Tech Articles

Coordinate Systems in MSC Nastran


Tech Articles ID    KB8023447
Status:    Published
Published date:    05/04/2018
Updated:    05/04/2018
Reported In:   MSC Nastran
Created with Version:   MSC Nastran
 

Abstract (Question)

Understanding Grid Point Coordinate Systems in MSC Nastran

Description (Answer)

Basic Coordinate System: MSC Nastran has built in rectangular cartesian system called the basic coordinate system, also called the default coordinate system. By default it is used to define the geometry of structure.

Global Coordinate System: The union of all the coordinate systems defined on all the CD fields on all the GRID entries is called the global coordinate system in MSC.Nastran. The global coordinate system in MSC.Nastran is, therefore, not a single unique system. Note that the basic coordinate system may also, but not necessarily, be a part of the global coordinate system.The term “global coordinate system” in some other codes has a meaning that is identical to that of MSC.Nastran’s basic coordinate system. The global stiffness matrix is assembeled in global system.

Local Coordinate System: In addition to basic coordinate system, MSC Nastran supports three types of local coordinate systems—rectangular, cylindrical, and spherical. A local coordinate system can reference either the basic coordinate system or another local coordinate system.
Depending on the structure, it may be more convenient to input model geometry in other types of coordinate systems. Therefore, MSC.Nastran allows the construction of alternate (local) coordinate systems. As an example, consider the farm silo in below figure. It is cylindrical with a hemispherical dome, and is offset from the the origin of the basic coordinate system. Creating local cylindrical (r,θ,z ) and spherical (r,θ,ø ) systems makes entry of model geometry, or reviewing displacement results from the analysis, far more convenient.




Reference Coordinate System: Location of grid points are always defined by three components(X1, X2 and X3 coordinates)
However, when you define a location of the grid point in space using these components, you are inherently defining the point relative to a coordinate system. By default grid points are defined with respect to basic  coordinate. This basic coordinate or any local coordinate defined on the CP field of grid is called reference coordinate system.

Displacement Coordinate System: The output coordinate system that user specify in the CD field for a particular grid point is the coordinate system used to apply the constraints, solve for the displacements,degrees of freedom, some applied loads, solution vectors and output the results associated with that grid.
The connectivity relationships e.g., SPCs, MPCs, RBAR etc.,are written in the coordinate system defined on the CD field not CP field.


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