Frequency Annotations

This guide explains how to use the frequency annotation system in Remote OpenFAST Plotter, which allows you to mark and label important frequencies in FFT plots.

Why Use Annotations?

Frequency annotations are valuable for:

  • Marking known excitation frequencies (1P, 3P, etc.)

  • Highlighting system natural frequencies

  • Creating consistent labels across multiple analyses

  • Documenting findings in exported plots

  • Comparing frequency peaks with expected values

Using the Annotation System

Adding Annotations

In the FFT tab, you’ll find the annotation controls:

  1. Enter a frequency value (in Hz)

  2. Add a descriptive label for the annotation

  3. Click “Add” to place the annotation on the plot

  4. Optionally select a color for better categorization

Annotations appear as vertical lines on the FFT plot with attached labels.

Managing Annotations

The application provides several tools for managing annotations:

  • Save Annotations: Store a set of annotations with a name

  • Load Annotations: Apply previously saved annotation sets to new plots

  • Remove: Delete a specific annotation

  • Clear All: Remove all annotations from the current plot

Annotation sets are saved between sessions, making them available for future use.

Common Annotation Practices

Wind Turbine Analysis

For wind turbine analysis, typical annotations include:

  • 1P: Once-per-revolution frequency (rotor rotation frequency)

  • 3P: Three-times-per-revolution (for three-bladed turbines)

  • Tower Frequencies: First and second modes for fore-aft and side-side

  • Blade Frequencies: First and higher flapwise and edgewise modes

  • Drivetrain Frequencies: Torsional modes

  • Grid Frequency: 50 Hz or 60 Hz depending on region

Naming Conventions

Consistent naming helps with interpretation:

  • Use a short, descriptive name (“1P”, “Tower FA”)

  • Include the frequency value when helpful (“1P (0.2 Hz)”)

  • Use standardized color coding for different types of frequencies

  • Be consistent across analyses for easier comparison

Example Workflow

Using annotations effectively:

  1. Create FFT Plot: * Load your OpenFAST files * Configure FFT parameters * Generate frequency plots

  2. Add Known Frequencies: * Add 1P frequency based on rotor speed * Add relevant multiples (3P, 6P, etc.) * Mark tower and blade modes if known

  3. Save Annotation Set: * Name the set (e.g., “5MW Baseline”) * Save for future use

  4. Compare with New Data: * Load new simulation data * Apply the saved annotation set * Identify shifts or new peaks

  5. Export for Documentation: * Export the annotated FFT plot as HTML * All annotations are included in the export

Best Practices

For effective use of annotations:

  • Create standard annotation sets for project types

  • Document the meaning of annotations in reports

  • Use color coding consistently (e.g., red for critical frequencies)

  • Include both excitation and natural frequencies

  • Compare annotated frequencies with design values