Acoustics and vibro-acoustics > Simcenter Nastran FEM acoustics > Modeling exterior acoustics using automatically matched layers

Modeling exterior acoustic problems using automatically matched layers

When modeling exterior acoustics, you can define an exterior boundary and apply an impedance (characteristic impedance given by the product of density and speed of sound) using acoustic absorbers. With this method, to prevent artificial reflections, this exterior boundary must be several wavelengths away from the vibrating source. As a result, these models tend to be large, and perfect absorption is difficult.

With the Simcenter Nastran FEM acoustic and vibro-acoustic environment, you can use an automatically matched layer (AML) to represent the non-reflective acoustic boundary condition. The AML boundary condition uses a reflectionless artificial layer that absorbs outgoing waves regardless of their frequency and angle of incidence.

For exterior acoustic problems such as radiation, you can model an AML region close to the vibrating structure or acoustic source, resulting in much smaller FE models. The AML surface must be convex.

AML is not limited to acoustic radiation problems. For example, you can model an infinite duct by introducing an anechoic duct termination at the free faces of a duct end section. You can use an AML to model this anechoic termination.

The following example demonstrates how an AML can be specified to represent the radiation from a vibrating gearbox.

Structural Mesh Fluid Mesh AML	AML applied on free element faces of a fluid mesh that surrounds a structural mesh

Defining an AML

You define an AML boundary condition using the Automatically Matched Layer simulation object.

You can create multiple Automatically Matched Layer simulation objects in a solution. When you create an Automatically Matched Layer simulation object, you define an AML Region by selecting free element faces in the fluid mesh or by selecting polygon geometry faces. When you define multiple AML simulation objects, be careful to avoid overlapping AML regions. For more information, see Meshing for FEM acoustic analysis.

The thickness of the AML determines its absorptive capacity. During the solve, the solver dynamically extrudes the AML to a PML (perfectly matched layer) volume using the AML thickness and the number of layers that you specify in the AML definition. The solver also adjusts the thickness of the AML layer for each of the solution frequencies.

Specifying the surfaces to compute acoustic results outside the fluid mesh

You can use a microphone mesh to request acoustic results in the far field. To compute these results, the solver uses the computed pressure and velocities from the radiation surfaces that you specify in the AML definition. You can choose to use the AML surface itself or the entire physical boundary, which is all free fluid element faces with the exception of faces on the AML and the infinite planes.

For acoustic results on microphones inside the fluid mesh, the results at fluid nodes are interpolated. To compute results at microphone points outside the fluid mesh, the software computes the acoustic solution using the pressure and velocity on the AML and a boundary integral.

For more information about microphone meshes, see Using microphone mesh to capture analysis results.

Representing reflecting surfaces using infinite planes

When you specify a radiation surface, you can also define infinite planes that represent reflecting surfaces, such as a hard wall, ground, or a pressure release surface (for underwater acoustics). At the infinite plane, the acoustic particle velocity is zero, because fluid does not cross the boundary. You can use infinite planes to define a symmetric (zero velocity, reflective) or anti-symmetric (zero pressure, non-reflective) boundary condition. The plane itself is not symmetric or anti-symmetric.

Because acoustic symmetry implies zero normal velocity, an infinite plane that is used to specify a symmetric boundary condition is acoustically equivalent to the presence of a rigid, reflecting floor.

If you are modeling a situation where the sound-radiating structure is located on a hard floor, such as the concrete floor of a semi-anechoic chamber, you can represent the presence of this floor by using a Rigid type of infinite plane.

Conversely, an infinite plane that is used to represent an anti-symmetric boundary condition is equivalent to the presence of a pressure-release surface. You can define this type of plane to specify free or non-reflecting surfaces. For example, to model the acoustic radiation into water from a submarine at a certain depth, you can model the presence of the sea surface above the submarine by defining a Pressure Release type of infinite plane.

The example below illustrates an AML with a radiation surface and a Rigid type of infinite plane.

Structural mesh Acoustic fluid mesh AML surface Radiation surface on fluid exterior		The AML surface is also used as radiation surface to predict the sound outside of the acoustic fluid mesh. You also request to compute the acoustic power through the radiation surface.

Infinite plane with zero velocity acting as a rigid reflecting surface		The radiation surface can also be defined on the physical surface. The results should be very close to the previous setup.

Checking the validity of an AML

To check the validity of an AML, run the Model Setup Check command when you solve the acoustic solution. A valid AML:

Must contain at least one fluid element free face.
Must contain only free faces of the selected fluid elements.
Must be defined on a convex surface.
If the AML contains more than one fluid free faces, the AML must be contiguous. The contiguity of the AML is only through the free edges, and not through the nodes.
Cannot overlap another AML.
Cannot have loads, constraints, acoustic absorbers, or surface-to-surface gluing.

When you use AMLs with infinite planes:

An AML free edge cannot lie in more than one infinite plane.
AML free edges must be closed by infinite planes.
Infinite planes must be perpendicular to each other and must be perpendicular to one of the global axes (X, Y, or Z).
AMLs cannot intersect and overlap an infinite plane.

When you specify radiation surfaces, only one AML can use the physical boundary (all fluid faces without the AML surface and infinite plane) as the radiation region.

For more information see, Model checks for acoustics analysis.

Requesting output for an AML

You can request the acoustic power (radiated power) to be computed for an AML by selecting the Acoustic Power check box in the Acoustic Output Requests or Vibro-Acoustic Output Requests for the solution. Simcenter Nastran computes acoustic power in SORT2 format, that is, one power function per AML as results for each subcase. You can plot the acoustic power SORT2 function results for an AML using the XY Function Navigator.

For more information on requesting output for acoustic power and plotting the acoustic power results for an AML, see Compute acoustic power on AML radiation surfaces and Plot acoustic power functions for an AML.

Where do I find it?

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 typeSOL 108 Direct Frequency Response or SOL 111 Modal Frequency Response as the solution type
Simulation Navigator	Right-click the Simulation Object Container node→New Simulation Object→Automatically Matched Layer

How do I

Create an Automatically Matched Layer (AML)

Compute acoustic power on AML radiation surfaces

Plot acoustic power functions for an automatically matched layer

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Automatically Matched Layer dialog box

Region dialog box

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