FLUID80
3D Contained
Fluid
FLUID80 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. |
FLUID80 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 eight nodes having three degrees of freedom at each node: translation in the nodal x, y, and z directions. See FLUID80 - 3D Contained Fluid for more details about this element. See FLUID79 for a 2D version of this element.
Note: This element cannot be used in a modal analysis.
FLUID80 Input Data
The geometry, node locations, and the coordinate system for this element are shown in Figure 80.1: FLUID80 Geometry. The element input data includes eight 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. A typical viscosity value for water is 1.639 x 10-7 lb-sec/in2.
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 80.1: FLUID80 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.
The element also includes special surface effects, which may be thought of as gravity springs used to hold the surface in place. This is performed by adding springs to each node, with the spring constants being positive on the top of the element, and negative on the bottom. Gravity effects (ACEL) must be included if a free surface exists. For an interior node, the positive and negative effects cancel out, and at the bottom, where the fluid must be contained to keep the fluid from leaking out, the negative spring has no effect (as long as all degrees of freedom on the bottom are fixed). If the bottom consists of a flexible container, or if the degrees of freedom tangential to a curved surface are released, these negative springs may cause erroneous results and "negative pivot" messages. In this case, use of KEYOPT(2) = 1 is recommended.
These surface springs, while necessary to keep the free surface in place, artificially reduce the hydrostatic motion of the free surface. The error for a tank with vertical walls, expressed as a ratio of the computed answer over the correct answer is 1.0/(1.0 + (bottom pressure/bulk modulus)), which is normally very close to 1.0. Hydrodynamic results are not affected by this overstiffness.
A summary of the element input is given in "FLUID80 Input Summary". A general description of element input is given in Element Input.
FLUID80 Input Summary
- Nodes
I, J, K, L, M, N, O, P
- Degrees of Freedom
UX, UY, UZ
- 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-L-K), face 2 (I-J-N-M), face 3 (J-K-O-N), face 4 (K-L-P-O), face 5 (L-I-M-P), face 6 (M-N-O-P)
- Body Loads
- Temperatures --
T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P)
- 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 Z = 0.0 plane (elements must not have positive Z coordinates)
FLUID80 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 80.1: FLUID80 Element Output Definitions
The pressure and temperature are evaluated at the element centroid. 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 80.1: FLUID80 Element Output Definitions
| Name | Definition | O | R |
|---|---|---|---|
| EL | Element Number | Y | Y |
| NODES | Nodes - I, J, K, L, M, N, O, P | Y | Y |
| MAT | Material number | Y | Y |
| VOLU: | Volume | Y | Y |
| XC, YC, ZC | Location where results are reported | Y | 1 |
| PRES | Pressures P1 at nodes J, I, L, K; P2 at I, J, N, M; P3 at J, K, O, N; P4 at K, L, P, O; P5 at L, I, M, P; P6 at M, N, O, P | Y | Y |
| TEMP | Temperatures T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P) | Y | Y |
| TAVG | Average temperature | Y | - |
| PAVG | Average pressure | Y | Y |
Available only at centroid as a *GET item.
Table 80.2: FLUID80 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 80.2: FLUID80 Item and Sequence Numbers:
- Name
output quantity as defined in the Table 80.1: FLUID80 Element Output Definitions
- Item
predetermined Item label for ETABLE command
- E
sequence number for single-valued or constant element data
- I,J,...,P
sequence number for data at nodes I,J,...,P
FLUID80 Assumptions and Restrictions
Zero volume elements are not allowed.
Elements may be numbered either as shown in Figure 80.1: FLUID80 Geometry or may have the planes IJKL and MNOP interchanged.
The element may not be twisted such that the element has two separate volumes. This occurs most frequently when the elements are not numbered properly.
Structures are usually modeled with the Z-axis oriented in the vertical direction and the top surface at Z = 0.0.
The element temperature is taken to be the average of the nodal temperatures.
Elements should be rectangular (brick shaped) whenever possible, as results are known to be of lower quality for some cases using nonrectangular shapes.
The nonlinear transient dynamic analysis should be used instead of the linear transient dynamic analysis for this element.
The amount of flow permitted is limited to that which will not cause gross distortions in the element.
The large deflection option should not be used with this element.
When used for a static application, the free surface must be input flat. Gravity must be input if there is a free surface. The element gives valid nodal forces representing hydrostatic pressure and also valid vertical displacements at the free surface. Other nodal displacements, which may be large, represent energy-free internal motions of the fluid.
Fluid element at a boundary should not be attached directly to structural elements but should have separate, coincident nodes that are coupled only in the direction normal to the interface.
Arbitrarily small numbers are included to give the element some shear and rotational stability.
Only the lumped mass matrix is available.