Evan Gaertner University of Massachusetts, Amherst IGERT Seminar Series October 1st, 2015 Floating Offshore Wind Turbine Aerodynamics and Optimization Opportunities

2 Agenda  Floating Wind Turbine Aerodynamics  Dynamics Stall  Design Optimization

3 Floating Offshore Wind Turbines Advantages:  Access to deeper water More useable area Further from onshore lines of site Reduce impact to important near shore habitats  Simplified installation Tow-out installation Reduce environmental impacts from pile driving

4 Platform Motion  Wind and wave loading  Non-rigid mooring system  Complex platform motion 6 transitional and rotational Degrees of Freedom  Adverse Affects: Increased aerodynamic complexity Stronger cyclical loading Requires more sophisticated controls

5 Velocity from Platform Motion Skewed flow  From pitch or yaw  Blade moves Toward wind: increased velocity Away from wind: decreased velocity  Occurs at rotational frequency Wake interaction  From pitch or surge  Rotor moves through its own wake  Can causes flow reversals and turbulence  Occurs at platform motion frequency

6 Wake Induced Dynamic Simulator (WInDS)  A free-vortex wake method Developed to model rotor-scale unsteady aerodynamics  By superposition, local velocities are calculated from different modes of forcing  Previously neglected blade section level, unsteady viscous effects [2]

Blade Scale Unsteadiness

8 Quasi-Steady Aerodynamics  Aerodynamic properties of airfoils determined experimentally in wind tunnels  Lift increases linearly with angle of attack ( α )  At a critical angle, flow separates and lift drops “Stall”  WInDS used quasi-steady data

9 Dynamic Stall

10 Dynamic Stall Flow Morphology Stage 1Stage 2Stage 2-3Stage 3-4Stage 5 [3] Lift Coef, C L Drag Coef, C D Moment Coef, C M Angle of Attack, α (°)

11 Modeling Dynamic Stall: Leishman-Beddoes (LB) Model  Semi-empirical method Use simplified physical representations Augmented with empirical data  Model Benefits Commonly used, well documented Minimal experimental coefficients Computationally efficient [3]

12 Example 2D LB validation: S809 Airfoil, k = 0.077, Re = 1.0×10 6 LB model validated against 2D pitch oscillation data

13 WInDS-FAST Integration  WInDS was originally written as a standalone model in Matlab Decouples structural motion and the aerodynamics  Integrated into FAST v8 by modifying the aerodynamic model, AeroDyn Fully captures the effects of aerodynamics and hydrodynamics on platform motions  changes the resulting aerodynamics OC3/Hywind Spar Buoy

Design Optimization

15 Rotor Design Design Process  Start with known optimal blade shape  Modify for practical structural and manufacturing concerns Problem  Uses ideal conditions for aerodynamic analysis: uniform, steady, non-skewed flow Typical optimization projects in the literation:  More sophisticated models  More design variabl es

16 Research Goal  Inform design process with realistic probability distributions of steady and unsteady condition Operating conditions are never ideal!  Include minimization of load variability as a design goal

17 Integrated Design of Offshore Wind Turbines Process:  Sequential design of subsystems Problem:  Optimized subsystems  Sub-optimal global system Solution:  Multi-objective, multi- disciplinary, iterative optimization Turbine Design Platform Design Controls

18 Interdisciplinary Opportunities Additional design goals could include:  Lower tip speed ratios Reduce risk of bird strikes  Larger turbine rotors Allow smaller wind farms with fewer seafloor disturbances  Optimization for deeper waters farther from shore Reduce competition for use or view-shed concerns Open to suggestions for other interdisciplinary objects!

Questions? Evan Gaertner This work was supported in part by the NSF-sponsored IGERT: Offshore Wind Energy Engineering, Environmental Science, and Policy and by the Edwin V. Sisson Doctoral Fellowship Thank You!

Supplemental Slides

21 Span-wise Unsteadiness  AoA predominately varying cyclically with rotor rotation, driven by: Mean platform pitch: ~4-5° Rotor shaft tilt: 5°

22 Dynamic Stall