An acoustic analysis calculates either the propagation properties of pure acoustic waves in the given environment or the coupled acoustic structural interaction (FSI) using either the Helmholtz or convective wave equation. The energy diffusion solution predicts the sound-level distribution in room acoustics. The full linear Navier-Stokes equations (FLNS) are solved in viscous-thermal acoustics for devices with narrow or thin acoustic paths. Biot’s theory is applied in calculating the wave propagation in porous media. Either the Westervelt or the Kuznetsov equation is used to simulate nonlinear acoustic wave propagation in the medium.

Support is available for time-harmonic, modal, and transient acoustic analysis. Steady-state analysis is also valid for room acoustics.

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

FLUID129 is a companion to the other 2D elements. It is used to envelop a model made of FLUID243 or FLUID244 finite elements for the truncation of an infinite propagating domain.

Use these fluid elements for 3D simulation:

FLUID130 is a companion to the other 3D elements. It is used to envelop a model made of FLUID30, FLUID220, or FLUID221 finite elements for the truncation of an infinite propagating domain.

Use these acoustic elements 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 wave equation, has been discretized with the coupling of acoustic pressure and structural motion at the interface. For theoretical background, see Acoustics in the Mechanical APDL Theory Reference.

The degree-of-freedom set varies, depending on the element behavior as specified by KEYOPT(2):

The elements can 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 structural elements to perform symmetric, unsymmetric, or damped modal (MODOPT) analyses, as well as full, and mode superposition harmonic (HROPT), and full transient analyses (TRNOPT). Static analysis is supported to analyze room acoustics, and several types of spectrum analyses are supported. For details on the supported analysis types, see Acoustic Analysis Solution Settings in the Acoustic Analysis Guide.

The geometry, node locations, and coordinate system for each acoustic element are shown in the individual element descriptions: see FLUID30, FLUID220, FLUID221, FLUID243, and FLUID244. The elements are defined by their nodes, a reference pressure, and the isotropic material properties.

All inputs described here are valid for the 3D elements. Most of the inputs are also valid for the 2D elements, except where noted.

The reference pressure (real constant 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 MP,SONC where k is the bulk modulus of the fluid (Force/Area) and ρ_o is the mean fluid density (Mass/Volume) (input as MP,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 TB,PERF command also defines the poroelastic acoustic material for mixed (u-P) poroelasticity problems. The material properties, thermal conductivity (MP,KXX), heat coefficient at constant volume per unit of mass (MP,CVH), the dynamic viscosity (MP,VISC), bulk viscosity (MP,BVIS), 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^-5 N•s/m² for elements associated with the TB,PERF command. For all other elements, the viscosity is assumed to be zero when undefined. For the poroelastic material, the elastic moduli is input by MP,EX, the shear moduli is input by MP,GXY and the Poisson’s ratio is input by MP,NUXY.

The TB,PERF command also defines the transfer admittance matrix for the equivalence of complex perforated structures, including plates with hole arrays. The parameters set via the TB,PERF command can be frequency dependent; use the TBFIELD command to define frequency as a field variable.

The TB,AFDM command, combined with the TBFIELD command, defines the frequency-dependent acoustic materials. Both the boundary layer impedance model (SF,,BLI) (not valid for 2D elements) and the low-frequency reduced model (LRF model defined by the TB,AFDM command) are available to simulate the interaction between an acoustic fluid and a rigid wall in a viscous-thermal medium. For room acoustics, the TB,AFDM command defines the diffusion properties. For nonlinear acoustics, the TB,AFDM command defines the diffusivity of sound and the dimensionless nonlinearity coefficient.

Element loads are described in Element Loading. A discussion of element loads for acoustics follows.

Fluid-structure interfaces (FSIs) can be applied by surface loads (SF, SFA, SFE) at the element faces as shown by the circled numbers in Figure 30.1: FLUID30 Geometry, Figure 220.1: FLUID220 Geometry, Figure 221.1: FLUID221 Geometry, Figure 243.1: FLUID243 Geometry, and Figure 244.1: FLUID244 Geometry. Use the command SFDELE, SFADELE, or SFEDELE remove surface loads.

Available surface loads are described below.

Temperatures can be input as element body loads at the nodes (BF,,TEMP). 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 (mass/length³/time) can be defined (BF,,MASS) in a harmonic analysis or in a transient analysis solved with the velocity potential formulation. The mass source rate (partial time derivative of mass source in units of mass/length³/time²) can be defined (BF,,MASS) in a transient analysis solved with the pressure formulation. For harmonic response analyses, both the amplitude and initial phase can be applied so that the inhomogeneous Helmholtz equation is solved. In an energy diffusion solution, the power source is also defined by the BF,,MASS command.

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).

Nonuniform velocity (BF,,VELO) can be defined in a harmonic analysis or in a transient analysis solved with the velocity potential formulation. BF,,VELO is also used to define nonuniform acceleration in a transient analysis solved with the pressure formulation.

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 and modal analyses (not valid for 2D elements). The force potential (BF,,UFOR) represents the body force when the convective wave equation (mean flow) is solved (not valid for 2D elements). In viscous-thermal acoustics and poroelastic acoustics, the shear force on the exterior surface is defined by BF,,SFOR. The volumetric heat source can be defined by BF,,HFLW, although the heat source is usually ignored in viscous-thermal acoustics.

The F command is used to define the volumetric body force in viscous-thermal acoustics.

In a transient room acoustics analysis, the initial condition of the acoustic energy density is defined by the IC,,ENKE command.

The D command defines the constraints on the degrees of freedom as the Dirichlet boundary condition. Defining the constraint on the auxiliary pressure DOF in viscous-thermal acoustics is invalid.

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 an .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. As an alternative to creating the .asi file during the structural analysis solution, you can issue the ASIFILE command during postprocessing (/POST1) of the structural analysis to write the structural results at the FSI to the .asi file. In this case, the command SF,,FSIN,1 must be used to define the coupling interface in the acoustic model.

If you are using Ansys Workbench, you can map the structural results on the selected interfaces to the acoustic model linking the projects in the 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 (not valid for 2D 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(1) specifies algorithm options:

KEYOPT (2) controls the acoustic element type:

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

For the 2D elements, KEYOPT(3) = 1 specifies a 2D axisymmetric model.

KEYOPT(4) specifies the existence of perfectly matched layers (PML) or irregular perfectly matched layers (IPML) to absorb the outgoing sound waves. PML and IPML are supported in modal, harmonic, and transient acoustic analyses. The pressure on the exterior enclosure of PML (KEYOPT(4) = 1 or 3) or IPML (KEYOPT(4) = 2 or 4) 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 Acoustic Analysis Guide and Artificially Matched Layers in the 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 or a modal 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 or modal solution of the convective wave equation (LDREAD command). The force potential (BF,,UFOR) represents the body force in the mean flow effect. (The mean flow effect is not valid for 2D elements.)

Use the DFSWAVE command to define a diffuse sound field consisting of an infinite number of plane waves for random acoustics. (The diffuse sound field is not valid for 2D analysis.). 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 scattered sound field generated by the incident diffuse sound field projecting on the objects is simulated without the surface element SURF154.

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 3D model, the 2D rotated extrusion model, or the 2D model. (The axisymmetric model is simulated via a slice of the 3D model with rotation or via 2D axisymmetric elements). The sound power data, including transmission loss, can be calculated on the defined ports by the PRAS and PLAS commands. The following quantities can 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).