1. Wind Energy Program School of Aerospace Engineering Georgia Institute of Technology Computational Studies of Horizontal Axis Wind Turbines PRINCIPAL INVESTIGATOR: Lakshmi N. Sankar NREL/SNL TECHNICAL MONITORS: Peter Tu (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.

2. Wind Energy Program SCHEDULE AND STATUS: This is a three year effort, spanning the period May 6, 1997 - December 31, 1999. Year 1 Goal: –Develop and validate a first-principles based method for the prediction of horizontal axis wind turbine aerodynamics. Year 1 Status: –Completed. Representative results for CER Phase II and Phase III rotors will be presented today. Year 2 Goal: –Incorporate tower shadow effects, atmospheric boundary layer effects, boundary layer transition models, and validate against available data. Year 3 Goal: –Reduce CPU time and turn-around time by use of distributed computing methods; make computer codes available to interested researchers and industries.

3. Wind Energy Program BUDGET: Year 1: $ 56, 805 –Covers 200 hours of P. I. Time and a graduate student. Year 2: $ 59,267 Year 3: $ 61,883

4. Wind Energy Program TECHNICAL RESULTS: Outline of the present methodology Sample results using the present methodology for Phase II and Phase III rotors and comparisons with experiments Comparisons with lifting line methods and full Navier- Stokes simulations

5. Wind Energy Program Existing Methodologies Aerodynamic Methodologies for modeling HAWT rotor aerodynamics may be classified into: –Lifting Line, Lifting Surface, Panel Methods – Navier-Stokes Methods – Hybrid methods which combine the desirable features of Lifting Line/Surface/Panel methods and the Navier-Stokes Methods. The present Method is a hybrid method.

6. Wind Energy Program Why do we need a hybrid methodology? Lifting line methods are very efficient, are invaluable to designers, are ideal for multidisciplinary design. They, however, require empirical input (airfoil C l and C d table, dynamic inflow and dynamic stall models). Navier-Stokes methods require no empirical input, except for turbulence modeling purposes. They, however, are very costly, since the flow field consists of Million or more cells! Hybrid methods use Navier-Stokes equations only in a small region near the rotor (~100,000 grid cells). The rest of the flow is modeled using potential flow, and a Lagrangean representation of the tip vortices.

7. Wind Energy Program 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

8. Wind Energy Program 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.

9. Wind Energy Program SAMPLE RESULTS - Phase III Rotor

10. Wind Energy Program Sample results - Phase II Rotor

11. Wind Energy Program The hybrid code rapidly converges to steady state when one exists (19 seconds/iteration on a HP Model 750 Workstation)

12. Wind Energy Program Flow Field May be Examined for Interesting Features Upper surface of 20 m/s for Phase II rotor.

13. Wind Energy Program Flow Pattern over the Upper Surface of the Phase II Rotor at 20 m/s

14. Wind Energy Program Research Plan for Year 2 Model the effects of the tower using an overset grid methodology. Model the atmospheric boundary layer effects, and yaw effects as upstream velocity boundary conditions. Determine if these effects may be modeled as a combination of potential flow field, and discrete vortex filaments. Model random turbulence as a rapidly decaying velocity field that only affects boundary conditions, as done in aircraft- turbulent gust simulations. Model transition to turbulence using existing engineering models based on momentum thickness, Reynolds number, and roughness.

15. Wind Energy Program Modeling Tower Effects (An Overset grid Will Be Used; Codes and Methods are in place)

16. Wind Energy Program Overset grid Method uses Separate Grids for Tower and Blades

17. Wind Energy Program Modeling Inflow Turbulence and Yaw Effects Present Methodology allows the user to specify inflow conditions upstream of the rotor and the tower. A steady cross flow, a boundary layer profile, or an unsteady freestream condition may be prescribed, with minor change to the present code. The method will capture these features and convect them towards the tower and the rotor, if Navier-Stokes methods are used. The vortical disturbances, if any, may be specified as pockets of vorticity, and convected using Lagrangean methods.

18. Wind Energy Program CONCLUSIONS: A first-principles based methodology for modeling 3-D unsteady aerodynamics of HAWT rotors has been proposed and validated. This methodology is less expensive than Navier-Stokes methods, but retains much of the essential physics, and does not require empirical input. A formulation is in place for modeling tower effects, atmospheric boundary layer effects, and transition from laminar to turbulent flow.

19. Wind Energy Program FUTURE PLANS: Tower effects will be modeled using an overset grid methodology, where the tower and the rotating blades are modeled on separate grids. Information is transferred between grids using an interpolating scheme. Transition effects will be initially modeled using existing engineering models, which rely on boundary layer thickness, Reynolds number and surface roughness. Yaw effects, and atmospheric turbulence/unsteadiness will be modeled as upstream boundary conditions in the flow solver. Preliminary results expected by this time next year.