3.3 Coordinate Systems
The ANSYS program uses several types of coordinate systems, each used for a different purpose: The working plane, which is separate from the coordinate systems, is for locating geometric primitives during the modeling process. 3.3.1 Global and Local Coordinate Systems
Global and local coordinate systems are used to locate geometry items. When a node or a keypoint is defined, its coordinates are interpreted in the global Cartesian system by default. For some models, however, it may be more convenient to define the coordinates in a system other than global Cartesian. Input the geometry in any of three predefined (global) coordinate systems, or in any number of user defined (local) coordinate systems. 3.3.1.1 Global Coordinate Systems
A global coordinate system (Fig. 3.4) can be thought of as an absolute reference frame. Three predefined global systems are available: Cartesian, cylindrical, and spherical. All three of these systems are right-handed and, by definition, share the same origin. They are identified by their coordinate system (CS) numbers: 0 for Cartesian, 1 and 5 for cylindrical, and 2 for spherical.
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Fig. 3.4 Global Coordinate Systems (a) Cartesian (X, Y, Z components) coordinate system 0 (C.S.0)
(b) Cylindrical (R, θ, Z components) coordinate system 1 (C.S.1)
(c) Spherical (R, θ, φ components) coordinate system 2 (C.S.2)
(d) Cylindrical (R, θ, Y components) coordinate system 5 (C.S.5) 3.3.1.2 Local Coordinate Systems
In many cases, it may be necessary to establish own coordinate system, whose origin is offset from the global origin, or whose orientation differs from that of the predefined global systems. Such user defined coordinate systems, known as local coordinate systems, can be created in the following ways: When a local coordinate system is defined, it becomes the active coordinate system. As a local system is created, assign it a CS identification number (which must be 11 or greater). Local coordinate systems can be created or deleted in any phase of the ANSYS session. Use Utility Menu> WorkPlane> Local Coordinate Systems> Delete Local CS to delete a local system. Use Utility Menu> List> Other> Local Coord Sys to view the status of all global and local coordinate systems.
The local coordinate systems can be Cartesian, cylindrical, or spherical, similar in form to the three predefined global systems. The analyst may define local cylindrical and spherical coordinate systems in either circular or elliptical configuration. Additionally, a toroidal local coordinate system can be defined, as illustrated in Fig. 3.5.
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Fig. 3.5 Types of Coordinate System 3.3.1.3 Active Coordinate System
The analyst may define as many coordinate systems as s/he likes, but only one of these systems may be active at a time. The choice of active coordinate system is determined as follows: Initially, the global Cartesian system is active by default. Each time a local coordinate system is defined, that newly-defined system then automatically becomes the active one. Use one of the following methods to activate one of the global coordinate systems or some other previously defined coordinate system: A coordinate system can be activated in any stage of the ANSYS session. That same coordinate system will remain active in all later stages until it is changed intentionally. 3.3.2 Display Coordinate System
By default, a listing of nodes or keypoints always shows their global Cartesian coordinates, even if they were defined in a different coordinate system. The display coordinate system used in such listings can be changed by one of the following methods: Changing the display coordinate system will also affect the graphical displays. Unless a specific effect is advantageous in the displays, reset the display coordinate system to the global Cartesian system before issuing any graphics display action commands. 3.3.3 Element Coordinate Systems
Every element has its own coordinate system, the element coordinate system, that determines the direction of orthotropic material properties, applied pressures, and results for that element. All element coordinate systems are right-handed orthogonal systems.
The default orientations for most elements' coordinate systems fit the following patterns: Many element types have key options that permit to change the default element coordinate system orientation. For area and volume elements, the analyst can also change the orientation to align the element coordinate system with a previously defined local system by using: Main Menu> Preprocessor> Meshing> Mesh Attributes> Default Attribs or Main Menu> Preprocessor> Modeling> Create> Elements> Elem Attributes. 3.3.4 Results Coordinate System
Results data are calculated during solution and consist of displacements (UX, UY, ROTX, etc.), gradients (TGX, TGY, etc.), stresses (SX, SY, SZ, etc.), strains (EPPLX, EPPLXY, etc.), etc. These data are stored in the database and on the results file in either the nodal coordinate system or the element coordinate system. However, results data are generally rotated into the active results coordinate system for displays, listings, and element table data storage.
The active results coordinate system can be changed to another system, or to the coordinate systems used during solution. On listing, displaying, or operating on the results data, they are rotated to this results coordinate system first. Use Main Menu> General Postproc> Options for Output or Utility Menu> List> Results> Options to change the results coordinate system.