Lecture 7 Solution and Simulation Controls 14.5 Release
ANSYS LS-DYNA in Mechanical APDL
Objectives •
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This lecture concerns Solution and Simulation Controls. The tools used to optimize and control explicit analyses are discussed. Topics:
A. B. C. D. E. F. G. H. I. J. K. L.
Basic ANSYS LS-DYNA Solution Controls Controlling LS-DYNA Binary Output Files Controlling LS-DYNA ASCII Output Files Visualization of Small Time Steps Mass Scaling Additional Time Controls Adaptive Meshing Simulation Control Editing the LS-DYNA Input File LS-DYNA Batch Launcher LS-DYNA Solver Precision Solution and Simulation Controls Workshop
A. Basic ANSYS ANSYS LS-DYNA LS-DYNA Solution Controls •
Many of the solution control parameters specified in an explicit exp licit analysis are similar to those encountered during an implicit run:
– Termination Time – Integration Point Output – Results Output and Restart Frequency
Termination Time : TIME •
The termination time is the actual time for time for which the physical process is being simulated. In an explicit dynamic analysis, this time is usually of very short duration – duration – often often in milliseconds.
– Solution > Time Controls > Solution Time
... Basic ANSYS LS-DYNA Solution Controls Integration Point Output : EDINT •
The EDINT command is used to specify the number of shell and beam integration points for which results data will be written for. The actual number of integration points used in the calculations is controlled by the NIP real constant associated with the element.
– Solution > Output Controls > Integ Pt Storage Default values shown
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To adequately capture plastic effects in shells, the NIP real constant needs to be at least 3, but 5 is recommended if a Gauss distribution is used. For the trapezoidal formulation, NIP > 20 is recommended. For beam elements, POST1 requires that integration point data be saved in order to plot the elements (EDINT > 0). Resultant beams lack integration point data, and are, therefore, not plotted in POST1.
... Basic ANSYS LS-DYNA Solution Controls Results Output and Restart Frequency : EDRST, EDHTIME, and EDDUMP •
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The EDRST command specifies how often results for the entire model are written to the binary Jobname.RST file. Typically, only 10 to 100 result sets are saved to this POST1 file, due to the large volume of data. The Jobname.RST file can be postprocessed in POST26, but the Jobname.HIS file (see below) is generally used in POST26 instead. The EDHTIME command specifies how often results for a subset of the model are written to the binary Jobname.HIS file. Typically, 1,000 to 100,000 results sets are saved to this POST26 time history file.
– If more than 1,000 sets are requested, the /CONFIG,NRES command must be issued to allocate the necessary model space.
– The output data is restricted to those nodal and element components specified via the EDHIST command. Reissue EDHIST, as needed…
– The EDHTIME command also controls the frequency for which the LS-DYNA ASCII output files are written (discussed later).
... Basic ANSYS LS-DYNA Solution Controls Results Output and Restart Frequency (continued): •
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Similarly, the EDDUMP command specifies how often the binary restart files are written (d3dump01, d3dump02, d3dump03, etc.). The EDSTART command (discussed later) is used to restart an analysis from one of these files.
The frequency for which results and restart files are written is based on the number of desired data sets or the actual time interval desired:
– Solution > Output Controls > File Output Freq > Number of Steps – Solution > Output Controls > File Output Freq > Time Step Size •
Default values shown for the N u m b e r o f S t ep s input format.
B. Controlling LS-DYNA Binary Output Files •
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Since LS-PREPOST, the LS-DYNA postprocessor, is provided for free with ANSYS LSDYNA, the user can create and review the LS-DYNA binary results files d3plot and d3thdt in addition to the ANSYS LS-DYNA binary results files, Jobname.RST and Jobname.HIS. Please note that the LS-PREPOST postprocessor is not supported by ANSYS, Inc. The EDOPT command determines which binary results files are output:
– Solution > Output Controls > Output File Types – The corresponding LS-DYNA Keyword is *DATABASE_FORMAT … •
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File options include ADD, DELETE, and LIST a file. Output can be produced for ANSYS only (.RST and .HIS), LS-PREPOST only (d3plot and d3thdt), or both postprocessors.
C. Controlling LS-DYNA ASCII Output Files •
In addition to LS-DYNA binary results files, the user can output a series of LS-DYNA ASCII output files that contain specialized information about an analysis: GLSTAT BNDOUT RWFORC DEFORC MATSUM NCFORC RCFORC DEFGEO SPCFORC SWFORC RBDOUT GCEOUT SLEOUT JNTFORC NODOUT ELOUT
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Global statistics data (contents controlled by EDENERGY) Boundary condition forces and energy Rigid wall forces Discrete element forces Material energies summary (on a Part ID basis) Nodal interface forces Resultant interface forces Deformed geometry data Single point constraint reaction forces Nodal constraint reaction forces (spotwelds & rivets) Rigid body data Geometry contact entities Sliding interface energies data Joint force data Node data Element data
... Controlling LS-DYNA ASCII Output Files •
The EDOUT command controls which ASCII files are written: –
Solution > Output Controls > ASCII Output
Select individual ASCII files desired (multiple selections permitted).
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Additional options include:
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Write ALL ASCII output files LIST output files selected DELETE all specifications
For s o m e of the ASCII output files, data is written only for a s u b s e t of the model. The EDHIST command specifies which nodal and element components data will be written for: –
Solution > Output Controls > Select Component •
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Output frequency controlled by EDHTIME command. Multiple selections allowed.
D. Visualization of Small Time Steps •
The LS-DYNA solver automatically calculates the minimum time step for each element based on its characteristic length, density, etc.
– The smallest of these element time steps is called the critical time step. – The actual time step used during solution is the product of the current critical time step and a stability factor (usually 0.90). As elements distort during the analysis, their time steps are recalculated, and the subsequent actual time step is written to the glstat file.
– At the beginning of the run, the initial 100 smallest time steps (and their corresponding element numbers) are written to the d3hsp file. •
The EDTP command is used to visualize elements with the smallest time steps before the LS-DYNA solver is invoked.
– – – –
Elements with smallest time steps are plotted in red. Elements with intermediate time steps are plotted in yellow. A translucency option is available along with a time step listing option. Re-meshing and mass scaling decisions can be made before the solution is initiated, but the EDTP macro is rather slow for large models .
... Visualization of Small Time Steps Solution > Time Controls > Time Step Prediction
EDTP , OPTION, VALUE1, VALUE2 OPTION = 1, 2, or 3:
1 = element plot of VALUE1 smallest element time steps 2 = #1 above + element listing of these time step values 3 = #2 above + VALUE2 translucency of remaining elements VALUE1 = plot/list limit for “smallest” designation (red elements decide size) VALUE2 = translucency ( 0 = no translucency, 1 = maximum, 0.9 = default level)
E. Mass Scaling •
An element’s time step is calculated based on its material properties (EX, NUXY, and DENS) and characteristic length: element 1
2
3
l 1
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l 2
t min l 3
c
l min c
l 2 c
E
(1 2 )
The equation can be rearranged to find the required density of each element for a desired time step size. By adding the corresponding mass to these elements, the solution time will be reduced. 2
t specified (1 2 ) i E li t
specified
i
2
E
l i2 (1 2 )
for element i
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This procedure is known as m a s s . However, care must be scaling taken as to not add so much mass as to invalidate the results…
... Mass Scaling •
Mass scaling is specified via the EDCTS command: –
Solution > Time Controls > Time Step Ctrls •
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Desired minimum time step size, , before applying the scale DTMS (stability) factor, TSSFAC . Stability factor (default = 0.90)
EDCTS, DTMS, TSSFAC
– If DTMS is a POSITIVE value, the mass of all elements in the model will be adjusted to achieve the desired time step size. This is only useful when inertial effects are insignificant.
– If DTMS is a NEGATIVE value, the mass scaling will only be applied to those elements for which the calculated element time step size is less than the desired time step size.
– Obviously, DTMS should be the quotient of the desired minimum time step size and the time step scale factor, TSSFAC.
... Mass Scaling Mass scaling (EDCTS) example: •
Car crash model – 140 parts – 42981 nodes – 1580 bricks – 60 beams – 35170 shells – Termination time 150 ms
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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
... •
Governing time step is 0.44612e-6 – Adding mass to those elements requiring the smallest time steps will raise the governing time step and reduce the CPU time …
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
... Mass Scaling •
Without mass scaling: –
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Initial time step = smallest time step in model:
t = 0.44612E-06 seconds
With mass scaling: –
Desired time step = 0.6534E-06 seconds.
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Use negative DTMS & compensate for TSSFAC : EDCTS, -0.726E-06
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Initial time step =
t = 0.90 X 0.726E-06 = 0.6534E-06 seconds
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CPU time reduced to 68% of time required without mass scaling
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Error in mass:
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Physical mass 1.26 metric tons
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Added mass 0.000027 metric tons (27 grams) => recorded in d 3 h s p file
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Error in mass 0.002% (insignificant)
The coordinates of the mass center have changed slightly, too.
F. Additional Time Controls •
There are several other solution controls that involve time:
– Time Step Scale Factor – CPU Control – Subcycling
Time Step Scale Factor : EDCTS, DTMS, TSSFAC •
Default stability factor (TSSFAC = 0.90) is normally sufficient
– Lowering TSSFAC may stabilize models with high beta damping – Decreasing TSSFAC may also improve contact behavior – Solution > Time Controls > Time Step Ctrls •
Default stability factor for high explosives (unsupported) is 0.67
… Additional Time Controls CPU Control : EDCPU, CPUTIME •
Terminates analysis after CPU limit reached (default CPUTIME =
)
– Useful for large models that are being run at an outside data center – Solution > Analysis Options > CPU Limit
Subcycling : EDCSC, K ey •
Subcyling enables different time steps to be used in large models. –
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In theory, elements with large time steps are updated less often. In practice, it only works for small, simple models, and therefore, is n o t recommended. It has been known to just “hang” an analysis. Solution > Time Controls > Subcycling
G. Adaptive Meshing •
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The automatic regeneration of a SHELL163 mesh is possible during solution to maintain a uniform bound on the distortion error in the analysis. Highly distorted shells are sub-divided , thereby allowing for more accurate results. Adaptive meshing is particularly useful in stamping and sheet metal forming problems where there is substantial plastic deformation. It should not be confused with the ALE method (see Lecture 13), which does not create additional elements, but rather smoothes the existing mesh so that the elements are less distorted.
Refined Mesh
... Adaptive Meshing •
There are two steps required to have adaptive meshing: – Specify Part ID’s to be Re-meshed – Set Adaptive Meshing Controls
Specify Part ID’s to be Re-meshed : EDADAPT, Part, Key •
First, specify which Part ID’s will be subject to adaptive meshing. – Solution > Analysis Options > Adaptive Meshing > Apply to a Part
Set Adaptive Meshing Controls : EDCADAPT, … m a n y o p t i o n s … •
Next, globally set adaptive meshing controls for all Part ID’s specified. –
Solution > Analysis Options > Adaptive Meshing > Global Settings
... Adaptive Meshing EDCADAPT, FREQ, TOL, OPT, MAXLVL, BTIME, DTIME, … FREQ = time interval (real time) between adaptive mesh refinements ( no default ) TOL = adaptive angle (degrees) based on original (OPT=1) or incremental (OPT=2) mesh MAXLVL = maximum number of mesh refinement levels
BTIME/DTIME = birth/death times when adaptive meshing is active in model
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Large d e f a u l t tolerance (TOL) on angle change prevents adaptive meshing… OPT sets angle tolerance to be relative or absolute
... Adaptive Meshing EDCADAPT, …, LCID,ADPSIZE,ADPASS,IREFLG,ADPENE,ADPTH,MAXEL LCID = data curve identifying interval of remeshing ADPSIZE = minimum element size to be adapted based on element edge length ADPASS = 1 or 2 pass adaptivity IREFLG = uniform refinement level ADPENE = flag to start adaptivity when approaching or penetrating tooling surface ADPTH = absolute shell thickness level below which adaptivity will be terminated MAXEL = maximum number of elements at which adaptivity terminated
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Most of these options are not required and may be skipped.
... Adaptive Meshing •
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The LS-DYNA solver automatically refines the mesh according to the criteria specified on the EDCADAPT command for the Part ID’s flagged with the EDADAPT command. Each adapted mesh has a separate Jobname.RS01, Jobname.RS02, … (POST1) and Jobname.HI01, Jobname.HI02, … (POST26) results file, due to the needed file header changes. POST1 animations are possible across different results files with the ANMRES macro: Utility Menu > PlotCtrls > Animate > Over Results … •
Note: The filename a d a p t should not be used, as LS-DYNA uses this root name for many of its files. The analysis can bomb out with no real indication from LS-DYNA as to why!
H. Simulation Control •
S en s e S w i t c h C o n t r o l s allow the user to interrupt the solution
process and to check the actual status, as well as write out binary files for the current state.
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To activate a sense switch control, type CTRL-C into the output window of ANSYS on Unix platforms or the separate LS-DYNA output window on Windows platforms. It interrupts the explicit solver and waits for an input in the output window of ANSYS. ‒
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Type s w 1 to terminate the run. A restart file will be written. Type s w 2 into the output window to receive global statistics of the current state. ANSYS LS-DYNA will then continue. Type s w 3 into the output window to write out a restart file for the current time. ANSYS LS-DYNA will then continue. Type s w 4 to write out results files. ANSYS LS-DYNA will then continue.
... Simulation Control •
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The f i r s t e s t i m a t i o n of the required CPU time is usually t o o h i g h . After a while, issue CTRL-C and type s w 2 for a better CPU estimate...
The LS-DYNA solver writes all important messages (errors, warnings, failed elements, contact problems, etc.) to the ANSYS output window (separate window on Windows O.S.) an d to the file d 3 h s p .
... Simulation Control •
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The LS-DYNA solver also writes data to the m e s s a g file. This file contains a brief summary of the time step information, warnings, errors, and a few other things that are written to the d 3 h s p file. The warnings and errors are detected by ANSYS and the user is instructed to view the m e s s a g file for more details:
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S o l u t io n i s d o n e ! message only appears for no errors/warnings
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This message implies that a successful run w as achieved
I. Editing the LS-DYNA Input File •
Most general LS-DYNA capabilities are supported by the ANSYS LS-DYNA interface. However, there are several additional features of LS-DYNA, that cannot be directly accessed through the ANSYS LS-DYNA interface. Some examples include:
– Material models: Fabric, unified creep, Drucker-Prager – Elements: Air bags, seat belts, explosives – Constraints: Spherical, revolute, cylindrical, etc. joints •
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Although these unsupported LS-DYNA capabilities cannot be directly accessed, a user familiar with LS-DYNA keyword input can still use any feature indirectly by editing the LS-DYNA input file that is generated by the ANSYS LS-DYNA interface. LS-PREPOST, which is also not supported , can always be used to validate the results, but using the ANSYS postprocessors POST1 and POST26 may not be possible, depending on the changes.
... Editing the LS-DYNA Input File •
To access these additional LS-DYNA features, do the following: 1.
Save the Database
2.
Generate the Input File
3.
Exit the ANSYS LS-DYNA Interface
4.
Edit the LS-DYNA Keyword Input File
5.
Submit the Job to the LS-DYNA Solver
6.
Re-enter the ANSYS LS-DYNA Interface and Review the Results
Save the Database : SAVE •
You will need to save the database before exiting the program…
Generate the Input File : EDWRITE •
The EDWRITE command creates the LS-DYNA keyword ASCII input file, J o b n a m e . K, from the information stored in the database.
... Editing the LS-DYNA Input File Generate the Input File (continued): •
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The EDWRITE command also creates the headers to the J o b n a m e . RST and J o b n a m e . HIS files. This header information includes the node and element definitions, so if the model’s basic entity information is then changed, these files could contain erroneous results. Therefore, you should also write out the LS-DYNA results files used by LS-PREPOST. The SOLVE command should n o t be used, since it not only issues the EDWRITE command (overwriting any existing J o b n a m e . K file), it also immediately submits the input file to the LS-DYNA solver without allowing the desired changes to be made. •
Solution > Write Jobname.K •
Default (ANSYS ) option only has results written to ANSYS .RST and .HIS files. Choose an option that includes LS-DYNA results files …
... Editing the LS-DYNA Input File Exit the ANSYS LS-DYNA Interface : /EXIT •
In order to free up the license, you will need to exit the program.
Edit the LS-DYNA Keyword Input File : •
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vi J o b n a m e . K
or
notepad …
Using an ASCII text editor (like v i or n o t e p a d ), edit the J o b n a m e . K file and add the desired features. The input file generated by the interface is in fixed format, so be careful when changing the data. The LS-DYNA Keyword User’s Manual can be downloaded from the ANSYS Customer Portal for free. When adding an unsupported material, it is best to use a dummy material when building the model and then just replace the dummy material information in the input file with the desired material data.
... Editing the LS-DYNA Input File Submit the Job to the LS-DYNA Solver : •
In the same directory where the J o b n a m e . K, J o b n a m e . RST, and J o b n a m e . HIS files reside, execute the LS-DYNA s c r i p t . For ANSYS/Multiphysics/LS-DYNA on a UNIX operating system, issue: /ansys_inc/v110/ansys/bin/lsdyna110 i=J o b n a m e . K pr= ANE3FLDS Add:
m=drelax MEMORY=#
for an implicit-to-explicit sequential solution. (in words) for large jobs (see EDSTART).
R=d 3 d u m p n n for small and full restarts (nn = 01, 02, 03, etc.)
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On the PC, use double quotes to enclose the entire command string (due to the spaces) to execute the LS-DYNA s c r i p t :
“C:\Program Files\Ansys Inc\V110\ANSYS\bin\intel\lsdyna110 ”
i= J o b n a m e . K …
... Editing the LS-DYNA Input File Submit the Job to the LS-DYNA Solver (continued): •
Yes! There is an easier way to submit LS-DYNA jobs directly. Please refer to the next slide for details about the LS-DYNA launcher …
Re-enter the ANSYS LS-DYNA Interface and Review the Results : •
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When done, resume the database and postprocess the ANSYS results files in POST1 and POST26. LS-DYNA continuously appends to these files during solution, but if there is an abnormal termination (e.g., full disk or power interruption), the pointer information will not be saved to the files and the results will not be available. However, the LS-PREPOST postprocessor can still read the d3plot and d3thdt files, if they were requested (see EDWRITE and EDOPT commands). Note: Editing the Jobname.K file is n o t s u p p o r t ed by ANSYS, Inc.
J. LS-DYNA Batch Launcher •
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The A NSYS LS-DYNA Int erface both creates the .K (keyword) ASCII input file and submits it to the LS-DYNA solver when the SOLVE command is issued. As previously noted, the LS-DYNA solver may also be launched directly from the command line by specifying the appropriate arguments, including the existing .K input file name. The A N S Y S P r o d u c t L a u n c h e r may also be used to submit existing .K input files to the LS-DYNA solver without having to remember the command format shown on the previous slides. On the PC, the ANSYS Launcher is accessed from the Start button: –
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On UNIX machines, the ANSYS Launcher is accessed by typing: –
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Start > Programs > ANSYS 11.0 > ANSYS Product Launcher
launcher110
The ANSYS Product Launcher searches for the available licenses before bringing up the GUI window.
... LS-DYNA Batch Launcher •
In the top GUI area:
– Select “LS-DYNA Solver” as the Simulation Environment
– Use a License that supports LS-DYNA
– Pick the desired Analysis Type •
The File Management tab input includes:
– Working Directory – Keyword Input File – Restart Dump File (if a restart analysis) •
Before picking Run ...
... LS-DYNA Batch Launcher •
Under Customization Preferences, specify:
– Memory (in words) for LS-DYNA (see EDSTART command)
– Number of CPUs (if license supports it)
– Consistency checking (improved accuracy for multiple CPU analyses)
– Double Precision version of LS-DYNA (if highest amount of accuracy required) •
Selecting the “Run” button will launch the LS-DYNA solver
K. LS-DYNA Solver Precision •
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By default, ANSYS LS-DYNA executes the S i n g l e P r e c i s i o n (SP) version of LS-DYNA. It is very fast and accurate enough for most simulations. However, a D o u b l e Pr e c i s i o n (DP) version is also available for most platforms and resides in the same directory as the SP version. Both the DP and SP versions support SMP (Shared Memory Parallel) execution. The DP version may be up to 20% slower than the SP version, but the additional accuracy it provides may be needed for long-duration events. Tracking the motion of a bouncing block is an example in which a little error at the beginning of the analysis can result in large amounts of compounded error by the end of the simulation. Both the SP and the DP versions use Build 7600.398 of LS971 R2 in ANSYS LS-DYNA 11.0 and Build 7600.1116 of LS971 R2 in 11.0 SP1.
... LS-DYNA Solver Precision •
The Double Precision version is activated by several different methods: –
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EDDBL, Double command (Solution > Analysis Options > Double Precision )
/CONFIG,dyna_dbl,1 command Selecting “Enable double precision analysis” from the ANSYS Product Launcher for the “LS-DYNA Solver” Simulation Environment (see earlier slide) -dp command line option: lsdyna110 -dp i=Jobname.K •
The m e s s a g file will indicate that the double precision version was used. –
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Letter “d” in version name (an “s” is used for the single precision version) Precision specification
