1. UPWIND, Aerodynamics and aero-elasticity
Rotor aerodynamics in atmospheric shear flow
N. N. Sørensen1+2 and J. Johansen2
1Department of Wind Energy, Risø National Laboratory, DTU, Denmark
2 Department of Civil Engineering, Aalborg University, Denmark

2. Introduction
The basic idea is to provide new insight about rotor operation in shear, with the aim to improve engineering models.
The problem of modeling the atmosphere and the rotor using CFD with RANS type turbulence modeling
How to account for the shear
How to avoid excessive eddy viscosity
What will we look for
Azimuth variation of the rotor loads at different radial positions
Azimuth variation of the inflow velocity
The wake behavior downstream of the rotor
Disturbance of the upstream flow due to the rotor loading

3. Extreme wind gradient over flat terrain
During day time the velocity gradient over flat terrain will typically be < 2 m/s over a 100 meter rotor
RANS equation are not well suited to model the atmosphere and the rotor in a single computation for day time conditions
During night time the velocity may vary as much as 5-6 m/s over a rotor diameter of 100 meters.
During night time the stable stratification suppress the mechanical turbulence and makes the flow low turbulent

4. Modeling the wind shear
We will take advantage of the fact that the flow is nearly laminar during stable stratification at night
The computational domain is spherical
The velocity profile is approximated by a power law

5. Rotor computations EllipSys3D Navier-Stokes solver
Moving mesh technique
Nacelle and tower are neglected
The grid consists of 14 million cells
Steady state/transient computations
Time step ~ 2.d-3
The Upwind turbine
Hub height: 90 meter
Rotor diameter 126 meter

6. Power and Thrust
Deviation due to airfoil data

7. Azimuth variation of low speed shaft torque
The shaft torque is not constant, and the load even though ’periodic’ possesses some phase lag

8. Spanwise variation of the axial force
The hysteresis of the spanwise forces between the 90 and 270 degree position is clearly visible
The blade remember the load history

9. Phase lag of axial force and velocity

10. Phase lag of axial force and velocity

11. Phase lag of axial force and velocity

12. Vertical profile of axial-velocity

13. Wake patterns The wake is tilted due to the vertical velocity gradient
The non-uniform azimuth loading is reflected in the wake pattern
The expansion of the wake is non-uniform

14. Conclusion
3D CFD rotor computations of a wind turbine in severe atmospheric shear have been performed
The time history of the mechanical power exhibits a clear 3P variation, even though the amplitude is very low
The computation show a large phase lag in the axial and tangential forces especially at central part of the rotor blades and decreasing towards the blade tips
The phase lag observed in the forces is also observed for the upstream axial velocities especially at central part of the rotor blades and decreasing towards the blade tips
The azimuth variation of the blade loads are clearly reflected in the rotor wake

15. Future work
We plan to compute and compare to measurements form Høvsøre tests showing the extreme wind shear for a real turbine
Using LIDAR the up- and downstream effect of the rotor can be measured
Knowledge about the velocity field around the turbine at high shear may be important for power curve measurements
We hope to provide detailed data that can be used to improve the BEM type methods