FLUID130
3D Infinite
Acoustic
FLUID130 Element Description
FLUID130 has been developed as a companion element to FLUID30, FLUID220, and FLUID221. It is intended to be used as an envelope to a model made of FLUID30, FLUID220, and FLUID221 finite elements. It simulates the absorbing effects of a fluid domain that extends to infinity beyond the boundary of the finite element domain that is made of FLUID30, FLUID220, and FLUID221 elements. FLUID130 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 3D fluid regions and as such, it is a plane surface element. It has four nodes with one pressure degrees of freedom per node. FLUID130 may be used in transient, harmonic, and modal analyses. Typical applications include structural acoustics, noise control, underwater acoustics, etc. See FLUID30, FLUID220, and FLUID221 in the Mechanical APDL Theory Reference for more details about this element.
FLUID130 Input Data
The geometry, node locations, and the coordinate system for this element are shown in Figure 130.1: FLUID130 Geometry. The element is defined by four nodes (I, J, K, L) or eight nodes (I, J, K, L, M, N, O, P), the material property SONC (speed of sound), and the real constants shown in "FLUID130 Input Summary". A triangular element may be formed by defining duplicate K and L node numbers. The element must be at the spherical boundary of an acoustic fluid domain, meshed using FLUID30, FLUID220, and FLUID221 elements, 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 symmetric stiffness and a damping matrix.
In a typical meshing procedure you should mesh the interior fluid domain that is bounded by a spherical boundary with FLUID30, FLUID220, and FLUID221 elements, select the nodes on the spherical boundary, select the type associated with the FLUID130 and then issue the ESURF command. The latter will automatically add the FLUID130 elements on the boundary of the finite domain.
FLUID130 Input Summary
- Nodes
I, J, K, L, M, N, O, P
- Degrees of Freedom
PRES
- Real Constants
RAD - Radius X0 - Center of enclosing circle, X value Y0 - Center of enclosing circle, Y value Z0 - Center of enclosing circle, Z value - Material Properties
MP command: SONC (velocity of sound), DENS (mass density)
- Surface Loads
None
- Body Loads
None
- Special Features
None
- KEYOPT(1)
FLUID130 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 130.1: FLUID130 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 130.1: FLUID130 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J, K, L | Y | Y |
| MAT | Material number | Y | Y |
| AREA: | AREA | 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 130.2: FLUID130 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 130.2: FLUID130 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 130.1: FLUID130 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
FLUID130 Assumptions and Restrictions
FLUID130 must lie on a boundary spherical in shape and should completely enclose the domain meshed with FLUID30, FLUID220, and FLUID221 elements.
The radius RAD of the spherical boundary of the finite domain should be specified as a real constant. If the coordinates (X0, Y0, Z0) of the center of the sphere are not supplied through the real constant input, the center will be assumed to be at the origin of the global coordinate system. The center of the sphere should be as close to the center of the model as possible.
The enclosing spherical 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 spherical shell of diameter D, the radius of the enclosing boundary, RAD, should be at least (D/2) + 0.2*lambda.
FLUID130 uses extra (internal) degrees of freedom, labeled XTR1 and XTR2, that are not available externally. These degrees of freedom are solely for Mechanical APDL's use, although they may appear in degree-of-freedom listings or in program messages.
The damped method is the only valid modal analysis method.
FLUID130 assumes constant density and speed of sound values without losses in the “infinite” domain. Consequently, the acoustic fluid elements adjacent to FLUID130 must not have viscosity (MP,VISC), non-uniform acoustic media, or the Johnson-Champoux-Allard model defined.