3.15. Absorption Element

An exterior acoustics problem typically involves an infinite, homogenous, inviscid fluid surrounding a given geometric entity. The pressure waves generated by this geometric entity must satisfy the radiation condition on the surrounding space. To simulate this infinite fluid, you enclose your model with the Absorption Element boundary condition. This approach truncates the unbounded domain by introducing a second-order absorbing boundary condition so that an outgoing pressure wave reaching the boundary of the model is "absorbed" with minimal reflections back into the fluid domain.

Note:

  • For a 3-D acoustic analysis, the absorbing element must be scoped to spherical faces having the same radius and center and that are connected to bodies with the same material assignment. For more information, refer to Absorbing Boundary Condition (ABC) section in the Mechanical APDL Acoustic Analysis Guide.

  • Currently, the Absorption Element boundary condition is not supported if you wish to scope it to an Acoustic region that is defined using frequency dependent material properties. However, you can resolve this issue by specifying the TBCOPY command using a Commands object at the Solution level.

Analysis Types

The Absorption Element boundary condition is available for the following analysis types:

Common Characteristics

This section describes the characteristics of the boundary condition, including the application requirements, support limitations, and loading definitions and values.

Types Supported

  • 3D Simulation: Supported.

  • 2D Simulation: Not Supported.

Geometry Types: Geometry types supported for the Absorption Element boundary condition include:

  • Solid: Supported.

  • Surface/Shell: Not Supported.

  • Wire Body/Line Body/Beam: Not Supported.

Topology: The following topology selection options are supported for Absorption Element.

  • Body: Not Supported.

  • Face: Supported (spherical face only).

  • Edge: Not Supported.

  • Vertex: Not Supported.

  • Nodes: Not Supported

  • Element: Not Supported.

  • Element Face: Not Supported.

Boundary Condition Application

To apply an Absorbing Element:

  1. On the Environment Context tab: click Acoustic Boundary Conditions>Absorption Element. Or, right-click the Environment tree object or in the Geometry window and select Insert>Acoustics>Absorption Element.

  2. Define the Scoping Method.

    Note:  The application calculates the radius of the sphere internally based upon the geometry you have selected. Material property requirements are derived based on the assigned material using the MPCOPY command.

Details View Properties

The selections available in the Details view are described below.

CategoryFields/Options/Description
Scope

Scoping Method, options include:

  • Geometry Selection: Default setting, indicating that the boundary condition is applied to a geometry or geometries, which are chosen using a graphical selection tools.

    • Geometry: Visible when the Scoping Method is set to Geometry Selection. Displays the type of geometry (Face, Edge, etc.) and the number of geometric entities (for example: 1 Face, 2 Edges) to which the boundary has been applied using the selection tools.

      These geometry selections must be made on bodies specified in one of the acoustic Physics Regions.

  • Named Selection: Indicates that the geometry selection is defined by a Named Selection.

    • Named Selection: Visible when the Scoping Method is set to Named Selection. This field provides a drop-down list of available user–defined Named Selections.

Definition

Type: Read-only field that describes the object - Absorption Element.

Suppressed: Include (No - default) or exclude (Yes) the boundary condition.

Mechanical APDL References and Notes

The Absorption Element is applied using the FLUID130 element.