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Order of Nodes

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The order of the nodes defining a shell is important for a few reasons relating to numerical behavior, load application and interpretation of results.

 

To satisfy numerical behavior, it is important to define quad shells by either numbering their nodes sequentially clockwise or counter-clockwise.  Specifying a quad shell by using a crisscross numbering order will result in a "warped" shell.  These look like a bowtie, and they do not perform properly.

 

Shells have a top and a bottom which have meaning in terms of the output.  To determine the orientation of a shell, use the fingers of your right hand to trace around the nodes of the shell in order from Node 1 to node 4.  Your right thumb will naturally point in the direction of the "top" of the plate.  The shell below was drawn from node 1 to 2 to 3 to 4, so we are looking at its top.

 

Shell with nodes

 

Do not be too concerned with the orientation of shells during the modeling process, because there are some simple ways to orient groups of plates consistently once they have been modeled.  These will be discussed in the section on Editing Model Geometry.