ANSYS Mechanical APDL Acoustic Analysis Guide

ANSYS Mechanical APDL Acoustic Analysis

Guide

Release 15.0ANSYS, Inc.

November 2013Southpointe

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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

iiiRelease 15.0 - SAS IP, Inc. All rights reserved. - Contains proprietary and confidential information

of ANSYS, Inc. and its subsidiaries and affiliates.

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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Acoustic Analysis Guide

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3910.1. 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:

F e F e F e F

&& &+ + =

where [MF], [CF], and [KF] are the mass, damping, and stiffness matrices, respectively, and {fF} is the ex-

ternal excitation vector in the acoustic fluid.

In acoustic-structural interaction application, the program solves for the fully coupled finite element

dynamic matrix equation:

S

T

S

0

{ }{ }

+

&&

&&

{ }{ }

+

{ }{ }

&

&

S

=

S

where [MS], [CS], and [KS] are the mass, damping, and stiffness matrices, respectively, and {fS} is the ex-

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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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 (Lw), directive gain, or target strength (TS) is usually investigated. The

programs 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

CommentsTasksStep

An acoustic model generally consists of fluid domain,

structural parts, FSI interfaces, sound excitations, and

the truncation of the infinite domain.

Build the model.1.

ANSYS Workbench may help in building the model

easily.

Acoustic analysis is supported by the FLUID29, FLU-

ID30, FLUID220, and FLUID221 elements.

Set up the model envir-

onment.

2.

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.

Defining the material properties for an acoustic analysis

is no different from any other analysis. Use the MP or

Define material proper-

ties.

3.

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.

Use meshing commands to mesh the different parts

of the model. Certain areas may require more detailed

meshing or special considerations.

Mesh the model.4.

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.

Define the boundary conditions using the D or SF

command. The absorbing element FLUID130 or the

Define the boundary

conditions.

5.

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Introduction to Acoustic Analysis

Perfectly Matched Layers (PML) can achieve better ac-

curacy for an open domain problem. For more inform-

ation refer to Acoustic Boundary Conditions, Absorbing

Boundary Condition (ABC), or Perfectly Matched Layers

(PML) in the Mechanical APDL Theory Reference.

Define the loads and excitations (D, SF, or BF).Define the loads and ex-

citations.

6.

If the analytic wave sources are required, issue the

AWAVE command.

The trim element with transfer admittance matrix is

defined by the TB command for perforated structures.

For more information, see Defining Material Properties

in the Basic Analysis Guide and Sophisticated Acoustic

Media in the Mechanical APDL Theory Reference.

Use the SF command to account for the acoustic fluid-

structural interaction (FSI) effect. The solution for FSI

with the strong coupled matrix is performed.

Account for the FSI ef-

fect.

7.

For more information, see Acoustic Fluid-Structural

Interaction (FSI) in theMechanical APDL Theory Refer-

ence.

The solution phase of an acoustic analysis adheres to

standard ANSYS conventions, although the FSI coupled

Solve the model.8.

matrices may not be symmetric. Modal, harmonic, and

transient analyses may be performed.

You may choose the symmetric algorithm for coupled

matrices in a modal or harmonic analysis.

The pure scattered pressure formulation is also avail-

able for the analytic incident wave, for more informa-

tion see Pure Scattered Pressure Formulation in the

Mechanical APDL Theory Reference.

You can use structural results as the acoustic excitation

source using one-way coupling process (ASIFILE).

You can take the nonlinear static analysis into account

and use a morphed mesh for the acoustic-structural

coupled solution via a linear perturbation scheme.

Use the POST1 general postprocessor and the POST26

time history postprocessor to review results.

Postprocess the acoustic

analysis.

9.

Specific commands are available in POST1 for near-

and far-field parameters (PRNEAR, PLNEAR, PRFAR,

PLFAR, *GET) and for sound power data (SPOWER).



Overview of the Acoustic Analysis Process

Chapter 2: Using the Acoustic Analysis Tools

The following tools and resources are available to facilitate an acoustic analysis:

2.1. Elements Used in an Acoustic Analysis

2.2. Commands Used in an Acoustic Analysis

2.3. Understanding Acoustic Analysis Terminology

2.4. Acoustic Analysis Resources and Examples

2.1. Elements Used in an Acoustic Analysis

These elements are available for acoustic analysis: FLUID29, FLUID129, FLUID30, FLUID130, FLUID220,

and FLUID221. Element properties are as follows:

Table 2.1: Acoustic Element Properties

Degrees of Freedom per NodeAttributeElement

UX, UY, PRES (coupled element)2-D 4-node quadrilateral

with triangle degeneracy

FLUID29

PRES (uncoupled element)

PRES2-D 2-node lineFLUID129

UX, UY, UZ, PRES (coupled element)3-D 8-node hexagonal with

prism with tetrahedral and

pyramid degeneracy

FLUID30


UX, UY, UZ, PRES (coupled modal symmetric

matrix)

PRES3-D 4- or 8-node quadrilater-

al surface with triangle de-

generacy

FLUID130

UX, UY, UZ, PRES (coupled element)3-D 20-node hexagonal with

pyramid and prism degener-

acy

FLUID220



matrix)

UX, UY, UZ, PRES (coupled element)3-D 10-node tetrahedralFLUID221



matrix)

One element shape (hexahedral, wedge, or tetrahedral), or any combination of shapes, can be used in

a 3-D acoustic model. The pyramid elements are transitional elements between the hexahedral and

tetrahedral elements.



Example 2.1: Creating Mixed Hexahedral and Wedge Elements

/prep7et,1,220 ! define hexahedral elementet,11,200,5 ! define 2-D 6-node triangle mesh elementet,12,200,7 ! define 2-D 8-node quadrilateral mesh elementrect,0,1,0,1 ! create area 1rect,1,2,0,1 ! create area 2aglue,all ! glue areas togetheresize,0.25 ! define the element sizeasel,s,loc,x,0,1 ! select area 1type,11 ! select triangle element typemshape,1 ! define the triangle element shapeamesh,all ! mesh area 1 with triangle mesh elementasel,s,loc,x,1,2 ! select area 2type,12 ! select quadrilateral mesh element typemshape,0 ! define the element quadrilateral shapeamesh,all ! mesh area 2 with quadrilateral mesh elementallsesize,,4 ! define element operationtype,1 ! select hexahedral elementasel,s,loc,z,0 ! select 2-D elementvext,all,,,0,0,1 ! create 3-D elements by extruding 2-D elementsfini

Example 2.2: Creating Mixed Hexahedral, Pyramid, and Tetrahedral Elements

/prep7ch=10.16e-3cw=22.86e-3cl=2.e-2h=2.e-3et,1,220,1 ! define hexahedral elementet,2,221,1 ! define tet elementblock,-cw/2,0,-ch/2,ch/2,0,cl/2 ! create volume 1block,-cw/2,0,-ch/2,ch/2,cl/2,cl ! create volume 2vglue,all ! glue volumes togetheresize,h ! define element sizetype,1 ! select hexahedral element typemshape,0,3d ! define hexahedral meshmshkey,1 ! use mapped meshvmesh,1 ! mesh first volumemshape,1,3d ! define tetrahedral elementmshkey,0 ! use free meshingvmesh,3 ! mesh second meshtchg,220,221,2 ! covert degenerated brick into tetfini

Although a geometrically complex structure can be meshed with tetrahedral elements, it may require

many elements and lead to a more computationally expensive simulation. Even a regularly shaped

volume may require many tetrahedral elements if it has a very large aspect ratio. In such a case, hexa-

hedral or wedge elements are a better choice.

2.2. Commands Used in an Acoustic Analysis

The following commands are commonly used in an acoustic analysis:

Table 2.2: Primary Acoustic Analysis Commands

CommentsCommand

Defines writing or reading the one-way acoustic-structural

coupling data to or from a file.

ASIFILE

Specify an acoustic harmonic analysis with the scattered

pressure formulation.

ASOL


Using the Acoustic Analysis Tools

Output control for an acoustic scattering analysis.ASCRES

Defines the analytic acoustic incident wave source.AWAVE

Defines spatial angles of a spherical radiation surface for

an acoustic radiator.

HFANG

Indicates the presence of symmetry planes for the compu-

tation of an acoustic field in the near- or far-field domains

(beyond the finite element region).

HFSYM

Plots acoustic far-field parameters.PLFAR

Plots the acoustic parameters in the near zone exterior to

the equivalent source surface.

PLNEAR

Defines perfectly matched layers (PML).PMLOPT

Determines the number of PML layers.PMLSIZE

Prints acoustic far-field parameters.PRFAR

Prints the acoustic parameters in the near zone exterior to

the equivalent source surface.

PRNEAR

Calculates sound power data of a multi-port network.SPOWER

Table 2.3: Secondary Acoustic Analysis Commands

CommentsCommand

Defines the acoustic body loads.BF

Defines the frequency range in the harmonic analysis, in-

cluding the octave bands.

HARFRQ

Specify the morphing operation in an acoustic linear per-

turbation.

MORPH

Prints the solution results for nodes.NSOL

Plots the acoustic solution results for nodes.PLNSOL

Prints the acoustic solution results for nodes.PRNSOL

Defines the acoustic surface loads and flags.SF

2.3. Understanding Acoustic Analysis Terminology

The following common terms are used throughout this guide:

Interior problem The sound wave oscillates in an enclosure or propagates

to the infinity in a constrained structure.

Exterior problem The sound wave radiates or is scattered into the infinite

open space.

FSI Acoustic fluid-structural interaction.

Coupled element Acoustic element with FSI interface.

Uncoupled element Acoustic element without FSI interface.

PML Perfectly matched layers.



Understanding Acoustic Analysis Terminology

Sound-hard surface A surface on which particle normal velocity is zero.

Sound-soft surface A surface on which sound pressure is constrained.

Transparent port An exterior surface on which incident pressure is launched

into the acoustic model and the reflected pressure wave

is fully absorbed by a defined matched impedance that

represents the infinity.

Vibro port An exterior surface on which incident pressure is launched

into the acoustic model by the vibration of the structural

surface.

2.4. Acoustic Analysis Resources and Examples

The following additional ANSYS, Inc. documentation is available for further information about acoustics

and related rotational phenomena:

Acoustics in the Mechanical APDL Theory Reference

Elements for Acoustic Analysis in the Element Reference

Acoustic Analysis of a Small Speaker System in the Technology Demonstration Guide

Also see Acoustic Analysis Examples (p. 63) in this guide.

Finally, the Mechanical APDL Verification Manual contains the following acoustics cases:

VM157 - 3-D Acoustic Modal Analysis with Temperature Change

VM177 - Natural Frequency of a Submerged Ring

VM242 - Johnson-Chamoux-Allard Equivalent Fluid Model

VM282 - Mode Superposition Response Analysis of a Piston-Fluid System

VM283 - Low Reduced Frequency Model for Visco-thermal Fluid with Thin Structure

VMR083-CA1 - Sound Radiation of a Vibrating Sphere

VMR083-CA2 - Sound Radiation of a Cylinder with Vibrating Lateral Surface


Using the Acoustic Analysis Tools

Chapter 3: Modeling for an Acoustic Analysis

Use either the preprocessor (/PREP7) or ANSYS DesignModeler to create the model geometry. The

model-building process is common to most analyses. For more information, see the Modeling and

Meshing Guide.

To begin, specify a jobname and title for your analysis.



Chapter 4: Defining the Acoustic Modeling Environment

The following topics describe the features and applications of the 3-D elements used in an acoustic

analysis:

4.1. Defining Element Types

4.2. Specifying the System of Units

4.1. Defining Element Types

To specify element type numbers and key options for acoustic elements, issue the ET command.

The key options (KEYOPTs) shown in the following two tables define the acoustic element properties:

Table 4.1: Key Options for FLUID30, FLUID220, and FLUID221

Element DefinitionsOptionsKEYOPT

No specification0

KEYOPT(1) Symmetric modal formulation1

Symmetric harmonic formulation2

Coupled element0KEYOPT(2)

Uncoupled element1

Normal element0KEYOPT(4)

Perfectly Matched Layers (PML) element1

Table 4.2: Key Options for FLUID130

Element DefinitionsOptionsKEYOPT

4-node surface element0KEYOPT(1)

8-node surface element2

The following figure shows an example of coupled acoustic-structural interaction:



Figure 4.1: Acoustic-Structural Interaction Model

Example 4.1: Defining Element Types

The following example input defines second-order coupled, uncoupled, and PML elements:

et,1,220,,0,,0 ! coupled brick element with DOF: UX,UY,UX,PRESet,1,220,,1,,0 ! uncoupled brick element with DOF: PRESet,1,220,,1,,1 ! uncoupled PML brick element with DOF: PRES

4.2. Specifying the System of Units

Acoustic analysis uses various unit systems (/UNITS), but uses the SI system as the default.

For more information about available units systems, see System of Units in the Coupled-Field Analysis

Guide.

The following table lists the physical quantities used for an acoustic analysis in the SI units system:

Table 4.3: SI Units

Expressed in terms of other SI

units

QuantityUnit SymbolUnit Name

lengthmmeter

masskgkilogram

timessecond

thermodynamic temperatureKkelvin

frequencyHzhertz

kgm/s2

force, weightNnewton

N/m2

pressurePapascal

Nmenergy, heatJjoule

J/spowerWwatt

kg/m3

mass density


Defining the Acoustic Modeling Environment

m/ssound speedc

Pasdynamic viscosity

W/mKthermal conductivity

J/Kheat coefficient at a constant

pressure per mass

cp

J/Kheat coefficient at a constant

volume per mass

cv

Ns/m3

impedanceZ

Support for defining a custom system of units is available via the /UNITS command. Typically, the length

and mass conversion factor, as well as the offset of the temperature, are defined. The program converts

the remaining quantities, including the static pressure, reference pressure, and reference power. The

matrices may have matrix conditions with other systems of units than either the SI or MKS system for

acoustic fluid-structural interaction (FSI) models.

Example 4.2: Defining the System of Units

/batch ! batch mode/units,cgs ! cgs units



Specifying the System of Units

Chapter 5: Defining Acoustic Material Properties

The following topics describing how to define acoustic material properties are available:

5.1. Basic Material Parameters of Acoustic Media

5.2. Non-Uniform Ideal Gas Material

5.3. Equivalent Fluid Model of Perforated Material

5.4.Viscous-Thermal Materials

5.1. Basic Material Parameters of Acoustic Media

Several primary commands are available to define basic acoustic material properties:

MP

TB,AFDM,,,,MAT

TBFIELD

TBDATA,1,DENS,SONC,VISC,KXX,CPH,CVHTBDATA,7,BVIS

Acoustic analyses require the mass density (MP,DENS) and sound speed (MP,SONC) of the acoustic

fluid.

For viscous-thermal material, the dynamic viscosity (MP,VISC), bulk viscosity (MP,BVIS), thermal conduct-

ivity (MP,KXX), heat coefficient at constant volume (MP,CVH), and heat coefficient at constant pressure

(MP,C) are defined.

Example 5.1: Defining Acoustic Material Properties (MP)

The following example input defines the acoustic material properties of air via the MP command:

mp,dens,1,1.21 ! mass densitymp,sonc,1,343 ! sound speedmp,visc,1,1.827e-5 ! dynamic viscositymp,bvis,1,1.096e-5 ! bulk viscositymp,kxx,1,0.0257 ! thermal conductivitymp,cvh,1,0.718 ! heat coefficient at a constant volume per massmp,c,1,1.005 ! heat coefficient at a constant pressure per mass

Frequency-dependent material properties are defined via the TB,AFDM,,,,MAT command. Material values

defined by TB,AFDM override the values defined by MP.

Frequency-dependent material properties are interpolated if the working frequency is not one of the

frequencies defined via the TBFIELD command.

Example 5.2: Defining Frequency-Dependent Acoustic Material Properties (TB)

The following example input defines frequency-dependent acoustic material properties via the TB

command:

tb,afdm,1,,,mat ! basic acoustic materialstbfield,freq,f1 ! table at frequency f1tbdata,1,dens1,sonc1,visc1,therm1,cph1,cvh1 ! material parameters table



tbdata,7,bvis1tbfield,freq,f2 ! table frequency f2 tbdata,1,dens2,sonc2,visc2,therm2,cph2,cvh2 ! material parameters tabletbdata,7,bvis2

Listing the Defined Material Properties

The MPLIST command lists the material properties defined via the MP command.

The TBLIST command lists the frequency-dependent material properties defined via the TB command.

5.2. Non-Uniform Ideal Gas Material

In non-uniform acoustic media, both mass density and sound speed vary with the spatial position and

are derived by the ideal gas law.

To set up a non-uniform ideal gas model:

1. Define the temperature T0 via the TREF command (defaults to 22 C).

2. Define the density 0 and sound speed C0 at the reference temperature T0 via the MP,DENS and MP,SONC

commands.

3. Define the reference static pressure Psref via the R,,Psref command.

4. Define the spatial static pressure Ps via the BF,,CHRGD command.

5. Define the spatial temperature via the BF,,TEMP command.

The ideal gas model does not support the MPTEMP and MPDATA commands for mass density and

sound speed. Mass density and sound speed are defined at a reference temperature, and the ideal gas

model calculates the temperature-dependent nodal mass density and sound speed.

Example 5.3: Defining a Non-Uniform Ideal Gas Model

mp,dens,1,1.21 ! mass densitymp,sonc,1,343 ! sound speedr,1,pref,psref ! static reference pressuretref,22 ! reference temperaturensel,s,loc,x ! nodes at x=0bf,all,chrgd,101325 ! static pressure on nodesbf,all,temp,100 ! temperature on nodes

For more information, see Non-uniform Acoustic Media in the Mechanical APDL Theory Reference.

5.3. Equivalent Fluid Model of Perforated Material

Several equivalent fluid models are available to approximate the perforated material with the rigid

skeleton.

The equivalent model uses the wave equation with complex effective density and velocity.


Defining Acoustic Material Properties

Define an equivalent fluid model via the TB,PERF,,,,TBOPT command. The following table shows thevalid TBOPT values and input parameters necessary for defining equivalent fluid models:

Table 5.1: Equivalent Fluid Models of Perforated Material

Input ParametersModelTBOPT

Fluid Resistivity , Porosity , Tortuosity

, Viscous Characteristic Length ,

Thermal Characteristic Length

Johnson-Champoux-AllardJCA

Fluid Resistivity (0.01 < f/

tbdata,1,denr1,deni1,cr1,ci1 ! complex density and velocity at f1tbfield,freq,f2 ! table at f2tbdata,1,denr2,deni2,cr2,ci2 ! complex density and velocity at f2

Trimming the perforated structures to a transfer admittance matrix avoids dense mesh and creates ex-

cellent numerical accuracy. See Trim Element with Transfer Admittance Matrix (p. 38).

For more information, see Equivalent Fluid of Perforated Materials in the Mechanical APDL Theory Refer-

ence.

5.4. Viscous-Thermal Materials

The following topics related to viscous-thermal materials in an acoustic analysis are available:

5.4.1. Acoustic Propagation in the Viscous Fluid

5.4.2. Boundary Layer Impedance (BLI) Model

5.4.3. Low Reduced Frequency (LRF) Model

5.4.1. Acoustic Propagation in the Viscous Fluid

An acoustic propagating wave in a viscous media is dampened due to the viscosity of the fluid. The

interaction between the acoustic pressure wave in a viscous fluid and a rigid wall is not taken into ac-

count.

Define the viscosity of a fluid via the MP,VISC command.

Example 5.6: Defining a Viscous Material

mp,dens,1,1.21 ! mass densitymp,sonc,1,343 ! sound speedmp,visc,1,1.827e-5 ! dynamic viscosity

For more information, see Acoustic Fundamentals in the Mechanical APDL Theory Reference.

5.4.2. Boundary Layer Impedance (BLI) Model

The interaction between an acoustic pressure wave in a viscous fluid and a rigid wall is taken into account.

Specify a rigid wall as a boundary layer via the SF,Nlist,BLI command.

BLI models are supported in full harmonic acoustic analyses only.

Example 5.7: Defining a BLI Model

mp,dens,1,1.21 ! mass densitymp,sonc,1,343 ! sound speedmp,visc,1,1.827e-5 ! dynamic viscositymp,bvis,1,1.096e-5 ! bulk viscositymp,kxx,1,0.0257 ! thermal conductivitymp,cvh,1,0.718 ! heat coefficient at a constant volume per massmp,c,1,1.005 ! heat coefficient at a constant pressure per mass

nsel,s,ext ! select exterior nodessf,all,bli ! flag boundary layer

For more information, see Boundary Layer Impedance (BLI) Model in the Mechanical APDL Theory Reference.


Defining Acoustic Material Properties

5.4.3. Low Reduced Frequency (LRF) Model

The interaction between an acoustic pressure wave in a viscous fluid and a rigid wall is taken into account

for specific structures according to low reduced frequency (LRF) approximation.

Define the LRF model via the TB,AFDM,,,,TBOPT command.

The following table shows the valid TBOPT values and input parameters for the LRF model in a viscous-thermal fluid:

Table 5.2: Low Reduced Frequency Models

Input ParametersCommentsTBOPT

Thickness of the layerThin layer between two rigid platesTHIN

Width and height of the rectangleA tube with a rectangular cross sectionRECT

Radius of the circleA tube with a circular cross sectionCIRC

LRF models are supported in full harmonic acoustic analyses only.

Example 5.8: Defining an LRF Model

The following example input defines a low reduced frequency model with a thin layer:

mp,dens,1,1.21 ! mass densitymp,sonc,1,343 ! sound speedmp,visc,1,1.827e-5 ! dynamic viscositymp,kxx,1,0.0257 ! thermal conductivitymp,cvh,1,0.718 ! heat coefficient at a constant volume per massmp,c,1,1.005 ! heat coefficient at a constant pressure per mass

tb,afdm,1,,,thin ! basic acoustic materialstbfield,freq,f1 ! table at frequency f1tbdata,1,thick1 ! material parameters tabletbfield,freq,f2 ! table frequency f2 tbdata,1, thick2 ! material parameters table

For more information, see Low Reduced Frequency (LRF) Model in the Mechanical APDL Theory Reference.



Viscous-Thermal Materials

Chapter 6: Specifying Acoustic Analysis Region Attributes and

Meshing

Attributes assigned to a model prior to meshing include element type and material number. Assign

these attributes to a region (VATT) for volumes to be meshed with 3-D elements (FLUID30, FLUID220,

and FLUID221). Specify different material ID numbers for the various material regions.

For an acoustic FEA formulation, the mesh must be fine enough to minimize numerical dispersion effects

from finite discretization. In general, the mesh should have at least ten low-order elements and five

high-order elements per propagating or resonant wavelength of the material.

If different materials are present in the model, the mesh should have a smooth transition from the

dense to the coarse. To obtain better accuracy and efficiency, use your best judgment and knowledge

of the sound pressure field to determine appropriate mesh density. For example, if the pressure varies

sinusoidally along the wide side of a rectangular cross section and is uniform along the narrow side,

you might choose to mesh the cross section as shown in this figure:

Figure 6.1: Sound Pressure Distribution and FEM Model

Do not mix low-order and high-order elements together in a model. Unlike low-order elements, high-

order elements have midside nodes to follow the curvature of a model.

If using MESH200 elements to generate 3-D acoustic elements FLUID30, FLUID220, or FLUID221 (VEXT),

select one of the following MESH200 options:

KEYOPT(1) = 4 for 3-D triangle elements, used for FLUID30

KEYOPT(1) = 5 for 3-D triangle elements with midside nodes, used for FLUID221

KEYOPT(1) = 6 for 3-D quadrilateral elements, used for FLUID30

KEYOPT(1) = 7 for 3-D quadrilateral elements with midside nodes, used for FLUID220

Example 6.1: Generating FLUID220 Elements

The following input example uses MESH200 elements to generate FLUID220 elements:

et,1,220,,1et,11,200,7

Rect,0,d,0,dtype,11amesh,allasel,s,loc,z,0



etype,1esize,,2vext,all,,,,,dzaclear,alletdele,11

fini


Specifying Acoustic Analysis Region Attributes and Meshing

Chapter 7: Applying Boundary Conditions in an Acoustic Analysis

Most boundary conditions can be applied to an acoustic analysis, either on the solid model entities or

on the finite element model entities. Applying boundary conditions to the solid model is advantageous

in that they are independent of the underlying finite element mesh.

The following related topics are available:

7.1. Applying Boundary Conditions

7.2. Absorbing Boundary Condition (ABC)

7.3. Perfectly Matched Layers (PMLs)

7.1. Applying Boundary Conditions

The following table shows the boundary conditions available for an acoustic analysis:

Table 7.1: Acoustic Boundary Conditions

FE Model EntitiesSolid Model EntitiesBoundary Condition

NodesLines or AreasPressure (Sound-Soft Boundary, SSB)

None requiredNone requiredRigid Wall (Sound-Hard Boundary, SHB)

NodesAreasImpedance Boundary Condition (IBC)

NodesAreasFree Surface (Sloshing Effect)

NodesAreasAbsorbing Boundary Condition (ABC)

ElementsNot ApplicablePerfectly Matched Layers (PML)

For general information about applying boundary conditions, see Loading in the Basic Analysis Guide.

7.1.1. Pressure Boundary

The pressure boundary is a Dirichlet boundary with p = p0. To apply pressure to the nodes of a finite

element model, issue the D,Node,PRES command.

Example 7.1: Applying Pressure to Nodes

nsel,s,loc,z,0.0 ! select the nodes d,all,pres,dispr,dispi ! complex pressure

If using coupled acoustic elements (KEYOPT(2) = 0), avoid zero-pivot warning messages by setting the

displacement degrees of freedom (UX, UY, and UZ) at the element nodes not on the interface to zero.

Example 7.2: Applying Displacement to Nodes

nsel,s,loc,z,0.0 ! select the nodes d,all,ux,0 ! zero ux d,all,uy,0 ! zero uy d,all,uz,0 ! zero uz



7.1.2. Rigid Wall Boundary

The rigid wall boundary is a Neumann boundary with =^

applied. It is not necessary to specify

a rigid wall boundary condition in an FEM acoustic analysis, as it is a natural boundary condition.

If the pressure spatial distribution can be predicted, the Neumann boundary can be used on the sym-

metric plane of the model to reduce the model size.

7.1.3. Surface Impedance Boundary

Table 7.2: Surface Impedance Boundary Conditions (p. 24) shows surface impedance boundary conditions

available for acoustic analysis. The sound pressure is damped on the impedance boundary and you can

use it to approximate infinity.

Table 7.2: Surface Impedance Boundary Conditions

SF Command LabelDefinitionBoundary Condition

INFZ=0C0Infinite Radiation Boundary

ATTN=

+

0

Boundary with Absorption

Coefficient

IMPDZ=Zr+jZiImpedance Boundary

The infinite radiation boundary assumes the ratio of the pressure and outward normal velocity is equal

to Z0 = 0C0. When the radiation boundary is close to the objects or the radiators, the outgoing pressure

wave may no longer hold the ratio Z0 and a numerical error may occur. Using either an absorbing

boundary element or a Perfectly Matched Layers (PML) is more accurate for modeling the far-field radi-

ation boundary. An infinite radiation boundary can be applied to the nodes of the finite element

model via the SF,Nlist,INF command:

Example 7.3: Defining an Infinite Radiation Boundary

nsel,s,ext ! select exterior node on selected elementssf,all,inf ! infinite radiation boundary

The absorption coefficient is often used to measure the absorption of a surface in acoustic applications.

The surface impedance with real value can deviate from the defined absorption coefficient, as shown

in Table 7.2: Surface Impedance Boundary Conditions (p. 24). The absorption coefficient of the surface

can be applied to nodes of the finite element model via the SF,Nlist,ATTN,VALUE command:

Example 7.4: Defining Boundary Absorption Coefficient

nsel,s,ext ! select exterior node on selected elementssf,all,attn,0.5 ! boundary absorption coefficient

A more flexible complex surface impedance represents the specific ratio between pressure and normal

particle velocity on the surface. Surface impedance can be applied to nodes on the finite element

model via the SF,Nlist,IMPD,VALUE,VALUE2 command:

Example 7.5: Applying the Impedance BC in an Acoustic Radiation or Scattering Analysis

Apply the impedance boundary condition to the exterior surface of the model in an acoustic radiation

or scattering analysis.


Applying Boundary Conditions in an Acoustic Analysis

Apply the impedance boundary condition to the inlet and outlet surface for the transparent port in an

acoustic propagating analysis.

For example, in a transmission loss analysis of a muffler, you might define the following:

nsel,s,loc,z,0 ! select nodes on inletsf,all,impd,z01 ! impedance on inletsf,all,shld,vn ! normal velocity on inletsf,all,port,10 ! transparent portnsel,s,loc,l ! select nodes on outletsf,all,impd,z02 ! impedance on outlet

If a complex value is applied to a surface (SF,Nlist,IMPD,VALUE,VALUE2) in an acoustic modal ana-lysis, a negative conductance of admittance is input as VALUE and the quotient of susceptance to theangular frequency is input as VALUE2.

Do not use the SF,Nlist,IMPD command to define the radiation boundary (SF,Nlist,INF) if the purescattered formulation is selected (ASOL,SC) unless the impedance value is different from the mediacharacteristic impedance Z0 = 0C0.

7.1.4. Free Surface (Sloshing Effect)

The free surface (sloshing effect) is taken into account by flagging the plane as a free surface

(SF,Nlist,FREE) and defining gravitational acceleration (ACEL).

The free surface must be aligned with the coordinate plane in the global Cartesian coordinate system.

The gravitational acceleration input is always positive regardless of how the model is set up.

Example 7.6: Defining the Sloshing Effect

nsel,s,loc,z,0 ! select the nodes on the free surfacesf,all,free ! flag the nodes on free surfaceallsacel,,,9.85 ! gravity acceleration in z-direction

For more information, see Acoustic Fluid-Structural Interaction (FSI) in the Mechanical APDL Theory Ref-

erence.

7.2. Absorbing Boundary Condition (ABC)

An exterior acoustics problem typically involves an infinite, homogenous, inviscid fluid. The pressure

wave must satisfy the Sommerfeld radiation condition. A typical approach to such a problem involves

truncating the unbounded domain by introducing a second-order absorbing element (FLUID130 or

FLUID129) on the boundary a at some distance from the structure.

For a 3-D acoustic analysis, the absorbing boundary must be a spherical enclosure centered at the origin

(x0,y0,z0) with radius ra.

For a 2-D acoustic analysis, the absorbing boundary must be a circle centered at the origin (x0,y0,0) with

radius Ra.

Absorbing elements have a mesh conforming to the underlying elements. The 4-node element FLUID130

(KEYOPT(1) = 0) is applied to the top of the 8-node element FLUID30. The 8-node element FLUID130

(KEYOPT(1) = 2) is applied to the top of the 20-node element FLUID220 or the 10-node element FLUID221.

The 2-node element FLUID129 is connected to the 4-node element FLUID29.

Generate the absorbing elements as follows:



Absorbing Boundary Condition (ABC)

1. Mesh the model enclosed by a spherical surface with FLUID30, FLUID220, or FLUID221 elements.

2. Select the underlying nodes and elements on the spherical boundary surface.

3. Generate a surface absorbing element with the ESURF command.

Example 7.7: Generating an Absorbing Element

et,1,220,,1 ! FLUID220et,2,130,2 ! second-order FLUID130r,1,1e-6r,2,DIM_DISTANCE ! radius of sphere mp,sonc,1,1500 ! sound speedmp,dens,1,1000 ! densitympcopy,,1,2 ! copy material 1 to 2 ! create 1/8 spheresphere,,DIM_DISTANCE,0,90 ! spherevsbw,allvsel,s,loc,z,0,-DIM_DISTANCEvdele,all,,,1allsel,allesize,DIM_ESIZEvmesh,all ! mesh sphere with FLUID220csys,2 ! activate spherical coordinateasel,s,loc,x,DIM_DISTANCE ! select area on the spherecsys,0 ! activate Cartesian coordinatensla,s,1 ! nodes attaching to area ! create surface meshtype,2 ! FLUID130real,2 ! real constant 2mat,2 ! material 2esurf ! generate surface elementallsel,all

Absorbing elements accommodate the symmetry of the model. If the radiated acoustic field has no

significant effect on the excitation source entity, the spherical absorbing boundary can locally enclose

the open space near the radiation outlet, as shown in this figure:

Figure 7.1: Spherical ABC for Symmetry and Near the Radiation Outlet

FLUID130 and FLUID129 can be used for modal, harmonic, and transient analyses.

For more information, see Absorbing Boundary Condition (ABC) in the Mechanical APDL Theory Reference.



7.3. Perfectly Matched Layers (PMLs)

An absorbing boundary condition (ABC) absorbs the outgoing pressure wave so that there are no re-

flections back into the FEA computational domain. Perfectly matched layers (PMLs) are the layers of

pressure waves absorbing elements designed for the mesh truncation of an open FEA domain in a

harmonic analysis.

PML is an artificial anisotropic material. It is transparent and heavily lossy to incoming pressure waves.

PML can reduce the size of the computational domain significantly with very small numerical reflections.

A PML region is backed by a soft-sound boundary condition (p = 0).

If the pressure wave should be absorbed in only one direction, such as in a traditional tube, construct

a 1-D PML region in the global Cartesian coordinate system or a local Cartesian coordinate system, as

shown in this figure:

Figure 7.2: 1-D Multiple PMLs for Pipes

To define PML elements, issue the ET command. Set KEYOPT(4) = 1 for FLUID30, FLUID220, or FLUID221

prior to meshing the PML region. Use any element shape to mesh the PML block.

More than one 1-D PML regions can exist in a model. The element coordinate system (ESYS) uniquely

identifies each PML region. Define a Cartesian coordinate system (LOCAL) with one axis in the wave-

propagating direction, then assign the coordinate system to the elements in the PML region (VATT or

ESYS prior to meshing, or EMODIF after meshing).

Example 7.8: Defining PML Elements in a Local Coordinate System

et,11,200,6 ! mesh elementet,1,30,,1 ! normal fluid30et,2,30,,1,,1 ! pml fluid30

local,11,0,2,3,4,50,-60,135 ! local coordinatewpcsys,,11rect,0,l,-d/2,d/2 ! area in local csrect,l,l+dpml,-d/2,d/2 ! area in local csaglue,allesize,htype,11amesh,all ! mesh area with mesh200asel,allasel,u,,,3eslatype,1 ! normal elementmat,1esize,,2vext,all,,,0,0,d, ! 3d mesh with normal elementesys,11 ! activate local element coordinateasel,s,,,3type,2, ! pml element



Perfectly Matched Layers (PMLs)

mat,1esize,,2vext,all,,,0,0,d, ! 3d mesh with pml element in esys 11

nsel,s,loc,x,l+dpml ! nodes backed to pml d,all,pres,0. ! zero pressure on boundary

A 3-D PML region consists of layers of elements extending from the interior volume towards the open

domain, as shown in this figure:

Figure 7.3: PML Enclosure

Construct a block about the origin in the global Cartesian coordinate system or a local Cartesian coordin-

ate system. Align the edges of the 3-D PML region with the axes of the Cartesian coordinate system.

To optimize the absorbing efficiency of the PML, construct the PML regions and apply the following

parameters carefully:

Thickness of the PML region

Number of PML elements

Attenuation parameters

Number of buffer elements between the PML region and objects or discontinuities

The attenuation from the PML interface to the PML exterior surface is a parabolic distribution that

minimizes numerical reflections from the PML elements, as shown in this figure:



Figure 7.4: Attenuation Distribution

The numerical reflection is caused by the discretization of a continuous distribution of material from

element to element. To obtain satisfactory numerical accuracy, use at least two layers of PML elements.

The PML thickness may need to be greater than 1/10 of a wavelength.

Because a PML region acts as an infinite open domain, any boundary conditions and material properties

must be carried over to the PML region. Material properties such as mass density and sound speed in

the PML region must be identical to those of the adjacent interior region.

A sound-soft Dirichlet boundary with p = 0 must back all exterior surfaces of the PML region, except

for symmetric surfaces with the rigid wall boundary condition. To specify a sound-soft boundary condition

on the outer surfaces of the PML region, use the D command for a finite element model. The sound-

soft or sound-hard boundary conditions can be applied on symmetric surfaces of a PML region.

Include at least two buffer elements between the PML region and a discontinuity or object in the domain,

as shown in this figure:

Figure 7.5: Buffer Elements

The PML can then absorb the outgoing wave effectively and minimize numerical reflections.

Because PML is an artificial anisotropic material, excitation sources are prohibited in the PML region.

The attenuation of the pressure wave in a PML region can be controlled. If desired, specify the normal

reflection coefficient (harmonic) for propagating waves as follows:

PMLOPT,ESYS,Lab,Xminus,Xplus,Yminus,Yplus,Zminus,Zplus

The direction designations are Xminus, Yminus, Zminus, Xplus, Yplus, and Zplus. The minus andplus refer to the negative and positive directions along the Cartesian coordinate axes, respectively.

If the propagating wave is absorbed in only one direction, define a 1-D PML region (Lab = ONE). In thiscase, only the Xminus argument is necessary.




For a 3-D PML region, a different normal reflection coefficient can be defined for each direction (Xminus,Yminus, Zminus, Xplus, Yplus, Zplus). Normal reflection coefficients default to 10-3 (-60 dB) fora harmonic analysis. Normal reflection coefficients should be less than 1.0. If only a very few PML layers

are used (for example, two or three), specifying a very small normal reflection coefficient (such as -100

dB) may lead to significant numerical reflection.

Example 7.9: Defining 3-D PML Parameters

The following example input defines 3-D PML parameters as illustrated in Figure 7.3: PML Enclos-

ure (p. 28):

pmlxm=0 ! no pml in x directionpmlxp=-40 ! -40 dB in +x directionpmlym=0 ! no pml in y directionpmlyp=-40 ! -40 dB in +y directionpmlzm=-60 ! -60 dB in z directionpmlzp=-60 ! -60 dB in +z directionpmlopt,0,three,pmlxm,pmlym,pmlzm,pmlxp,pmlyp,pmlzp ! define 3-d pml

Repeat the PMLOPT command for additional PML regions. The PML may have a different number of

elements in each direction.

The number of PML layers determines the absorbing efficiency of the PML region. An excessive number

of PML elements significantly increases computational requirements. The number of PML layers (n) for

acceptable numerical accuracy is determined by the following command:

PMLSIZE,FREQB,FREQE,DMIN,DMAX,THICK,ANGLE

The following figure shows the relationship between DMIN, DMAX, and THICK:

Figure 7.6: Distance Between Source/Objects and PML Region

If n < 2, the number of layers is set to 2 to reduce numerical reflection. If n > 20, the number of layers

is set to 20 to avoid an excessive number of PML elements.

Before meshing the model, issue the PMLSIZE command. If the thickness of the PML region is known,

the command specifies an element edge length. If the thickness of the PML region is unknown, it specifies

the number of layers (n). For further information, see the PMLOPT and PMLSIZE commands in the

Command Reference, and Perfectly Matched Layers (PML) in the Mechanical APDL Theory Reference.

PML is used only in acoustic harmonic analysis.

PML may be necessary in cases where:



The impedance is unknown on exterior surfaces of the model, such as complex scatters.

Multiple propagating modes on the outlet surface are excited by discontinuities in the structure so that

the defined impedance may not absorb all outgoing propagating modes.

Using the spherical second-order ABC leads to numerous elements or lesser accuracy.

High absorbing rate is required for greater numerical accuracy.

In most acoustic radiation and scattering applications, the open domain is fully enclosed by 3-D PML,

as shown in Figure 7.3: PML Enclosure (p. 28). If the radiated acoustic field has no significant effect on

the excitation source entity, however, the 3-D PML can be used to locally enclose the open space near

the radiation outlet, as shown in this figure:

Figure 7.7: PML Near the Radiation Outlet

It is necessary to separate the PML region and sound-propagating region with the rigid wall, as the

PML connects only to the infinity.

For more information, see Perfectly Matched Layers (PML) in the Mechanical APDL Theory Reference.




Chapter 8: Applying Excitation Sources and Loads in an Acoustic

Analysis

The following topics are available:

8.1. Applying Acoustic Excitation Sources

8.2. Applying Acoustic Loads

8.1. Applying Acoustic Excitation Sources

Sound excitation sources are fundamental to an acoustic analysis. You can introduce sound excitation

sources via:

Specified pressure at nodes (D)

Normal velocity (harmonic analysis) or acceleration (transient analysis) on the exterior surface of the domain

(SF)

Arbitrary velocity (harmonic analysis) or acceleration (transient analysis) on the exterior surface of the

domain (BF)

Analytic incident wave sources (AWAVE)

Mass source (harmonic analysis) or mass source rate (transient analysis) at nodes, along lines, on surfaces,

or in volumes (BF)

The following table shows all excitation sources available for acoustic analysis:

Table 8.1: Acoustic Excitation Sources

FE Model EntitiesExcitation Sources

NodesPressure

NodesOutward normal velocity (acceleration)

Nodes or elementsArbitrary nodal velocity (acceleration)

Not applicableAnalytic incident wave sources

Nodes or elementsMass sources

Excitation sources can be applied on the finite element model entities.

The following detailed descriptions of the available excitations are available:

8.1.1. Pressure Excitation

8.1.2. Outward Normal Velocity (Acceleration) Excitation

8.1.3. Arbitrary Velocity (Acceleration) Excitation

8.1.4. Analytic Incident Wave Sources

8.1.5. Mass Source (Mass Source Rate)

For general information about applying loads, see Loading in the Basic Analysis Guide.



8.1.1. Pressure Excitation

Pressure excitation (D,Node,PRES) behaves as a Dirichlet pressure boundary (Pressure Boundary (p. 23)).

When applying pressure excitation, the pressure is enforced to a given value. Sound pressure reflected

by other objects back to the excitation point cannot be taken into account.

Pressure excitation can be used only under conditions where the effect of reflected sound pressure is

not required.

8.1.2. Outward Normal Velocity (Acceleration) Excitation

Outward normal velocity or acceleration can be applied to the exterior surface of the model

(SF,Nlist,SHLD).

Apply a minus sign of the outward normal velocity if an inward normal velocity is required.

For a harmonic analysis, a complex normal velocity to the surface is defined by the amplitude and phase

angle. The program solves for the pressure on the normal velocity excitation surface.

If the reflected sound pressure waves passing through the normal velocity excitation surface are taken

into account, apply the impedance boundary condition to the excitation surface so that the PML is not

backed to the excitation surface. In such a case, the normal velocity surface load is applied to the non-

PML element surface with doubled values that are applied to the exterior surface of the model.

Normal velocity excitation exists either on the structural surface or on the transparent pressure wave

port on which the incident wave propagates into the acoustic domain and the reflected wave backs to

the infinity. To absorb the reflected wave on the transparent port, apply the impedance boundary to

the port surface (SF,Nlist,IMPD or SF,Nlist,INF) along with the velocity excitation. To distinguishthe transparent wave port from the structural surface, specify the transparent port surface

(SF,Nlist,PORT).

The following command applies outward normal velocity or acceleration to the nodes of the FE model:

SF,Nlist,SHLD,Value,Value2

Example 8.1: Defining the Normal Velocity and Impedance BC on a Transparent Wave Port

nsel,loc,z,0 ! select nodes at z = 0sf,all,shld,vn,ang ! complex normal velocitysf,all,impd,z0 ! impedance boundarysf,all,port,1 ! transparent port

Example 8.2: Defining the Normal Velocity and Impedance BC on a Structural Surface

nsel,loc,z,0 ! select nodes at z = 0sf,all,shld,vn,ang ! complex normal velocitysf,all,impd,z0 ! impedance boundary

Example 8.3: Defining the Frequency Dependency of the Normal Velocity of Acceleration

Use tables (*DIM) in the SF command to define the frequency dependency of the normal velocity of

acceleration, as shown:

*dim,vn,TABLE,2,1,1,FREQ ! normal velocity table*dim,ang,TABLE,2

ANSYS Mechanical APDL Acoustic Analysis Guide

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