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Read Restart Usage

NetDEM supports two restart mechanisms to facilitate flexible and efficient continuation of large-scale discrete element simulations. Users can choose between VTK-based and JSON-based restart options depending on the fidelity required and the intended use (e.g., visualization vs. simulation continuation).

Overview of Restart Mechanisms

VTK-Based Restart

The VTK format is used to export basic simulation results through the DataDumper module. It supports reading geometry information such as particles and walls but does not currently support restoring contact information.

JSON-Based Restart

The JSON format is generated when dump_read_restart is enabled in DataDumper. It contains comprehensive simulation state data, including particle properties, wall status, and contact information. This mechanism is recommended for high-fidelity restart scenarios where exact simulation continuation is required.

Usage Instructions

A restart can be initiated using the following command:

sim.scene.AutoReadRestart("packing/", 40000, 0, "json")

Where: - "packing/" is the directory containing the saved restart data. - 40000 specifies the target cycle or time step. - 0 is the subdomain ID (used in parallel simulations). - "json" or "vtk" sets the data format to be loaded.

If the "vtk" format is selected, please note that contact information will not be recovered. Use "json" if full state recovery is needed.

Resetting Simulation Time and Step

After reading restart data, the simulation clock and cycle count can be reset using:

sim.SetTimeAndCycles(0, 0)

This is useful for cleanly defining the start of a new simulation stage and for consistent time tracking in post-processing.

Alternatively, users may set the original time and cycle values (e.g., from a previous run) using:

sim.SetTimeAndCycles(4.0, 40000)

This allows the simulation to resume from the actual interruption point, ensuring continuity in both numerical progression and temporal tracking. It is especially important when simulation results need to be seamlessly integrated across different stages.

When restarting from a previous simulation, it is often necessary to recover the time series used by ParaView for animation and analysis. This can be done via:

data_dumper.ReadTimeSeries("dem/")

This function reads the existing .series file to restore consistent visualization timelines. Without this, the file may be overwritten during restart.

Configuring DataDumper for Restart

To enable restart functionality and data output during the initial simulation, configure the DataDumper module accordingly.

VTK Output

To export results in VTK format for visualization in ParaView:

data_dumper.SetSaveByTime(0.01)  # Save VTK files every 0.01 seconds

This generates geometry and field data at fixed time intervals, but does not include full simulation state (e.g., contact info). Use this option when your goal is primarily visualization and post-processing.

JSON Output for Simulation Restart

To save full simulation state for accurate restart:

data_dumper.dump_read_restart = True
data_dumper.SetSaveReadRestartByTime(1.0)  # Save restart data every 1.0 seconds

These settings enable periodic dumping of complete simulation snapshots in JSON format, including contact models, particle states, wall conditions, and other metadata required for exact restart.

Restarting of the CFD Side (OpenFOAM)

On the CFD side (e.g., using OpenFOAM), restart is configured via the controlDict file located in the system/ directory. To resume from a previously saved simulation state, modify the startFrom and startTime entries as follows:

startFrom       latestTime;

Where: - startFrom latestTime tells OpenFOAM to automatically start from the latest available time folder in the case directory. - You may alternatively replace latestTime with a specific time value, such as 4.0, to restart from that point explicitly.

Note: If you manually set startTime, make sure the corresponding time folder exists. Otherwise, OpenFOAM will return an error.

This configuration enables smooth continuation of a CFD simulation either from the latest available time or a chosen earlier time step, and is especially useful when coupling with DEM restart logic in hybrid simulations.

Restart Execution Script

In restart scenarios, it is recommended not to use the default Allrun-parallel or Allclean scripts, as they may overwrite or delete necessary data such as decomposed time directories or previous simulation logs.

Instead, use a minimal execution script (e.g., named All-restart) based on Allrun-parallel, with the following modifications: - Skip mesh generation and decomposition steps. - Back up the previous solver log. - Directly launch the solver using the existing decomposed case.

Example: All-restart script

#!/bin/sh
cd "${0%/*}" || exit                                # Run from this directory
. ${WM_PROJECT_DIR:?}/bin/tools/RunFunctions        # Tutorial run functions
#------------------------------------------------------------------------------

# Backup previous log
mv log.$(getApplication) log.$(getApplication)-bak

# Run the solver in parallel (assumes case is already decomposed)
runParallel $(getApplication)

#------------------------------------------------------------------------------

Note: Ensure that the case has already been decomposed (i.e., decomposePar has been run previously), and that the necessary processor*/ directories exist before executing this script.

This streamlined script ensures a clean restart without overwriting mesh or field files, and is especially useful in CFD-DEM coupled simulations where DEM restart is coordinated in parallel.

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