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

Paper Number: 165498

Influence of Lift Force’s Span-Wise Correlation and Reynolds Number on the Stochastic, Flow-Induced Instability of Wind Turbine Blades 

Flow-induced stability of wind turbine blades is relevant to the design of wind turbine structures. Critical flutter rotor speed can be an issue during operational stages for very long blades, such as the ones used in offshore wind turbines. From a reliability standpoint, flutter probability can be of the order of 1E-3, i.e., a potentially concerning issue. Consequently, it is important to examine the non-deterministic flutter problem.

This study continues recent numerical investigations, using a numerical reduced-order model of a wind turbine blade. The model utilizes stochastic calculus methods through implementation of the theory of (largest) Moment Lyapunov Exponent to study flow-induced instability of the blades. The formulation is inspired by an analogous model, used for rotorcraft dynamic stability under inflow turbulence perturbations. Inflow turbulence is simulated by (inverse Fourier) transforming a rotationally sampled turbulence spectrum to the time domain. The load correlation effects are simulated by introducing a reduced-magnitude turbulence spectrum, which accounts for the loss of spanwise correlation of the turbulence; the concept is inspired by “the equivalent wind spectrum technique”. The Reynolds number effects are reproduced by suitably varying the slope of the sectional aerodynamic lift coefficient curve as a function of mean angle of attack. Variations of the slope are deduced from typical experimental (literature) results, over a wide range of Reynolds number, and used within the aeroelastic load formulation.

The final, dynamical system is assembled as a stochastic differential equation that depends on two scalar Wiener processes. The state vector includes flapwise and torsional modal responses (generalized variables), aeroelastic states, rotationally sampled turbulence and a Gaussian variable that simulates various error sources in the aeroelastic loads. The coupled-mode flutter of the NREL (National Renewable Energy Laboratory) 5-MW wind turbine blade with rotor radius 61.5 m is studied; the third flapwise and first torsional modes are considered. The solution is found numerically by numerical integration of the equations.

Inspection of the simulations confirms that, since the stochastic problem is non-linear, it is amplitude dependent. Amplitude effects are investigated by randomly varying the magnitude of the generalized modal coordinates at the initial time. The introduction of the Gaussian modeling error (alone) tends to delay the de-stabilizing aeroelastic effects. First, numerical results will be presented to first examine the influence of turbulence and spanwise correlation on the loads. Second, the simulations will be expanded to consider the concurrent effect of partially correlated turbulence and Reynolds number on the onset of flow-induced instability.

This work was supported in part by the USA National Science Foundation, grant CMMI-2421490, collaborative research between the University of Massachusetts – Amherst and Northeastern University.

Presenting Author: Luca Caracoglia Northeastern University

Presenting Author Biography: Luca Caracoglia is currently a Full Professor in the Department of Civil and Environmental Engineering of Northeastern University, Boston, Massachusetts, USA. Luca Caracoglia’s research and professional interests are in structural dynamics, random vibration, wind engineering, fluid-structure interaction of civil engineering structures, nonlinear cable network dynamics, wind energy and wind-based energy harvesting systems. He has been author/co-author of 100+ peer-reviewed journal publications and book chapters (published or in press) and more than 140 conference proceedings / presentations. He has taught courses at the undergraduate and graduate levels in: Statics/Solid Mechanics, Structural Analysis, Steel Structure Design, Pre-stressed Concrete, Bridge Design, Wind Engineering and Wind Energy Systems.

Authors:

Luca Caracoglia Northeastern University