Boundary conditions overview (FEM acoustics)

You can define acoustic loads and boundary conditions in the Simcenter Nastran solver environment, for acoustic and vibro-acoustic analysis that use SOL108 (Direct Frequency Response) and SOL111 (Modal Frequency Response) solutions.

You can define the following acoustic boundary conditions:

• Acoustic monopole

• Acoustic dipole

• Acoustic plane wave

• Transfer admittance and acoustic continuity

• Acoustic panel normal velocity

• Surface dipole

• Fan noise

• Enforced acoustic pressure

• Force loading from external result files (vibro-acoustic analysis)

• Force loading from a turbulent boundary layer (TBL) model

• Acoustic absorber (impedance/admittance)

Acoustic monopole

An acoustic monopole is a pulsating sound source that radiates sound (acoustic energy) equally in all directions.

Acoustic monopole source

Note:

The plus sign means that the monopole source expands.

You can position an acoustic monopole by selecting an arbitrary point (coordinate) or a fluid node.

You can define an acoustic monopole by specifying amplitude values, or by specifying acoustic power and a phase angle.

• Amplitude—You can specify:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

• Acoustic power—You can specify constant acoustic power and a phase angle of the equivalent monopole amplitude values. You can also enter field values for different monopole powers for different frequencies.

Acoustic dipole

A dipole is a pair of monopoles of equal strength and opposite phases. You can consider a dipole as a pair of sound sources, except that one is a source and the other is a sink.

The directivity pattern has a maximum along two opposite poles and no radiation at the poles perpendicular to the maximum axis. Therefore, a dipole is a directional sound source.

Acoustic dipole source

You can define an acoustic dipole by specifying magnitude and direction or by specifying its 3 directional components.

• Magnitude and direction—You can specify:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

• Components—You can specify:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value for all three Dx, Dy, and Dz complex variables (components).

You can use an acoustic dipole source to simulate simplified fans or speakers.

Acoustic plane wave

A plane wave is an acoustic wave that radiates in a single direction. The propagation direction is the same as the direction of excitation. An acoustic plane wave generates a plane wave only on one side of the source, along the positive direction of the source vector.

(1) Incident field(2) Vector(3) Position(4) Non-incident field	Acoustic plane wave source

You can specify:

• A constant complex excitation with real and imaginary parts.

• A constant complex excitation with magnitude and phase angle.

• A frequency dependent complex field value.

You can use an acoustic plane wave for panel transmission loss as well as random vibro-acoustic analysis of spacecraft.

Surface Dipole

In aeroacoustic simulations, you can use the Surface Dipole boundary condition to create the equivalent of a surface dipole acoustic source. A surface dipole is an acoustic source that you use to model the sound generated by rigid surfaces located in a low-speed fluid flow application. In Simcenter 3D, surface dipoles are based on the Neumann formulation, which assumes that the mass density and speed of sound around the rigid surfaces are uniform.

To model the sound generated by the interaction of a compressible or incompressible flow with a rigid surface, define the Surface Dipole boundary condition with fluid pressures.

The aerodynamic or hydrodynamic fluid pressures are typically computed by a computational fluid dynamics (CFD) software, such as Simcenter STAR-CCM+ and processed by a Model and Load Pre-processing solution process in Pre/Post for later use by Simcenter Nastran.

For example, you may want to examine sound generated by the interaction of low-speed flow with surrounding surfaces, such as HVAC duct walls, or an automotive side mirror. In contrast to solving the entire acoustics problem with volumetric flow information and generating an equivalent point source for each CFD cell, you use a surface dipole source on, for example, a side mirror surface as a boundary condition in your analysis. This reduces the data transfer from the CFD software to Pre/Post, and consequently the computational effort by Simcenter Nastran.

For more information on creating acoustic sources from CFD results with the Model and Load Pre-processing solution process, see Creating acoustic sources from CFD results.

You can combine a surface dipole and the following:

• Fan Noise—In the same simulation with no special considerations. The loading from the Fan Noise is on free coordinates, so no overlapping is possible.

• Absorbent panel property—Even when the two application regions overlap.

• Other boundary conditions—As long as the application regions do not overlap.

Acoustic panel normal velocity

You can apply acoustic panel velocity to acoustic fluid elements using an Acoustic Panel Normal Velocity boundary condition.

You can specify:

• A constant complex excitation with real and imaginary parts.

• A constant complex excitation with magnitude and phase angle.

• A frequency-dependent complex field value.

You use this boundary condition to represent rigid planes, perfectly reflective surfaces, hard surfaces, or a ground plane.

You can use acoustic panel normal velocity for muffler transmission loss.

Enforced acoustic pressure

You can use acoustic pressure boundary conditions to model the free surface conditions, an opening in a cavity, and so on.

You can apply acoustic pressure to the acoustic fluid elements with the Enforced Acoustic Pressure boundary condition. You can specify a constant, complex excitation with real and imaginary parts, complex excitation defined by magnitude and phase angle, or a complex field value that is frequency dependent.

Force loading from external result files or a TBL model

You can use the Model and Load Pre-processing solution process to transform data from external files or a TBL model for acoustic pressure loading. For more information, see Model and Load Pre-processing solution process.

Transfer admittance

The transfer admittance is a Simulation Object that lets you define a relationship linking the velocity values (V1, V2) and pressure values (P1, P2) on both sides of a surface or object. You define a transfer admittance between two sides of a perforated plate, or between an inlet and outlet.

When specifying the two sides of a transfer admittance, you can select compatible or incompatible faces:

• Compatible—Where all the nodes and elements on the two faces are exactly the same.

• Incompatible—Where the nodes and elements do not match perfectly.

When you select compatible faces, direct projection is possible in the transfer admittance, which results in greater accuracy. When you select incompatible faces, the software must perform an additional projection from the source mesh to target mesh.

You can select the faces using the Automatic Pairing Creation method or the Manual Pairing Selection method.

You can define transfer admittance using these methods:

• Admittance and source coefficients—The most general form of Transfer Admittance formula is given below; it requires 6 alpha (α) values as 4 complex admittance coefficients and 2 complex source coefficients.where:P1 = Inlet pressureP2 = Outlet pressureVn1 = Inlet velocityVn2 = Outlet velocityα = admittance and source coefficients

• Rigid transfer admittance—You can specify:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

• Geometric parameters—Mechel’s formula is used to calculate the admittance value based on the geometric details of the perforated plates.

• Transfer matrix—The transfer matrix method defines a relationship between the acoustic velocities and pressures on one side of a surface, and the acoustic velocities and the pressures on the other side. The transfer matrix formulation is given by:where:P1 = Inlet pressureP2 = Outlet pressureV1 = Inlet velocityV2 = Outlet velocityT = transfer matrix parameters

Acoustic continuity

The acoustic continuity is a Simulation Object that lets you define a transfer relationship between two selected free faces. The free faces must be acoustic faces. You can define an acoustic continuity using the Continuity method (available for both Standard Finite Element Method and for Finite Element Method Adaptive Order) or using the Glue method (available only for Standard method).

You can define an acoustic continuity using the Automatic Pairing Creation method or using the Manual Pairing Selection method.

Acoustic absorbers to model admittance/impedance

You can use an acoustic absorber to model impedance and admittance.

You can define an acoustic absorber in two ways:

• Damping and stiffness coefficients—You can define an equivalent structural damping and equivalent structural stiffness.

• Admittance/impedance coefficients—You can specify:A constant complex excitation with real and imaginary parts.A constant complex excitation with magnitude and phase angle.A frequency dependent complex field value.

Acoustic load set

An acoustic load set is a container of acoustic sources.

You can create the following types of acoustic loads in a load set:

• Acoustic monopole

• Acoustic dipole

• Acoustic plane wave

• Acoustic panel normal velocity

• Enforced acoustic pressure

• Fan noise

• Surface dipole

For information on creating an acoustic load set, see Defining acoustic load sets, sources, and pressures (FEM acoustics).

Where do I find it?

Creating an acoustic monopole, an acoustic dipole, an acoustic plane wave, fan noise, or surface dipole source in Simcenter Nastran

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverFEMAO as the specified method for fan noise and surface dipoleAcoustic monopole, acoustic dipole, acoustic plane wave, or fan noise sourceAcoustic or Vibro-Acoustic as the specified analysis typeSOL 108 Direct Frequency ResponseSOL 111 Modal Frequency ResponseSurface dipole sourceAcoustic or Vibro-Acoustic as the specified analysis typeSOL 108 Direct Frequency ResponseVibro-Acoustic as the specified analysis typeSOL 111 Modal Frequency Response
Simulation Navigator	Expand the solution step node of an active solution→Loads→right-click the Loads container→New Load→Acoustic Monopole or Acoustic Dipole, Acoustic Plane Wave, Fan Noise, or Surface Dipole.

Force loading from external result files or a TBL model

Application	Pre/Post
Prerequisites	A Simulation file as the work and displayed partFluid, acoustic, or force loads transformationsSimcenter Nastran as the specified solverStructural, Acoustic, or Vibro-Acoustic as the specified analysis typeSOL 108 Direct Frequency ResponseSOL 111 Modal Frequency ResponseForce loads based on TBL modelsSimcenter Nastran as the specified solverAcoustic or Vibro-Acoustic as the specified analysis typeSOL 108 Direct Frequency ResponseSOL 111 Modal Frequency ResponseSubcase-Random as the specified solution step
Simulation Navigator	Right-click a Simulation file→New Solution Process→Model and Load Pre-processing

Creating a transfer admittance or acoustic continuity in Simcenter Nastran

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverVibro-Acoustic as the specified analysis type
Simulation Navigator	Right-click the Simulation Objects container of an active solution→New Simulation Object→Transfer Admittance or Acoustic Continuity

Creating an acoustic panel normal velocity load in Simcenter Nastran

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or Vibro-Acoustic as the specified analysis type
Simulation Navigator	Expand the Solution Step node of an active solution→Loads→right-click the Loads container→New Load→Acoustic Panel Normal Velocity

Creating a transfer admittance-coefficients in Simcenter Nastran

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or Vibro-Acoustic as the specified analysis type
Menu	Insert→Modeling Objects
Location in dialog box	Modeling Objects Manager dialog box→Type list→Transfer Admittance-Coefficients

Creating an acoustic transfer matrix-coefficients in Simcenter Nastran

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverAcoustic or Vibro-Acoustic as the specified analysis type
Menu	Insert→Modeling Objects
Location in dialog box	Modeling Objects Manager dialog box→Type list→Acoustic Transfer Matrix

How do I

Create an acoustic source load

Create an acoustic absorber

Learn more

Defining acoustic load sets, sources, and pressures (FEM acoustics)

Creating acoustic constraints

Random loading for acoustic and vibro-acoustic analysis (FEM acoustics)

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Boundary conditions overview (FEM acoustics), Simcenter 3D 2021.1 Series

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