2-D 8-Node
Diffusion Solid
PLANE238 Element Description
PLANE238 is a 2-D, 8-node diffusion element. The element has one degree of freedom, concentration (CONC), at each node. The 8-node elements have compatible concentration shapes and are well suited to model curved boundaries.
This element is applicable to steady-state and transient diffusion analyses. The element can also account for transport effects from a velocity field. See PLANE238 - 2-D 8-Node Diffusion Solid in the Mechanical APDL Theory Reference for more details about this element.
PLANE238 Input Data
The geometry, node locations, and the coordinate system for this element are shown in Figure 238.1: PLANE238 Geometry. The element is defined by eight nodes and orthotropic material properties. A triangular-shaped element may be formed by defining the same node number for nodes K, L and O.
Orthotropic material directions correspond to the element coordinate directions. The element coordinate system orientation is as described in Coordinate Systems. Properties not input by default are as described in the Material Reference. When saturated concentration (MP,CSAT) is specified, the meaning of the CONC degree of freedom is normalized concentration. The actual concentration is evaluated at the element centroid and output as SMISC,1. When unspecified, the saturated concentration (MP,CSAT) defaults to 1.0 and the nodal degree of freedom CONC is actual concentration. See Diffusion in the Mechanical APDL Theory Reference for more information on the diffusion analysis.
Nodal loads are defined with the D (Lab = CONC) and F (Lab = RATE) commands. The nodal forces, if any, should
be input per unit of depth for a plane analysis and on a full 360°
basis for an axisymmetric analysis.
The temperature (for material property evaluation only) body loads may be input based on their
value at the element's nodes or as a single element value [BF,
BFE]. In general, unspecified nodal values of temperatures default to the
uniform value specified with the BFUNIF or TUNIF
commands. The transport velocity components (in the global coordinate system) are input with
the BF (Lab = VELO) command.
The material properties (DXX, DYY, DZZ, CSAT) and loads (DFLUX, DGEN, VELO) can be input as numerical values or as functions of primary variables by using tabular input. The possible primary variables are time (TIME), x, y, and z location (X, Y, and Z) in local or global coordinates, temperature (TEMP), and the concentration (CONC) degree of freedom. For more information and examples of using tabular input, see Defining Materials Using TABLE Type Array Parameters in the Mechanical APDL Basic Analysis Guide, Applying Loads Using TABLE Type Array Parameters in the Mechanical APDL Basic Analysis Guide, and the *DIM command in the Command Reference.
A summary of the element input is given in "PLANE238 Input Summary". A general description of element input is given in Element Input. For axisymmetric applications see Harmonic Axisymmetric Elements.
PLANE238 Input Summary
- Nodes
I, J, K, L, M, N, O, P
- Degrees of Freedom
CONC
- Real Constants
THK - Thickness (used only if KEYOPT(3) = 3)
- Material Properties
MP command: DXX, DYY, CSAT
- Surface Loads
- Diffusion Flux (DFLUX) --
Face 1 (J-I), face 2 (K-J), face 3 (L-K), face 4 (I-L)
- Body Loads
- Temperature (TEMP)--
T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P)
- Diffusing Substance Generations (DGEN) --
DG(I),DG(J), DG(K), DG(L), DG(M), DG(N), DG(O), DG(P)
- Transport Velocity --
VELO (input transport velocity components with the BF command)
- Special Features
- KEYOPT(1)
Diffusion damping matrix:
- 0 --
Consistent
- 1 --
Diagonalized
- KEYOPT(2)
Implementation of transport effects:
- 0 --
Matrix implementation; produces an unsymmetric matrix. In a linear analysis, a response to transport effects is achieved after one iteration.
- 1 --
Load vector implementation; produces a symmetric matrix. Requires at least two iterations to achieve a response to transport effects.
- KEYOPT(3)
Element behavior:
- 0 --
Plane
- 1 --
Axisymmetric
- 3 --
Plane with thickness input, specified via real constant THK.
PLANE238 Output Data
The solution output associated with the element is in two forms:
Nodal degrees of freedom included in the overall nodal solution
Additional element output as shown in Table 238.1: PLANE238 Element Output Definitions.
The element output directions are parallel to the element coordinate system. 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 number refers to a table footnote that describes when the item is conditionally available, and “-” indicates that the item is not available. |
Table 238.1: PLANE238 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 | Location where results are reported | Y | 2 |
| TEMP | Temperatures T(I), T(J), T(K), T(L), T(M), T(N), T(O), T(P) | Y | Y |
| LOC | Output location (X, Y) | 1 | - |
| CG:X, Y, SUM | Concentration gradient components and vector magnitude | - | 1 |
| DF:X, Y SUM | Diffusion flux components and vector magnitude | - | 1 |
| CONC | Element concentration [3] | - | 1 |
The solution value is output only if calculated (based on input data). The element solution is at the centroid.
Available only at centroid as a *GET item.
Actual concentration obtained by multiplying the saturated concentration (MP,CSAT) and the normalized concentration evaluated at the element centroid. For more information, see Normalized Concentration Approach in the Mechanical APDL Theory Reference.
Table 238.2: PLANE238 Item and Sequence Numbers lists output available through the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Mechanical APDL Basic Analysis Guide and The Item and Sequence Number Table in this reference for more information. The following notation is used in Table 238.2: PLANE238 Item and Sequence Numbers:
- Name
Output quantity as defined in Table 238.1: PLANE238 Element Output Definitions.
- Item
Predetermined Item label for ETABLE command.
- E
Sequence number for single-valued or constant element data.
PLANE238 Assumptions and Restrictions
The area of the element must be positive.
The element must lie in a global X-Y plane as shown in Table 238.2: PLANE238 Item and Sequence Numbers, and the Y-axis must be the axis of symmetry for axisymmetric analyses.
An axisymmetric structure should be modeled in the +X quadrants.
A face with a removed midside node implies that the concentration varies linearly, rather than parabolically, along that face. See Quadratic Elements (Midside Nodes) in the Mechanical APDL Modeling and Meshing Guide for more information about the use of midside nodes.
CSAT cannot be temperature-dependent.