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1、Lesson content:IntroductionNear-Field and Far-Field EffectsFully Coupled AnalysisSequentially Coupled AnalysisAcoustic-to-Structural SubmodelingCoupled Acoustic-Structural SubstructuresBoundary ImpedancesCreating ASI elements on geometryCreating ASI elements on orphan meshesWorkshop 3: Workshop 3: T

2、ruck Cab Acoustic Analysis (IA)Workshop 3: Workshop 3: Truck Cab Acoustic Analysis (KW)Lesson 4: Coupled Structural-Acoustic Analysis2 hoursBoth interactive (IA) and keywords (KW) versions of the workshop are provided. Complete only one.Introduction (1/3)Structural-acoustic couplingIf the acoustic m

3、edium 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 accelerat

4、ion 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.Introduction (2/3)SimplificationsIn air the forces on structur

5、es 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

6、 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 modu

7、le).A distributed volumetric acceleration can be specified (*INCIDENT WAVE INTERACTION in an input file or use the Abaqus/CAE Interaction module).Introduction (3/3)Interface impedancesThe structural-acoustic interface can have an impedance, Z, of its own.This means that the relationship between the

8、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.Near-Field and Far-Field Effects (1/4)Near and far fieldsVibrating machineAirNear fi

9、eldFar fieldNear-Field and Far-Field Effects (2/4)Near fieldThe 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 qu

10、ickly).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.Near-Field and Far-Field Effects (3/4)Far fieldBeyond one wavelength the complexities of the near field diminish.The near-field me

11、sh 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 st

12、atic 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.Near-Field and Far-Field Effects (4/4)Speaker with finely meshed air (left) and coarsely meshed air (right)Fully Coupled Analysis (1/10)When is f

13、ully 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 Acousti

14、c pressureA drawback of coupled analysis is that it can be unnecessarily expensive for large problems where the acoustic pressure has little effect on the structure (e.g., steel-air interaction). In these cases mode-based analysis following uncoupled frequency extraction generally yields solutions w

15、ith acceptable accuracy with low solution times. Minimal changes to the model are required.Another possibility is to use sequentially coupled analysis (discussed in the next section). Additional model setup is required.Fully Coupled Analysis (2/10)Coupling with ASI (acoustic-structural interface) el

16、ementsIf 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, the surface-based approach is generally mended (discussed later). The surface-based approach does not require AS

17、I elements. Structure and acoustic domain are coupled internally with tie constraints.ASI elements lining machine surface Vibrating machineAirFully Coupled Analysis (3/10)CouplingAccelerations at the ASI nodes induce acoustic pressures in the acoustic mesh. Acoustic pressures at the ASI nodes induce

18、 accelerations in the structural mesh.Degrees of freedomASI 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 (deg

19、rees of freedom 4, 5, 6) are not coupled.Fully Coupled Analysis (4/10)Element normalsASI 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 elem

20、ents the normal direction must be specified on the data line of the *INTERFACE option.You can specify impedance conditions at the interface to include surface treatment effects.Fully Coupled Analysis (5/10)Abaqus/CAE and ASI elementsASI elements can be created directly in Abaqus/CAE (using skin rein

21、forcements). The mended 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.Fully Coupled Analysis (6/10)Surface-based interfaces Surface-based interfac

22、es do not require the structural and acoustic meshes to match at the interface.Separate surfaces on the structural and acoustic meshes have to be defined at the interface.The interface can be modeled directly in Abaqus/CAE: Create an assembly with an acoustic part and a solid part, and use tie const

23、raints in the interaction module (shown in the figure).The structural and the acoustic element normals have to be defined such that they point towards each other. Fully Coupled Analysis (7/10)Usage: You define a tie constraint between the two surfaces using: *TIE, NAME=tie_interaction_nameslave surf

24、ace, master surfaceAutomatically 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 (d

25、egrees of freedom 1, 2, 3) can be applied to the surface of the structural mesh.Fully Coupled Analysis (8/10)Example: Acoustic radiation of a muffler*TIE, NAME=MUFFLER_AIRINT_AIR, MUFFLER_INTOUT_AIR, MUFFLER_EXTOUT_AIR (only half the surface is shown)INT_AIRMUFFLER_INT: interiorMUFFLER_EXT: exterior

26、The material with the lower wave speed generally should be more refined and, therefore, should be the slave surface.Fully Coupled Analysis (9/10)Example: Truck cab analysisCAB-INSIDEInside-air*TIE, POSITION TOLERANCE=0.01, ADJUST=NO Inside-air, CAB-INSIDEFully Coupled Analysis (10/10)Master and slav

27、e surfacesEither 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 su

28、rface.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. A

29、t least one of the surfaces of the solid must be a master surface.Sequentially Coupled Analysis (1/12)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.

30、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 roomVibrating machineAirSequentially Coupled Analysis (2/12)Sequentially coupled analysisIn these cases the structural analysis can be

31、 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.Sequentially Coupled Analysis (3/12)Submode

32、lingSubmodeling 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 differ

33、ent.The first analysiswhich includes the structuresupplies the global model.The second analysiswhich includes an acoustic fluidsupplies the submodel.Sequentially Coupled Analysis (4/12)Global modelThis analysis includes the structure.Example: In the case of the vibrating machine the global model con

34、tains the machine only.Global model: vibrating machine onlySequentially Coupled Analysis (5/12)Example: Acoustic radiation of a mufflerThe global model contains the interior air of the muffler and the muffler structure.The two domains are coupled using TIE constraints.The relevant material propertie

35、s 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 airShell model of the mufflerSequentially Coupled Analysis (6/12)The displacement results of the gl

36、obal 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

37、 structure.*NSET, NSET=muffler :*OUTPUT, FIELD *NODE OUTPUT, NSET=muffler U,For .fil file output use:*NODE FILE, NSET=muffler U,Sequentially Coupled Analysis (7/12)SubmodelIn the submodel analysis only the acoustic domain is modeled.The interface with the location of the structural model (modeled in

38、 the previous, global analysis) is lined with ASI elements.ASI elements (in red) driving the exterior acoustic domain (in green)Sequentially Coupled Analysis (8/12)The mesh of the acoustic fluid need not match the mesh of the structure in the previous, global analysis. The submodeling capability int

39、erpolates 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)Sequentially Coupled Analysis (9/12)Example (contd)In submodel analysis:*NSET, NS

40、ET=muffler*driven nodes must be on the ASI*SUBMODEL, EXTERIOR TOLERANCE=0.05muffler*BOUNDARY, SUBMODEL, STEP=1muffler, 1, 3Sequentially Coupled Analysis (10/12)Execution procedure for submodel analysis:abaqus job=submodel job name globalmodel=global job name (with either .odb or .fil extension)Seque

41、ntially Coupled Analysis (11/12)Fully coupled analysisSequentially coupled analysisNormalized CPU time: 1 (NT)Normalized CPU time: 0.13+ 0.19 = 0.32 (NT)Acoustic pressure in internal airInduced displacements in muffler bodyAcoustic pressure in external airGlobalmodelSubmodelExcitation frequency 170

42、HzSequentially Coupled Analysis (12/12)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 o

43、f the muffler analysis the peak pressure in the fully and sequentially coupled results differs by approximately 10%.Fully coupledSequentially coupledAcoustic-to-Structural Submodeling (1/2)Another application of submodeling in acoustics involves situations where the structural response is of primary

44、 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 analysisThe submodel is an uncoupled structural force-displacement analysisInterpolated acoustic pressures are converted to concent

45、rated loadsDriven nodes are specified by defining an element-based surfaceShell surfaces can be driven on both sides by different acoustic domainsUsage:*SUBMODEL, ACOUSTIC TO STRUCTURE surface_name*BOUNDARY,SUBMODEL, STEP=n node_set, 8Acoustic-to-Structural Submodeling (2/2)Example: Submerged submar

46、ineThe acoustic field obtained from the global coupled analysis drives submodel structural analysis.Detailed analysis can then be performed of critical components, e.g., for shock testing.The method is suited for design sensitivity analysis: the fluid mesh needed only once.Global model With fluid me

47、shSubmodel (exterior)- no fluid meshSubmodel (interior)Coupled Acoustic-Structural Substructures (1/2)Substructures may be generated in Abaqus/Standard from models which include acoustic elements.The retained degrees of freedom must be displacements and rotations.At right, a simple tire (orange) and

48、 air (blue) system forms a substructure with retained nodes.Retained nodesAirTireCoupled Acoustic-Structural Substructures (2/2)In a static analysis involving a substructure containing acoustic elements, the results will differ from the results obtained in an equivalent static analysis without subst

49、ructures. The acoustic-structural coupling is taken into account in the substructure (leading to hydrostatic contributions of the acoustic fluid), while the coupling is ignored in a static analysis without substructures.Coupled acoustic-structural substructures should not be used in geometrically no

50、nlinear analyses.Limitations:During substructure generation the effect of structural-acoustic coupling must be included in the retained eigenmodes, (use acoustic coupling=ON during substructure generation)The uncoupled eigenmodes calculated by Lanczos with SIM or with AMS can not be used as retained

51、 modes.When a substructure load case definition includes acoustic loading during a substructure generation procedure in which retained modes are specified: the contribution of the singular (constant pressure) acoustic modes is not taken into account in the generated load caseBoundary Impedances (1/6

52、)Default behavior (no boundary impedance) The exterior boundary of the fluid is assumed rigid.The fluid particle and the structural motions at the interface are equal.The fluid pressures act directly on the structural body.The structural displacements/accelerations directly induce pressure gradients

53、 on the fluid and vice versa.Energy and momentum losses are zero.Boundary Impedances (2/6)Boundary impedance (Z)Specifies the relationship between the pressure of the acoustic medium and the normal motion at the interface: where 1/ k1 is the proportionality coefficient between the pressure and the d

54、isplacement normal to the surface, 1/ c1 is the proportionality coefficient between the pressure and the velocity normal to the surface and is the angular frequency.Boundary Impedances (3/6)Nonzero boundary impedancesProvide a relationship between the pressure and the particle velocity on the acoust

55、ic fluid side of the interface. The model is analogous to a spring and dashpot in series between the fluid medium and the structure.Energy dissipates in proportion to c1.Phase lag between velocity and pressure is in proportion to k1. Can model the surface effects of small-amplitude “sloshing” of the

56、 liquid medium.Vibrations in the “air” side of the interface dissipate additional energy associated with displacing the liquid against gravity.Boundary impedance can be used to model this effect by setting the impedance equal to the density times the gravity constant, g.Can model the effect of a com

57、pressible, possibly dissipative, lining (for example, carpet) between the acoustic medium and the structure.Boundary Impedances (4/6)Impedance propertiesThe impedance material properties are based on measurements of the absorbing characteristics of the structural surface. They are applied using *IMP

58、EDANCE PROPERTY and then *IMPEDANCE for use with ASI elements or *SIMPEDANCE for use with surface-based interfaces.For steady-state analysis the impedance interface properties are given as functions of frequency.For transient analysis the impedance interface properties are given as single values.Bou

59、ndary Impedances (5/6)Usage: ASI-based interfaceModel data:*ELSET, ELSET=CARPET1element numbers*IMPEDANCE PROPERTY, NAME=CARPET_PROP1/k1, 1/c1, frequency 11/k1, 1/c1, frequency 2.History data:*IMPEDANCE, PROPERTY=CARPET_PROPCARPET1, InBoundary Impedances (6/6)Usage: Surface-based interfaceModel data

60、:*SURFACE, NAME=CARPET2carpet element set name*IMPEDANCE PROPERTY, NAME=CARPET_PROP1/k1, 1/c1, frequency 11/k1, 1/c1, frequency 2.History data:*SIMPEDANCE, PROPERTY=CARPET_PROPCARPET2,Creating ASI elements on geometry (1/3)Steps for creating ASI elements on geometry In the Property module, create a