FLUID81 Element Description

FLUID81 is a modification of the axisymmetric structural solid element (PLANE25). The element is used to model fluids contained within vessels having no net flow rate. It is defined by four nodes having three degrees of freedom at each node: translations in the nodal x, y, and z directions. The element is used in a structural analysis as an axisymmetric ring element.

The element is a generalization of the axisymmetric version of FLUID79, the 2D fluid element, in that the loading need not be axisymmetric. Various loading cases are described in Harmonic Axisymmetric Elements with Nonaxisymmetric Loads. 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. See FLUID81 - Axisymmetric-Harmonic Contained Fluid for more details about this element. Another fluid element (FLUID116) is available to model fluids flowing in pipes and channels.

Figure 81.1: FLUID81 Geometry

FLUID81 Input Data

The geometry, node locations, and the coordinate system for this element are shown in Figure 81.1: FLUID81 Geometry. The element input data includes four nodes, the number of harmonic waves (MODE on the MODE command), the symmetry condition (ISYM on the MODE command), and the isotropic material properties. If MODE = 0 and ISYM = 1, the element behaves similar to the axisymmetric case of FLUID79. The MODE and ISYM parameters are discussed in detail in Harmonic Axisymmetric Elements with Nonaxisymmetric Loads. 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. Density (DENS) must be input as a positive number.

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 regardless of the value of MODE. Harmonically varying nodal forces, if any, should be input on a full 360° basis.

Element loads are described in Element Loading. Harmonically varying pressures may be input as surface loads on the element faces as shown by the circled numbers on Figure 81.1: FLUID81 Geometry. Positive pressures act into the element.

Harmonically varying 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 "FLUID81 Input Summary". A general description of element input is given in Element Input.

FLUID81 Output Data

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

The pressure and temperature are evaluated at the element centroid. Nodal forces and reaction forces are on a full 360° basis.

In the displacement printout, the UZ component is out-of-phase with the UX and UY components. For example, in the MODE = 1, ISYM = 1 loading case, UX and UY are the peak values at  θ = 0° and UZ is the peak value at  θ = 90°. Printout for combined loading cases may be obtained from the POST1 routine. We recommend that you always use the angle field on the SET command when postprocessing the results. For more information about harmonic elements, see Harmonic Axisymmetric Elements with Nonaxisymmetric Loads.

A general description of solution output is given in Solution Output. See the Basic Analysis Guide for ways to view results.

The following notation is used in Table 81.1: FLUID81 Element Output Definitions:

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 81.2: FLUID81 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 81.2: FLUID81 Item and Sequence Numbers:

FLUID81 Assumptions and Restrictions

FLUID81 Product Restrictions

There are no product-specific restrictions for this element.