Working with FE models, the term “restart” is often used in a concept of continuing a simulation from an already solved state of the model. The reason could either be because of the simulation crashing at a later stage, or because of nonlinearities (e.g. yield in the material) combined with loads which are to be investigated on a case-by-case basis and not be combined. Another subcase of the latter example has to do with preloads in the structure: Perhaps you have time-consuming stages where fastener loads and contacts are to develop before any other loads are applied and several independent load cases to analyse. For these cases restarts are great for saving simulation time using the nonlinear solutions SOL401 and SOL402 in Simcenter Nastran.
For SOL101-SOL200 the restart capability is not the same as for the nonlinear solutions SOL401 and SOL402. Restarting from a non-converged simulation is possible for these solutions as well, but a caveat is that neither bolt preloads nor contacts are supported in a SOL101-SOL200 restart (but glue interfaces are). However, restarting from a solution can be a handy tool nonetheless. Let’s go through how you can use it in SC3D.
Using restarts can be very convenient if you have a completed simulation, let’s use SOL101 as an example: Imagine that when you are writing the report you realise that some result output is missing; or perhaps you would like to investigate some more load cases on the structure. Well, then you are in luck, as this is something that restart runs in SOL101 will do excellently for you.
Time gains by performing a restart can vary depending on system, FE model and the solution to be solved, as well as the restart procedure. Because of this, a general percentage in reduced time spent cannot be given. However, for e.g. SOL101 the decomposition of the stiffness matrix can be skipped when performing a restart instead of a new cold start (the first run generating restart files) and this operation is one of the most time consuming when solving.
In the case that you need to change parts of the FE model – like adding or removing elements or constraints, then the time gains by performing a restart will be small since this will trigger Nastran to decompose the stiffness matrix once again. If this is not the case, then you better keep on reading as a restart can save you substantial time. Remember, what we will discuss here is only valid for SOL101 to SOL200 in Simcenter Nastran.
To perform one of these restarts in SC3D you will need to perform these two steps:
- Create and solve a cold start solution and create restart files
- The input deck needs to be edited for the restart solution
1. Setting up the Cold Start Solution
The first of these steps is to obtain files to be used for restart using the scratch=No/Mini option. Assuming you have a SOL101 simulation set up, it is found under:
<active solution>→Edit Solver Parameters→nastran Command Keywords (group)
This will save .DBALL and .MASTER files in the simulation directory, which will enable a new solution, the restart, to build upon the cold start solution. Using scratch=mini will only enable new output requests in the restart run, with the benefit of smaller database files (around 75 % smaller than full database files).
2. Manipulating the Input Deck
The second step consists of telling Nastran that a restart is to be made, what file that is to be used for restart, and then edit the Case Control section and BULK data section for changes and removal of duplicate information.
Telling Nastran to perform a restart, and with which file, is done via the File Management section statements RESTART and ASSIGN.
Depending on whether you are performing a read-only restart or a read-write restart the ASSIGN statement will look slightly different. In this post we will only show a read-only version for the commands. These statements need to be written to the input deck (.dat file) of the solution and would look like in the image below:
As can be seen in the image the commands are added to the input deck as “keyin” text (user defined text) in SC3D. This is done via:
<active solution>→ Edit→File Management→User Defined Text (Edit or create new)→Type in the input or open the Keyin Text window→Write the restart statements
The only thing left to do before solving is to change the output request, or to add subcases, and then edit the input deck so that no duplicate BULK entries are sent to the solver. An easy way to do this in SC3D is to:
Solve (active solution)→Submit = Write, Edit & Solve Input File
Pressing OK will open up your text editor where the input deck can be edited before it is sent to Nastran. Saving and then closing the editor will send the solution to be solved. If you have an Enterprise Nastran license you can edit the input deck directly outside of SC3D and either execute Nastran from the command prompt or in SC3D by switching from a Desktop license to an Enterprise license under:
<the active solution>→Edit Solver Parameters→License Type→Enterprise
The File Management section was treated with the user defined text in the beginning of this step. The Executive Control Section will look the same as in the cold start run. For the Case Control Section care must be taken:
- Do not change any statements which are still valid from the cold start run (subcases, load identifiers etc.). Only add new statements. (This is something which SC3D will take care of for you in the majority of the cases)
- Output request can be altered entirely. E.g. if strain is wanted as extra output this can replace or be added to the output request in the cold start solution.
Note that when placing e.g. constraints in subcase containers in SC3D a new SPC BULK entry will be created for that SPC per subcase. This will also increment the ID. For restart cases where only loads are to be changed the constraints should be defined on a solution level for simpler edit.
3. Summary and Examples
As a final note: If you have big models where you either want to change the output requests from a solution; Or want to investigate additional load cases where only loads change, then you have a high chance of saving time solving your simulation by using restarts in e.g. SOL101.
Taking the example with the cube in the below videos: If the mesh size is decreased from 0.25 mm, as seen in the videos, to 0.1, mm the solution time for a cold start will be around 90 seconds. Restarting to just obtain new output can take about 4 seconds. Restarting with an additional subcase where only new loads are defined can take 10 seconds!
Lastly, it was mentioned in the beginning that contacts cannot be used in restart cases and that is true. There are ways of still modelling the influence from the contact which is described in an interesting Knowledge Base article, KB-PL8610319, in the Siemens Support Center.
Below you can see two examples of firstly: when new output is requested, and secondly: when a new subcase is defined in a restart solution. Both run in SC3D version 2412.
New Output Restart
New Subcase Restart
Hope you will have use of this blogpost and if you have any questions of how to set this up yourself reach out to us at support@volupe.com.
Viktor Hultgren, M.Sc.
Contact: support@volupe.com
+46 704 21 06 61
