NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

FAST and AeroDyn Enhancements

Sandia Blade Workshop

Khanh Nguyen

May 30-June 1, 2012

2

Outline

• FAST Structural Modelo Motivationo Current FAST modelo Structural model enhancement

• AeroDyno AeroDyn Overhaul o BEMT numerical solutionso Generalized dynamic wake modelo Dynamic stall model

• Conclusions

3

CAE ToolsFAST-AeroDyn-HydroDyn Coupling

CAE: Computer-Aided Engineering

4

Motivation

Trends in blade design:o Future blades tend to be longer and more flexibleo Blade torsion important in aeroelastic responseso Aeroelastic coupling designs to reduce loads

– Material coupling (anisotropic beam)– Geometric coupling (sweep, pre-bend)

o Advanced controls with blade mounted actuatorso Aeroelastic stability could be a design driver

5

Current FAST

FAST is an aero-hydro-servo-elastic code for HAWT Key features

o FAST formulation based on multi-body dynamics with elastic elements represented by structural modes

o Controls implementation with subroutines, DLLs, or Simulink®

o Coupled to AeroDyn for aeroelastic and HydroDyn for hydro-elastic simulations

o Land-based or sea-based with monopiles or floating platforms

Current blade structural modelo Modal representation of straight blade with isotropic materialso No axial or torsion deflections, coupled flap and edge modes

– Include radial shortening, centrifugal, Coriolis, gyroscopic effects o Modes imported from external source, such as BModeso No blade-pitch actuator dynamics

6

Structural Model Enhancement

• Geometric Exact Beam (Hodges 2006)o Timoshenko beamo Nonlinear, curved, anisotropic beam modelo Developed with VABS (Variational Asymptotic Beam Section

Analysis), a cross-sectional analysis• GEBT (Geometric Exact Beam Theory)

o Open-source software developed by Prof. Yu based on Hodges’ geometric exact beam theory

o Mixed formulation based on Hamilton’s extended principleo Implemented with linear finite elements

• GEBT is planned for FAST structural enhancemento FEMo Modal representation for efficient computation

7

AeroDyn - Overhaul

• Overhaul plan based ono AeroDyn Overhaul Meeting 2008o Code reviews

– Leishman– Peters

• v13.00.01a-bjj, released February 2012o Improved interfaces with FAST and TurbSimo Added ability to read-in HAWC wind format (Mann turbulence)o Several minor changes and bug fixeso Initiate code modularization

8

BEMTNumerical Solutions

• Blade element momentum theory provides quasi-static solutions to axial and tangential inductions a, a’

• Current BEMT solutions, based on fixed-point iteration, not robust

• Use relaxation improves convergenceai+1 = f*ai + (1-f)*F(ai)a’i+1 = g*a’i + (1-g)*G(a’i)where f, g are relaxation factors ∈ [0,1)

o Number of iterations can be high • Using Brent’s method reduces

computing time and eliminates convergence problems

0 2 4 6 8 10 12 140

2

4

6

8

10

12

14

16

18

20

Tip Speed Ratio

Bla

de P

itch,

deg

Non-convergent cases (f, g=0)

0 2 4 6 8 10 12 14 16 180

10

20

30

40

50

60

70

80

Blade Station Index

No.

of I

tera

tion

Fixed PointBrent

781

9

Generalized Dynamic Wake

• GDW models inflow dynamics based on acceleration potential of actuator disk (Peters & He)

• AeroDyn review by Prof. Peters:o Possible to include tangential induction directlyo Identify source of low speed instability, associated with

transition through vortex ring stateo Include yaw rate effects with skew wake

Burton et al. (2001)

10

Dynamic Stall

• Dynamic stall is characterized by stall delays and large lift and moment overshoots over static values

• AeroDyn includes a modified version of Leishman modelo Main difference is the modeling of static airfoil behaviorso Need to validate modified version with original model

• Leishman dynamic stall model (below) implemented based on Gupta & Leishman Wind Energy 2006 paper

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

10 15 20 25 30Angle of Attach, deg

CL

OSU DataModel

α = 20 + 5sinωt, k=0.07, Re=106

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

010 15 20 25 30

Angle of Attach, deg

CM

OSU DataModel

11

AeroDynCurrent & Planned Work

• BEMTo Update numerical solution with a robust root finding method

• Dynamic Stallo Validate AeroDyn implementation with original modelo Revise algorithms per recommendation of Leishman

• GDWo Revise algorithm per Peters’ recommendationo Include tangential inductiono Resolve problems with low speed instabilityo Include yaw rate effects

• Wakeso Include vortex wake methods (prescribed and free wakes)o Add a Dynamic Wake Meandering model (with UMass & DTU Wind)

• General:o Improve modularization:

– Create separate modules for wind inflow, airfoil aerodynamics, and inductiono Interface AeroDyn with the ECN AWSM free-wake vortex codeo Interface AeroDyn with the DTU Wind HAWC2 aero module

12

Conclusions

• FASTo Progress under way to implement new blade

structural model for FAST

• AeroDyno Improved interfaces with FAST and TurbSimo Modularize codeo Improve BEMT numerical solutions o Verify dynamic stall modelo Identify enhancements to GDW

14

OverhaulReasons for Overhaul

• Trouble developing, maintaining, and using AeroDyn• Common request from users• Desire to have improved:

o Functionalityo Usabilityo Code readability

• Eliminate problems• Make it easier to include additional aerodynamic theories• Develop a standardized & streamlined interface to structural

dynamic analysis programs• Important because proper aerodynamic modeling is critical for

accurate performance, loads, & stability analyses

15

OverhaulRecent Work (Wind Inflow Module in v13.00.00)

• All wind-inflow routines & variables are contained in a separate module with clear interface

• Can read TurbSim’s binary full-field “.bts” & tower “.twr” files• Full-field wind files are relative to the ground, not the turbine

hub-height• Can be used outside of AeroDyn, e.g. the module has been

made into a MATLAB mex function (allows easy access to the wind file data)

16

Extra

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.1

0.15

0.2

0.25

0.3

0.35

Blade Radius

Axi

al In

duct

ion

BrentFixed Point

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.01

0.02

0.03

0.04

0.05

0.06

Blade Radius

Tang

entia

l Ind

uctio

n

BrentFixed Point

• Compare numerical solutions of a, a’ using Brent’s method and fixed-point iteration