The element has the capability to include damping of sound absorbing material at the interface. The element can be used with other 2-D structural elements to perform unsymmetric or damped modal, full harmonic response and full transient method analyses (see the description of the TRNOPT command). When there is no structural motion, the element is also applicable to static, modal and reduced harmonic response analyses. See Section 14.29 in the ANSYS Theory Reference for more details about this element.
Figure 4.29-1 FLUID29 2-D Acoustic Fluid
) in the fluid is input by SONC where k is the
bulk modulus of the fluid (Force/Area) and 
is the mean fluid density
(Mass/Volume) (input as DENS). The dissipative effect due to fluid viscosity is
neglected, but absorption of sound at the interface is accounted for by
generating a damping matrix using the surface area and boundary admittance
at the interface. Experimentally measured values of the boundary admittance
for the sound absorbing material may be input as material property MU (with
values from 0.0 to 1.0). MU=0.0 represents no sound absorption and MU=1.0
represents full sound absorption. DENS, SONC and MU are evaluated at the
average of the nodal temperatures.
Nodal flow rates, if any, may be specified using the F command where both the real and imaginary components may be applied. Nodal flow rates should be input per unit of depth for a plane analysis and on a 360° basis for an axisymmetric analysis.
Element loads are described in Section 2.7 Fluid-structure interfaces (FSI) may be flagged by surface loads at the element faces as shown by the circled numbers on Figure 4.29-1. Specifying the FSI label (without a value) [SF, SFA, SFE] will couple the structural motion and fluid pressure at the interface. Deleting the FSI specification [SFDELE, SFADELE, SFEDELE] removes the flag. The flag specification should be on the fluid elements at the interface. The surface load label IMPD with a value of unity should be used to include damping that may be present at a structural boundary with a sound absorption lining. A zero value of IMPD removes the damping calculation. The displacement degrees of freedom (UX and UY) at the element nodes not on the interface should be set to zero to avoid zero-pivot warning messages.
Temperatures may be input as element body loads at the nodes. The node I temperature T(I) defaults to TUNIF. If all other temperatures are unspecified, they default to T(I). For any other input pattern, unspecified temperatures default to TUNIF.
KEYOPT(2) is used to specify the absence of a structure at the interface and, therefore, the absence of coupling between the fluid and structure. Since the absence of coupling produces symmetric element matrices, a symmetric eigensolver [MODOPT] may be used within the modal analysis. However, for the coupled (unsymmetric) problem, a corresponding unsymmetric eigensolver [MODOPT] must be used.
A summary of the element input is given in Table 4.29-1. A general description of element input is given in Section 2.1.
Table 4.29-1 FLUID29 Input Summary
| Element Name
|
FLUID29
|
| Nodes
|
I, J, K, L
|
| Degrees of Freedom
|
UX, UY, PRES if KEYOPT (2) = 0 PRES if KEYOPT (2) = 1
|
| Real Constants
|
PREF
|
| Material Properties
|
DENS, SONC, MU
|
| Surface Loads
|
Fluid-structure Interface Flag: face 1 (J-I), face 2 (K-J), face 3 (L-K), face 4 (I-L) Impedance: face 1 (J-I), face 2 (K-J), face 3 (L-K), face 4 (I-L)
|
| Body Loads
|
Temperatures: T (I), T (J), T (K), T (L)
|
| KEYOPT(2)
|
0 - Structure present at interface (unsymmetric element matrix) 1 - No structure at interface (symmetric element matrix)
|
| KEYOPT(3)
|
0 - Planar 1 - Axisymmetric
|
The following notation is used in Table 4.29-2:
A colon (:) in the Name column indicates the item can be accessed by the Component Name method [ETABLE, ESOL] (see Section 2.2.2). The O and R columns indicate the availability of the items in the file Jobname.OUT (O) or in the results file (R), a Y indicates that the item is always available, a number refers to a table footnote which describes when the item is conditionally available, and a - indicates that the item is not available.
Table 4.29-2 FLUID29 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 |
| VOLU:
|
Volume
|
Y | Y |
| CENT: X, Y
|
Global location XC, YC
|
Y | Y |
| TEMP
|
Temperatures T(I), T(J), T(K), T(L)
|
Y | Y |
| PRESSURE
|
Average pressure
|
Y | Y |
| PG( X, Y, SUM)
|
Pressure gradient components and vector sum
|
Y | Y |
| VL( X, Y, SUM)
|
Fluid velocity components and vector sum
|
1 | 1 |
| SOUND
PR.LEVEL
|
Sound pressure level (in decibels)
|
1 | 1 |
Table 4.29-3 lists output available through the ETABLE command using the Sequence Number method. See Chapter 5 of the ANSYS Basic Analysis Procedures Guide and Section 2.2.2.2 of this manual for more information. The following notation is used in Table 4.29-3:
| Name
|
Item
|
E
|
| PGX
|
SMISC
|
1 |
| PGY
|
SMISC
|
2 |
| VLX
|
SMISC
|
3 |
| VLY
|
SMISC
|
4 |
| PRESSURE
|
NMISC
|
1 |
| PGSUM
|
NMISC
|
2 |
| VLSUM
|
NMISC
|
3 |
| SOUND PR. LEVEL
|
NMISC
|
4 |
The acoustic pressure in the fluid medium is determined by the wave equation with the following assumptions:
1. The fluid is compressible (density changes due to pressure variations).
2. Inviscid fluid (no dissipative effect due to viscosity).
3. There is no mean flow of the fluid.
4. The mean density and pressure are uniform throughout the fluid. Note that the acoustic pressure is the excess pressure from the mean pressure.
5. Analyses are limited to relatively small acoustic pressures so that the changes in density are small compared with the mean density.
The lumped mass matrix formulation [LUMPM,ON] is not allowed for this element.