FLUID79
2D Contained
Fluid
FLUID79 Element Description
| Although this archived element is available for use in your analysis, Ansys, Inc. recommends using a current-technology element such as FLUID29, FLUID30, FLUID220, or FLUID221. |
FLUID79 is used to model fluids contained within vessels having no net flow rate. Another fluid element (FLUID116) is available to model fluids flowing in pipes and channels. The fluid element is particularly well suited for calculating hydrostatic pressures and fluid/solid interactions. Acceleration effects, such as in sloshing problems, as well as temperature effects, may be included.
The fluid element is defined by four nodes having two degrees of freedom at each node: translation in the nodal x and y directions. The element may be used in a structural analysis as a plane element or as an axisymmetric ring element. See FLUID79 - 2D Contained Fluid for more details about this element. See FLUID80 for a 3D version of this element.
Note: This element cannot be used in a modal analysis.
FLUID79 Input Data
The geometry, node locations, and the coordinate system for this element are shown in Figure 79.1: FLUID79 Geometry. The element input data includes four nodes and the isotropic material properties. EX, which is interpreted as the "fluid elastic modulus", should be the bulk modulus of the fluid (approximately 300,000 psi for water). The viscosity property (VISC) is used to compute a damping matrix for dynamic analyses (typical viscosity value for water is 1.639 x 10-7 lb-sec/in2). The use of KEYOPT(2) for gravity springs is discussed in "FLUID80 Input Data". Vertical acceleration (ACELY on the ACEL command) is needed for the gravity springs.
Element loads are described in Element Loading. Pressures may be input as surface loads on the element faces as shown by the circled numbers on Figure 79.1: FLUID79 Geometry. Positive pressures act into the element. 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.
A summary of the element input is given in "FLUID79 Input Summary". A general description of element input is given in Element Input. For axisymmetric applications see Harmonic Axisymmetric Elements.
FLUID79 Input Summary
- Nodes
I, J, K, L
- Degrees of Freedom
UX, UY
- Real Constants
None
- Material Properties
MP command: EX, ALPX (or CTEX or THSX), DENS, VISC, ALPD, BETD, DMPR
- Surface Loads
- Pressures --
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)
- Special Features
None
- KEYOPT(2)
Location of gravity springs:
- 0 --
Place gravity springs on all sides of all elements
- 1 --
Place gravity springs only on face of elements located on Y = 0.0 plane (elements must not have positive Y coordinates)
- KEYOPT(3)
Element behavior:
- 0 --
Plane
- 1 --
Axisymmetric
FLUID79 Output Data
The solution output associated with the element is in two forms:
Degree of freedom results included in the overall nodal solution
Additional element output as shown in Table 79.1: FLUID79 Element Output Definitions
The pressure and temperature are evaluated at the element centroid. Nodal forces and reaction forces are on a full 360° basis for axisymmetric models. 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 79.1: FLUID79 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 |
| XC, YC | Location where results are reported | Y | 1 |
| PRES | Pressures P1 at nodes J, I; P2 at K, J; P3 at L, K; P4 at I, L | Y | Y |
| TEMP | Temperatures T(I), T(J), T(K), T(L) | Y | Y |
| TAVG | Average temperature | Y | - |
| PAVG | Average pressure | Y | Y |
Available only at centroid as a *GET item.
Table 79.2: FLUID79 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 for more information. The following notation is used in Table 79.2: FLUID79 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 79.1: FLUID79 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
- I,J,...,L
sequence number for data at nodes I,J,...,L
FLUID79 Assumptions and Restrictions
The area of the element must be positive.
The fluid element must lie in an X-Y plane as shown in Figure 79.1: FLUID79 Geometry and the Y-axis must be the axis of symmetry for axisymmetric analyses.
An axisymmetric structure should be modeled in the +X quadrants.
Radial motion should be constrained at the centerline.
Usually the Y-axis is oriented in the vertical direction with the top surface at Y = 0.0.
The element temperature is taken to be the average of the nodal temperatures.
Elements should be rectangular whenever possible, as results are known to be of lower quality for some cases using nonrectangular shapes.
Axisymmetric elements should always be rectangular.
The nonlinear transient dynamic analysis should be used instead of the linear transient dynamic analysis for this element.
A very small stiffness (EX x 1.0E-9) is associated with the shear and rotational strains to ensure static stability. See FLUID80 for more assumptions and restrictions.
Only the lumped mass matrix is available.