COMBI214 Element Description

COMBI214 has longitudinal as well as cross-coupling capability in 2D applications. It is a tension-compression element with up to two degrees of freedom at each node: translations in any two nodal directions (x, y, or z). The element has two nodes plus one optional orientation node. No bending or torsion is considered.

For user-defined characteristics (KEYOPT(1) = 0), the spring, damping, or mass capability may be removed from the element.

When used as a cylindrical journal bearing or squeeze film damper (KEYOPT(1) ≠ 0), the fluid film pressure forces based on finite length assumption are calculated. Only static and nonlinear transient analyses are supported. Static analysis can be used to determine pressure forces at a known equilibrium position. Stiffness/damping characteristics can also be obtained by specifying a perturbation increment in the element real constants.

A longitudinal spring/damper with torsion capabilities is available via the COMBIN14 element. A general spring or damper is also available in the stiffness, damping or mass matrix element MATRIX27. Another spring-damper element having its direction of action determined by the nodal coordinate directions is COMBIN40.

For more information about this element, see COMBI214 - 2D Spring-Damper Bearing in the Mechanical APDL Theory Reference.

Figure 214.1: COMBI214 Geometry for KEYOPT(1) = 0 and KEYOPT(2) = 0

I and J can be coincident.

Figure 214.2: COMBI214 Geometry for KEYOPT(1) > 0 and KEYOPT(2) = 0

I is attached to the bearing journal.

J is attached to the rotor.

COMBI214 Input Data

The geometry, node locations, and coordinate system for this element are shown in Figure 214.1: COMBI214 Geometry for KEYOPT(1) = 0 and KEYOPT(2) = 0. The element is defined by two nodes.

Depending on the element technology (KEYOPT(1)) specified, the inputs involve stiffness characteristics K₁₁, K₂₂, K₁₂ and K₂₁, damping characteristics C₁₁, C₂₂, C₁₂ and C₂₁, and mass characteristics M₁₁, M₂₂, M₁₂, and M₂₁ or bearing geometry data.

KEYOPT(2) = 0 through 2 options define the element plane. The element operates in the nodal coordinate system. The nodal coordinate systems of nodes I and J must be the same. (1) and (2) indicate the first and second axis of the element plane defined by KEYOPT(2). For example, if KEYOPT(2) = 0 and there is no nodal coordinate system defined for nodes I and J, then (1) is the global X-axis and (2) is the global Y-axis.

A summary of the element input is given in "COMBI214 Input Summary". A general description of element input is given in Element Input.

User-Defined Stiffness/Damping Characteristics (KEYOPT(1) = 0)

The stiffness coefficients should have units of Force/Length, and the damping coefficient units are Force*Time/Length. The damping capability is not used for static or undamped modal analyses. The mass coefficients should have units of Force*Time²/Length.

For stiffness, damping, and mass real constants, either numerical values or tabular array inputs can be specified. If specifying tabular inputs, enclose the table name within % characters (%tabname%).

These real constants can vary with the amplitude of the rotational velocity vector (defined via the OMEGA or CMOMEGA command). Use the *DIM command and the primary variable OMEGS to dimension the table and identify the variable. Because the amplitude of the rotational velocity vector is an absolute value, only positive values of OMEGS in the table parameter are valid.

Stiffness and damping real constants can also vary with the eccentricity (the position of the center line of the rotor) and/or the phase shift between the displacements in the two nodal directions. Use the *DIM command and the primary variable ECCENT and/or THETA to dimension the table and identify the variables. The phase shift values unit (THETA) is the degree. ECCENT and THETA only apply to nonlinear analyses.

For more information about using tabular inputs, see Array Parameters in the Ansys Parametric Design Language Guide, Applying Loads Using Tabular Input in the Basic Analysis Guide, and Performing a Thermal Analysis Using Tabular Boundary Conditions in the Thermal Analysis Guide.

The KEYOPT(3) = 0 and 1 options specify whether or not the element is symmetric. When symmetric, cross-coupling terms in stiffness, damping, and mass coefficients are equal (that is, K₁₂ = K₂₁, C₁₂ = C₂₁, and M₁₂ = M₂₁).

In a full nonlinear transient analysis (NROPT,FULL), if the stiffness or damping properties are not isotropic, periodic terms are generated (for details, see The Case of Rotating Reference Frame Dynamics in the Theory Reference).

Reynolds Equation Integration (KEYOPT(1) = 1 or 2)

Calculating all element results (KEYOPT(1) = 2) may be computationally expensive. KEYOPT(1) = 1 can be used to calculate a limited number of element outputs. See Table 214.2: COMBI214 Element Output Definitions for KEYOPT(1) = 1 or 2.

The bearing geometry is defined by real constants: radial clearance (C), length (L), and rotor radius (R).

In a static analysis, the location of the rotor center is specified using degree of freedom constraints (D command) and the translational velocities of the rotor are defined by real constants (Veloc1 and Veloc2). To obtain the stiffness and damping characteristics, the non-dimensional perturbation increment (real constant PertInc) must be specified along with KEYOPT(1) = 2.

The integration of Reynolds equations is performed with a circumferential step of 2 degrees. It can be modified using real constant ThetaInc.

In the particular case of a squeeze film damper in synchronous precession (when the rotor center describes an orbit having the same frequency as the rotational velocity), the real constant OmgPrec is used.

COMBI214 Input Data (KEYOPT(1) = 1 or 2)

COMBI214 Output Data

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

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates 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 a - indicates that the item is not available.

Figure 214.3: COMBI214 Angle Definition

Table 214.3: COMBI214 Item and Sequence Numbers for KEYOPT(1) = 0 lists output available via 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 document for more information. The following notation is used in Table 214.3: COMBI214 Item and Sequence Numbers for KEYOPT(1) = 0:

Output data for COMBI214 consists of the following:

COMBI214 Assumptions and Restrictions

COMBI214 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

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