OBJECTIVE(S):

1
School of Aerospace Engineering Georgia Institute
of Technology
Computational Studies of Horizontal Axis Wind
Turbines PRINCIPAL INVESTIGATOR Lakshmi N.
Sankar NREL/SNL TECHNICAL MONITORS Alan Laxson
(NREL), Scott Schreck (NREL), Walter
Wolfe (SNL)

• OBJECTIVE(S)
• Develop a first-principles based methodology for
  the prediction of horizontal axis wind turbine
  performance.
• Use the methodology to study the effects of tower
  shadow, atmospheric turbulence and yaw angle on
  rotor blade loads.
• Reduce the computational cost of modeling the 3-D
  viscous flow field, through the use of
  phenomenological models.

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SCHEDULE AND STATUS

• This is a three year effort, spanning the period
  May 1997 - May 2000.
• Year 1 Goal
• Develop and validate a first-principles based
  method for the prediction of horizontal axis wind
  turbine aerodynamics.
• Status Completed.
• Year 2 Goal
• Incorporate Atmospheric boundary layer effects,
  boundary layer transition models, 1-equation
  turbulence models, and validate against available
  data.
• Status Completed. Results to be presented today.
• Year 3 Goal
• Incorporate tower effects Examine existing stall
  models and tip loss models in light of computed
  data Make computer codes available to interested
  researchers and industries.

3
BUDGET

• Year 1 56, 805
• Covers 200 hours of P. I. Time and a graduate
  student.
• Year 2 59,267
• Year 3 61,883

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TECHNICAL RESULTS

• Outline of the present methodology
• Recap of Results for Phase II and Phase III
  rotors
• Transition Model Studies
• Effects of Yaw on Rotor Loads and Power Generation

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PRESENT HYBRID METHODLOGY

• The flow field is made of
• a viscous region near the blade(s)
• A potential flow region that propagates the blade
  lift 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.
• Method is unsteady, compressible, and does not
  have singularities near separation lines.
• Method described in AIAA Journal of Aircraft,
  Vol. 34, No.5, 1997, pp. 635-640.

N-S zone
Potential Flow Zone
Tip Vortex
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SAMPLE GRID

• A fully automated grid algebraic generation
  procedure has been developed.
• User only needs to specify the airfoil shape and
  twist distribution at a few radial locations.
• The grid generator automatically divides the
  zones into Navier-Stokes and Viscous Zones, based
  on user input.

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SAMPLE RESULTS - Phase III Rotor
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Sample results - Phase II Rotor
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The hybrid code rapidly converges to steady
state when one exists(19 seconds/iteration on a
HP Model 750 Workstation)
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Transition Models

• Transition to turbulent boundary layers can have
  a dramatic effect on the flow over the rotor, and
  power generation.
• A number of Engineering models are available in
  literature.
• These models were developed from 2-D steady flow
  applications, and may be applied in 3-D flows
  using a strip theory approximation.
• Two transition models were incorporated into the
  hybrid code
• Eppler Model, Michels Model

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Epplers Model
This model is in wide use in many of the airfoil
analyses and design codes used by the wind
turbine industry. Transition is said to occur
when laminar flow separates, or when
Roughness Factor
Ratio of energy thickness over momentum thickness
Reynolds number based on momentum thickness
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Michels Model
This model is in wide use in fixed wing aircraft
industry.
Reynolds No based on momentum thickness
Reynolds Number based on distance from leading
edgeu? x/n
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Transition Line on the Rotor Upper SurfacePhase
III Rotor, 6 m/s wind
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Transition Line on the Rotor Lower SurfacePhase
III Rotor, 6 m/s wind
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Transition line on the Upper SurfacePhase III
Rotor, 8m/s Wind
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Conclusions on Transition Model Study

• On both the upper and the lower surface, Epplers
  model predicts a transition location that is
  either comparable to, or upstream of Michels
  predictions.
• On the lower surface, the pressure gradients are
  favorable. This leads to a thinner boundary
  layer. Both these criteria predict that
  transition will occur aft of the corresponding
  upper surface locations.
• Transition line location appears insensitive to
  the turbulence model used, except at inboard
  stations.

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Modeling Inflow Turbulence and Yaw Effects

• Present Methodology has been modified to account
  for inflow turbulence and yaw on the rotor loads
  and power generation.
• A steady cross flow, a boundary layer profile, or
  an unsteady freestream condition may be
  prescribed, with minor change to the present
  code.
• This information impacts the outer flow
  (potential) field
• far away from the rotor as follows

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Typical Wind Conditions for the Phase IV Rotor
(NREL Database)
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(No Transcript)
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Variation of Computed Power over an Entire
Revolution10 m/s, 20 Degree Yaw
A 3 per rev variation was dominant. Caused by the
120 degree phase difference between the three
blades.
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CONCLUSIONS

• A first-principles based methodology for
  Predicting Power Generation by HAWT rotors has
  been proposed and validated.
• Two turbulence models (Baldwin-Lomax,
  Spalart-Allmaras) and two transition models have
  been implemented.
• A formulation is in place for modeling yaw
  effects and inflow turbulence. The code correctly
  predicts the expected 3/rev variation in power.
• The computed power generation in the presence of
  yaw effects is in overall agreement with measured
  data.

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FUTURE PLANS

• Tower effects will be modeled using an overset
  grid methodology, where the tower, nacelle and
  the rotating blades are modeled on separate
  grids.
• The existing theories for static and dynamic
  stall delay, and for tip loss effects will be
  examined in light of computed data.
• Additional simulations for the NREL Phase IV
  Rotor under yaw conditions will be done and
  compared with NREL data.
• Results for these calculations will be presented
  this time next year.