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MultiStage Dependent or Independent Simulations using *CASE


A powerful feature of LS-DYNA v970 and beyond that has yet to gain widespread popularity is the *CASE keyword. Here are two examples to highlight the use of *CASE.

Sequential and Independent Simulations
Ex: Head Impact (FMVSS 201)

*keyword
$
$ Impact 1
$
*case_begin_1
*title
position 1 impact
*include
vehicle_model.i
*include
head_position_1.i
*case_end_1
$
$ Impact 2
$
*case_begin_2
*title
position 2 impact
*include
vehicle_model.i
*include
head_position_2.i
*case_end_2
$
$ ....
*end
Sequential and Dependent Simulations
Ex: Cumulative Bumper Damage

*keyword
$
$ Impact 1
$
*case_begin_1
*title
position 1 impact
*include
vehicle_model.i
*include
impactor_position.i
*case_end_1
$
$ Impact 2
$
*case_begin_2
r=1.d3dump01
*title
position 2 impact
*include
vehicle_model.i
*include
impactor_position_2.i
*case_end_2
$
$ ....
*end
  • Amir says:

    Hi Suri,

    am I right?

    -every KEYWORD defined outside the *CASE cards are global and are active during the total simulation time

    and

    -every KEYWORD inside is only gilty for the sequence between *CASE_N begin and *CASE_N end like *LOAD_, CONSTRAINT, *MAT_, …

    Regards
    Amir

  • Suri Bala says:

    Hi Amir,

    Yes.

    Regards,
    Suri Bala

  • Nihar says:

    Hi Suri,

    This is really helpful in Headrest performance test on Seat.

    How can we define a multistage loading in the same direction incase of Implicit. Many times a single step loading creates convergence problem.
    e.g.
    Case -1 : Loading 0 – 100N
    Case – 2 : Loading 100 – 200 N and so on,
    by this we can slow down the rate of loading at particular level where there is a chance of convergence ( change the slope of loading curve )

    Is this possible with *CASE ?

    Thanks
    Nihar

  • Suri Bala says:

    Hi Nihar,

    To improve convergence, I would suggest breaking down the step size at the point of difficulty or to switch to Explicit for a short number of cycles. You can view the post on automatic time step control for implicit calculations for more information.

    CASE may also be applied but may not be necessary since the load as you described can be achieved within a single definition of a load curve.

    Hope this helps.

    Suri

  • Tiru says:

    Hi,
    I am trying to use case command with different termination times and different control conditions for different cases for a single model. I am not able to solve using *CASE keyword. I am getting an error as ” can find neither internal nor external lscasedriver program”. Can some one help in this regard.
    Regards,
    -Tirumala.

  • Suri Bala says:

    Tiru,

    Hopefully you are using 971 as some 970 executables may not have the necessary driver. If you are using 971, I would contact your local distributer to get the appropriate version which has the necessary driver.

    The case driver is the heart of this feature which is either embedded inside the code or works as an external program which should be available under /usr/bin/ or /usr/lstc directory.

    Suri

  • Nihar says:

    Hi Suri,

    I am also getting a message that either Internal or external lscasedriver program.
    I am running Dyna 971 7600.398 on PC, I checked and could find lscasedriver.exe file along with Dyna executable.

    How can I run, Do I have to set some Env. variable ?

    Thanks.
    Nihar

  • Suri Bala says:

    Hi Nihar,

    If you already have the executable, perhaps you can place it under /usr/bin/ and also at /usr/lstc directory. I am not sure of any environment variable that can be set to indicate LS-DYNA where to look for the driver executable.

    Suri

  • Ranjith says:

    dear suri,

    i have never tried this feature
    but the first doubt coming to my mind is how dyna will be writing out the ascii and binary databases for the two cases in a Sequential and Independent Simulations ? sequentially ? like for the first case d3plot t0 d3plot12 and for the second case d3plot13 to d3plot 25. how about ascii files?
    hope my question makes sense

    thnks
    ranjith

  • Suri Bala says:

    Ranjith,

    It would be based on the ID specified in the *CASE keyword. Ex. 1.d3plot, 2.d3plot, 1.glstat, 2.glstat, where 1 and 2 are the case ids.

    Suri

  • Nihar says:

    Hi Tiru,

    To get rid of � can find neither internal nor external lscasedriver program�. Error Do the following, ( On Windows PC )

    1) Create a directory with same name of your Dyna executable, i.e. ls971_7600_d_xxx in the same Directory of Dyna executables files are there.

    2) Copy lscasedriver.exe in the newly created directory
    3) Run your simulation through DOS Windows.

    I was also facing the same problem, and with the above mentioned steps I could solve a problem using *CASE.

    Regards,
    Nihar

  • Joe says:

    I am trying to do a static analysis in 2 steps on a structure. The first step applies the gravity load. In the second step, I want to apply external loads; the effects of gravity has to be retained. How does one go about setting up *CASE for such a 2-step analysis? How does the second case know that the gravity load should be retained. In Abaqus, this is fairly easy using the *STEP command (I am assuming that you know some Abaqus). Basically, I want to do something similar. Thanks.

  • suri bala says:

    Joe,

    In this case, I would recommend dynamic relaxation using IMDRFLG=5 in CONTROL_DYNAMIC_RELAXATION.
    You need to apply gravity with a load curve (SIDR=1) that ramps up the gravitational load linearly. You need to redefine the gravitational load to be constant this time (SIDR=0) for your external loads.

    You can also accomplish this using *CASE in which the second run would then be a dependent run which could read the stress/displacements from the first case using a restart file (r=case1.runrsf).

    I would try the DR approach first.

    Thanks
    Suri Bala

  • Joe says:

    Thanks,

    I am using the Windows version of LS-DYNA and I am running my analysis using the LS-DYNA manager, not through command lines. It says in the keyword manual that “the *CASE keyword requires a stand alone program that is started by the LS-DYNA executable” – what does this mean? Maybe my question is kind of dumb. Sorry.

  • suri bala says:

    Joe,

    You can download any new version of LS-DYNA (not the manager) and it comes with the lscasedriver executable.

    Thanks,
    Suri Bala

  • KK says:

    Suri,
    When I try this “Sequential and Dependent Simulations”, I am getting error saying “case1.d3dump01 not fund”. Any suggestion?
    Thanks
    SS

    • admin says:

      KK,

      After the first run is completed (case1), can you verify if the file does exist ?
      If not, perhaps they are removed especially in the case of MPP when the nodump option is specified.

      Suri

  • fzbm says:

    The very good information

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