2-D Axisymmetric Harmonic Acoustic Fluid

Compatible Products: – | – | – | Enterprise | Ent PP | Ent Solver | –

FLUID29 Element Description

FLUID29 is used for modeling the fluid medium and the interface in fluid/structure interaction problems. Typical applications include sound wave propagation and submerged structure dynamics. The governing equation for acoustics, namely the 2-D wave equation, has been discretized taking into account the coupling of acoustic pressure and structural motion at the interface. The element has four corner nodes with three degrees of freedom per node: translations in the nodal x and y directions and pressure. The translations, however, are applicable only at nodes that are on the interface. Acceleration effects, such as in sloshing problems, may be included.

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 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 and modal analyses. See FLUID29 in the Mechanical APDL Theory Reference for more details about this element.

Figure 29.1:   FLUID29 Geometry

FLUID29 Geometry

FLUID29 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 29.1:  FLUID29 Geometry. The element is defined by four nodes, the number of harmonic waves (MODE on the MODE command), the symmetry condition (ISYM on the MODE command), a reference pressure, and the isotropic material properties. The MODE and ISYM parameters are discussed in detail in Harmonic Axisymmetric Elements with Nonaxisymmetric Loads. The reference pressure (PREF) is used to calculate the element sound pressure level (defaults to 20x10-6 N/m2). The speed of sound () in the fluid is input by SONC where k is the bulk modulus of the fluid (Force/Area) and ρo 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. We recommend MU values from 0.0 to 1.0; however, values greater than 1.0 are allowed. 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 Nodal Loading. Fluid-structure interfaces (FSI) can be flagged by surface loads at the element faces as shown by the circled numbers on Figure 29.1:  FLUID29 Geometry. Specifying the FSI label (without a value) (SF, SFA, SFE) couples 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.

Vertical acceleration (ACELY on the ACEL command) is needed for the gravity regardless of the value of MODE, even for a modal analysis.

A summary of the element input is given in "FLUID29 Input Summary". A general description of element input is given in Element Input. For axisymmetric applications see Harmonic Axisymmetric Elements.

FLUID29 Input Summary


I, J, K, L

Degrees of Freedom
UX, UY, PRES if KEYOPT (2) = 0
PRES if KEYOPT (2) = 1
Real Constants

PREF (reference pressure)

Material Properties

MP command: 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)

Mode Number

Input mode number on MODE command

Loading Condition

Input for ISYM on MODE command

1 -- 

Symmetric loading

-1  --

Antisymmetric loading

Special Features



Structure at element interface:

0 -- 

Structure present at interface (unsymmetric element matrix)

1 -- 

No structure at interface (symmetric element matrix)


Element behavior:

0 -- 


1 -- 


2 --



Free surface effect:

0 -- 

Do not include sloshing effect

1 -- 

Include sloshing effect on face of elements located on Y = 0.0 plane (elements must not have positive Y coordinates)

FLUID29 Output Data

The solution output associated with the element is in two forms:

Solution Output gives a general description of 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 number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available.

Table 29.1:  FLUID29 Element Output Definitions

ELElement NumberYY
NODESNodes - I, J, K, LYY
MATMaterial numberYY
XC, YCLocation where results are reportedY 2
TEMPTemperatures T(I), T(J), T(K), T(L)YY
PRESSUREAverage pressureYY
PG( X, Y, SUM)Pressure gradient components and vector sumYY
VL( X, Y, SUM)Fluid velocity components and vector sum 1 1
SOUND PR.LEVELSound pressure level (in decibels) 1 1

  1. Output only if ANTYPE,HARMIC

  2. Available only at centroid as a *GET item.

Table 29.2: FLUID29 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 29.2: FLUID29 Item and Sequence Numbers:


output quantity as defined in the Table 29.1: FLUID29 Element Output Definitions


predetermined Item label for ETABLE command


sequence number for single-valued or constant element data

Table 29.2:  FLUID29 Item and Sequence Numbers

Output Quantity Name ETABLE and ESOL Command Input

FLUID29 Assumptions and Restrictions

  • The area of the element must be positive.

  • The element must lie in a global X-Y plane as shown in Figure 29.1:  FLUID29 Geometry.

  • All elements must have 4 nodes. A triangular element may be formed by defining duplicate K and L nodes (see Degenerated Shape Elements).

  • The acoustic pressure in the fluid medium is determined by the wave equation with the following assumptions:

    • The fluid is compressible (density changes due to pressure variations).

    • Inviscid fluid (no dissipative effect due to viscosity).

    • There is no mean flow of the fluid.

    • The mean density and pressure are uniform throughout the fluid. Note that the acoustic pressure is the excess pressure from the mean pressure.

    • 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.

FLUID29 Product Restrictions

There are no product-specific restrictions for this element.

Release 18.2 - © ANSYS, Inc. All rights reserved.