FLUID129
2D Infinite Acoustic
FLUID129 Element Description
FLUID129 has been developed as a companion element to FLUID243 and FLUID244. It is intended to be used as an envelope to a model made of FLUID243 or FLUID244 finite elements. It simulates the absorbing effects of a fluid domain that extends to infinity beyond the boundary of FLUID243 or FLUID244 finite element domain. FLUID129 realizes 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. The element can be used to model the boundary of 2D (planar or axisymmetric) fluid regions and as such, it is a line element. It has two or three nodes with one pressure degree of freedom per node. FLUID129 may be used in transient, harmonic, and modal analyses. Typical applications include structural acoustics, noise control, underwater acoustics, etc. See FLUID129 in the Mechanical APDL Theory Reference for more details about this element.
FLUID129 Input Data
The geometry, node locations, and the coordinate system for this element are shown in Figure 129.1: FLUID129 Geometry. The element is defined by two or three nodes (I, J, K), the material properties and the real constants (defined in "FLUID129 Input Summary"). The element must be circular with radius RAD and center located at or near the center of the structure. The radius RAD should be supplied through the real constants.
The element is characterized by a pair of symmetric stiffness and damping matrices.
In a typical meshing procedure, you should mesh the interior fluid domain that is bounded by a circular boundary with FLUID243 or FLUID244 elements, select the nodes on the circular boundary, select the type associated with the FLUID129 and then issue the ESURF command. The latter will automatically add the FLUID129 elements on the boundary of the finite domain.
FLUID129 Input Summary
- Nodes
I, J, K
- Degrees of Freedom
PRES
- Real Constants
RAD - Radius X0 - Center of enclosing circle, X value Y0 - Center of enclosing circle, Y value - Material Properties
MP command: SONC (velocity of sound)
- Surface Loads
None
- Body Loads
None
- Special Features
None
- KEYOPT(1)
Element behavior:
- KEYOPT(3)
Element behavior:
- 0 --
Planar
- 1 --
Axisymmetric
FLUID129 Output Data
The solution output associated with the element is in two forms:
Nodal degrees of freedom included in the overall nodal solution
Additional element output as shown in Table 129.1: FLUID129 Element Output Definitions
A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.
The Element Output Definitions table uses the following notation:
A colon (:) in the Name column indicates that the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file jobname.out. The R column indicates the availability of the items in the results file.
In either the O or R columns, “Y” indicates that the item is always available, a letter or number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.
Table 129.1: FLUID129 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J | Y | Y |
| MAT | Material number | Y | Y |
| LINE: | Length | Y | Y |
| XC, YC | Location where results are reported | Y | 1 |
| SONC | Speed of sound | Y | Y |
Available only at centroid as a *GET item.
Table 129.2: FLUID129 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 129.2: FLUID129 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 129.1: FLUID129 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
FLUID129 Assumptions and Restrictions
FLUID129 must lie on a boundary circular in shape and should completely enclose the domain meshed with FLUID243 or FLUID244 elements.
The radius RAD of the circular boundary of the finite domain should be specified as a real constant. If the coordinates (X0, Y0) of the center of the circle are not supplied through the real constant input, the center will be assumed to be at the origin. The center of the circle should be as close to the center of the model as possible.
The enclosing circular boundary should be placed at a distance of at least 0.2*lambda from the boundary of any structure that may be submerged in the fluid, where lambda = c/f is the dominant wavelength of the pressure waves, c is the speed of sound (SONC) in the fluid, and f is the dominant frequency of the pressure wave. For example, in the case of a submerged circular cylindrical shell of diameter D, the radius of the enclosing boundary, RAD, should be at least (D/2) + 0.2*lambda.
FLUID129 uses an extra (internal) degree of freedom, labeled XTR1, that is not available externally. This degree of freedom is solely for Mechanical APDL's internal use, although it may appear in degree-of-freedom listings or in program messages.
The damped method is the only valid modal analysis method.