ABAQUS声学分析-acoustics-lecture4.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Copyright 2005 ABAQUS,Inc.,Structural-Acoustic Analysis with ABAQUS,L4.,*,Structural-Acoustic Analysis with ABAQUS,Coupled Structural-Acoustic Analysis,Lecture 4,深圳,ABAQUS,培训,深圳,ANSYS,培训,深圳,ANSYS,深圳,ABAQUS,深圳有限元培训,ABAQUS,培训,ANSYS,培训,Structural-Acoustic Analysis with ABAQUS,Overview,Introduction,Near-Field and Far-Field Effects,Fully Coupled Analysis,Sequentially Coupled Analysis,Acoustic-to-Structural Submodeling,Coupled Acoustic-Structural Substructures,Boundary Impedances,Creating ASI elements on geometry,Creating ASI elements on orphan meshes,Structural-Acoustic Analysis with ABAQUS,Introduction,Structural-Acoustic Analysis with ABAQUS,Introduction,Structural-acoustic coupling,If the acoustic medium adjoins a structure,structural-acoustic coupling occurs at the interface.,The pressure field in the acoustic medium creates a normal surface traction on the structure.,The acceleration field in the structure creates the natural forcing term at the fluid boundary.,Recall that,volumetric acceleration,is the conjugate of acoustic pressure in an acoustic analysis.Volumetric acceleration is what you apply as a,concentrated load,in an acoustic analysis.,Generically,the acoustic and structural fields must be solved for simultaneously.,Structural-Acoustic Analysis with ABAQUS,Introduction,Simplifications,In air the forces on structures caused by the air are usually weak.,In such a case,rather than performing a coupled analysis,you might model the effect of the structure on the acoustic fluid using boundary conditions or loads on the acoustic model.,The value of acoustic pressure can be specified directly(,*BOUNDARY,in an input file or use the ABAQUS/CAE Load module).,This includes the case of complicated boundary pressure fields computed in a previous structural analysis see Sequentially Coupled Analysis below.,A concentrated volumetric acceleration can be specified(,*CLOAD,in an input file or use the ABAQUS/CAE Load module).,A distributed volumetric acceleration can be specified using the,*INCIDENT WAVE,options.,Requires the use of the,Keywords Editor,in ABAQUS/CAE.,Structural-Acoustic Analysis with ABAQUS,Introduction,Interface impedances,The structural-acoustic interface can have an impedance,Z,of its own.,This means that the relationship between the structural motion and the acoustic pressure need not be continuous:,Used to include easily the effects of thin interface layers,such as carpet on the floor of a car,or the absorptive acoustic coating of a submarine.,Structural-Acoustic Analysis with ABAQUS,Near-Field and Far-Field Effects,Structural-Acoustic Analysis with ABAQUS,Near-Field and Far-Field Effects,Near and far fields,Vibrating machine,Air,Near field,Far field,Structural-Acoustic Analysis with ABAQUS,Near-Field and Far-Field Effects,Near field,The region within one wavelength of the interface is generally considered the,near field,.,Near-field region shrinks with increasing frequency.,The near-field solution tends to be complicated.,Includes“evanescent”effects(effects that fade quickly).,In the near field the mesh has a strong effect on accuracy.,The element size on both sides of the interface is governed by the medium that requires the finer mesh.,Structural-Acoustic Analysis with ABAQUS,Near-Field and Far-Field Effects,Far field,Beyond one wavelength the complexities of the near field diminish.,The near-field mesh has limited effect on far-field results.,Whether or not the near-field mesh is refined often does not strongly affect the results away from the interface.,The mesh in the far field needs to be appropriate only for the material and the simulation requirements,as discussed in Lecture 3.,Unlike in static problems,in acoustic simulations the far field is oscillatory,not constant.,The far-field mesh still needs to be fine enough to capture these waves.,Structural-Acoustic Analysis with ABAQUS,Near-Field and Far-Field Effects,Speaker with finely meshed air(left)and coarsely meshed air(right),Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,When is fully coupled analysis appropriate?,Fully coupled analysis is the most general approach to structural-acoustic problems.,All problems can be solved appropriately using the fully coupled approach.,Makes no assumptions about which direction has the strongest coupling effects.,Structural deformation,A,coustic pressure,A,drawback,of coupled analysis is that it can be unnecessarily expensive for large problems where the acoustic pressure has little effect on the structure.,In these cases sequentially coupled analysis can be more efficient(discussed in the next section).,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Coupling with ASI(acoustic-structural interface)elements,If the structural mesh and the acoustic mesh share nodes at their interface,lining the interface with ASI elements enforces the required coupling.,While this approach is available,but,the surface-based approach is generally recommended,.,ASI,elements lining machine surface,Vibrating machine,Air,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Coupling,Accelerations at the ASI nodes induce acoustic pressures in the acoustic mesh.,Acoustic pressures at the ASI nodes induce accelerations in the structural mesh.,Degrees of freedom,ASI element degrees of freedom include translations(degrees of freedom 1,2,3)and acoustic pressure(degree of freedom 8).,Boundary conditions can be applied to any of these degrees of freedom at the interface.,Local structural rotations(degrees of freedom 4,5,6)are,not,coupled.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Element normals,ASI element normals must point,into,the acoustic medium.,For user-specified two-and three-dimensional ASI elements the normal directions are implicit in the node numbering.,For one-dimensional user-specified ASI elements the normal direction must be specified on the data line of the,*,INTERFACE option.,You can specify impedances(,*,IMPEDANCE,*,SIMPEDANCE)at the interface to include surface treatment effects.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,ABAQUS/CAE and ASI elements,ASI elements cannot be created directly in ABAQUS/CAE.However,A special technique using skins and the,Keywords Editor,can be employed to create ASI elements from within ABAQUS/CAE for axisymmetric and 3D geometries.,2D planar geometries must be addressed manually.,If the underlying solid elements are tetrahedral,the triangular skin elements must be converted into degenerated quadrilateral elements.This may done done,e.g.,with a script.,This details of this technique will be outlined later.,The recommended approach when using ABAQUS/CAE,however,is to use surface-based TIE constraints to enforce the coupling.,This is discussed next.,The exception is acoustic submodeling,where ASI elements are required.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Surface-based interfaces,Do not require the structural and acoustic meshes to match at the interface.,The structural and acoustic meshes have separate nodes at the interface.,You define separate surfaces on the structural and acoustic meshes at the interface.,Essentially the same approach as defining contact between two surfaces.,Can be modeled directly in ABAQUS/CAE:Create an assembly with an acoustic part and a solid part,and use TIE constraints.,The internally generated ASI elements will have the correct normal directions automatically.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Usage:,You define a TIE constraint between the two surfaces using:,*TIE,NAME=,tie_interaction_name,1,slave surface,master surface,Automatically couples the structural accelerations and the acoustic pressures at the interface in the same way as ASI elements.,Acoustic pressure boundary conditions(degree of freedom 8)can be applied to nodes on the surface of the acoustic mesh.,Translation boundary conditions(degrees of freedom 1,2,3)can be applied to the surface of the structural mesh.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Example:Acoustic radiation of a muffler,*TIE,NAME=MUFFLER_AIR,INT_AIR,MUFFLER_INT,OUT_AIR,MUFFLER_EXT,OUT_AIR(only half the surface is shown),INT_AIR,MUFFLER_INT:interior,MUFFLER_EXT:exterior,The material with the lower wave speed generally should be more refined and,therefore,should be the slave surface.,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Example:Truck cab analysis,CAB-INSIDE,Inside-air,*TIE,POSITION TOLERANCE=0.01,ADJUST=NO,Inside-air,CAB-INSIDE,Structural-Acoustic Analysis with ABAQUS,Fully Coupled Analysis,Master and slave surfaces,Either surface can be slave or master,but the choice affects the accuracy of the solution.,Mesh refinement depends on the wave speeds of the two materials meeting at the interface.,The material with the lower wave speed generally should be more refined and,therefore,should be the slave surface.,If solution details near the interface are important,the meshes on either side should be refined equally corresponding to the requirements of the lower wave speed material.,In this case choice of the slave and master are arbitrary.,Exception,:Fluids coupled to both sides of a shell or beam.At least one of the surfaces of the solid must be a master surface.,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,When is sequentially coupled analysis appropriate?,When the normal surface traction exerted on the structure created by the acoustic fluid is negligible compared to the other forces on the structure.,Example:Vibrating machine radiates sound to the air,but the reaction pressure of the air on the machine may be insignificant to the analysis.,Machine vibrating in a room,Vibrating machine,Air,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Sequentially coupled analysis,In these cases the structural analysis can be performed first(uncoupled from the fluid).,The acoustic analysis follows,driven by the structure at the interface.,Solving the problem in two distinct analyses decouples the solution.,The decoupling reduces computational cost,especially for large problems.,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Submodeling,Submodeling is the approach used to drive the acoustic analysis with the results of the structural analysis.,The term submodeling refers to the technique of using a coarse global solution to drive a refined local analysis.In acoustics we use the same technique,although the application is different.,The first analysiswhich includes the structuresupplies the,global,model,.,The second analysiswhich includes an acoustic fluidsupplies the,submodel,.,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Global model,This analysis includes the structure.,Example:In the case of the vibrating machine the global model contains the machine only.,Global model:vibrating machine only,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Example:Acoustic Radiation of a Muffler,The global model contains the interior air of the muffler and the muffler structure.,The two domains are coupled using TIE constraints.,The relevant material properties and boundary conditions used in the fully-coupled analysis model are also used in this model.,A single step invoking the direct steady-state dynamics procedure is used.,Acoustic model of the internal air,Shell model of the muffler,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,The displacement results of the global analysis must be saved to the output database(,.odb,)or results(,.fil,)file at the structural-acoustic interface.,Examples:,Vibrating machine analysis:The nodes at the interface are the exterior nodes of the machine.,Muffler analysis:The nodes at the interface are all nodes on the muffler shell structure.,*NSET,NSET=muffler,:,*OUTPUT,FIELD,*NODE OUTPUT,NSET=muffler,U,For,.fil,file output use:,*NODE FILE,NSET=muffler,U,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Submodel,In the submodel analysis only the acoustic domain is modeled.,The interface with the location of the structural model(modeled in the previous,global,analysis)is lined with,ASI elements,.,The surface-based approach is not available to drive a sequentially coupled analysis.,A technique to create ASI elements from within ABAQUS/CAE will be described later.,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,The mesh of the acoustic fluid need not match the mesh of the structure in the previous,global analysis.,The submodeling capability interpolates structural displacements saved from the global,structural analysis and applies them to the driven nodes in the submodel analysis.,Global model(solid or structural elements),Submodel(acoustic and ASI elements),Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Example(contd),In,submodel,analysis:,*NSET,NSET=muffler,*driven nodes must be on the ASI,*SUBMODEL,EXTERIOR TOLERANCE=0.05,muffler,*BOUNDARY,SUBMODEL,STEP=1,muffler,1,3,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Execution procedure for submodel analysis:,abaqus job=,submodel job name,globalmodel=,global job name(with either,.odb,or,.fil,extension),Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Fully coupled analysis,Sequentially coupled analysis,CPU time:465 sec(NT),CPU time:60+88=148 sec(NT),Acoustic pressure in internal air,Induced displacements in muffler body,Acoustic pressure in external air,Global,model,Submodel,Excitation frequency 170 Hz,Structural-Acoustic Analysis with ABAQUS,Sequentially Coupled Analysis,Is sequential analysis appropriate?,If sequential analysis is appropriate for a given problem,the effect of the global model on the submodel is nearly the same as the effect of the structure on the acoustic fluid in a fully coupled single analysis.,In the case of the muffler analysis the peak pressure in the fully and sequentially coupled results differs by approximately 10%.,Fully coupled,Sequentially coupled,Structural-Acoustic Analysis with ABAQUS,Acoustic-to-Structural Submodeling,Structural-Acoustic Analysis with ABAQUS,Another application of submodeling in acoustics involves,situations where the structural response is of primary interest and the presence of the(heavy)fluid is required mainly for the application of the load onto the structure.,The global model is a coupled structural-acoustic analysis,The submodel is an uncoupled structural force-displacement analysis,Interpolated acoustic pressures are converted to concentrated loads,Driven nodes are specified by defining an element-based surface,Shell surfaces can be d