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A Computational Study of the Aerodynamics and Aeroacoustics of a Flatback Airfoil Using Hybrid RANS-LES. Christopher Stone Computational Science and Engineering Matthew Barone Sandia National Laboratories C. Eric Lynch and Marilyn J. Smith Georgia Institute of Technology.

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A Computational Study of the Aerodynamics andAeroacoustics of a Flatback Airfoil Using HybridRANS-LES

Christopher Stone

Computational Science and Engineering

Matthew Barone

Sandia National Laboratories

C. Eric Lynch and Marilyn J. Smith

Georgia Institute of Technology

ASME Wind Energy Symposium

Orlando, FL

5 January 2009


Outline
Outline

  • Motivation

    • Flatback airfoils

    • Goals of the present CFD analysis of flatback airfoils

  • Method

    • CFD codes and models

    • Aeroacoustic prediction methods

  • Results

    • Aerodynamic performance

    • Vortex-shedding noise

    • Wake visualization and interpretation

  • Conclusions

2009 ASME Wind Energy Symposium


Flatback airfoils background
Flatback Airfoils: Background

  • Flatback airfoil shapes have been proposed for the inboard region of wind turbine blades.

    • Thickness is added about a given camber line – different from “truncated” airfoil.

  • Benefits

    • Structural benefit of larger sectional area and larger moment of inertia for a given maximum thickness.

    • Aerodynamic benefits of larger sectional Clmax , larger lift curve slope, and reduced aerodynamic sensitivity to leading edge soiling.

  • Drawbacks

    • Increased drag due to separated base flow.

    • Introduction of an aerodynamic noise source due to trailing edge vortex shedding.

References: C.P. van Dam et al., SAND 2008-2008, SAND 2008-1782, J. Solar Energy Eng., 128:422, 2006 .

2009 ASME Wind Energy Symposium


Overview of available cfd methods
Overview of Available CFD Methods

  • The “industry-standard” method for high-Reynolds number CFD is Reynolds-Averaged Navier-Stokes (RANS)

    • Turbulent fluid flow is modeled using a turbulence model

    • Typically a steady-state solution is found

    • Necessary for affordable calculation of thin turbulent boundary layers

  • Drawbacks of RANS

    • Inaccurate for massively separated flow (flatback wake)

    • No prediction of unsteady turbulent fluctuations (flatback noise sources)

  • Large Eddy Simulation (LES)

    • Directly computes the large scale features of a turbulent flow

    • Models the small-scale, or “sub-grid” features of turbulence

  • Hybrid RANS/LES

    • Combines RANS in the near-wall turbulent boundary layer region with LES in regions of separated flow

2009 ASME Wind Energy Symposium


Goals and approach
Goals and Approach

  • Goal: Assess the ability of hybrid RAN/LES methods to:

    • Predict flatback airfoil lift and drag

    • Simulate vortex-shedding in the wake for aeroacoustic predictions.

  • Approach

    • Use multiple CFD codes with hybrid RANS/LES capability

    • Use measured flatback airfoil shape from Virginia Tech wind tunnel experiment

    • Run simulations independently on different meshes and using different RANS/LES models (not a rigorous code-to-code comparison)

    • Compare two different methods for aeroacoustic predictions

2009 ASME Wind Energy Symposium


Flatback airfoil definition
Flatback Airfoil Definition

  • TU-Delft DU97-W-300, 30% thick, 1.5% chord trailing edge thickness

  • DU97-flatback: 10% chord base thickness

  • DU97-flatback with splitter plate

2009 ASME Wind Energy Symposium


Cfd codes
CFD Codes

  • Multi-block, structured grid finite volume code

  • Stable low-dissipation finite volume scheme

  • 2nd order implicit time advancement

  • Spalart-Allmaras RANS model

  • Detached Eddy Simulation hybrid model

OVERFLOW

SACCARA

FUN3D

  • Overset grid finite difference code

  • 4th order finite difference scheme

  • 2nd order implicit time advancement

  • Spalart-Allmaras, SST RANS models

  • GT-HRLES hybrid model

  • Unstructured grid finite volume code

  • Node-centered finite volume scheme

  • 2nd order implicit time advancement

  • SST RANS model

  • GT-HRLES hybrid model

2009 ASME Wind Energy Symposium


Computational meshes
Computational Meshes

OVERFLOW

  • 3D Domain: Span-wise extent/number of grid points

    • OVERFLOW: 0.5c with 33 grid points

    • SACCARA: 0.2c, 0.4c, 0.8c with 64 grid cells

    • FUN3D: two-dimensional only

    • Periodic boundary conditions in spanwise direction

SACCARA

FUN3D

2009 ASME Wind Energy Symposium


Flatback wake visualizations
Flatback Wake Visualizations

3D GT-HRLES

2D DES

2009 ASME Wind Energy Symposium


Aerodynamic Results

  • Chord Reynolds number = 3 million

  • Angle of attack = 4 and 10 degrees

  • Boundary layer transition at x/c = 0.05/0.10 on upper/lower surfaces

2009 ASME Wind Energy Symposium


Time averaged du97 flatback lift
Time-averaged DU97-flatback Lift

  • AOA = 4 deg., All simulations: 0.84 < CL < 0.91

    • Experimental CL = 0.81 ± 0.04

  • AOA = 10 deg., All simulations: 1.56 < CL < 1.66

    • Experimental CL = 1.57 ± 0.07

  • Decent agreement between simulation and experiment

  • Time-averaged lift relatively insensitive to code, grid, spanwise domain size, turbulence model

2009 ASME Wind Energy Symposium


Lift fluctuation spectrum
Lift Fluctuation Spectrum

  • Experimental acoustic Strouhal number, : St = 0.24 ± 0.01

  • All simulation results within the range 0.18 < St < 0.23

  • 3D hybrid RANS/LES results within the range 0.18 < St < 0.20

  • Overflow splitter plate simulation: Strouhal number shifted from 0.20 to 0.28

    • Experimental acoustic Strouhal number shifted from 0.24 to 0.30

Overflow, AOA = 4 deg.

2009 ASME Wind Energy Symposium


Time averaged du97 flatback drag
Time-averaged DU97-flatback Drag

  • Experimental free transition CD = 0.06 ± 0.005

  • Steady RANS: 0.034 < CD < 0.047

  • Unsteady 2D RANS: 0.052 < CD < 0.144

  • 3D Hybrid RANS/LES: 0.056 < CD < 0.110

  • Results for drag are sensitive to: code, turbulence model, grid, spanwise domain size

  • Best agreement: FUN3D unsteady RANS (SST model) and Overflow GT-HRLES, CD = 0.055 – 0.056

2009 ASME Wind Energy Symposium


Drag from 3d hybrid rans les
Drag from 3D Hybrid RANS/LES

0.4c/64

0.2c/64

0.4c/64

0.8c/64

0.5c/33

0.5c/33

2009 ASME Wind Energy Symposium


Effect of splitter plate overflow results
Effect of Splitter Plate (Overflow results)

  • Splitter plate lengthens wake recirculation zone and decreases intensity of the turbulent Reynolds stress

  • Drag is reduced by 18-27 %, compared to 45-50% reduction observed in experiment

2009 ASME Wind Energy Symposium


Aeroacoustic Results

2009 ASME Wind Energy Symposium


Aeroacoustic prediction methods
Aeroacoustic Prediction Methods

1. Approximate vortex-shedding theory assuming a compact line source imperfectly correlated along the span.

  • Computes far-field noise using integration of airfoil surface pressures

Sectional lift coefficient spanwise correlation:

Correlation length

Centroid of correlation area

L

L

2. PSU WOP-WOP Aeroacoustic Prediction Tool (courtesy of James Erwin, Penn State)

2009 ASME Wind Energy Symposium


Peak vortex shedding noise
Peak Vortex-Shedding Noise

Simulation results were for tripped b.l.

2D

2D

0.2c/64,

0.4c/64

0.4c/64

0.5c/33

0.8c/64

2009 ASME Wind Energy Symposium


Wake visualization saccara aoa 4 deg
Wake VisualizationSACCARA, AOA=4 deg.

Z=0.4c, Nz=64

CD = 0.11, SPL = 98.6

Z=0.8c, Nz=64

CD = 0.084, SPL = 89.9

Z=0.8c, Nz=128

2009 ASME Wind Energy Symposium


Wake visualization overflow aoa 10 deg
Wake VisualizationOVERFLOW, AOA=10 deg.

2009 ASME Wind Energy Symposium


Conclusions
Conclusions

  • Time-averaged lift insensitive to code, grid, turbulence model, 2D/3D and agreement with experiment is decent

  • Strouhal number is relatively insensitive to the above parameters and is consistently lower than experimental values

  • Overflow simulations predicted qualitative effects of splitter plate: reduction in drag, reduction in noise, increase in shedding frequency

  • Time-averaged drag and noise is sensitive to the above parameters

  • 3D simulations exhibit 2 different vortex-shedding behaviors depending on spanwise domain size and grid resolution

    • well-defined 2D rollers with streamwise braid vortices : higher drag and louder tone

    • distorted 2D rollers breaking down into randomly oriented vortices : lower drag and quieter tone

  • More domain size and resolution studies are needed

2009 ASME Wind Energy Symposium


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