Session: 08-25-01: Flow-Induced Vibrations of Energy Systems

Paper Number: 164890

Comparison of 2D Hansen Model and OpenFAST to Analyze Classical Flutter Instability of Wind Turbine Blades 

Wind turbine blades have drastically grown in size in recent decades, becoming longer and increasingly slender with each new turbine to be built. In the last 20 years, especially due to the extensive development of offshore farms, the rotor diameter of wind turbines has nearly doubled. As offshore wind (OSW) turbines continue to exist in regions with variable wind patterns, understanding potential aeroelastic instabilities–namely dynamic stall and classical flutter–has been necessary in the design of wind turbine blades. Nonlinear analysis packages such as OpenFAST and HAWCStab2 are capable of predicting these aeroelastic instabilities for different turbines under various operating and extreme wind conditions. However, these complex models require longer computational time and setup. Meanwhile, the Hansen model, as first suggested by M.H. Hansen in 2004, utilizes a 2D, simplified eigenvalue model for evaluating the critical flutter speeds of wind turbine blades. The Hansen model serves as the basis for HAWCStab, the predecessor of HAWCStab2. Although comparisons have been conducted relating the aeroelastic stability calculations of OpenFAST and HAWCStab2, little work has been done to assess the adequacy of simplified 2D models such as Hansen’s for identifying classical flutter conditions for recent, larger turbine blades such as the IEA 15 MW reference turbine (rotor diameter ~117m). The 2D eigenvalue model, in cases of early-stage preliminary design, could be useful in identifying aeroelastic instability problems. In this paper, we compare the classical flutter instability results for the IEA 15 MW turbine in both OpenFAST and with Hansen’s 2D model. In understanding these results,  we assess the applicability of Hansen’s model for early-stage analysis of aeroelastic instabilities for OSW turbines, highlighting its usefulness and limitations for future designs.

The project is conducted by operating OpenFAST in its “linearization” mode to compare flutter speed results to the simplified Hansen approach. Other recent papers have utilized OpenFAST to analyze flutter on the IEA 15 MW turbine; therefore, a similar approach to one of those studies is used. MATLAB is employed to replicate the approach outlined in Hansen’s 2004 and 2007 papers to create a linear 2D eigenvalue model. Operating conditions as well as extreme wind speeds (>25 m/s) are examined based on IEC DLC standards. The results that are compared are the flutter frequencies and critical flutter speeds.

The preliminary results of this study indicate the validity of simplified models for future generations of offshore wind turbines. Demonstrating the perennial applicability of 2D models to reliably approximate flutter conditions can reduce computational demand in the early design phase of offshore wind turbines. Hansen’s Model can provide adequate estimations for flutter conditions in large-scale turbines, but limitations are exhibited for more complex aeroelastic systems.

Presenting Author: Max Raha Northeastern University

Presenting Author Biography: Max Raha is currently a PhD student at Northeastern University advised by Dr. Andrew T. Myers and Dr. Nathan Post. He has a B.S. in Civil Engineering from the University of Connecticut. His research focuses on structural reliability of offshore wind turbine blades as well as their response to tropical and non-tropical storms.

Authors:

Max Raha Northeastern University
Andrew T. Myers Northeastern University
Nathan L. Post The Roux Institute, Northeastern University
Luca Caracoglia Northeastern University