitude (Pa) vs. the inverse of the Strouhal number, for different Stokes number...... 141
5-5 Pressure contours in the cavity and orifice (Case 2) from numerical simulations..143
5-6 Pressure contours in the cavity and orifice (Case 3) from numerical simulations..144
5-7 Cavity pressure probe locations in a ZNMF actuator from numerical simulations...... 145
5-8 Normalized pressure inside the cavity during one cycle at 15 different probe locations from numerical simulation results...... 146
2 5-9 Cavity pressure normalized by ρVj vs. phase from numerical simulations corresponding to the experimental probing locations...... 147
5-10 Contours of pressure phase inside the cavity by numerically solving the 3D wave equation using FEMLAB...... 148
5-11 Cavity pressure vs. phase by solving the 3D wave equation using FEMLAB and corresponding to the experimental probing locations...... 149
5-12 Log-log frequency response plot of Case 1 (Gallas et al.) as the cavity volume is decreased from the LEM prediction...... 153
5-13 Log-log frequency response plot of Case 1 (CFDVal) as the cavity volume is decreased from the LEM prediction...... 154
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5-14 Experimental log-log frequency response plot of a ZNMF actuator as the cavity volume is decreased for a constant input voltage...... 158
5-15 Close-up view of the peak locations in the experimental actuator frequency response as the cavity volume is decreased for a constant input voltage...... 158
5-16 Normalized quantities vs. phase of the jet volume rate, cavity pressure and centerline driver velocity...... 160
5-17 Experimental results of the ratio of the driver to the jet volume velocity function of dimensionless frequency as the cavity volume decreases...... 164
5-18 Experimental jet to driver volume flow rate versus actuation to Helmholtz frequency...... 166
5-19 Current divider representation of a piezoelectric-driven ZNMF actuator...... 168
5-20 Frequency response of the power conservation in a ZNMF actuator from the lumped element model circuit representation for Case 1 (Gallas et al.)...... 169
6-1 Control volume for an unsteady laminar incompressible flow in a circular orifice, from y/h = -1 to y/h = 0...... 172
6-2 Numerical results for the contribution of each term in the integral momentum equation as a function of phase angle during a cycle...... 176
6-3 Definition of the approximation of the orifice entrance velocity from the orifice exit velocity...... 178
6-4 Momentum integral of the exit and inlet velocities normalized by Error! Objects cannot be created from editing field codes. and comparing with the actual and approximated entrance velocity...... 179
6-5 Total momentum integral equation during one cycle, showing the results using the actual and approximated entrance velocity...... 181
6-6 Numerical results of the total shear stress term versus corresponding lumped linear resistance during one cycle...... 183
6-7 Numerical results of the unsteady term versus corresponding lumped linear reactance during one cycle...... 184
6-8 Numerical results of the normalized terms in the integral momentum equation as a function of phase angle during a cycle...... 187
6-9 Comparison between lumped elements from the orifice impedance and analytical terms from the control volume analysis...... 188
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6-10 Experimental results of the orifice pressure drop normalized by the dynamic pressure based on averaged velocity versus St⋅ h d for different Stokes numbers...... 191
6-11 Experimental results of each term contributing in the orifice pressure drop coefficient vs. St⋅ h d ...... 192
6-12 Experimental results of the relative magnitude of each term contributing in the orifice pressure drop coefficient vs. intermediate to low St⋅ h d ...... 193
6-13 Experimental results for the nonlinear pressure loss coefficient for different Stokes number and orifice aspect ratio...... 196
6-14 Nonlinear term of the pressure loss across the orifice as a function of St⋅ h d from experimental data...... 197
6-15 Implementation of the refined LEM technique to compute the jet exit velocity frequency response of an isolated ZNMF actuator...... 201
6-16 Comparison between the experimental data and the prediction of the refined and previous LEM of the impulse response of the jet exit centerline velocity. Actuator design corresponds to Case I from Gallas et al...... 203
6-17 Comparison between the experimental data and the prediction of the refined and previous LEM of the impulse response of the jet exit centerline velocity. Actuator design corresponds to Case II from Gallas et al...... 205
6-18 Comparison between the experimental data and the prediction of the refined and previous LEM of the impulse response of the jet exit centerline velocity. Actuator design is from Gallas and is similar to Cases 41 to 50...... 207
6-19 Comparison between the refined LEM prediction and experimental data of the time signals of the jet volume flow rate...... 209
7-1 Spanwise vorticity plots for three cases where the jet Reynolds number Re is increased...... 212
7-2 Spanwise vorticity plots for three cases where the boundary layer Reynolds number is increased...... 213
7-3 Comparison of the jet exit velocity profile with increasing...... 214
7-4 Pressure contours and streamlines for mean A) inflow, and B) outflow through a resonator in the presence of grazing flow...... 218
7-5 LEM equivalent circuit representation of a generic ZNMF device interacting with a grazing boundary layer...... 224
xvi
7-6 Schematic of an effort divider diagram for a Helmholtz resonator...... 230
7-7 Comparison between BL impedance model and experiments from Hersh and Walker as a function of Mach number for different SPL...... 233
7-8 Experimental setup used in Jing et al...... 236
7-9 Comparison between model and experiments from Jing et al...... 237
7-10 Effect of the freestream Mach number on the frequency response of the ZNMF design from Case 1 (CFDVal) using the refined LEM...... 239
7-11 Effect of the freestream Mach number on the frequency response of the ZNMF design from Case 1 (Gallas et al.)...... 240
7-12 Schematic of the two approaches used to develop the scaling laws from the jet exit velocity profile...... 244
7-13 Methodology for the development of the velocity profile based scaling law...... 245
7-14 Nonlinear least square curve fit on the decomposed jet velocity profile for Case I...... 247
7-15 Nonlinear least square curve fit on the decomposed jet velocity profile for Case III...... 248
7-16 Nonlinear least square curve fit on the decomposed jet velocity profile for Case V...... 249
7-17 Nonlinear least square curve fit on the decomposed jet velocity profile for Case VII...... 250
7-18 Nonlinear least square curve fit on the decomposed jet velocity profile for Case IX...... 251
7-19 Nonlinear least square curve fit on the decomposed jet velocity profile for Case XI...... 252
7-20 Nonlinear least square curve fit on the decomposed jet velocity profile for Case XIII...... 253
7-21 Comparison between CFD velocity profile, decomposed jet velocity profile, and modeled velocity profile, at the orifice exit, for four phase angles during a cycle...... 255
7-22 Test case comparison between CFD data and the scaling law based on the velocity profile at four phase angles during a cycle...... 260
7-23 Methodology for the development of the integral parameters based scaling law...262
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7-24 Comparison between the results of the integral parameters from the scaling law and the CFD data for the test case...... 268
7-25 Example of a practical application of the ZNMF actuator reduced-order model in a numerical simulation of flow past a flat plate...... 271
8-1 POD analysis applied on numerical data for ZNMF actuator with a grazing BL...278
8-2 Use of quarter-wavelength open tube to provide an infinite impedance...... 280
8-3 Representative MEMS ZNMF actuator...... 281
8-4 Predicted output of MEMS ZNMF actuator...... 281
A-1 Acoustic test duct and siren showing a liner panel test configuration...... 285
A-2 Schematic of test apparatus used in Hersh and Walker...... 286
A-3 Apparatus for the measurement of the acoustic impedance of a perforate used by Kirby and Cummings...... 288
A-4 Sketch of NASA Grazing Impedance Tube...... 290
B-1 Helmholtz resonator...... 291
C-1 Rectangular slot geometry and coordinate axis definition...... 295
D-1 Orifice details with coordinate system...... 303
F-1 Schematic of A) the sharp-interface method on a fixed Cartesian mesh, and B) the ZNMF actuator interacting with a grazing flow...... 328
F-2 Typical mesh used for the computations. A) 2D simulation. B) 3D simulation...329
F-3 Example of 2D and 3D numerical results of ZNMF interacting with a grazing boundary layer...... 329
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LIST OF SYMBOLS AND ABBREVIATIONS
c0 isentropic air speed of sound [m/s]
2 2. 4 CaC cavity acoustic compliance = ∀ ρc0 [s m /kg]
2. 4 CaD diaphragm short-circuit acoustic compliance = ∆∀ P [s m /kg] Vac =0
CD orifice discharge coefficient [1]
2 C f skin friction coefficient = τρwj0.5 V [1]
Cµ momentum coefficient during the time of discharge [1]
C n successive moments of jet velocity profile [1] φ12 d orifice diameter [m]
dH hydraulic diameter = 4( area) ( wetted perimeter) [m]
D orifice entrance diameter (facing the cavity) [m]
Dc cavity diameter (for cylindrical cavities) [m] f actuation frequency [Hz]
fd driver natural frequency [Hz] f Helmholtz frequency = 12π cSh′∀ = 12π MMC+ [Hz] H ( ) 0 n ( ()aN aRad aC )
fn natural frequency of the uncontrolled flow [Hz]
f0 fundamental frequency [Hz]
f1 , f2 synthetic jet lowest and highest resonant frequencies, respectively [Hz]
xix
h orifice height [m]
h′ effective length of the orifice = hh+ 0 [m]
h0 “end correction” of the orifice = 0.96 Sn [m]
H cavity depth (m) / boundary layer shape factor = θ δ ∗ [1]
I0 impulse per unit length [1]
-1 k wave number = ω c0 [m ]
Kd nondimensional orifice loss coefficient [1]
L0 stroke length [m]
2π R2 2 4 M aD diaphragm acoustic mass = ρ A ⎡⎤wr() rdr [kg/m ] ∆∀2 ∫0 ⎣⎦
4 M aN orifice acoustic mass due to inertia effect [kg/m ]
4 M aO orifice acoustic mass = MMaN+ aRad [kg/m ]
4 M aRad orifice acoustic radiation mass [kg/m ] p′ acoustic pressure [Pa]
P differential pressure on the diaphragm [Pa]
Pi incident pressure [Pa]
Pw Power [W] q′ acoustic particle volume velocity [m3/s]
3 Qc volume flow rate through the cavity = QQjd− [m /s]
3 Qd volume flow rate displaced by the driver = jω∆∀ [m /s]
3 Qj volume flow rate through the orifice [m /s]
xx
3 Qj time averaged orifice volume flow rate during the expulsion stroke [m /s] r radial coordinate in cylindrical coordinate system [m]
R radius of curvature of the surface [m]
4. RaD diaphragm acoustic resistance = 2ζ M aDC aD [kg/m s]
4. RaN viscous orifice acoustic resistance [kg/m s]
4. RaOlin linear orifice acoustic resistance = RaN [kg/m s]
4. RaOnl nonlinear orifice acoustic resistance [kg/m s]
2. R0 specific resistance [kg/m s]
Re jet Reynolds number = Vdj ν [1] s Laplace variable = jω [rad/s]
S Stokes number = ωd 2 ν [1]
St jet Strouhal number = ωdVj [1]
2 Sc cavity cross sectional area [m ]
2 Sd driver cross sectional area [m ]
2 Sn orifice neck area [m ] u′ acoustic particle velocity [m/s]
ub bias flow velocity through the orifice [m/s]
u∗ wall friction velocity [m/s]
U∞ freestream mean velocity [m/s]
vCL centerline orifice velocity [m/s]
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