Phase Analysis

This example demonstrates the phase analysis capabilities of Remote OpenFAST Plotter for examining phase relationships between signals.

Phase Analysis Fundamentals

Phase analysis helps you understand the timing relationships between different signals. In wind turbine analysis, phase relationships can reveal:

  • Structural dynamics and resonance conditions

  • Control system response characteristics

  • Interactions between different components

  • Potential design issues where forces and responses are out of phase

Using the Phase Analysis Tab

The phase analysis tab provides specialized tools for examining signal phase relationships:

  1. Access the Phase Tab:

    • Click on the “Phase” tab in the main navigation

  2. Configure Analysis:

    • Select a reference signal

    • Choose one or more signals to compare against the reference

    • Set analysis parameters if available

  3. Generate Phase Plots:

    • Click “Calculate Phase” to generate the analysis

    • View both magnitude and phase plots

  4. Interpret Results:

    • Examine phase differences across frequency ranges

    • Look for 0°, 90°, or 180° phase relationships at key frequencies

    • Identify resonance points where phase shifts occur

Example: Tower-Blade Phase Relationships

Here’s a step-by-step example analyzing the phase relationship between tower and blade motions:

  1. Load Files:

    • Load your OpenFAST output files containing tower and blade measurements

  2. Setup Phase Analysis:

    • Select “TwrBsMxt” (Tower base fore-aft moment) as the reference signal

    • Select “RootMycX1” (Blade 1 root out-of-plane moment) as the comparison signal

  3. Calculate Phase:

    • Click “Calculate Phase” to generate the analysis plots

  4. Analyze Results:

    • Look for frequencies where phase crosses -90° (indicates resonance)

    • Examine the magnitude plot to see if there are peaks at these frequencies

    • Identify frequencies where the phase is near 0° (in-phase motion) or 180° (counter-phase motion)

Interpreting Phase Plots

Phase plots show how the phase angle between signals varies with frequency. Key aspects to examine:

  1. Phase Crossing -90°:

    • Indicates a resonance condition

    • The structure is most responsive at these frequencies

    • Important to check if these align with common excitation frequencies (1P, 3P)

  2. 0° Phase:

    • Signals are in phase - moving together

    • Can indicate rigid body motion or strong coupling

  3. 180° Phase:

    • Signals are in counter-phase - opposing motion

    • Can indicate certain types of mode shapes or control interactions

  4. Phase Curve Slope:

    • Steeper slopes around crossings indicate more damped systems

    • Gentler slopes may indicate less damping

Advanced Phase Analysis Techniques

For more sophisticated analysis:

  1. Multiple Signal Comparison:

    • Compare phase relationships across multiple components

    • Look for patterns in how different parts of the structure respond

  2. Operational Condition Comparison:

    • Compare phase relationships under different operating conditions

    • Identify how structural dynamics change with wind speed, control settings, etc.

  3. Combining with FFT Analysis:

    • Use FFT analysis to identify frequencies of interest

    • Then examine phase relationships at those specific frequencies

Example: Detecting Resonance Conditions

A practical example for identifying potential resonance issues:

  1. Initial Setup:

    • Load simulation results from a turbine operational analysis

    • Navigate to the Phase tab

  2. Reference Selection:

    • Select a forcing function as reference (e.g., “RotSpeed” or “WindVxi”)

    • Select structural responses to analyze (tower motions, blade moments)

  3. Analysis Focus:

    • Look particularly at frequencies near known excitation sources: * 1P (once-per-revolution) * 3P (three-times-per-revolution for three-bladed turbines) * Control frequencies

  4. Identifying Issues:

    • Look for -90° phase crossings that align with excitation frequencies

    • Check magnitude plots for amplification at these frequencies

    • Document any potential resonance conditions for further investigation

Tips for Effective Phase Analysis

  • Signal Quality: Ensure signals have sufficient resolution and length for accurate phase calculation

  • Frequency Range: Focus on the most relevant frequency range for your analysis (often 0-5 Hz for wind turbines)

  • Multiple References: Try different reference signals to understand the system from various perspectives

  • Documentation: Export plots with clear labels for inclusion in reports or presentations