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Reference CID and Analysis CID

ron.kunz
Dear all,
I am trying to understand an example model with a bearing.
The two nodes which are used to constrain the bearing give for the Node Location Information:
Reference CID = Global Rectangular
Analysis CID = 4 Rectangular
 
All other nodes are referring also to Global Rectangular CID for the Analysis CID.
I would like to know what this means 4 Rectangular?
The RBE2s which are connected to these points have no reference to other CID so I assume they take the global CID.
 
In the BDF-file there are some CORD2R defined:
CORD2R* 4                              0.             0.
*       0.            1.              0.            0.
*      0.              0.            -1.
*
CORD2R* 5                              0.             0.
*       0.            -1.             0.            0.
*      0.              0.            1.
*
CORD2R* 6                              0.             0.
*       0.            1.              0.            0.
*      0.              0.            -1.
*
CORD2R* 7                              0.             0.
*       0.            -1.             0.            0.
*      0.              0.            1.
*
CORD2R* 12                            -160.81759643554368.5
*      -226.72961425781-160.81759643554691.1295443819 -556.83722631104
*      -168.57967172422698.56095064975695.8543142001753
 
I don`t see a special CID.
Can you explain the 4 Rectangular?
Thanks
 
 
 
 
  • 0
    "4 Rectangular" means that it references CID 4, which is a rectangular coordinate frame. This is CORD2R 4 in your bdf.
  • 0
    Ok thanks and in addition I found this on the different coordinate systems.
    Does that mean, one can check with the analysis CID how your nodes are behaving?
    Just a control check? Because the reference CID is the physical CID?
     
    The Reference Coordinate System is the coordinate system in which the location of nodes are defined.
     
    • In Patran, this known as the Reference Coordinate System.
    • In MSC Nastran, this is Field 3 or CP in the GRID bulk data entry.
     
    The Analysis Coordinate System is the coordinate system in which the displacements of the nodes are observed. In addition to observed displacements, the following may be observed: degrees-of freedom, constraints, and solution vectors are defined at the grid point. The Analysis Coordinate System is also referred to as the Displacement Coordinate System.
     
    • In Patran, this is known as the Analysis Coordinate System.
    • In MSC Nastran, this is Field 7 or CD in the GRID bulk data entry.
    • Note: Once you perform an analysis and have displacements, these oserved results may be transformed to different coordinate systems.
     
  • 0
    What Patran refers to with the term "Analysis Coordinate System", is translated to Nastran terminaology as "Displacement Coordinate System". Other FE codes may use similar but different terminology.
     
    For Nastran:
    In linear static analysis, each grid point can undergo at most three orthogonal
    translational and three orthogonal rotational components of displacement.
    Each component is called a degree of freedom and adds one unknown to the
    system of simultaneous linear equations representing the structure.
     
    Each grid point refers to two coordinate systems:
    • One system is used to locate the grid point (CP in field 3)
    • The other is used to establish the grid point’s displacement (output) coordinate system (CD infield 7).
    The displacement coordinate system defines the direction of displacements, constraints, and other grid
    point related quantities such as reaction forces. The basic (default) coordinate system is indicated by a zero or blank in the CP and CD fields. CD and CP do not have to be the same coordinate system.
     
    so the analysis system is not so much "one can check with the analysis CID how your nodes are behaving?" it IS the system that the analysis dof (a particular term in the matrix that is being solved) is with reference to.
  • 0
    The reference CID defines the physical position of the node. The analysis coordinate system defines the directions of the degrees of freedom of that node, and thus the directions in which the node results are output. Your CID 4 has a different orientation than CID 0:
    ScreenShot2147
    This means that, for example, a Z displacement reported for that node would be an X displacement in global (CID 0).
     
    The analysis CID is also commonly used to define the directions of constraints applied to a node: so a constraint in X on that node would act in a different direction than a constraint in X on a node with CID 0 as its analysis CID.
     
    There is some useful discussion this topic in the Nastran Linear Static Analysis User's Guide, Chapter 3.
  • 0
    PS I was quoting some bits from the "MSC Nastran Getting Started Guide". Chapter 5 Model Geometry has details on these topics.
    best regards
  • 0
    Thanks Arthur and Darrel,
    Regarding Darrels last remark. In the model I examine, the node for which it is used is the midpoint of a circular rbe2. So the transformation from CID 0 to CID4 does not matter because it is radial. so x and z are the same and y is tangent to this, so I don`t understand why it is used. But ok that is now more in detail of the model you both don`t have.
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