An acoustic analysis calculates either the propagation properties of pure acoustic waves in the given environment or the coupled acoustic structural interaction (FSI).

Support is available for time-harmonic, modal and transient acoustic analysis.

Use the following fluid elements to simulate 2-D acoustic or coupled acoustic problems:

Use these fluid elements for 3-D simulation:

Use FLUID30, FLUID220 and FLUID221 to model the fluid medium and the interface in fluid/structure interaction problems. Typical applications include sound wave propagation and submerged structure dynamics. The governing equation for acoustics, namely the 3-D wave equation, has been discretized taking into account the coupling of acoustic pressure and structural motion at the interface. The element node has four degrees of freedom per node: translations in the nodal x, y and z directions, and pressure. The translations are applicable only at nodes on the interface. Acceleration effects like those in sloshing problems can be included. The mean flow effect or incompressible fluid can be simulated. For more information, see Acoustics in the Mechanical APDL Theory Reference.

The elements have the capability to include damping of sound-absorbing material at the interface as well as damping within the fluid. The elements can be used with or without other 3-D structural elements to perform symmetric, unsymmetric or damped modal (MODOPT), full harmonic (HROPT), and full transient method analyses (TRNOPT).

See FLUID30 in the Mechanical APDL Theory Reference for more information about the low-order element. See FLUID220 for more information about the high-order hexahedral element and FLUID221 for more information about the high-order tetrahedral element.

The geometry, node locations, and coordinate system for each 3-D acoustic element is shown in Figure 30.1: FLUID30 Geometry, Figure 220.1: FLUID220 Geometry, and Figure 221.1: FLUID221 Geometry. The elements are defined by eight nodes (FLUID30), 20 nodes (FLUID220), or 10 nodes (FLUID221), a reference pressure, and the isotropic material properties. The reference pressure (PREF) is used to calculate the element sound pressure level (defaults to 20x10^-6 N/m²). The speed of sound () in the fluid is input by SONC where k is the bulk modulus of the fluid (Force/Area) and ρ_o is the mean fluid density (Mass/Volume) (input as DENS). The dissipative effect due to fluid viscosity can be included (input as MP,VISC). DENS, SONC, and VISC are evaluated at the average of the nodal temperatures.

The TB,PERF command defines the equivalent fluid of the perforated material using the Johnson-Champoux-Allard, Delany-Bazley, Miki, impedance-propagating constant, or complex density-velocity model. The material properties, thermal conductivity (MP,KXX), heat coefficient at constant volume per unit of mass (MP,CVH), the dynamic viscosity (MP,VISC) (default 1.84x10^-5 N•s/m²), and specific heat (MP,C) can be defined for the Prandtl number calculation (default 0.713) and the specific heat ratio (default 1.4) if necessary. Viscosity defaults to 1.84x10^-5s for elements where the TB,PERF command has been issued. For all other elements, the viscosity is assumed to be zero when undefined.

The TB,PERF command also defines the transfer admittance matrix for the equivalence of complex perforated structures, including plates with hole arrays. The TB,AFDM command defines the frequency-dependent acoustic materials combined with the TB,FIELD command. The parameters set via the TB,PERF command can be frequency-dependent along with the TB,FIELD command. Both the boundary layer (SF,BLI) and the low-frequency reduced model (TB,AFDM) are available to simulate the interaction between acoustic fluid and rigid wall in visco-thermal medium

Element loads are described in Element Loading. Fluid-structure interfaces (FSIs) can be flagged by surface loads at the element faces as shown by the circled numbers in Figure 30.1: FLUID30 Geometry, Figure 220.1: FLUID220 Geometry, and Figure 221.1: FLUID221 Geometry. Specifying the FSI label (without a value) (SF, SFA, SFE) couples the structural motion and fluid pressure at the interface. Deleting the FSI specification (SFDELE, SFADELE, SFEDELE) removes the flag. The flag specification should be on the fluid elements at the interface.

The fluid-structure interface (FSI) can be flagged automatically if acoustic elements are adjacent to solid structural elements (except for shell elements) and FSIs have not been flagged manually. Use the surface load label IMPD with a given complex impedance value to include any damping present at a structural boundary with a sound absorption lining. These impedance boundary conditions can also be applied to a flagged FSI interface. A zero IMPD value removes the damping calculation. If using the coupled acoustic elements (KEYOPT(2) = 0), set the displacement degrees of freedom (UX, UY and UZ) to zero at the element nodes not on the interface to avoid zero-pivot warnings. The surface load label SHLD with a given amplitude and initial phase angle defines a normal sound speed in a harmonic analysis or a normal sound acceleration in a transient analysis on the exterior surface. The sloshing surface that must be parallel to the coordinate plane of the global Cartesian system can be flagged via the FREE surface load label. When near- or far- field parameters are required, apply the surface load label MXWF to the equivalent source surface. The label MXWF can be applied automatically to a PML-acoustic medium interface or exterior surface via the INF label INF (if MXWF surfaces have not been flagged manually). The surface load label ATTN with the absorption coefficient defines an absorbing surface in modal and harmonic response analyses. The surface load label BLI is applied on the rigid wall to introduce the boundary layer model in viscous-thermal fluid. The surface load label PORT defines the network ports. When the transfer admittance matrix is used, define a pair of ports on the opposite faces in the same element. Surface loads with load label IMPD, ATTN and SHLD can be frequency- or time-dependent using tabular inputs. The label RIGW flags the rigid walls.

Temperatures can be input as element body loads at the nodes. The node I temperature T(I) defaults to TUNIF. If all other temperatures are unspecified, they default to T(I). For any other input pattern, unspecified temperatures default to TUNIF.

Mass source (in units of mass/length³/time) can be defined in a harmonic analysis and mass source rate (partial time derivative of mass source in units of mass/length³/time²) can be defined in a transient analysis (BF,,MASS). For harmonic response analyses, both the amplitude and initial phase can be applied so that the inhomogeneous Helmholtz equation is solved. The impedance sheet inside a fluid can be defined (BF,,IMPD or BFA,,IMPD) in a harmonic analysis. For a nonuniform acoustic medium, define the reference temperature T₀ (TREF) and the reference static pressure (real constant PSREF). PSREF defaults to 101325, the standard atmospheric pressure in units of N/m². Nodal temperatures are input via body load commands. Nodal static pressure can also be input (BF,,SPRE). You can define nonuniform velocity in a harmonic analysis or nonuniform acceleration in a transient analysis (BF,,VELO). Body loads with labels TEMP, MASS or VELO can be frequency- or time-dependent using tabular inputs. The interior port is defined with BF,,PORT. You can also define the Floquet periodic boundary condition (BF,,FPBC) in harmonic and modal analyses. The mean flow velocity is introduced by BF,,VMEN in harmonic analysis.

One-way coupling from structure to acoustics is more computationally efficient, while the acoustic effect on the structure can be neglected. The structural solution is performed first. If a conforming mesh is used on the FSI interface, you can flag the FSI in the structural model (SF,FSIN) and write the structural results on the FSI to a .asi (default) file (ASIFILE). The velocities or accelerations on the FSI are loaded into the sequential acoustic solution with multiple frequencies or time steps corresponding to the previous structural solution. If a nonconforming mesh is used on the FSI, the ASIFILE,,,,MAP command efficiently maps the structural results on the FSI interface to the acoustic model during the sequential acoustic solution. You can also map the structural results on the selected interfaces to the acoustic model linking the projects in Workbench Project Schematic, then perform an acoustic solution.

One-way coupling from the ANSYS Fluent CFD Solver to Mechanical APDL acoustics is available for noise prediction in the enclosure modeled by acoustic and structural elements. The surface element SURF154 must be generated on top of the structural solid or shell elements and flagged (SF,FSIN) for a one-way coupling interface. The coupled vibro-acoustic FSI interface is also flagged (SF,FSI). The fast Fourier transformation (FFT) data of the transient CFD solution is written to a .cgns file with the one-sided peak complex pressure values in the CFD postprocessor. Mechanical APDL reads the CFD result (FLUREAD) and maps the complex pressure to the one-way coupling interface, then performs harmonic solutions at multiple FFT frequencies within the defined frequency range (HARFRQ).

The nonlinear static analysis for the structural deformation can be performed before performing a coupled or pure acoustic solution. Because of the structural deformation, the mesh in the acoustic domain is morphed during the static structural solution (MORPH). The linear perturbation scheme (ANTYPE and PERTURB) is used to perform a sequential acoustic solution with the updated mesh. If a morphing failure occurs during mesh morphing (typically due to large deformations), automatic time stepping will bisect the solution, if possible. The nonlinear static structural analysis is also efficiently performed without mesh morphing in the acoustic domain if the structural deformation can be ignored (see ANTYPE).

KEYOPT(2) = 1 specifies the absence of a structure at the interface and the absence of coupling between the fluid and structure. Because the absence of coupling produces symmetric element matrices, a symmetric eigensolver (MODOPT) can be used within the modal analysis. KEYOPT(2) = 0 (the default) specifies a coupled (unsymmetric) problem, requiring a corresponding unsymmetric eigensolver (MODOPT). If KEYOPT(1) = 2, specifying symmetric algorithms in the presence of FSI coupling, a symmetric linear equation solver can be used for full harmonic analysis. The FLUID130 element is compatible with the symmetric options during the solution.

To reduce the size of the Jobname.emat file, issue the ECPCHG command. This command converts acoustics element adjacent to solid structural elements (or flagged with FSI) to coupled elements, and converts coupled acoustic elements to uncoupled elements.

KEYOPT(4) specifies the existence of perfectly matched layers (PML) or irregular perfectly matched layers (IPML) to absorb the outgoing sound waves. The pressure on the exterior enclosure of PML (KEYOPT(4) = 1) or IPML (KEYOPT(4) = 2) must be constrained to zero, unless the pressure is on the symmetric planes. The PML must be defined in a PML coordinate system (PSYS). The rigid walls must be flagged (SF,,RIGW) if the zero pressure on the exterior surface of the IPML is set by the program. For more information about using PML and IPML, see Artificially Matched Layers in the Mechanical APDL Acoustic Analysis Guide and Artificially Matched Layers in the Mechanical APDL Theory Reference.

KEYOPT(5) = 1 specifies the non-morphed element during the static structural solution (MORPH) for efficient meshing.

KEYOPT(6) = 1 specifies the incompressible fluid in which the sound speed equivalently tends to infinite.

If free surface effects are present (SF,,FREE), vertical acceleration (ACEL,,,ACEL_Z) is necessary to specify gravity, even for a modal analysis.

A harmonic analysis can be performed over the octave band, the 1/2, 1/3, 1/6, 1/12, or 1/24 octave bands, or the general frequency band with logarithm frequency span (HARFRQ,,,,LogOpt).

An analytic acoustic mode can be launched in the rectangular, circular, and coaxial acoustic duct (APORT). The incident planar wave on the port can also be defined.

The mean flow effect is taken into account by solving the convective wave equation with a defined mean flow velocity (BF,,VMEN) in a harmonic analysis. The static mean flow can be solved with the mean flow velocities defined on the inlet and outlet in an upstream static analysis and stored in the Jobname.rmf file. The static mean flow is loaded into the model during the sequential harmonic solution of the convective wave equation (LDREAD command).

Use the DFSWAVE command to define a diffuse sound field consisting of an infinite number of plane waves for random acoustics. The surface element, SURF154, must be defined on the top of the structural panel meshed by structural or shell elements. The half space where the diffuse sound field exists is not meshed, and the radiation space (receiver side) is meshed with the acoustic element and truncated by the PML or absorbing elements. Multiple solutions at the same frequencies (MSOLVE) are necessary to obtain a stable, average solution for the physical samplings. The transmission loss of the structural panel is calculated and displayed by the PRAS and PLAS commands.

The angle sweep in the harmonic analysis with the Floquet periodic boundary condition is controlled by the MSOLVE command, and the results are displayed by the PRAS and PLAS commands.

For acoustic scattering analysis, acoustic incident waves can be specified outside of the model (AWAVE). These incident waves can be combined with a PML or Robin boundary surface (SF,INF). Either the total field or pure scattered field formulation can be used. When using the pure scattered formulation, the scattered formulation is required (ASOL), and acoustic incident waves can also be specified inside the model (AWAVE).

The acoustic near- and far-field parameters can be calculated (PRNEAR, PLNEAR, PRFAR, or PLFAR) for the full 3-D model or the 2-D rotated extrusion model. (The axisymmetric model is simulated via a slice of the 3-D model with rotation). The sound power data, including transmission loss, can be calculated on the defined ports by the PRAS and PLAS commands. The following quantities can be also be calculated and displayed by these two commands:

The acoustic parameters can be obtained by the *GET,,ACUS command. The acoustic far-field parameters radiated from a structural panel are also calculated based on the Rayleigh integral principle (PRFAR or PLFAR).

Element input for FLUID30, FLUID220, and FLUID221 is given in 3-D Acoustic Element Input Summary. A general description of element input is given in Element Input.

The solution output associated with the elements consists of the following:

A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file Jobname.OUT. The R column indicates the availability of the items in the results file.

In either the O or R columns, “Y” indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 6.2: 3-D Acoustic Element Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table of this reference for more information. The following notation is used in Table 6.2: 3-D Acoustic Element Item and Sequence Numbers:

No product-specific restrictions exist for 3-D acoustic elements.