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Solution and Simulation Controls
Chapter 7
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Chapter Objectives
• Upon completion of this chapter, students will be able to specify solution and simulation controls in the ANSYS/LS-DYNA program.
– 1. Describe the basic solution control options in ANSYS/LS-DYNA– 2. Describe how to control the writing of LS-DYNA binary and ASCII
output files– 3. Define and describe how to use mass scaling– 4. Describe how simulation control is used– 5. Describe how a user can edit the LS-DYNA input stream– 6. Given step-by-step guidance, simulation controls will be
demonstrated in a beam buckling example.
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• In many aspects, the solution control parameters specified in an explicit analysis are very similar to those encountered during an implicit run.
• The basic parameters to be specified in an explicit solution are:– 1. Time: the actual time for which the physical process is being
simulated [TIME]
Solution: Time Controls -> Solution Time…The actual solution time should be of very short duration, often in milliseconds.
Basic Solution Controls
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• The basic parameters to be specified in an explicit solution (continued)
– 2. Number integration points for shell and beam elements [EDINT]
Solution: Output Controls -> Integ Pt Storage…
– 3. Number of time steps results will be written to the .rst and .his files [EDRST, EDHTIME] . Output frequency times also available.
Solution: Output Controls -> File Output Freq ->Number of Steps...
• The .rst file records results for the entire model for general postprocessing. It should typically have 10-100 output steps to reduce disk space (100 default).
• The .his file records results for a subset of the model for time history postprocessing. It generally has 1000-100,000 output steps (1000 default).
• The output frequency for restarts can also be input.
3-5 integration points are required for shell elements to adequately capture plastic effects
Basic Solution Controls
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File Options include ADD, DELETE, and LIST a file
Output can be produced for ANSYS only (.rst and .his) LS-TAURUS and LS-POST only (d3thdt, d3plot) or both.
Controlling Binary Output Files
• Since LS-TAURUS, the LS-DYNA postprocessor, is provided with ANSYS/LS-DYNA, the user has the ability to create the LS-DYNA results files d3plot and d3thdt in addition to the ANSYS .his and .rst binary files.
• Determining which binary results files are output by the program is controlled by the EDOPT command:
Solution: Output Controls -> Output File Type…
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Training ManualControlling ASCII Output Files
• In addition to LS-TAURUS binary files, the user can also output a series of ASCII output files that contain specialized information about an analysis.
• The following is a list and description of the files available:GLSTAT Global model data (global statistics) BNDOUT Boundary condition forces and energyRWFORC Rigid wall forcesDEFORC Discrete element forcesMATSUM Material energies data (on a part basis)NCFORC Nodal interface forcesRCFORC Resultant interface forcesDEFGEO Deformed geometry dataSPCFORC Single point constraint forcesSWFORC Nodal constraint reaction forces (spotwelds)RBDOUT Rigid body dataGCEOUT Geometry contact entitiesSLEOUT Sliding interface energies dataJNTFORC Joint force dataNODOUT Node dataELOUT Element data
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The EDOUT command controls which ASCII files are written:Solution: Output Controls -> ASCII Output...
Simply select which individual ASCII file that output data should be generated for (e.g., MATSUM)
The following OPTIONS can also be selected:
ALL - write all ASCII output files
LIST - list all time history output specifications
DELE - Delete all ASCII output specifications
• It is important to note that the information contained in some of the ASCII files is for a small subset of the model. The EDHIST command controls the nodes or elements for which the ASCII files will be written:
Solution: Output Controls -> Select Component…The information contained in the ASCII output files will be for the nodal or element component specified
The time interval for output of the ASCII files is controlled by the EDHTIME command.
Multiple specifications are permitted on different components
(continued)
Controlling ASCII Output Files
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• In ANSYS/LS-DYNA, there are three advanced solution controls that are commonly used:
– 1. CPU Control: Specifies CPU limit for an ANSYS/LS-DYNA analysis.– 2. Mass Scaling: Adjusts element mass to increase the time step size.– 3. Sub-cycling: Tunes model to decrease CPU time (not
recommended).
Specify the CPU time limit (in seconds) to terminate an analysis. A value of 0 (default) will set no time limit.
CPU ControlThe overall CPU time limit is specified using the EDCPU command:
Solution: Analysis Options->CPU Limit…
Advanced Solution Controls
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Specify the minimum time step size to be used and the scale factor to be applied.
Mass Scaling
• Mass scaling is used to adjust elements to a specified time step by adjusting the density of each element.
• As previously discussed, the time step depends on EX, NUXY and DENS, as well as the element size.
• With mass scaling, it is possible to adjust the density of any element to a specified time step depending on the size of the element.
• Mass scaling is specified with the EDCTS command:
Solution: Time Controls->Time Step Ctrls...
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(continued)
Mass Scaling
There are two mass scaling alternatives:
• Have the same time step size for all elements by adjusting the density. This is only useful when inertial effects are insignificant.
– EDCTS, DTMS – where DTMS is the POSITIVE value of the necessary time step size
(multiplied with 1.111)
• Mass scaling applies only to elements with Dt < Dt specified– EDCTS, DTMS– where DTMS is the NEGATIVE value of the necessary time step size
(multiplied with 1.111)
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element 1 2 3
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(continued) Mass Scaling
• Mass scaling for minimum time step size:
• Adjust the density to a user defined time step size:
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Training ManualMass Scaling - Example
• An example for the use of mass scaling (EDCTS)
– Car crash model
• 140 parts
• 42981 nodes
• 1580 bricks
• 60 beams
• 35170 shells
• Termination time 150 ms
• 100 smallest element time-steps (see LS-DYNA output file d3hsp):
element time-step
shell 151018 0.44612E-06
shell 150894 0.46867E-06
shell 52321 0.48682E-06
shell 51321 0.48682E-06
shell 16923 0.52225E-06
shell 16458 0.52225E-06
...
shell 152483 0.70112E-06
shell 92708 0.70113E-06
shell 92308 0.70114E-06
shell 38547 0.70223E-06
shell 38047 0.70223E-06
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(continued) Mass Scaling - Example
• Without mass scaling:
– Initial time step size depends on the smallest element
t = 4.46115E-07 sec
• With mass scaling:
– Time step wanted is 6.534E-07 sec. Give the negative value of 111% of the time step to the EDCTS command:
– EDCTS, -7.26E-07
– Main Menu > Preprocessor > Output Ctrls > Mass scaling > ...
– Initial time step size
t = 6.534E-07 sec
• CPU time is reduced to 68%
• Error in mass:
– Physical mass 1.26 metric tons
– Added mass 0.000027 metric tons (27 grams)
– Error in mass 0.002%
• The coordinates of the mass center have changed, too.
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Training ManualSimulation Control
• Sense Switch Controls allow the user to interrupt the solution process and to check for the actual state.
• Do the following to use sense switch controls:
– Type CTRL-C into the output window of ANSYS on Unix platforms or the separate LS-DYNA output window on NT platforms. It interrupts the explicit solver and waits for an input in the output window of ANSYS.
• Type sw1 to terminate. A restart file will be written.
• Type sw2 into the output window to receive a global statistic of the actual state. LS-DYNA continues.
• Type sw3 into the output window to receive a restart file for the actual time. LS-DYNA continues.
• Type sw4 to write a results data set. LS-DYNA continues.
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Training ManualSolution Control
• The LS-DYNA solver writes all important messages (errors, warnings, failed elements, contact problems) to the ANSYS output window (separate window on NT) and to the file d3hsp.
• The first estimation of CPU time is usually too high. Use CTRL-C to interrupt the solver and type sw2 for the actual values of the global statistics.
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Training ManualVisualization of Time Step Size
• The LS-DYNA solver automatically calculates the minimum time step size of each element based on the length and density.
• The actual time step size used by LS-DYNA is the smallest of these values.
• The EDTP command can be used to visualize the elements with the smallest time steps before the LS-DYNA solver is invoked.
• This information allows the user to evaluate a mesh and take appropriate action (ie. Remeshing or using mass scaling)
EDTP, OPTION, VALUE1, VALUE2 where:
OPTION = 1, 2, or 3 as described below:1 = element plot of VALUE1 smallest element time steps2 = #1 above + element listing of these time step values3 = #2 above + VALUE2 translucency of remaining elements
VALUE1 = plot/list limit for “smallest” designation (red elements decide size)VALUE2 = translucency ( 0 = no translucency, 1 = max., 0.9 = default level)
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Training ManualVisualization of Small Elements
• EDTP maps element time step sizes before SOLVE– The smallest elements in the model will control the CPU time– Smallest time step elements plotted in red (intermediate time yellow) – Translucency option and time step size listing options available– Meshing/mass scaling decisions based on estimated time step sizes.
Solution: Time Controls>Time Step Predic….
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Training ManualAdaptive Meshing
• Automatic regeneration of SHELL163 mesh can be performed during solution to maintain a uniform bound on the distortion error in the analysis
– Adaptive meshing is specified on a part basis (EDADAPT command)– The EDCADAPT command controls how often, on what basis, to what
extent, and when adaptive meshing occurs– Each adapted mesh has separate POST1 (Jobname.rs01, Jobname.rs02,
…) and POST26 (Jobname.hi01, Jobname.hi02, …) results file– Adaptive meshing is particularly useful in stamping and sheet metal
forming problems where there is substantial plastic deformation
EDADAPT, PART, Key ! Set Key=On to activate for part PART
EDCADAPT, FREQ, TOL, OPT, MAXLVL, BTIME, DTIME FREQ = time interval (real time) between adaptive mesh refinementsTOL = adaptive angle (deg) based on OPT=1 (original) or 2 (incremental) mesh MAXLVL = maximum number of mesh refinement levels BTIME/DTIME = birth/death times when adaptive meshing is active in model
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Training ManualAdaptive Meshing
• By the automatic sub-division of highly distorted shell elements, ANSYS/LS-DYNA ensures that more accurate results are obtained.
• Adaptive meshing is performed in two steps.
– 1. Select a Part for which adaptive meshing is to be turned on.
Solution: Analysis Options> Adaptive Meshing> Apply to Part
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• 2. Specify the appropriate adaptive meshing controls
Solution: Analysis Options> Adaptive Meshing> Global Settings
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Training ManualAdaptive Meshing
• Once adaptive meshing is specified for a given part, the LS-DYNA solver on the EDCADAPT command.
• Animations across different results files are not possible directly, but a macro can be written with FILE and /SEG commands to animate.
Refined
Mesh
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Training ManualEditing the LS-DYNA Input File
• Most general LS-DYNA capabilities are supported by the ANSYS/LS-DYNA program and are easily accessed in the GUI.
• There are several additional features of LS-DYNA, however, that cannot be directly accessed through the ANSYS GUI. Some examples include:
– Material models: Fabric, soil, geological cap
– Elements: Air bags, seat belts, explosives
– Constraints: Rigid body local coordinate systems
• Although these unsupported capabilities cannot be directly accessed, a user familiar with LS-DYNA input can still use any feature indirectly by editing the LS-DYNA keyword input file.
• However, post-processing in ANSYS may not be possible, depending on the changes. LS-TAURUS or LS-POST can always be used to validate the analysis.
• Editing the LS-DYNA input file is accomplished using a five step procedure.
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(continued)
Editing the LS-DYNA Input File
STEP 1: Issue the EDWRITE command
• After all of the general modeling has been completed in ANSYS/LS-DYNA, the EDWRITE command should be issued instead of the SOLVE command.
Solution: Write Jobname.k…
• The EDWRITE command writes the Jobname.k without starting the ANSYS/LS-DYNA solution process.
• Issuing the EDWRITE command also creates the headers to the .rst and .his files.
• EDWRITE is an action command and will immediately write the LS-DYNA input file specified.
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(continued)
Editing the LS-DYNA Input File
STEP 2: Exit the ANSYS/LS-DYNA Program
Utility Menu: File->Exit
STEP 3: Edit the input file Jobname.k
• Using a standard vi or text editor, add the additional features to the Jobname.k using the required LS-DYNA keyword format.
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(continued)
Editing the LS-DYNA Input FileSTEP 4: Execute the LS-DYNA Solver
• In the directory where the Jobname.rst and Jobname.his reside, directly execute the LS-DYNA script (which, in turn, executes the solver).
• The solution produced by LS-DYNA will automatically be appended to the ANSYS results files.
• The typical command for executing the LS-DYNA script is:
/ansys57/bin/lsdyna57 i=Jobname.k p= ANSYS product variable (see install. guide)
Additionally, m=drelax is needed for an implicit-to-explicit solution.
Also, MEMORY=# (in words) is used for large jobs (see EDSTART).
Further, R=D3dumpnn for small restarts
STEP 5: Re-enter the ANSYS/LS-DYNA Program
• Re-enter the ANSYS/LS-DYNA program after the LS-DYNA solution is done.
• The results can be viewed in both the general (Jobname.rst) and time history (Jobname.his) postprocessors.
NOTE: The nodes and elements cannot be modified when using this procedure. Further, editing the Jobname.k file is unsupported.
March 15, 2001
Inventory #001458
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Training ManualBeam Buckling Exercise
• The exercise for this chapter begins on page E7-1 of Volume II.
• Solution and simulation controls are demonstrated in an example of a beam buckling under axial load.
