ANSYS Mechanical APDL Acoustic Analysis Guide

94 Pages·2013·1.16 MB·English

ANSYS Mechanical APDL Acoustic Analysis
Guide
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Table of Contents
1. Introduction to Acoustic Analysis........................................................................................................... 1
1.1.The General Acoustic Equations........................................................................................................ 1
1.2. Overview of the Acoustic Analysis Process......................................................................................... 1
2. Using the Acoustic Analysis Tools........................................................................................................... 5
2.1. Elements Used in an Acoustic Analysis............................................................................................... 5
2.2. Commands Used in an Acoustic Analysis........................................................................................... 6
2.3. Understanding Acoustic Analysis Terminology.................................................................................. 7
2.4. Acoustic Analysis Resources and Examples........................................................................................ 8
3. Modeling for an Acoustic Analysis.......................................................................................................... 9
4. Defining the Acoustic Modeling Environment..................................................................................... 11
4.1. Defining Element Types.................................................................................................................. 11
4.2. Specifying the System of Units........................................................................................................ 12
5. Defining Acoustic Material Properties.................................................................................................. 15
5.1. Basic Material Parameters of Acoustic Media.................................................................................... 15
5.2. Non-Uniform Ideal Gas Material...................................................................................................... 16
5.3. Equivalent Fluid Model of Perforated Material................................................................................. 16
5.4.Viscous-Thermal Materials............................................................................................................... 18
5.4.1. Acoustic Propagation in the Viscous Fluid............................................................................... 18
5.4.2. Boundary Layer Impedance (BLI) Model.................................................................................. 18
5.4.3. Low Reduced Frequency (LRF) Model..................................................................................... 19
6. Specifying Acoustic Analysis Region Attributes and Meshing ............................................................ 21
7. Applying Boundary Conditions in an Acoustic Analysis....................................................................... 23
7.1. Applying Boundary Conditions....................................................................................................... 23
7.1.1. Pressure Boundary................................................................................................................. 23
7.1.2. Rigid Wall Boundary............................................................................................................... 24
7.1.3. Surface Impedance Boundary................................................................................................. 24
7.1.4. Free Surface (Sloshing Effect).................................................................................................. 25
7.2. Absorbing Boundary Condition (ABC).............................................................................................. 25
7.3. Perfectly Matched Layers (PMLs)..................................................................................................... 27
8. Applying Excitation Sources and Loads in an Acoustic Analysis.......................................................... 33
8.1. Applying Acoustic Excitation Sources.............................................................................................. 33
8.1.1. Pressure Excitation................................................................................................................. 34
8.1.2. Outward Normal Velocity (Acceleration) Excitation.................................................................. 34
8.1.3. Arbitrary Velocity (Acceleration) Excitation.............................................................................. 35
8.1.4. Analytic Incident Wave Sources.............................................................................................. 36
8.1.5. Mass Source (Mass Source Rate).............................................................................................. 37
8.2. Applying Acoustic Loads................................................................................................................. 38
8.2.1.Trim Element with Transfer Admittance Matrix........................................................................ 38
8.2.2. Impedance Sheet................................................................................................................... 41
8.2.3. Equivalent Surface Source...................................................................................................... 41
8.2.3.1. Flagging an Equivalent Source Surface........................................................................... 42
8.2.4. Surface Port........................................................................................................................... 42
9. Accounting for Acoustic Fluid-Structure Interaction (FSI).................................................................... 45
10. Solving an Acoustic Analysis............................................................................................................... 47
10.1. Acoustic Analysis Solution Settings............................................................................................... 47
10.1.1. Modal Analysis Settings........................................................................................................ 47
10.1.2. Harmonic Analysis Settings.................................................................................................. 48
10.1.2.1. Full Harmonic Analysis................................................................................................. 48
10.1.2.1.1. Setting the Analysis Frequencies......................................................................... 48
10.1.2.1.2. Specifying the Analysis Solver............................................................................. 49
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Acoustic Analysis Guide
10.1.2.1.3. Selecting the Scattered Formulation for Harmonic Analysis.................................. 49
10.1.2.2. Mode Superposition Harmonic Analysis....................................................................... 50
10.1.3.Transient Analysis Settings.................................................................................................... 50
10.2. Starting and Finishing the Solution................................................................................................ 51
11. Using Advanced Solution Techniques in an Acoustic Analysis........................................................... 53
11.1. One-Way Coupling from Structure to Acoustics.............................................................................. 53
11.2. Linear Perturbation in an Acoustic Application.............................................................................. 54
12. Postprocessing Acoustic Analysis Results.......................................................................................... 57
12.1. Helpful Postprocessing Commands............................................................................................... 57
12.2. Postprocessing a Harmonic Acoustic Analysis................................................................................ 58
12.2.1. Reviewing Analysis Results................................................................................................... 58
12.2.2. Calculating Near Fields, Far Fields, and Far-Field Parameters................................................... 59
12.2.2.1. Accounting for Model Symmetry.................................................................................. 59
12.2.2.2. Radiation Solid Angle.................................................................................................. 60
12.2.2.3. Near Sound Pressure Field............................................................................................ 60
12.2.2.4. Far Sound Pressure Field and Far-Field Parameters........................................................ 60
12.2.2.5. Far-Field Microphone................................................................................................... 60
12.2.3. Calculating Acoustic Propagation Parameters....................................................................... 61
12.3. Postprocessing a Modal or Transient Acoustic Analysis................................................................... 62
13. Acoustic Analysis Examples................................................................................................................ 63
13.1. Example: Acoustic-Structural Coupled Modal Resonance of an Annular Ring Submerged in Water
with a Harmonic Analysis...................................................................................................................... 63
13.2. Example: Sloshing Modes of a Cylindrical Cavity............................................................................. 65
13.3. Example: Resonant Frequencies in a Pipe with Ideal Gas................................................................. 66
13.4. Example: Acoustic Harmonic Response in a Room.......................................................................... 68
13.5. Example:Transmission Loss of a Muffler......................................................................................... 70
13.6. Example: Johnson-Champoux-Allard Model of a Perforated Material.............................................. 71
13.7. Example:Transfer Admittance Matrix in Fluid................................................................................. 72
13.8. Example: Boundary Layer Impedance Model of a Rigid Walled Waveguide with Viscous-Thermal
Fluid..................................................................................................................................................... 74
13.9. Example: Radiation from Two Waveguides..................................................................................... 75
13.10. Example: Radiation from a Dipole................................................................................................ 77
13.11. Example: Monopole Incident Wave Scattering of a Rigid Sphere................................................... 79
13.12. Example: Planar Incident Wave FSI Scattering of an Infinite Cylindrical Shell.................................. 80
13.13. Example: One-Way Coupling from Structure to Acoustics............................................................. 83
13.14. Example: Modal Analysis of an Acoustic-Structural Coupled Structure with Nonlinear Static Prestress
Using Linear Perturbation..................................................................................................................... 85
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List of Figures
4.1. Acoustic-Structural Interaction Model................................................................................................... 12
6.1. Sound Pressure Distribution and FEM Model......................................................................................... 21
7.1. Spherical ABC for Symmetry and Near the Radiation Outlet................................................................... 26
7.2. 1-D Multiple PMLs for Pipes................................................................................................................... 27
7.3. PML Enclosure...................................................................................................................................... 28
7.4. Attenuation Distribution....................................................................................................................... 29
7.5. Buffer Elements.................................................................................................................................... 29
7.6. Distance Between Source/Objects and PML Region............................................................................... 30
7.7. PML Near the Radiation Outlet.............................................................................................................. 31
8.1. Spherical Coordinates........................................................................................................................... 36
8.2.Trim Element with Transfer Admittance Matrix....................................................................................... 39
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List of Tables
1.1. Acoustic Analysis Steps........................................................................................................................... 2
2.1. Acoustic Element Properties................................................................................................................... 5
2.2. Primary Acoustic Analysis Commands..................................................................................................... 6
2.3. Secondary Acoustic Analysis Commands................................................................................................. 7
4.1. Key Options for FLUID30, FLUID220, and FLUID221................................................................................. 11
4.2. Key Options for FLUID130..................................................................................................................... 11
4.3. SI Units................................................................................................................................................. 12
5.1. Equivalent Fluid Models of Perforated Material...................................................................................... 17
5.2. Low Reduced Frequency Models........................................................................................................... 19
7.1. Acoustic Boundary Conditions.............................................................................................................. 23
7.2. Surface Impedance Boundary Conditions.............................................................................................. 24
8.1. Acoustic Excitation Sources................................................................................................................... 33
8.2. Acoustic Analytic Incident Wave Sources............................................................................................... 36
8.3. Acoustic Loads...................................................................................................................................... 38
8.4.Transfer Admittance Matrix Models of Perforated Structures:TB,PERF,,,,TBOPT....................................... 39
10.1. Acoustic Eigen Equations and Solvers.................................................................................................. 47
11.1. Acoustic Linear Perturbation Analysis Process...................................................................................... 55
12.1. Postprocessing Commands................................................................................................................. 57
12.2. Plotting Commands............................................................................................................................ 57
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Chapter 1: Introduction to Acoustic Analysis
The following topics introducing you to acoustic analysis are available:
1.1.The General Acoustic Equations
1.2. Overview of the Acoustic Analysis Process
1.1.The General Acoustic Equations
Acoustic analysis is available in the ANSYS Multiphysics and ANSYS Mechanical products.This type of
analysis simulates the generation and propagation properties of either the coupled acoustic-structural
interaction (FSI) or the uncoupled pure acoustic wave in the given environment. Support is available
for modal, time-harmonic, and transient acoustic analysis.
The program assumes that the fluid is compressible with zero mean flow. Only relatively small pressure
changes are allowed with respect to the mean pressure. An acoustic analysis usually involves modeling
the acoustic phenomena in an acoustic fluid and in a structure. A coupled acoustic-structural interaction
analysis takes the structural dynamics equation into account, along with the linearized Navier-Stokes
equations of fluid momentum and the flow continuity equation. A pure acoustic analysis models the
acoustics fluid.
In an acoustic analysis, one of the two following matrix equations is solved. For pure acoustic phenomena,
the program solves for this finite element dynamic matrix equation:
ɺɺ ɺ
+ + =
F e F e F e F
where [M ], [C ], and [K ] are the mass, damping, and stiffness matrices, respectively, and {f } is the ex-
F F F F
ternal excitation vector in the acoustic fluid.
In acoustic-structural interaction application, the program solves for the fully coupled finite element
dynamic matrix equation:
 {ɺɺ }  {ɺ }  − { }  
 S {ɺɺ(cid:1)}+ S {ɺ(cid:1)}+ S { (cid:1)}=S
ρ T          
0 (cid:0) (cid:1) (cid:0) (cid:1) (cid:0) (cid:1) (cid:0)
where [M ], [C ], and [K ] are the mass, damping, and stiffness matrices, respectively, and {f } is the ex-
S S S S
ternal force vector in the structure. [R] is the coupled matrix and represents the coupling conditions on
the interface between the acoustic fluid and the structure.
For more information about the matrices, see the Derivation of Acoustic Matrices and Acoustic Fluid-
Structural Interaction (FSI) sections in the Mechanical APDL Theory Reference.
1.2. Overview of the Acoustic Analysis Process
In general, the program simulates interior problems or exterior problems as well as FSI problems.
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Introduction to Acoustic Analysis
For interior problems, a sound wave propagates or oscillates in a closed structure.The oscillating fre-
quencies and transmission loss (TL) are usually investigated. For exterior problems, a sound wave radiates
into open space or it is scattered by a structural object in the open domain.
The radiation sound power level (L ), directive gain, or target strength (TS) is usually investigated.The
w
program’s applications include, but are not limited to, sonar, noise investigation, acoustic design of ar-
chitecture, underwater acoustics, and the design of acoustic devices.
An acoustic analysis involves most of the general steps found in any analysis. Following is the general
process for performing an acoustic analysis:
Table 1.1: Acoustic Analysis Steps
Step Tasks Comments
1. Build the model. An acoustic model generally consists of fluid domain,
structural parts, FSI interfaces, sound excitations, and
the truncation of the infinite domain.
ANSYS Workbench may help in building the model
easily.
2. Set up the model envir- Acoustic analysis is supported by the FLUID29,FLU-
onment. ID30,FLUID220, and FLUID221 elements.
The FLUID129 and FLUID130 elements can act as ab-
sorbing elements to truncate the infinite fluid domain.
The FLUID29 element may not support some 3-D ele-
ment features. See the documentation for that element
in the Element Reference.
3. Define material proper- Defining the material properties for an acoustic analysis
ties. is no different from any other analysis. Use the MP or
TB commands to define linear or nonlinear material
properties.
The equivalent fluid model is defined by the TB com-
mand. For more information, see Defining Material
Properties in the Basic Analysis Guide and Sophisticated
Acoustic Media in the Mechanical APDL Theory Refer-
ence.
4. Mesh the model. Use meshing commands to mesh the different parts
of the model. Certain areas may require more detailed
meshing or special considerations.
To ensure a reliable solution, either ten elements per
wavelength for low-order elements or five elements
per wavelength for high-order elements are required
at the highest working frequency.
For more information, see the Modeling and Meshing
Guide.
5. Define the boundary Define the boundary conditions using the D or SF
conditions. command.The absorbing element FLUID130 or the
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