This page provides operational recipes for common PICurv tasks. Each recipe includes what to change, why it matters, and a quick verification action.

1. Setup and Physics

1.1 Change Reynolds Number

What to change (in case.yml):

properties:
  scaling:
    length_ref: 0.1
    velocity_ref: 1.5
  fluid:
    density: 1000.0
    viscosity: 0.001

Why:

\[ Re = \frac{\rho U L}{\mu} \]

These values set the non-dimensional operating point consumed by solver controls.

Quick check:

rerun picurv validate ...,
inspect generated .control file for updated values.

1.2 Run in 2D

models:
  physics:
    dimensionality: "2D"
 
grid:
  mode: programmatic_c
  programmatic_settings:
    km: [3]

Why:

2D mode still uses a thin third dimension for structured-grid machinery,
small km is typically enough for planar scenarios.

Quick check:

confirm generated run uses expected Z resolution.

1.3 Increase Grid Resolution

programmatic_c: increase im/jm/km arrays,
file: use finer .picgrid,
grid_gen: increase generator resolution args.

Verification:

compare runtime memory/cost and key output metrics across resolutions.

2. Boundary Conditions

2.1 Set a Constant-Velocity Inlet and Walls

boundary_conditions:
  - face: "-Zeta"
    type: "INLET"
    handler: "constant_velocity"
    params: {vx: 0.0, vy: 0.0, vz: 1.5}
  - face: "-Eta"
    type: "WALL"
    handler: "noslip"
  # define all remaining faces explicitly

Why:

handler/type compatibility is validated,
all faces must be covered for each block.

Verification:

use validate first and check BC generation files under runs/<run_id>/config/.

2.2 Enable Periodicity in One Direction

models:
  domain:
    i_periodic: true

Use PERIODIC BC type on both paired faces with supported handlers:

geometric
constant_flux (requires target_flux)

Verification:

confirm paired-face consistency in validation output.

3. Running and Monitoring

3.1 Run in Parallel and Control DMDA Layout

./bin/picurv run -n 16 --solve ...

Optional partition hints:

grid:
  da_processors_x: 4
  da_processors_y: 2
  da_processors_z: 2

Note: da_processors_* are scalar globals, not per-block vectors.

3.2 Run on Slurm (Generate and Submit)

./bin/picurv run --solve --post-process \
  --case my_case/case.yml \
  --solver my_case/solver.yml \
  --monitor my_case/monitor.yml \
  --post my_case/post.yml \
  --cluster my_case/cluster.yml

Generate-only mode:

./bin/picurv run --solve --post-process \
  --case my_case/case.yml \
  --solver my_case/solver.yml \
  --monitor my_case/monitor.yml \
  --post my_case/post.yml \
  --cluster my_case/cluster.yml \
  --no-submit

Submit an already staged run later:

./bin/picurv submit --run-dir runs/<run_id>

Cancel a submitted run by directory:

./bin/picurv cancel --run-dir runs/<run_id> --stage solve

Generated Slurm solver jobs already enable an automatic runtime walltime guard. Override it only when you need a different warmup/headroom policy:

execution:
  walltime_guard:
    enabled: true
    warmup_steps: 10
    multiplier: 2.0
    min_seconds: 60
    estimator_alpha: 0.35

Ask Slurm for an early warning signal as fallback protection when you want PICurv to flush one last snapshot before walltime or preemption:

execution:
  extra_sbatch:
    signal: "USR1@300"

If the batch script launches mpirun directly, use signal: "B:USR1@300" and prefer exec mpirun ....

Verification:

inspect scheduler/*.sbatch and submission.json in run directory.
confirm the generated solver script exports PICURV_JOB_START_EPOCH and PICURV_WALLTIME_LIMIT_SECONDS.
confirm the generated cluster profile contains the intended signal fallback policy before submission.

3.3 Restart from a Saved Step

run_control:
  start_step: 500
  total_steps: 1000

Pass --restart-from on the CLI to point at the previous run:

./bin/picurv run --solve --post-process \
  --restart-from ../runs/flat_channel_20260303-120000 \
  --case restart_case/case.yml \
  --solver restart_case/solver.yml \
  --monitor restart_case/monitor.yml \
  --post restart_case/post.yml

Meaning:

If a run has completed through step 500, set start_step: 500.
The next run loads the saved state at step 500.
The first new step advanced is step 501.
total_steps is the number of additional steps to run.
In this example, the restarted run advances from step 501 through step 1500.
When --restart-from is given, picurv automatically resolves the previous run's restart directory from that run's config/monitor.yml and injects the correct -restart_dir into the new control file.

Typical full field restart (solver.yml):

operation_mode:

eulerian_field_source: "load"

Particle restart choices (case.yml):

Full restart of the existing particle swarm:

models:
  physics:
    particles:
      restart_mode: "load"

Restart the flow field but reseed particles from scratch:

models:
  physics:
    particles:
      restart_mode: "init"

Common combinations:

Full restart: start_step > 0, eulerian_field_source: load, restart_mode: load
Flow restart + fresh particles: start_step > 0, eulerian_field_source: load, restart_mode: init
Analytical mode is different: eulerian_field_source: analytical regenerates the analytical field at the requested (t, step) instead of loading restart files.

How to think about this workflow:

restart uses the normal run --solve path; there is no separate restart command,
the new run directory is a fresh run artifact,
the saved field state is loaded from existing restart/output files referenced by the current restart path contract,
--restart-from on the CLI is the preferred way to point picurv at the old run automatically,
use --continue as a shorthand when resuming from the most recent run of the same case,
the old run directory is not mutated in place by picurv.

Before launching a restart, verify:

the previous run actually wrote solver outputs for the target start_step,
the --restart-from path points to the intended previous run directory,
monitor.yml -> io.directories.restart points to the intended restart directory name,
monitor.yml -> io.directories.output matches where the prior run wrote field data,
restart source files for the requested step exist,
start_step matches an actual saved timestep, not just a desired number.

Common restart mistakes:

Setting start_step: 501 after a run that ended at 500. Use start_step: 500.
Forgetting solver.yml -> operation_mode.eulerian_field_source: load for a true field restart.
Forgetting to choose particles.restart_mode: load or init explicitly.
Trying to restart from a step that was never written to disk.

Verification:

confirm restart directory and step indices in run logs,
confirm the banner/load path shows the expected restart step,
if particles are enabled, confirm the log shows the intended particle restart mode.

3.4 Enable Targeted Debug Logging

logging:
  verbosity: "DEBUG"
  enabled_functions:
    - Projection
    - UpdatePressure

Use this for local diagnosis of instability or boundary anomalies. Prefer narrow function lists to keep logs manageable.

4. Post-Processing Recipes

4.1 Postprocess an Existing Run

./bin/picurv run --post-process \
  --run-dir runs/flat_channel_20240401-153000 \
  --post my_study/standard_analysis.yml

Use when solver outputs already exist and you are iterating only on analysis pipeline.

To catch up the same recipe in batches while the solver is still running, add --continue and keep the full desired window in post.yml:

./bin/picurv run --post-process --continue \
  --run-dir runs/flat_channel_20240401-153000 \
  --post my_study/standard_analysis.yml

Behavior notes:

if the same recipe already produced steps 0..60 and post.yml still asks for 0..100, PICurv launches only 70..100.
if source files currently exist only through 420, PICurv launches only the fully available contiguous prefix and exits successfully; a later --continue run picks up the newer steps.
if you change the recipe itself, PICurv starts from that recipe's configured start_step instead of inheriting completion from the earlier recipe.
if you omit --continue, PICurv honors the requested window exactly and treats the run as an explicit rerun.

For the broader run-directory lifecycle around restart, post-only reuse, and generated scheduler artifacts, see Run Lifecycle Guide.

4.2 Add Q-Criterion to Eulerian Pipeline

eulerian_pipeline:
  - task: nodal_average
    input_field: Ucat
    output_field: Ucat_nodal
  - task: q_criterion
 
io:
  eulerian_fields:
    - Ucat_nodal
    - Qcrit

Verification:

open VTK output and confirm Qcrit field is present.

4.3 Enable Statistics Output (MSD)

statistics_pipeline:
  output_prefix: "Stats"
  tasks:
    - task: msd

Verification:

check output/statistics/Stats_msd.csv by default, unless statistics_pipeline.output_prefix includes an explicit relative or absolute path.

5. Case Initialization and Binary Management

5.1 Initialize a New Case

picurv init flat_channel --dest my_case

This copies template files and writes metadata. Runtime binaries (simulator, postprocessor) are resolved from the project bin/ directory via PATH — no copies are placed in the case.

5.2 Pin Binaries for Reproducibility

picurv init flat_channel --dest my_case --pin-binaries

Use --pin-binaries when you plan to submit Slurm jobs and may rebuild the repo before the job executes. Case-local copies take precedence over bin/ originals at runtime.

Equivalent manual step after init:

picurv sync-binaries --case-dir my_case

5.3 Rebuild Safety

picurv (the Python conductor) can be updated at any time — it only launches jobs, it does not run during solver execution.
simulator and postprocessor in bin/ are overwritten by make all. If a queued Slurm job references them by absolute path, the running binary may change.
Use --pin-binaries or sync-binaries before submission to protect running jobs.

6. Sweep Studies

./bin/picurv sweep \
  --study my_study/study.yml \
  --cluster my_study/cluster.yml

What you get:

expanded case matrix,
scheduler array scripts,
aggregated metrics table (auto-collected after jobs complete),
optional plots.

If a case is killed (e.g. walltime), continue the study:

./bin/picurv sweep --continue --study-dir studies/<study_id>

Re-aggregate metrics manually:

./bin/picurv sweep --reaggregate --study-dir studies/<study_id>

See Sweep and Study Guide for full contract details.

7. Next Steps

Full capability map: Capabilities Summary
Config references: Configuration Reference: Case YAML, Configuration Reference: Solver YAML, Configuration Reference: Monitor YAML, Configuration Reference: Postprocessor YAML
Troubleshooting and quality: Common Fatal Errors and Fixes, Testing and Validation Guide

CFD Reader Guidance and Practical Use

This page describes User How-To Guides within the PICurv workflow. For CFD users, the most reliable reading strategy is to map the page content to a concrete run decision: what is configured, what runtime stage it influences, and which diagnostics should confirm expected behavior.

Treat this page as both a conceptual reference and a runbook. If you are debugging, pair the method/procedure described here with monitor output, generated runtime artifacts under runs/<run_id>/config, and the associated solver/post logs so numerical intent and implementation behavior stay aligned.

What To Extract Before Changing A Case

Identify which YAML role or runtime stage this page governs.
List the primary control knobs (tolerances, cadence, paths, selectors, or mode flags).
Record expected success indicators (convergence trend, artifact presence, or stable derived metrics).
Record failure signals that require rollback or parameter isolation.

Practical CFD Troubleshooting Pattern

Reproduce the issue on a tiny case or narrow timestep window.
Change one control at a time and keep all other roles/configs fixed.
Validate generated artifacts and logs after each change before scaling up.
If behavior remains inconsistent, compare against a known-good baseline example and re-check grid/BC consistency.

Table of Contents

1. Setup and Physics

1.1 Change Reynolds Number

1.2 Run in 2D

1.3 Increase Grid Resolution

2. Boundary Conditions

2.1 Set a Constant-Velocity Inlet and Walls

2.2 Enable Periodicity in One Direction

3. Running and Monitoring

3.1 Run in Parallel and Control DMDA Layout

3.2 Run on Slurm (Generate and Submit)

3.3 Restart from a Saved Step

3.4 Enable Targeted Debug Logging

4. Post-Processing Recipes

4.1 Postprocess an Existing Run

4.2 Add Q-Criterion to Eulerian Pipeline

4.3 Enable Statistics Output (MSD)

5. Case Initialization and Binary Management

5.1 Initialize a New Case

5.2 Pin Binaries for Reproducibility

5.3 Rebuild Safety

6. Sweep Studies

7. Next Steps

CFD Reader Guidance and Practical Use

What To Extract Before Changing A Case

Practical CFD Troubleshooting Pattern