Cyclic Symmetry

Two different methods:

If SOL 101 is used and MPC/SPC cards are generated, then the following parameter must be set to ensure that the model solves because it exhibits a tangential rigid body translation

Also, make sure to rotate the nodes into the cylindrical analysis coord system.


Last Reviewed: 03/16/01
Article ID: 4696
Problem Summary:

A few hints for using rotational cyclic symmetry in a normal mode analysis with patran-nastran (SOL 115):

– Define the cyclic symmetry in patran via:

– Create a cylindrical system where the z-axis is the axis of symmetry. Let’s call it Coord 1.

– Define the cyclic symmetry:
Finite elements – Create – Mpc – Cyclic Symmetry
Connect the independent nodes on one side of the segment to dependent nodes which are revolved around the z-axis over an angle of 360/N, where N is the number of segments. The dependent nodes need not be part of the segment. They do not even need to be attached to elements. They are just used to define the number of segments in the model . In most situations the dependent nodes are also part of the segment when the segment exactly represents a “piece of cake”.

Note that here the Mpc has nothing to do with a rigid element, but is only used to define the cyclic symmetry in the nastran input deck. The Mpc results in a CYSYM,N,ROT entry and two CYJOIN
entries in the nastran input deck, one for side 1; the independent nodes and one for side two; the dependent nodes.

If a node lies exactly on the z-axis, choose it both as dependent and as independent. This results in one CYAX entry. Make sure that the independent nodes have lower theta angle than the dependent nodes,
t.i. nastran will create the new segments using the right hand rule by rotating around the z-axis in positive direction.

In order to use the patran cyclic symmetry utility (see below), make sure all the nodes are defined in this cylindrical system, otherwise the in-plane eigenvectors are wrong!