Grid Point Force Summation
OVERVIEW
Using the "node_forces" mode, REPTILE can extract and sum grid point forces for many locations across many load cases and output files in a single step. This is useful for generating interface, section cut, or "freebody" loads and tabulating them in a user-friendly format.
NOTE: REPTILE extracts and processes the grid point forces as the force applied TO the node BY the element, since this is the way that NASTRAN stores the data and the way it is presented in the .f06 file. If the opposite sense is desired (force applied to element at node), then the "-FLIP" command line option can be used.
<< insert figure (including sign convention) >>
NOTE: REPTILE extracts and processes the grid point forces as the force applied TO the node BY the element, since this is the way that NASTRAN stores the data and the way it is presented in the .f06 file. If the opposite sense is desired (force applied to element at node), then the "-FLIP" command line option can be used.
<< insert figure (including sign convention) >>
Definition File
In order to perform a grid point force summation, REPTILE needs 4 things:
These items are provided to REPTILE with a "Definition File," which is a text file with a certain structure. Here is an example of a Definition File (keywords highlighted in cyan for clarity):
- The nodes whose forces we want to sum
- The elements that contribute to the free-body loads
- A summation point
- The coordinate system in which to calculate the summed loads
These items are provided to REPTILE with a "Definition File," which is a text file with a certain structure. Here is an example of a Definition File (keywords highlighted in cyan for clarity):
DEF EXAMPLE_CUT_1
NODES 10 11 14 16 18 20 84
ELEMS 41 42 43 44 45
SUMPT 19.27 -4.88 98.09
CSYS1 15.62 -1.34 104.61
CSYS2 1002
CSYS3 1003 1005
# OPTIONAL COMMENT LINE
DEF EXAMPLE_CUT_2
NODES 10 11 14:20:2 84
ELEMS 41:45
SUMPT 1001
COORD 201
The Definition File consists of blocks of input associated with each free-body cut. Indentation is not required, but it recommended purely for readability. Comments can be added to help document the input file as the user sees fit. Comment lines begin with the "#" character.
Each block begins with the "DEF" keyword followed by the name of the free-body cut. After that, the inputs for the cut follow (in any order).
The "NODES" and "ELEMS" keywords are followed by a list of nodes and elements (respectively) that are to be used in the free-body cut. REPTILE understands "Patran-Style" list syntax, which makes writing sequential lists more compact (e.g. "15:24" means all IDs from 15 to 24, "30:120:10" means IDs from 30 to 120 counting by 10's).
In addition to forces from elements, there are 3 special IDs recognized by REPTILE:
The "SUMPT" keyword defines the summation point. Summation points can be given in one of three ways:
The coordinate system can be given in one of two ways:
Each block begins with the "DEF" keyword followed by the name of the free-body cut. After that, the inputs for the cut follow (in any order).
The "NODES" and "ELEMS" keywords are followed by a list of nodes and elements (respectively) that are to be used in the free-body cut. REPTILE understands "Patran-Style" list syntax, which makes writing sequential lists more compact (e.g. "15:24" means all IDs from 15 to 24, "30:120:10" means IDs from 30 to 120 counting by 10's).
In addition to forces from elements, there are 3 special IDs recognized by REPTILE:
- Applied Load = -1
- Force of SPC = -2
- Force of MPC = -3
The "SUMPT" keyword defines the summation point. Summation points can be given in one of three ways:
- A set of X Y Z coordinates separated by spaces (these must contain decimal points, or REPTILE may get confused)
- A single node ID
- Two node IDs (REPTILE will use the midpoint of the two nodes)
The coordinate system can be given in one of two ways:
- The "COORD" keyword followed by the ID of a coordinate system in the finite element model (cylindrical coordinates are supported, by spherical currently are not)
- The X-direction is defined by the vector from the points from "CSYS1" to "CSYS2" and "CSYS3" is a point that lies in the XY-plane.
EXAMPLE Command LineS
REPTILE can be invoked from a batch script (recommended) or from a command prompt (not recommended), but in either case, the command lines look the same. See Getting Started for more information.
The Grid Point Force Summation mode requires 3 command line arguments:
So a minimal grid point force summation command line might look like this:
The Grid Point Force Summation mode requires 3 command line arguments:
- An Input File (as described above)
- An Output Filename (output is in CSV format by default)
- The XDB File(s) to process (wildcards are allowed)
So a minimal grid point force summation command line might look like this:
reptile --node_forces -i Input_File.def -o Output_File.csv file1.xdb
Of course, wildcards are allowed for the XDB Files, so if you have a lot of XDB Files, you might do this:
reptile --node_forces -i Input_File.def -o Output_File.csv file*.xdb
Now lets say you want to output to a SDIO and perform Load Combinations. First, we need to add a "-SDIO" option which tells REPTILE to generate a SDIO file after it creates the CSV file (See SDIO Output for more info). Note: It doesn't matter where in the command line you put a option like this. Second, instead of giving REPTILE one or more XDB Files, we use the "-COMB" option and give it the name of the file that defines the load combinations (this file will point to the appropriate XDB Files) (See Load Combinations for more info).
So the final command line will look like this:
So the final command line will look like this:
reptile --node_forces -i Input_file.def -o Output_File.csv -SDIO -COMB file.comb
Other Options
-NL
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Non-linear mode. This performs the summation accounting for the deflected position of the nodes. If this option is used, it is recommended that the coordinate system be defined using nodes, as this will allow the coordinate systems to track with the nodal displacements.
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-SES file.ses
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Writes out a Patran session file that creates groups for each free-body cut.
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-FBDY
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In conjunction with the "-SES" option, this option adds commands to the Patran session file to create a geometric point at the summation point and two vectors to define the coordinate system.
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-FLIP
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Swap the signs of the grid point forces to use the force applied to the element at the node, rather than the force applied to the node by the element (which is the default). Using "-FLIP" will cause the output to match the MSC.Patran "Freebody" convention.
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-LOC
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Write the coordinates of the summation point to the output file.
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