Wind Modeling Studies by Dr. Xu at Tennessee State University

Guanpeng Xu

Tennessee State University

Center of Excellence in Information System,

Engineering & Management

Overview of Presentation

• Wind Projects

• Methodologies

• Results and Conclusions

Wind Modeling Studies Computational Studies of Horizontal Axis Wind

Turbines Full NS Hybrid Methodology Overset Grid (CHIMERA)

2D/3D Icing Simulation 2D Icing 3D Icing

The first project is a part of my Ph.D. research. My advisor is Lakshmi Sankar at School of Aerospace Engineering,

Georgia Institute of Technology

Projects were supported by

National Renewable Energy Laboratory (NREL), DOE

Mathematical FormulaReynolds Averaged Navier-Stokes Equations in Finite Volume Representation:

Where q is the state vector. E, F, and G are the inviscid fluxes, and R, S, and T are the viscous fluxes

•A finite volume formulation using Roe’s scheme is used.

•The scheme is third order or fifth order accurate in space and second order accurate in time.

t

qdV Eˆ i Fˆ j G ˆ k n dS Rˆ i Sˆ j T ˆ k

n dSt

qdV Eˆ i Fˆ j G ˆ k n dS Rˆ i Sˆ j T ˆ k

n dS

The Hybrid Methodology The flow field is made of

– a viscous region near the blade(s)– A potential flow region that

propagates the blade circulation and thickness effects to the far field

– A Lagrangean representation of the tip vortex, and concentrated vorticity shed from nearby bluff bodies such as the tower

This method is unsteady, compressible, and does not have singularities near separation lines

N-S zone

Potential Flow Zone

Tip Vortex

The Overset Grid Methodology

– Body-fitted grids are used for rotating blades and tower.

– Each grid block is simulated using either a Navier-Stokes or hybrid method.

– The flow fields among the grid sets are linked by 3-D interpolation.

•Inclusion of tower effects requires modeling non-rotating and rotating components.

•Georgia Tech CHIMERA methodology has been modified for tower shadow effects of HAWT :

The Icing Simulation•Porous ice with liquid water content and air/vapor is assumed.

•The flow field and icing/melting are calculated using a modular approach.

•Grid is deformed with on-the-fly ice shape; NS solver is used for outer flow.

Configuration Studied

NREL has collected extensive performance data for three rotor configurations:– A rotor with rectangular planform, untwisted blade and S-809

airfoil sections, called the Phase II Rotor

– A twisted rotor, with rectangular platform and S-809

sections, called the Phase III Rotor

– A two bladed, tapered and twisted rotor, called the Phase VI

Rotor. Best quality measurements (wind tunnel) are

available.

Results and Discussion

Body fitted grid on Phase II rotor

•Size

11043402(380,000)•Viscous zone 6043202

(100,000)

--Sample Grid

OVERSET GRID

A very coarse grid was used for Proof of Concept

-10

-5

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0 5 10 15 20 25Wind Speeds[m/s]

Gen

erat

or

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wer

[kw

]

NREL experimentN-S SolverHybrid CodeLifting Line results

Results for the Phase II Rotor

Results for the Phase III Rotor

Results for the Phase VI Rotor

Upwind Configuration, Zero Yaw

0

1000

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3000

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5000

5 10 15 20 25 30

Wind Speed (m/s)

Ro

ot

Fla

p B

end

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Mo

men

t (N

m)

NREL

Present Methodologies

Flap Bending Moment for One Blade

8.0

8.5

9.0

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10.5

11.0

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0 2 4 6 8 10 12 14 16

Time(sec)

Me

asu

red

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d S

pe

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Inflow wind(3)inflow wind(4)

Typical Natural 10m/s Inflow Wind

Measured Power v.s. Time at 20 degree Yaw

•Average values well predicted

•Higher harmonics are not captured well, because we only model the first harmonic of the wind.

Wave Number Analysis for 10m/s Wind -20 deg Yaw

0

0.01

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Wave Number

Am

pli

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Harmonic Analysis of the Calculated Power

The Upper Surface of the Phase II Rotor at 20 m/s

Flow Field May be Examined for Interesting Features

Streamlines at a Typical Span Station of Phase II rotor at 20m/s

Ice Shape after Half an Hour

Tower Shadow Causes 15% Variation in Wind Speed

Portion of the Rotor Disk exposed to the tower wake

10m/s ~8.5m/s

•Code predicted this loss in dynamic pressure, but not the vortex shedding effects due to the sparse grid employed.

Improvement to a Tip Loss Model and a Stall Delay Model Using CFD as a Guide

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500

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wind speed

torq

ue

strip; no tip loss;no stall delayNREL NASA AmesNew Tip Loss ModelCorrigan Model; n = 1Corrigan Model; n = 1.85

Effects of Corrigan’s Model with Different values of n

Conclusions• The Hybrid method, which solves the HAWT flow using a zonal approach, has been developed for efficiently simulating fully three-dimensional viscous fluid flow around an HAWT. Good results have been obtained.

• A full Navier-Stokes methodology has also been developed. Two turbulence models and two transition prediction models have been integrated into above solvers. Consistent results have been obtained for above two solvers. An overset grid based version that can model rotor-tower interactions has been developed.

Conclusions

• The physics studied includes turbulence models, transition prediction models, yaw (unsteady) simulation, tower shadow, wind turbine flow states, stall delay, and tip losses.

• The complete research activities have been documented in Guanpeng Xu’s doctoral thesis, Journal of Solar Energy Engineering, and in AIAA papers, 1999-0042, 2000-0048, 2001-0682, 2001-7796, and are omitted here.