pyBModes

pybmodes is a pure-Python finite-element library for wind-turbine blade and tower modal analysis.

It is best understood as a modern interpretation of the legacy BModes Fortran workflow developed by NREL: the project keeps the familiar engineering inputs and reference behavior of BModes, while providing a cleaner Python API, automated regression tests, and direct integration with modern OpenFAST/ElastoDyn workflows.

Overview

pybmodes can:

• read BModes-style .bmi main input files and tabulated section-property files
• solve rotating blade modal problems
• solve onshore and offshore tower modal problems
• fit ElastoDyn-compatible 6th-order blade and tower mode-shape polynomials
• patch OpenFAST ElastoDyn input files with fitted coefficients
• plot FEM mode shapes and polynomial-fit quality

Supported tower workflows currently include:

• cantilevered onshore towers
• tension-wire-supported towers
• floating spar-type offshore towers
• bottom-fixed monopile towers

Why This Exists

BModes is still a valuable engineering reference, but its legacy Fortran form can make it harder to inspect, test, automate, and extend in modern Python-heavy workflows.

pybmodes does not try to replace the BModes lineage or hide it. Instead, it re-expresses that workflow in Python so it is easier to:

• validate against reference cases
• integrate into reproducible analysis pipelines
• generate ElastoDyn-ready mode-shape coefficients
• inspect intermediate results, fits, and classification decisions

This repository is therefore both:

• a practical Python modal-analysis library
• a validation-driven reinterpretation of legacy BModes behavior

Installation

Requires Python >= 3.11.

Install directly from GitHub:

pip install git+https://github.com/SMI-Lab-Inha/pybmodes.git

For development:

git clone https://github.com/SMI-Lab-Inha/pybmodes.git
cd pybmodes
pip install -e ".[dev]"

If you want plotting support as well:

pip install -e ".[dev,plots]"

Quick Start

Blade analysis

from pybmodes.models import RotatingBlade
from pybmodes.elastodyn import compute_blade_params

blade = RotatingBlade("my_blade.bmi")
result = blade.run(n_modes=10)
params = compute_blade_params(result)

print(result.frequencies[:3])
print(params.BldFl1Sh)

Tower analysis

from pybmodes.models import Tower
from pybmodes.elastodyn import compute_tower_params, compute_tower_params_report

tower = Tower("my_tower.bmi")
result = tower.run(n_modes=10)

params = compute_tower_params(result)
params2, report = compute_tower_params_report(result)

print(result.frequencies[:4])
print(report.selected_fa_modes)
print(report.selected_ss_modes)

Patch ElastoDyn files

from pybmodes.models import Tower
from pybmodes.elastodyn import compute_tower_params, patch_dat

result = Tower("my_tower.bmi").run(n_modes=10)
params = compute_tower_params(result)

patch_dat("ElastoDyn_tower.dat", params)

Examples

The examples/ directory contains compact end-to-end scripts:

• 01_blade_analysis.py: rotating blade modal analysis and ElastoDyn blade fits
• 02_tower_analysis.py: tower modal analysis and ElastoDyn tower fits
• 03_patch_elastodyn.py: patch existing ElastoDyn blade/tower files in place
• 04_plot_results.py: plot FEM mode shapes and fit quality

Run them from the repository root, for example:

python examples/01_blade_analysis.py
python examples/02_tower_analysis.py

Inputs and Outputs

Inputs

pybmodes reads BModes-style inputs without forcing a new file format:

• .bmi main input files
• tabulated section-property .dat files

Outputs

The high-level model APIs return:

• natural frequencies
• nodal mode shapes
• ElastoDyn-ready polynomial coefficients for blades and towers

For tower fitting, the companion reporting API can also expose which modal candidates were considered and why specific FA/SS family members were selected.

Validation

The codebase is validated against BModes reference outputs and offshore benchmark-style cases.

Reference cases currently exercised in the test suite:

Case	Configuration	Status
CertTest01	Non-uniform rotating blade	Passed
CertTest02	Blade with tip mass	Passed
CertTest03	Onshore cantilevered tower	Passed
CertTest04	Tension-wire-supported tower	Passed
OC3Hywind	Floating offshore spar	Passed
CS_Monopile	Bottom-fixed monopile	Passed

At the time of this README update, the full local suite passes with:

159 passed

The tests cover:

• input parsing and path resolution
• FEM building blocks
• model pipelines for blades and towers
• polynomial fitting
• ElastoDyn parameter generation and file patching
• regression checks against validated blade, tower, and offshore cases

Project Layout

src/pybmodes/
  io/         BModes-style input/output parsers
  fem/        Finite-element core
  models/     High-level blade and tower APIs
  fitting/    Mode-shape polynomial fitting
  elastodyn/  ElastoDyn parameter generation and file patching
  plots/      Plotting helpers for mode shapes and fit quality
examples/     End-to-end usage scripts
tests/        Validation and regression coverage

Development

# Run the full test suite
pytest

# Skip integration tests
pytest -m "not integration"

# Lint
ruff check src/ tests/

# Type check
mypy src/pybmodes

Relationship to BModes

This project is a modern Python reinterpretation of the legacy BModes program developed by NREL.

BModes remains the numerical and workflow reference point for much of the validation philosophy used here. pybmodes is an independent Python reimplementation and is not an official NREL release.

License

Released under the MIT License.