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 Formula
Reynolds Averaged Navier-Stokes Equations in Finite Volume
Representation:



ˆ
ˆ
ˆ
ˆ
ˆ
ˆ
qdV   Ei  Fj  Gk  ndS   Ri  Sj  Tk  n dS

t




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.
The Hybrid Methodology
N-S zone
Potential Flow
Zone

Tip Vortex

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
The Overset Grid Methodology
•Inclusion of tower effects requires modeling nonrotating and rotating components.
•Georgia Tech CHIMERA methodology has been
modified for tower shadow effects of HAWT :
–
–
–
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.
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
--Sample
Grid
•Size 11043402(380,000)
•Viscous zone 6043202
(100,000)
Body fitted grid on Phase II rotor
OVERSET GRID
A very coarse grid was used for Proof of Concept
Results for the Phase II Rotor
20
Generator Power[kw]
15
10
5
0
0
-5
-10
NREL experiment
N-S Solver
Hybrid Code
Lifting Line results
5
10
15
Wind Speeds[m/s]
20
25
Results for the Phase III Rotor
Results for the Phase VI Rotor
Upwind Configuration, Zero Yaw
Root Flap Bending Moment (Nm)
5000
4000
3000
2000
NREL
Present Methodologies
1000
0
5
10
15
20
25
Wind Speed (m/s)
Flap Bending Moment for One Blade
30
Typical Natural 10m/s Inflow Wind
11.5
Inflow wind(3)
inflow wind(4)
Measured Wind Speed
11.0
10.5
10.0
9.5
9.0
8.5
8.0
0
2
4
6
8
Time(sec)
10
12
14
16
Measured Power v.s. Time
at 20 degree Yaw
Phase IV Measured Power Vs. Time at 20 degree Yaw
(NREL data for a typical wind condition)
•Average values
well predicted
8.5
8.0
•Higher harmonics
are not captured
well, because we
only model the
first harmonic of
the wind.
Power
7.5
7.0
Present Calculations 10 m/s
6.5
Average Wind Speed 10.1m/s
6.0
0
45
90
135
180
t
225
270
315
360
Harmonic Analysis of the
Calculated Power
Wave Number Analysis for 10m/s Wind -20 deg Yaw
0.09
0.08
0.07
Amplitude
0.06
0.05
0.04
0.03
0.02
0.01
0
0
5
10
15
Wave Number
20
25
30
Flow Field May be Examined for
Interesting Features
The Upper Surface of the Phase II Rotor at 20 m/s
Streamlines at a Typical Span
Station of Phase II rotor at 20m/s
Ice Shape after Half an Hour
Ice Shape with smoothing
(AOA= 5deg, Re=10^6, LWC=1.5, T ST OP= 200s, T ot simulation time = 1800sec)
Y/C
0.20
After 200sec
After 1800sec
0.00
-0.5
-0.20
0
0.5
X/C
1
1.5
Tower Shadow Causes 15%
Variation in Wind Speed
10m/s
Portion of the Rotor
~8.5m/s Disk exposed to the
tower wake
•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
2000
strip; no tip loss;no stall delay
NREL NASA Ames
New Tip Loss Model
Corrigan Model; n = 1
Corrigan Model; n = 1.85
torque
1500
1000
500
0
0
5
10
15
20
25
wind speed
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, 20017796, and are omitted here.