SimNexus

A Python module for orchestrating complex simulations with native support for LS-DYNA, OpenRadioss, and OpenFOAM.

Overview

SimNexus enables the automation and coordination of multi-physics simulation workflows. The module is particularly suited for simulations that span multiple domains, such as combined structural and fluid dynamics analyses. It supports tasks from from input preparation and remote execution to results extraction and post-processing.

A design variant is defined using variables and evaluated using a directed graph of actions. The design variables can be user defined or defined inside the input decks of the supported solvers using e.g. a *PARAMETER keyword. An action is any processing step such as mesh creation, an FEA simulation, results extraction, or computations; while the graph specifies the dependencies between actions. An action therefore waits for the other actions it depends on.

Multiple designs can be evaluated in parallel. A simulation iterator evaluates the whole graph for several design points simultaneously, each in its own job directory. Within a single graph, independent branches can likewise run at the same time, each in its own process.

SimNexus has a native support for solvers like LS-DYNA, OpenRadioss, and OpenFOAM. In addition OpenRadioss using LS-DYNA input is supported as a special case.

Key Features

• Workflow Management: Define simulation workflows as directed acyclic graphs (DAGs) where actions are executed based on dependency relationships and completion status of prerequisite tasks
• Native Solver Support: Specify input parameter values and the results to extract for a supported solver. Currently implemented are LS-DYNA and OpenRadioss for structural analysis, and OpenFOAM for computational fluid dynamics
• Results Extraction: Read from the solvers' result databases in the graph. Supported are: LS-DYNA d3plot, OpenRadioss VTK and time-history CSV, and OpenFOAM fields and histories.
• Parallel Execution: Evaluate several design points of a study concurrently, each job in its own directory with its own progress bar; within one graph, independent branches run at the same time (asynch=True)
• Remote Execution: Submit computational subgraphs to remote computing resources while maintaining local workflow coordination
• Custom Actions: Add an operation of your own by subclassing WorkAction and writing solve(val_dict); it then behaves like any built-in action.
• Discoverability: Query any graph for its inputs and outputs without running it — solver actions read their parameterised input files to report variable names, types, and default values

Typical Workflow

1. Configure input files for target solvers
2. Define analysis actions and their dependencies
3. Execute simulations on designated compute resources (local or remote)
4. Extract relevant results from solver outputs
5. Aggregate and summarize findings

SimNexus streamlines the complexity of managing heterogeneous simulation environments, enabling researchers and engineers to focus on analysis rather than workflow orchestration.

Documentation

(The link provided is not yet active)

Online documentation is available here

Installation

pip install simnexus

SimNexus has so far only been tested on Linux and WSL; it may well work on other platforms, but that has not been verified.

The library uses lasso-python (including vortex-radioss and lasso.dyna) as well as dynakw for compatibility with LS-DYNA, and grpc for remote execution.

Usage

See also the documentation and the examples directory.

Note that OpenFOAM, LS-DYNA and OpenRadioss workflows follow the same pattern.

OpenRadioss

An example for OpenRadioss is given below. An example using LS-DYNA input is given the examples directory.

starter_deck = Path('models/cube_TYPE7_0000.rad')
engine_deck  = Path('models/cube_TYPE7_0001.rad')

# 1. Define RadiossAnalysis to run the simulation
run_rad = RadiossAnalysis( name='rad', 
                  starter_cmd='openradioss_starter',
                  starter_input_path=starter_deck,
                  engine_cmd='openradioss_engine',
                  engine_input_path=engine_deck,
                  create_d3plot=True )

# 2. Create a workflow and add the d3plot extraction actions
wf = WorkFlow( 'Radioss_WorkFlow' )
wf.add_action( run_rad )

d3p = d3plot_File( name='d3plot' )
d3p.NodalValue(name='n5', state=1, nid=5, component= 'node_displacement'  )
wf.add_action( d3p )

wrk_area = WorkArea( wf, copy_paths=[starter_deck,engine_deck] )

# Discover variables defined in input deck and other actions
discovered_vars = wrk_area.parameters()
print("Discovered variables:")
for v in discovered_vars:
        print(f"  {v}")

# 3. Execute the workflow. Provide values for the variables.
val_dict = { 'E': 210000.0, }

print("Starting workflow...")
print(f"Parameters: {val_dict}")
    
ret = wrk_area.solve( val_dict )
print("Available results.", ret.keys() )

OpenFOAM

An example for OpenFOAM is given below.

import logging
logging.basicConfig(level=logging.WARNING)

from pathlib import Path
from simnexus.args import JobType
from simnexus.graph_actions import WorkFlow, WorkArea
from simnexus.openfoam_actions import OpenFOAMAnalysis
from simnexus.openfoam_actions import OpenFOAM_Field, OpenFOAM_History

def create_openfoam_graph():

    case_dir = Path(__file__).parent.parent / "tests" / "openfoam_exa"
    if not case_dir.exists(): exit(f"Error: {case_dir} does not exist.")

    case_paths = [
        str(case_dir / "system"),
        str(case_dir / "constant"),
        str(case_dir / "0"),
    ]

    wf = WorkFlow('OpenFOAM_WorkFlow')

    wf.add_action( OpenFOAMAnalysis(
        name="my_job",
        job_flag=JobType.CREATE_MESH | JobType.RUN_SIMULATION,
        solve_cmd="icoFoam",
        mesh_cmd="blockMesh" ) )

    wf.add_action(  OpenFOAM_Field(  # Extract field of pressure values
        name='pressure',
        field_variable='p',
        time=0.5 ) )

    wa = WorkArea(wf, copy_paths=case_paths)

    return wa


of_graph = create_openfoam_graph()

# List graph variables and outputs
print("\n\nGraph variables:")
for v in of_graph.parameters():
        print(f" - {v}")

print("\n\nGraph outputs:")
for name, (eval_type, description) in of_graph.outputs().items():
    print(f' - {name}: {eval_type} — {description}')


# Run the job with new parameter values
print("\n\nRunning job.solve({'lidVelocity': 1.2, 'nCells': 6})...")
outcomes = of_graph.solve({"lidVelocity": 1.2, "nCells": 6})

# Print the computed field
print(f"Pressure field:", outcomes['pressure'] )

Inspecting a graph

A graph can be examined before it is run — what it is made of, what it will write to disk, what values it needs and what it will produce. Nothing here starts a solver.

# using itr -- an existing graph, work area or simulation iterator

itr.print_tree()                 # the actions, as a tree
itr.print_work_dir()             # the directories and files a run will create
itr.describe_workflow()          # both of the above, one after the other

SimulationIterator 'Radioss_WorkFlow_Iter'
└── WorkFlow 'Radioss_WorkFlow'
    ├── RadiossAnalysis 'rad'
    └── d3plot_File 'd3plot'
        └── _d3plot_NodalValue 'n5'

Pass describe=True to add each action's description:

SimulationIterator 'Radioss_WorkFlow_Iter'  — Simulation iterator for graph Radioss_WorkFlow
└── WorkFlow 'Radioss_WorkFlow'  — Workflow Radioss_WorkFlow
    ├── RadiossAnalysis 'rad'  — OpenRadioss analysis using input file models/cube_TYPE7_0000.rad
    └── d3plot_File 'd3plot'  — D3plot file reader for d3plot
        └── _d3plot_NodalValue 'n5'  — D3plot nodal field node_displacement at state 1

print_work_dir() predicts the run directory from the actions themselves, so it works before anything has run and without the solvers installed:

Radioss_WorkFlow/   (results root)
├── status.json   (run progress: current job, jobs done; see simnexus.progress)
├── jobs_index.json   (job -> variable values and group labels; see simnexus.simulation_iterator)
├── job_0/   (one directory per design evaluation)
│   ├── iter_variables.json   (this design's variable values)
│   ├── actions_output.pkl   (this design's action outputs)
│   ├── cube_TYPE7_0000.rad   (copied in)
│   ├── cube_TYPE7_0001.rad   (copied in)
│   ├── status.json   (live action states; see simnexus.progress)
│   ├── radioss_variables.json
│   ├── rad_run_file_0000.rad
│   ├── rad_run_file_0001.rad
│   ├── rad_run_file_0000.starter.stdout
│   ├── rad_run_file_0000.starter.stderr
│   ├── rad_run_file_0001.engine.stdout
│   ├── rad_run_file_0001.engine.stderr
│   └── d3plot*
└── job_1/ … job_N/

The inputs and the outputs are asked for in the same way. parameters() returns the variables the graph needs — solver actions read them out of their parameterised input files — and outputs() the results each action produces:

# Inputs: the variables the graph expects, with type, default and origin
for v in itr.parameters():
    print( ' -', v )

# Outputs: {action name: (data type, description)}
for name, (data_type, description) in itr.outputs().items():
    print( f' - {name}: {data_type} — {description}' )

 - Variable Name: E, Data Type: float, Value: 210000.0, Description: 'From 'cube_TYPE7_0000.rad''

 - rad: EvalType.NOT_SPECIFIED — OpenRadioss analysis using input file models/cube_TYPE7_0000.rad
 - n5: EvalType.NOT_SPECIFIED — D3plot nodal field node_displacement at state 1

Call parameters() on a WorkArea or SimulationIterator rather than on the bare graph: they copy copy_paths into a temporary directory first, so the solver actions can find the decks they have to read.

Design studies using parallel execution

A SimulationIterator runs the graph once per design point, each in its own job directory. Give it max_workers and solve_parallel evaluates a batch of design points that many at a time, one process per job:

# using wf -- an existing workflow 

# Create a simulation iteration that runs four jobs at the same time,
# each in its own job_N directory. cleanup=True deletes the bulk solver
# output (here the d3plot) of a job once its graph has run, so the
# extractions below still get their data but the study does not fill the
# disk.
itr = SimulationIterator( wf, copy_paths=[starter_deck, engine_deck],
                          max_workers=4, cleanup=True )

design_points = [ { 'E': 190000. },
                  { 'E': 210000. },
                  { 'E': 230000. },
                  { 'E': 250000. } ]

# run the jobs
evals = itr.solve_parallel( design_points  )

for vals, out in zip( design_points, evals ):
    print( vals, '->', out['d3plot']['n5'] )

solve_parallel returns one result dict per design point, in the order given — the same results a serial run produces, only faster. In a terminal it reports itself as a bar counting the jobs of the batch, and under it one bar per job running right now:

Radioss_WorkFlow_Iter:  25%|██▌       | 1/4 [00:14<00:43, 14.5s/job]
  job_1  rad 1 of 3: time 12.9 of 40 (97%)        32%|███████▎               |
  job_2  rad 1 of 3: time 11.4 of 40 (86%)        29%|██████▋                |
  job_3  d3plot 2 of 3                            67%|███████████████▍       |

Each job's bar is fed from the status.json that job writes, so it follows the job through its actions. The iterator's own bar, at the top, counts jobs: its percentage is how many of the batch have finished.

The bars use tqdm and are shown only when stderr is a terminal.

Independently of the bar, every run writes status.json files into its work directories, which another process can follow at any time.

Debug output

simnexus logs through the standard logging module, one logger per module under the simnexus name, so the usual configuration applies:

import logging
logging.basicConfig( level=logging.DEBUG )                # everything

logging.basicConfig( level=logging.WARNING )              # or simnexus alone
logging.getLogger( 'simnexus' ).setLevel( logging.DEBUG )

The solvers' own output is not logged: it is redirected to files in the run directory (*.stdout, *.stderr), and a job running in parallel writes what it prints to job_N/job.log so it does not disturb the progress bars.

Example problems

The examples directory holds runnable scripts, each demonstrating one part of the workflow. Run them from the project root.

• dyna_spring.py — an LS-DYNA workflow: the *PARAMETER values of a deck are set from the variables, the job is submitted, and nodal displacements and coordinates are read back from the d3plot.
• jinja_dyna.py — an LS-DYNA deck parameterised with Jinja markup (JinjaReplace) instead of *PARAMETER cards, run through OpenRadioss with RadiossUsingDynaInput.
• radioss.py — an OpenRadioss workflow: starter and engine decks, job submission, and results extraction from the d3plot it writes.
• openfoam_example.py — an OpenFOAM workflow: mesh creation and solve (blockMesh, icoFoam), then extraction of a pressure field.
• discover_graph.py — inspecting a graph before running it, with parameters() and outputs(). No solver needed.
• radioss_progress.py, radioss_progress_workarea.py, radioss_progress_nested.py — the same OpenRadioss run, which solves and then reads its results back from VTK, in three shapes: as the progress bars of a parallel study, as one run in a WorkArea followed from its status.json with StatusWatcher, and as a study whose jobs each hold a WorkArea in a subdirectory. Needs OpenRadioss.
• parallel_jobs.py — a design study run with solve_parallel: the same six design points one at a time and then three at a time, with the timings, the job bar, and the job_N directories it leaves behind. No solver needed.
• remote/ — remote execution over gRPC: a self-contained client and server in one script (remote_execution.py), and an OpenFOAM server in a container (openfoam_remote_server.py, openfoam_remote_example.py, Dockerfile.openfoam). See remote/README.md.

The LS-DYNA, OpenRadioss and OpenFOAM examples follow the same pattern, so an example written for one solver is easy to move to another. The decks they use are in tests and models; the solver itself has to be installed and on the path.

License

This project is licensed under the MIT License.