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How to create Automatically Match Layer (AML) property and AML surface ? What is it for ?

[ Edited ]

Im about to use it in vibro-acoustic analysis.

I just know that the AML is good but im not really sure about it.

I already read the information in the online help but im not really understand it.

 

Im appreciate if anyone can help me about it and explain the use of AML in analysis. 

Thanks

1 REPLY

Re: How to create Automatically Match Layer (AML) property and AML surface ? What is it for ?

Adapted from SC11 Help:

 

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 NX 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 acoustic 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.

 

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.

 

The thickness of the AML determines its absorptive capacity. During the solve, the solver dynamically (and automatically) 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.

 

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.