Copyright 2005 ABAQUS,Inc.Modeling Acoustic Problems Using ABAQUSLecture 3Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.2OverviewAcoustic Element TypesMaximum Element SizeExternal Meshed DomainBoundary ConditionsLoadsExterior Problems using Infinite ElementsAnalysis ProceduresOutputStructural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.Acoustic Element TypesStructural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.4Acoustic Element TypesAcoustic elements are available in linear and quadratic interpolation.ABAQUS/Standard has first-and second-order full integration elements:Quadrilateral and triangular for planar and axisymmetric domains(AC2D3,AC2D4,AC2D6,AC2D8,ACAX3,ACAX4,ACAX6,ACAX8)Hexahedral,tetrahedral,and wedge for three-dimensional domains(AC3D4,AC3D6,AC3D8,AC3D10,AC3D15,AC3D20)One-dimensional acoustic links to model acoustic channels(AC1D2,AC1D3)Examples:Piping,ducts,small openings between large regionsStructural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.5Acoustic Element TypesABAQUS/Explicit has first-order reduced integration elements:Quadrilateral and triangular for planar and axisymmetric domains(AC2D3,AC2D4R,ACAX3,ACAX4R)Hexahedral and tetrahedral for three-dimensional domains(AC3D4,AC3D8R)Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.6Acoustic Element TypesAcoustic-structural interface(ASI)coupling elements(ABAQUS/Standard only):Provide the coupling effects between an acoustic mesh and a structural mesh.Can be defined two different ways:User can explicitly create ASI elements.User can allow ASI elements to be generated internally and automatically,using TIE constraints between the fluid and solid surfaces.TIE constraints are available for both ABAQUS/Standard and ABAQUS/Explicit and will be discussed further in the next lecture.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.7Acoustic Element TypesAcoustic infinite elements:Available in both ABAQUS/Standard and ABAQUS/Explicit*.Provided for exterior problems.Have surface topology and are similar to structural infinite elements.Generally defined on a terminating surface of a region of acoustic finite elements.Formulation is considerably more accurate than impedance-type boundary conditions for complex wave fieldsninth order implementation.Can be coupled directly to a structural surface using TIE constraints.In general,acoustic infinite elements will provide higher accuracy when used instead of impedance-type boundaries at the same distance from the structure.*For ABAQUS/Explicit,this represents a significant advance over the published literature on infinite elements.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.8Acoustic Element Types Element selection Interpolation orderSecond-order elements are generally much more accurate than first-order acoustic elements for a given number of degrees of freedom.First-order elements are slightly faster and are included for convenience.Both first-and second-order acoustic meshes work well together with structural meshes.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.9Acoustic Element Types Element shapeQuad/hex elements are slightly more accurate than tri/tet elements.Often tri/tet elements require less effort in creating the mesh.General commentsThere are no problems associated with using different integration orders or different element shapes on the fluid and solid meshes.ABAQUS/Explicit offers reduced integration elements with automatic hourglass control,making hourglassing extremely unlikely.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.10Acoustic Element TypesAcoustic importAcoustic elements may be imported between ABAQUS/Standard and ABAQUS/Explicit.This enables coupled fluid-solid analyses such as:Eigenanalysis(ABAQUS/Standard)of post-shock(ABAQUS/Explicit)structures.Transient dynamic(ABAQUS/Explicit)analysis of systems under static preload(ABAQUS/Standard).The interface for acoustic import is identical to that for general import in ABAQUS.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.Maximum Element SizeStructural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.12Maximum Element SizeElement sizeRelated to maximum frequency(or minimum wavelength)Higher frequency response requires smaller elements.Wavelength decreases with increasing frequency,so there is an upper frequency limit for a given mesh.Maximum element sizeFor reasonable accuracy at least six representative internodal intervals(nmin)should fit into the shortest acoustic wavelength of interest.An“internodal interval”is defined as the distance from a node to its nearest neighbor in an element.Structural-Acoustic Analysis with ABAQUSCopyright 2005 ABAQUS,Inc.L3.13 Maximum Element SizeFor example,the maximum element size for a linear element is half the maximum element size for a quadratic element.Use the following relationships to determine the m