Modal super-element definition
- @MODAL_SUPERELEMENT_DEFINITION {
- @MODAL_SUPERLEMENT_NAME {MseName} {
- @EDGE_NAME {EdgeName}
- @NUMBER_OF_MODAL_COORDINATES {N_{δ}}
- @REDUCED_MASS_MATRIX_FILE_NAME {MassFileName}
- @REDUCED_STIFFNESS_MATRIX_FILE_NAME {StiffFileName}
- @REDUCED_DAMPING_MATRIX_FILE_NAME {DampFileName}
- @ASSOCIATED_CONNECTION_NAME {Asc1Name, Asc2Name, ...}
- @SHAPE_NAME {ShapeName}
- @GRAPHICAL_PARAMETERS_NAME {GrfParamName}
- @COMMENTS {CommentText}
- }
- }
Introduction
A modal based super-element is an elastic substructure whose stiffness and mass characteristics are represented in an approximate manner using Herting's modal reduction approach. The super-element takes into account the elastic deformations of the structure by means of a modal representation. In addition, the substructure is allowed undergo large rigid body displacements and rotations. The modal based super-element is associated with an edge, EdgeName used for the representation of multibody systems. The modal super-element can be connected to other components of the model at Vertex1Name, of the edge; this connection represents the first boundary point of the element. Additional connection points can be defined by means of associated connections. All the boundary points must feature six degrees of freedom: three displacements and three rotations. The first vertex, Vertex0Name, of the edge should not be connected to other bodies.
The geometry of the modal super-element is inherited from edge, EdgeName, which must have an associated triad, TriadName. The displacements and rotations components at the boundary points are all measured in this triad.
NOTES
- The number of boundary points, N_{bpt}, and modal coordinates, N_{δ}, determine the size of the reduced mass and stiffness matrices: 6N_{bpt} + N_{δ}. Typically, these matrices are obtained by applying Herting's transformation to the unconstrained mass and stiffness matrices computed by a finite element package such as NASTRAN.
- The entries of the reduced symmetric mass and stiffness matrices must be given in files named MassFileName and StiffFileName, respectively. These files are typically obtained by Herting's transformation. Optionally, a reduced damping matrix DampFileName can be specified.
- Additional boundary points can be defined for the modal super-element If the modal super-element features multiple boundary points, associated connections must be defined. Each associated connection will add a boundary point to the modal super-element.
- An external shape, ShapeName, can be optionally defined for the modal super-element. This external shape is exclusively used in the visualization post-processor and has no effect on the analysis procedure. Modal super-element shapes must be of the SURFACE type.
- It is possible to attach comments to the definition of the object; these comments have no effect on its definition.
Graphical parameters
The appearance of the modal super-element during the visualization phase of the analysis can be controlled by associating graphical parameters, GrfParamName to the element. The following representations are valid for modal super-elements: RepresentationType = MESH or SURFACE. The LINE representation depicts the FrameName as a line along the curve CurveName that defines the beam element. The other three representations are allowed only if a shape ShapeName is defined for the beam; CURVE shows the shape as a set of sectional curves at the beam nodes, MESH depicts the shape as an orthogonal mesh of curves, and SURFACE as an external surface. Default value: SURFACE if a ShapeName is defined, LINE otherwise. The following vector fields are valid for modal super-elements: VfdType = ANGULAR_VELOCITIES or VELOCITIES.
Sensors
- Sensors can be defined to extract information about modal super-elements. The following SensorType and associated FrameName specifications are allowed for modal super-elements: DISPLACEMENTS_1, EIGEN_DISPLACEMENTS_1, EIGEN_MODAL_COORDINATES, MODAL_COORDINATES, POSITIONS_1, VELOCITIES. (Default value: DISPLACEMENTS_1).
- For SensorType = DISPLACEMENTS_1, EIGEN_DISPLACEMENTS_1, POSITIONS_1 or VELOCITIES, no u or v values are accepted for the modal super-element. For SensorType = MODAL_COORDINATES, six of the modal coordinates are evaluated as determined by the u value. If u = 1, the first six modal coordinates are evaluated, if If u = 2, the next six are evaluated, etc. u is an integer such that 1 ≤ u ≤ N_{δ}/6. No v value is accepted for the modal super-element.