1. Wind Modeling Studies by Dr. Xu at Tennessee State University
Guanpeng Xu
Tennessee State University
Center of Excellence in Information System,
Engineering & Management

2. Overview of Presentation
Wind Projects
Methodologies
Results and Conclusions

3. 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

4. Mathematical Formula
Reynolds 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.

5. 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

6. The Overset Grid Methodology
Inclusion of tower effects requires modeling non-rotating 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.

7. 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.

8. 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.

9. 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

10. OVERSET GRID
A very coarse grid was used for Proof of Concept

11. Results for the Phase II Rotor

12. Results for the Phase III Rotor

13. Results for the Phase VI Rotor
Flap Bending Moment for One Blade

14. Typical Natural 10m/s Inflow Wind

15. 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.

16. Harmonic Analysis of the Calculated Power

17. Flow Field May be Examined for Interesting Features
The Upper Surface of the Phase II Rotor at 20 m/s

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

19. Ice Shape after Half an Hour

20. Tower Shadow Causes 15% Variation in Wind Speed
10m/s
Portion of the Rotor Disk exposed to the tower wake
~8.5m/s
Code predicted this loss in dynamic pressure, but not the vortex shedding effects due to the sparse grid employed.

21. Improvement to a Tip Loss Model and a Stall Delay Model Using CFD as a Guide
Effects of Corrigan’s Model with Different values of n

22. 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.

23. 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, , , , , and are omitted here.