AACTx R160 L-01 Introduction to Acoustics

Introduction to AcousticsAcoustics ACTx R160 1 © 2011 ANSYS, Inc. July 20, 2015 Acoustics ACTx 2 © 2011 ANSYS, Inc. July 20, 2015 Acoustics ACT Extension Expose 3D acoustic features in Mechanical without the need for APDL • Define acoustics properties • Apply acoustic boundary conditions & loads • Postprocess acoustic results 3 © 2014 ANSYS, Inc. July 20, 2015 AACTx R160 Enhancements The following enhancements have been exposed in the R160 version of this extension: – – – – – – – – – – – Input and output parameters definition Diffuse Sound capability Fluent pressure mapping via CGNS file Mass Source/Mass Source Rate support tabular data Acoustic element morphing option in linear perturbation analysis User defined forcing frequencies FSI Response Spectrum analysis Far Field Contour Plot Far Field option for Y axis rotated sector Cartesian coordinates input for microphone result Selected surface acoustics result (acoustic impedance. sound power…) – Frequency band sound pressure level – Pressure & SPL Probes – Importing/Exporting material properties 4 © 2014 ANSYS. force. 2015 . mechanical impedance. Inc. July 20. If you need more information about ACT please contact your sales representative.ACT: Application Customization Toolkit ACT allows customization in the Mechanical application : • Replace command snippets with interactive objects • Create customized Loads / BCs • Create customized Results • Ability to connect a third party solver in a standard Workbench process ACT Acoustics extension is a customization made with ACT to integrate ANSYS acoustics capabilities in Mechanical. 2015 . July 20. The extension consists of one XML file (Configures the UI content) and one python script (Implements the extension functionality). 5 © 2014 ANSYS. Inc. ACT Extension Library ANSYS Customer Portal • support.wbex file) or binary plus scripted format (python and XML files) • Scripted extensions are great examples • Links to ACT documentation and training material • Goals for an ACT developers forum 6 © 2013 ANSYS. 2015 .com • A library of helpful ACT extensions available to any ANSYS customer • Roughly one dozen available • More being added continually Great place to get started • Extensions made available in either binary format (. Inc. July 20.ansys. Download the Acoustics ACT Extension The Acoustics ACT Extension for ANSYS 16.ansys.com . 2015 david. Inc.roche@ansys. So to report an issue or provide your feedback in regards to this extension please contact: David Roche: 7 © 2013 ANSYS. July 20.0 is available for download on the ACT Extension Library of the Customer Portal: https://support.com/AnsysCustomerPortal/en_us/Downloads/Applic ation+Library Please pay attention to paragraph 9 of the CLICKWRAP SOFTWARE LICENSE AGREEMENT FOR ACS EXTENSIONS regarding TECHNICAL ENHANCEMENTS AND CUSTOMER SUPPORT (TECS): “TECS is not included with the Program(s)”. It will open a file dialog to select a “*.wbex” file 3. Select the “Install Extension …” option 2.Installing ACT Extensions Installing from WB Project page: 1. The extension is installed 8 © 2014 ANSYS. July 20. Inc. 2015 . July 20. 2015 .Acoustics ACT Extension The ACT Acoustics extension can be used to create acoustic boundary conditions and define fluid bodies (elements & material properties): 9 © 2014 ANSYS. Inc. July 20. Inc. 2015 .Introduction to Acoustics 10 © 2011 ANSYS. the acoustic counterpart of radar • Design of concert halls. and many other similar devices. Applications for acoustics include the following: 11 • Sonar . • Geophysical exploration © 2014 ANSYS. Inc. mufflers. July 20. and reflection of sound pressure waves in a fluid medium. where an even distribution of sound pressure is desired • Noise minimization in machine shops • Noise cancellation in automobiles • Underwater acoustics • Design of speakers. acoustic filters. 2015 . speaker housings. absorption. propagation.Applications Acoustics is the study of the generation. The pressure solution is the deviation from the mean pressure. Also. A coupled acoustic analysis takes the fluid-structure interaction into account. Typical quantities of interest are the pressure distribution in the fluid at different frequencies. diffraction. 12 © 2014 ANSYS. scattering.Acoustic Analysis An acoustic analysis. pressure gradient. usually involves modeling the fluid medium and the surrounding structure. available in the ANSYS Multiphysics and ANSYS Mechanical programs only. radiation. particle velocity. as well as. and dispersion of acoustic waves. not the absolute pressure. Inc. but allows only relatively small pressure changes with respect to the mean pressure. attenuation. transmission. 2015 . July 20. The program assumes that the fluid is compressible. the sound pressure level. the fluid is assumed to be non-flowing. An uncoupled acoustic analysis models only the fluid and ignores any fluidstructure interaction. generated by a vibrating surface. water. Sound propagates in the form of longitudinal (as opposed to transverse) waves. involving a succession of compressions and rarefactions in the elastic medium. solids). July 20. sound and noise constitute the same phenomenon of atmospheric pressure fluctuations about the mean atmospheric pressure. The differentiation is greatly subjective. When a sound wave propagates in air. or oscillations.g. 13 © 2014 ANSYS. Inc. From the acoustics point of view. in an elastic medium (e. 2015 .. Sound (or noise) is the result of pressure variations. the oscillations in pressure are above and below the mean pressure.Sound Noise can be defined as undesired or disagreeable sound. or turbulent fluid flow. air. it causes measurable fluctuations in pressure. For our purposes in acoustics and noise control. 2015 .t) meters/second Temperature Ttotal = T + τ(r.t) kg/m3 July 20. temperature and density. We can describe the physical state in terms of mean (steady state) values and small fluctuations about that mean.t) ° Celsius Density ρtotal = ρ + σ(r. 14 © 2014 ANSYS. Inc.Acoustics Variables As sound propagates through air (or any elastic medium). all we care about is the fluctuating portion. velocity.t) Pascals Velocity Utotal = U + u(r. Physical Quantity State Variable Units Pressure Ptotal = P + p(r. Each particle in the fluid moves back and forth about one position. Longitudinal Wave – The simplest type of wave is compressional (or longitudinal wave) where the particle oscillation is in the same direction as the energy transport. 15 © 2014 ANSYS. sound waves in any medium can be a mixture of longitudinal and shear waves. The disturbance propagates in the direction of the particle motion. In general. Inc. 2015 . you can have transverse shear and torsional waves. and water waves are a mixture of shear and longitudinal waves. Bending waves (in a beam or plate). depending primarily on the boundary conditions. July 20. In solids. This is the predominant mechanism in fluids and gases because shear stresses are negligible.Sound waves As sound travels in space there is energy transport but there is no net transfer of mass. Shear Wave – The particle motion direction is orthogonal (perpendicular) to direction in which the disturbance (and the energy) propagates. at audible frequencies. the pressure fluctuations occur under essentially adiabatic conditions (no heat is transferred between adjacent gas particles). The speed of sound then becomes: 𝒄= where : ϒ=Cp/Cv=1. sound travels at a speed of c: 𝒄= 𝟒 𝑲 + (𝟑)𝑮 𝝆 K = bulk modulus G = shear modulus ρ= density of the material In normal gases. Inc.4 for air and P =ρRT (Ideal Gas Law) 16 © 2014 ANSYS.Speed of sound For a longitudinal wave in an unbounded medium. 2015 𝜸𝑷 𝝆 . July 20. 1 17 © 2014 ANSYS.1 0 2 -1.x The time for wave to repeat (its period): T  1  2 f  Pressure Period T 0. 2015 8 10 12 .1 6 8 10 12 Distance .1 0 2 4 -1.Wave length In acoustics we define the wavelength λ as the distance between repeating features of the wave: c 2   f k Pressure Wavelength λ 0.t July 20. 4 6 Time .9 -0. Inc.9 -0. July 20. • • 18 Sound Intensity Level: Sound Pressure Level: © 2014 ANSYS.Units & Levels Human response to sound is roughly proportional to the logarithm of sound power. 2015 𝑳𝑰 = 𝟏𝟎𝒍𝒐𝒈𝟏𝟎 𝑳𝑷 = 𝟐𝟎𝒍𝒐𝒈𝟏𝟎 𝑰 𝑰𝒓𝒆𝒇 𝑷𝒓𝒎𝒔 𝑷𝒓𝒆𝒇 . A logarithmic level (measured in decibels or dB) is: 𝑳𝒘 = 𝟏𝟎𝒍𝒐𝒈𝟏𝟎 𝑾 𝑾𝒓𝒆𝒇 An increase in 1 dB is the minimum increment necessary for a noticeably louder sound. Inc. Other quantities of interest in acoustics are the sound power level and the sound pressure level. 2015 60 quiet 40 very quiet 20 . The chart below shows typical noise levels of common noise sources. whisper Recording studio 19 © 2014 ANSYS. Inside subway train 120 very noisy 100 Noisy urban daytime Noisy 80 Wrinkled newspaper Moderately noisy 70 Conversation at 1 meter Quiet urban nighttime. Inc. July 20.Sound Pressure Levels The range of audible sound ranges from approximately 1 to 140 dB. although everyday sounds rarely rise above about 120 dB. Artillery fire Qualitative Descriptions Sound Pressure level (dB) Intolerable 140 Rock group. Sound Sources Jet Take-off. Trail bike Discotheque. this effectively cuts off the lower and higher frequencies that the average person cannot hear.Frequency Weighting The human ear responds more to frequencies between 500 Hz and 8 kHz and is less sensitive to very low-pitch or high-pitch noises. to ensure that the meter is measuring pretty much what you actually hear. 2015 . July 20. 20 © 2014 ANSYS. Inc. The most common weighting that is used in noise measurement is A-Weighting. Like the human ear. The frequency weightings used in sound level meters are often related to the response of the human ear. July 20.Directivity Most sources do not radiate equally in all directions. Piston Define a directivity factor Q (called Dθ in some references): where: Pϑ = actual rms sound pressure at angle θ PS = rms sound pressure of a uniform point source radiating the same total power W as the actual source 21 © 2014 ANSYS. Example – a circular piston in an infinite baffle (which is a good approximation of a loudspeaker). Inc. 2015 . Acoustic Energy Dissipation Acoustic energy is lost when converted into thermal energy. Here. July 20. thin ducts. and small rooms) • Losses in the medium (important when the volume of fluid is large). Sources of dissipation are due to: 22 • Losses at the boundaries (relevant for porous materials. Acoustic energy dissipation is often very slow and it can be ignored for small distances or short times. 2015 . the losses are associated with: – viscosity – heat conduction © 2014 ANSYS. Inc. Inc. proportional to velocity. a natural choice for this measure is the ratio between pressure and velocity: Z p u p  Z   0 c0 If we define the reflection coefficient R: R    p Z   0 c0 The impedance with no reflection (of a plane wave) is thus: Z   0 c0 23 © 2014 ANSYS.Impedance & Reflection coefficient A useful quantity in acoustics is impedance. Since frictional forces are. by and large. 2015 . It is a measure of the amount by which the motion induced by a pressure applied to a surface is impeded. July 20. – panel resonators or – volume resonators 24 © 2014 ANSYS. A measure of that absorption is the absorption coefficient α.Sound Absorption As sound strikes a wall. July 20. while some is absorbed by the wall. some of it is reflected. Inc. defined as: I I I   absorbed  incident reflected I incident I incident Incident Transmitted Reflected While some of the absorbed sound is dissipated as heat in the material. 2015 . The amount of energy that gets into the next room is quantified by the transmission coefficient: I   transmited I incident Absorption can be obtained by three primary mechanisms: – porous materials. some re-radiates from the other side. The acoustic wave travels down the quarter wave tube and back. travelling half the wavelength which in turn experiencing 180 degree phase shift interfering with the incoming acoustic wave. pump pulsation abatement.Quarter Wave Resonator Quarter wave tubes are commonly used in applications such as air intake induction system on engines. and other narrow band noise mitigation applications. 2015 . destructively. July 20. 25 © 2014 ANSYS. abating the target noise. The length of a quarter wave tube is a quarter of a wavelength of the noise it is tuned to. Inc. The pressure in the cavity changes by the influx and efflux of fluid through the neck. The fluid in the cavity resembles a mechanical mass element.Helmholtz Resonator Helmholtz resonator is a side branch acoustic absorber. The break-up of vortices created in the shear layer dissipates energy. July 20. 2015 . making the cavity act as a spring element. Inc. It 0consists of a rigid cavity communicating with the external medium through a port (neck). acting as a damper. 26 © 2014 ANSYS. July 20. 2015 . Inc.Impedance & Absorption coefficient There are different ways to determine the impedance or the absorption coefficient of a material: 27 • Free field methods under anechoic conditions • Reverberant field methods • Impedance tube methods © 2014 ANSYS. To get the best of both worlds.STL or τ Good absorbing materials allow sound pressure fluctuations to enter their surface and dissipate energy by air friction. Fiberglass insulation makes a terrible barrier. it is common to see an absorbing layer laminated to a barrier material. a single homogeneous material will not be both a good absorber and a barrier. open cell foam. Inc. Good barrier materials reflect sound. for instance a layer of gypsum board and a layer of fiberglass. July 20. 2015 . steel. glass. or loaded vinyl laminated to open cell foam. such as fiberglass. or acoustical ceiling tiles. and a sealed concrete wall has virtually no absorption.α • Ability to reflect or block sound energy . and are dense and nonporous (concrete. They are generally porous and lightweight. In general. brick. gypsum board). 28 © 2014 ANSYS.What is the difference between an absorbing material and a barrier material? The two important noise-related properties of a material are: • Ability to absorb acoustic energy . lead. July 20. Inc. 2015 .Boundary Conditions There are three different types of boundary conditions in Acoustics: • Dirichlet condition: – Homogeneous (Open tube: Sound Soft Boundary): p  0 – Inhomogeneous (Applied pressure): p  P • Neumann condition: – Homogeneous (Closed tube: Sound Hard Boundary): – Inhomogeneous (Velocity excitation): • v V p 0 x v0 V  1 p j x Robin condition (Given admittance): p   j 0Yp x Note: By default the natural boundary condition in acoustics corresponds to a rigid wall (Neumann condition) 29 © 2014 ANSYS. 2015 . the liquid must have a free surface to constitute a slosh dynamics problem. and cargo slosh in ships and trucks transporting liquids (for example oil and gasoline). where the dynamics of the liquid can interact with the container to alter the system dynamics significantly. Inc. typically. Important examples include propellant slosh in spacecraft tanks and rockets (especially upper stages).Sloshing In fluid dynamics. Strictly speaking. also undergoing motion). July 20. sloshing refers to the movement of liquid inside another object (which is. 30 © 2014 ANSYS. Sloshing in a rigid tank Mode 1 31 © 2014 ANSYS.8819 Hz Mode 2 1. Inc.3213 Hz July 20.25 Hz 1.32 Hz 1.88 Hz 0.2496 Hz Mode 4 1.0506 Hz Mode 3 1. 2015 Mode 4 .05 Hz 1. Mode 2 Mode 3 Sloshing Modes Analytical Numerical Mode 1 0. Inc. July 20. 2015 . UX.Understanding Acoustic Analysis Terminology The following common terms are used throughout this guide: 32 • 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 infinite open space • Coupled element: Acoustic element with FSI interface (PRES. UZ DOF) • Uncoupled element: Acoustic element without FSI interface (PRES DOF) • PML: Perfectly matched layers • 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 infinity © 2014 ANSYS. UY. 2015 .Governing Equations 33 © 2011 ANSYS. Inc. July 20. © 2014 ANSYS. Inc. The discretized structural dynamics equation can be formulated using the structural elements. • There is no mean flow of the fluid.Governing Equations In acoustic fluid-structural interaction (FSI) problems. The fluid momentum (Navier-Stokes) equations and continuity equations are simplified to get the acoustic wave equation using the following assumptions: 34 • The fluid is compressible (density changes due to pressure variations). July 20. 2015 . the structural dynamics equation must be considered along with the Navier-Stokes equations of fluid momentum and the flow continuity equation. the wave equation is referred to as the lossy wave equation for propagation of sound in fluids. Harmonically varying pressure is given by:  pr . July 20. The discretized structural and the lossy wave must be considered simultaneously in FSI problems. t   Re pr e jt  The wave equation is reduced to the following inhomogeneous Helmholtz equation: 35 © 2014 ANSYS.Governing Equations Since the viscous dissipation has been taken into account using the Stokes hypothesis. 2015 . Inc. The acoustic pressure exerted on the structure at the FSI interface will be considered in the Derivation of Acoustics matrices to form the coupling stiffness matrix. The wave equation is multiplied by the testing function w and integrated over the volume of the domain with some manipulation to yield the following: 36 © 2014 ANSYS. 2015 .Governing Equations The finite element formulation is obtained by a testing wave using the Galerkin procedure. July 20. Inc. 2015 . the normal velocity on the boundary of the acoustic domain is given by: The “weak” form of equation is given by: 37 © 2014 ANSYS.Governing Equations From the equation of momentum conservation. Inc. July 20. Inc. July 20. 2015 . given by: After using the above equation: 38 © 2014 ANSYS.Governing Equations The normal acceleration of the fluid particle can be presented using the normal displacement of the fluid particle. July 20. The wave equation can be written in matrix notation to create the following discretized wave equation: 39 © 2014 ANSYS. 2015 .Derivation of Acoustic Matrices Other terms are defined in Acoustic Fundamentals. Inc. Inc.Analysis Types 40 © 2011 ANSYS. 2015 . July 20. Analysis Types In Acoustics we can currently perform four different types of analysis: • Modal Analysis (frequency domain) • Harmonic Response Analysis (frequency domain) • Transient Analysis (time domain) • Response Spectrum These analysis can be solved as pure acoustic problems as well as vibro-acoustic problems. 41 © 2014 ANSYS, Inc. July 20, 2015 Modal Analyses • The goal of modal analysis is to determine frequencies and standing wave patterns within a structure • We have the ability to include impedance and interaction with structure (FSI) • Block Lanczos, Damped, Subspace and unsymmetric eigensolvers are available Image on the right shows standing wave patterns in an acoustic cavity 42 © 2014 ANSYS, Inc. July 20, 2015 Harmonic Response Analyses 43 • The objective of a harmonic analysis is to calculate the response of the system as a function of frequency based on volumetric flow rate or pressure excitation • A plot of transmission loss is shown below left, sound waves in a room shown below right. • Full and mode-superposition (without damping) Harmonic are supported • Sparse, QMR and ICCG solvers are available. © 2014 ANSYS, Inc. July 20, 2015 acoustic waves generated from offshore pile driving is at right (courtesy of MENCK) • Only Full Transient is supported (not mode-sup transient) © 2014 ANSYS.Transient Analyses 44 • Transient simulations allow us to investigate the time-dependent response of a system • An example of a time-history pressure plot showing the beat phenomenon is at left. Inc. 2015 . July 20. Inc. July 20.Response Spectrum 45 • Response Spectrum analysis allows us to investigate the maximum response of a structure subject to a time-history loading function. • It requires an unsymmetric upstream modal analysis. 2015 . • Single Point Response Spectrum (base motion is uniform) and Random Vibration are supported. © 2014 ANSYS. 2015 . Inc.Acoustic Material Properties 46 © 2011 ANSYS. July 20. DENS & MP. One can then scope to the bodies representing the fluid domain and input the mass density and the sound velocity: Note: MAPDL commands: MP. Thus these properties have to be input for the acoustic domains. July 20. An acoustic domain is defined with the “Acoustic Body” object.Mass Density & Sound Speed The wave equation used in acoustic simulation requires mass density and sound velocity of the fluid media. Inc.. 2015 ..SONC 47 © 2014 ANSYS. 2015 .Acoustic Viscosity Viscosity is the degree to which a fluid resists flow. The acoustic media can sometimes be viscous (water. July 20. • Governing equation – Momentum and mass conservation equation  va 4   a  0  pa  (    B )(  va )    0  va t 3 t – 2nd –order acoustic pressure equation 1  2 pa 4 1 pa   ( pa )     [  ( )]  0 2 2 2 0 0c0 t 30 0c0 t 1 48 © 2014 ANSYS. Inc. In this case the wave equation must be modified to include the bulk viscosity term. oil…). 2015 . Inc. July 20.Acoustic Viscosity The viscosity introduces a dissipative effect in the acoustic media as illustrated below: Transmission Loss (dB) Transmission Loss 40 30 20 Without Viscosity 10 With Viscosity 0 500 1000 1500 Frequency (Hz) The dynamic viscosity (Pa-s) can be input in the detail properties of the “Acoustics Body”: Note: MAPDL command: MP.VISC 49 © 2014 ANSYS.. 50 © 2014 ANSYS. Then the created xml file can be imported using “Import” option on another “Acoustic Body” to automatically apply the material property values in the xml file. Inc. July 20.Importing/Exporting Material Properties It is possible to right click on an “Acoustic Body” and choose “Export” option in order to export the defined material properties into a given xml file. 2015 . July 20.Non Uniform Acoustic Media 51 © 2011 ANSYS. Inc. 2015 . 2015 . the ideal gas pressure has no effect on the speed of sound. only the medium's compressibility and density are important factors. Adiabatic compressibility is directly related to pressure through the heat capacity ratio (adiabatic index). In fluids. and pressure and density are inversely related at a given temperature and composition. because pressure and density (also proportional to pressure) have equal but opposite effects on the speed of sound. molecular composition. and heat capacity ratio) important. July 20. 52 © 2014 ANSYS.Non-uniform Acoustic Media The speed of sound is variable and depends on the properties of the substance through which the wave is travelling. and the two contributions cancel out exactly. At a constant temperature. Inc. thus making only the latter independent properties (temperature. Inc. 2015 .Non-uniform Acoustic Media In non-uniform acoustic media the mass density and sound speed vary with the spatial position. The wave equation in lossless media is written by: 1 1 2 p  Q (   (  p)   2   )  0 (r ) 0 (r )c (r ) t 2 t 0 (r ) According to the ideal gas law the equation of state and the speed of sound in an ideal gas are given by:   c 2 (r )  RT (r )    Pstate (r )   (r ) RT (r ) 53 © 2014 ANSYS. July 20. 2015 T ( x) T0 pstate ( x)  0T0 . Inc. July 20.Non-uniform Acoustic Media Assuming the density ρ0 and sound speed c0 at the reference temperature T0 (input as the Environment Temperature) and the reference static pressure (input in the properties of the “Acoustics Body”) casts the density and sound speed in media as follow: c( x)  c0 54 © 2014 ANSYS.  ( x)  T ( x) pstate . 0 . July 20. Inc. 2015 .Non-Uniform Acoustic Medium One can see the evolution of the sound speed in the acoustic media for a spatial variation of the temperature of the fluid: 55 © 2014 ANSYS. 6. Oberg.L.W. Modal analysis predicts correct results • C. Vol. Baer. No. 2015 . 56 © 2014 ANSYS. July 20. Inc. N.Non-Uniform Acoustic Medium • Step change in temperature is shown on left. AIAA Journal. pp 1131-1137.D. Ryan. 6. which affects propellant properties. A. A Study of T-Burner Behavior. TEMP.ACT Acoustics Extension The thermal condition can be applied on bodies using the “Acoustics Temperature” object available in the “Loads” drop down menu : Note: MAPDL command: BF. value 57 © 2014 ANSYS.CHRGD. 2015 . Inc.. July 20. value The static pressure can be applied on bodies using the “Acoustics Static Pressure” object available in the “Loads” drop down menu : Note: MAPDL command: BF.. Inc. 2015 .Mesh Requirement 58 © 2011 ANSYS. July 20. For linear elements. 2015 . while 6 elements per wavelength are needed for quadratic elements. Wavelength:  c f Also note that all acoustic domain must be mesh connected (nonconformed mesh is not supported for acoustic domains). July 20.Meshing Guidelines The mesh should be fine enough to capture the mode shapes of the structure. Inc. at least 12 elements per wavelength are needed. 59 © 2014 ANSYS. Solving Fluid-Structure Interaction (FSI) 60 © 2011 ANSYS. Inc. 2015 . July 20. July 20. transient (unsymmetric algorithm) and spectrum (unsymmetric algorithm). 61 © 2014 ANSYS. 2015 . The governing finite element matrix equations then become: The analyses available with FSI are modal (symmetric & unsymmetric algorithm). Then the interaction of the fluid and the structure at a mesh interface causes the acoustic pressure to exert a force applied to the structure and the structural motions produce an effective “fluid load“. Inc.Acoustic Structure Coupling If you want to take into account the fluid-structure interaction (pressure waves generated by the structure vibration or/and structure deformation due to fluid pressure) you can use coupled acoustic analysis. harmonic (symmetric & unsymmetric algorithm). is defined to be outward from the fluid mesh and in towards the structure.Acoustic Structure Coupling The interaction of the fluid and the structure at a mesh interface causes the acoustic pressure to exert a force applied to the structure and the structural motions produce an effective "fluid load. July 20. as the program uses it. Both the structural and fluid load quantities that are produced at the fluid-structure interface are functions of unknown nodal degrees of freedom. Inc. 2015 ." The governing finite element matrix equations then become: [R] is a "coupling" matrix that represents the effective surface area associated with each node on the fluid-structure interface (FSI). The coupling matrix [R] also takes into account the direction of the normal vector defined for each pair of coincident fluid and structural element faces that comprises the interface surface. Placing these unknown "load" quantities on the left hand side of the equations and combining the two equations into a single equation produces the following: The foregoing equation implies that nodes on a fluid-structure interface have both displacement and pressure degrees of freedom. The positive direction of the normal vector. 62 © 2014 ANSYS. Acoustic Structure Coupling There are different approaches to define acoustic structure coupling depending of the vibro-acoustic analysis: • For modal analyses & harmonic: Program Controlled Uncoupled. Uncoupled with symmetric algorithm. 63 © 2014 ANSYS. With unsymmetric matrices we require twice as much memory because we need to store the full matrix and not only the upper triangular half. July 20. the CPU time increases by about 50%. Inc. Program Controlled Coupled (Unsymmetric). 2015 . Also. The default approach is to use unsymmetric matrices but in V13 we introduced a more efficient symmetry formulation (for modal and harmonic). Program Controlled Coupled (Unsymmetric). The symmetric formulation preserves the symmetric nature of the matrices avoiding the performance issues above. Coupled with symmetric algorithm • For transient analyses: Program Controlled Uncoupled. • For spectrum analyses: Program Controlled Coupled (Unsymmetric). ACT Acoustics Extension The coupling algorithm is chosen in the properties of the “Acoustic Body” depending on the analysis type: 64 © 2014 ANSYS. July 20. 2015 . Inc. UY.uncoupled element (DOF: PRES.UZ.coupled element (DOF: UX.uncoupled element (DOF: PRES. Inc.uncoupled element (DOF: PRES.UZ. symmetric solver) • Program Controlled Coupled: KEYOPT(2) = 0 . SP01.UY.Acoustic Structure Coupling Below are descriptions of the different algorithms behavior: • Program Controlled Uncoupled: KEYOPT(2) = 1 .PRES. symmetric solver) • Coupled With Symmetric Algorithm: KEYOPT(2) = 0 . 2015 .coupled element (DOF: UX.UZ. symmetric solver) © 2014 ANSYS.UY. July 20. SP01.PRES. unsymmetric solver if any KEYOPT(2) = 0) Modal analysis: • Uncoupled With Symmetric Algorithm: KEYOPT(2) = 1 .coupled element (DOF: UX. symmetric solver) Harmonic analysis: 65 • Uncoupled With Symmetric Algorithm: KEYOPT(2) = 1 .PRES. symmetric solver) • Coupled With Symmetric Algorithm: KEYOPT(2) = 0 . It’s possible to use both coupled and uncoupled symmetric formulations. July 20. 66 © 2014 ANSYS. The best solution here in terms of number of DOF to compute is to create a single of layer of elements using coupled algorithm at the FSI boundary and use uncoupled algorithm for all other elements.Acoustic Structure Coupling • Symmetric formulation: All the elements in the model can use the symmetric formulation. The best solution here in terms of number of DOF to compute is to create a single of layer of elements using coupled algorithm at the FSI boundary and use uncoupled symmetric algorithm for all other elements. Inc. 2015 . • Unsymmetric formulation: In this case it’s possible to use both program controlled coupled and program controlled uncoupled formulations. Inc. it’s good practice to define FSI manually.FSI.. 2015 . Note: MAPDL command: SF. July 20. However. 67 © 2014 ANSYS.ACT Acoustics Extension The FSI interfaces correspond to the acoustic domain faces in contact with structure bodies (where pressure are transferred to the structure).1 It’s worth noting that if no FSI flags are defined. the MAPDL solver will try to automatically detect FSI surfaces (in case of conform mesh). The acoustic Fluid-Structure interface can be identified using the “FSI Interface” object available in the “Boundary Conditions” drop down menu. Inc. 2015 .Structure/Fluid domains connection The displacements of the structure can be transferred to the fluid domain using a connected mesh or contact regions. Please note that when you perform vibroacoustic analyses. it is strongly recommended to use a multibody part definition rather than contact. 68 © 2014 ANSYS. July 20. To get accurate results it is then important to follow the recommendations detailed in the following slides. Due to the complexity of the geometry and to facilitate the mesh generation contacts can however be an alternative. Inc. Uncouple U to ROT 69 © 2014 ANSYS. July 20. 2015 .Structure/Fluid domains connection To connect the structural components and the acoustic domains with contacts regions it is recommended the following contacts settings for a better accuracy: • Type: Bonded • Behavior: Asymmetric with contact on the fluid side and target on the structural side • Formulation: MPC • Detection Method: Nodal-Projected Normal From Contact For shell structural elements: • Constraint Type: Target Normal. Structure/Fluid domains connection To connect the structural components and the acoustic domains with contacts regions it is recommended the following contacts settings for a better accuracy: • Type: Bonded • Behavior: Asymmetric with contact on the fluid side and target on the structural side • Formulation: MPC • Detection Method: Nodal-Projected Normal From Contact For shell structural elements: • Constraint Type: Target Normal. Uncouple U to ROT 70 © 2014 ANSYS. July 20. 2015 . Inc. Inc. July 20. 2015 .Structure/Fluid domains connection It is possible to use “Convert Standard to FSI Recommended Contacts” object available from “Tools” menu to define the recommended settings: When the object is generated it will automatically modify the contacts regions located between structural and acoustic parts to define the recommended settings: 71 © 2014 ANSYS. July 20.Automatic creation of boundary conditions 72 © 2011 ANSYS. Inc. 2015 . 2015 . Inc. July 20. 73 © 2014 ANSYS.Structure/Fluid domains connection Clicking on this button will create an FSI Interface object scoped on all faces used by contact regions and belonging to the defined acoustic bodies (Fluid-Structure interface faces of the acoustics side when contact is used between acoustic and structural domains). When the button is pressed it’s checked for each named selection if it contains one of the following keywords. If it’s the case the corresponding object is created scoped on this named selection. 2015 . Keyword acousticbody normalvelocity normalacceleration masssource massrate surfacevelocity surfaceacceleration staticpressure impsheet temperature pressure impedance thermovisc free fsi radiation absorbingelem attenuation plot 74 © 2014 ANSYS. Inc. Corresponding Object Acoustic Body Normal Surface Velocity Normal Surface Acceleration Mass Source Mass Source Rate Surface Velocity Surface Acceleration Static Pressure Impedance Sheet Temperature Acoustic Pressure Impedance Boundary Thermo-viscous BLI Boundary Free Surface FSI Interface Radiation Boundary Absorbing Elements Attenuation Surface Acoustic Time_Frequency Plot July 20.Automatic creation of BC This tool is used to automatically create boundary conditions & loads based on the existing named selections. Inc. 2015 .Applications 75 © 2011 ANSYS. July 20. July 20. 2015 .Underwater Example 76 © 2014 ANSYS. Inc. July 20. Inc. 2015 .Speaker Example 77 © 2014 ANSYS.