Computational Aerodynamics of a Rotating Wind Turbine Blade Segment with Trailing Edge Flaps

Abstract

Rotor aerodynamics remains one of the most complex areas of engineering, as it governs the design and performance of important aerodynamic systems such as helicopters, drones, and wind turbines. Rotorcraft have the advantage of controlling outputs for flight maneuverability; in contrast, wind turbines are governed by wind speeds in nature to harvest energy. Dynamic inflow conditions such as wind shear and yaw misalignment introduce unsteady aerodynamic conditions that are difficult to predict and therefore solutions are limited. This thesis focuses on a CFD simulation technique that can capture the three-dimensional effects during blade rotation with a highly resolved blade mesh.

The numerical work first used a two-dimensional Reynolds Averaged Navier Stokes (RANS) CFD validation of the S833 airfoil, commonly used in wind turbine blades, with the addition of a trailing edge flap (TEF). This 2D CFD simulation was validated against wind tunnel experiments at identical conditions at a Reynolds number, Re, of 1.7 × 10⁵ using pressure taps that measured the loading profile across the blade and a surface oil visualization technique (SOFV) which showed very close agreement with the chordwise location of a laminar separation bubble (LSB). The mesh and tuning parameters were extended into a larger fluid domain that included a rotating blade element using mesh motion, an inflow region, and conical wake section. The simulation captured the centrifugal force induced radial flow, which was redirected into the chordwise direction through the Coriolis force and contributed to resisting stall. The simulation was validated against rotating wind turbine experiments at a tip speed ratio (TSR) of 3.5 to 5 and Re of 4.0 × 10⁵.

The CFD simulation was further applied to dynamic inflow conditions of wind shear and yaw misalignment to evaluate dynamic stall. Under linear wind shear, a time-varying angle of attack (AOA) range was produced that was comparable to 2D pitching experiments. The simulation captured leading edge vortex (LEV) formation along with the full nondimensional normal force (Cn) and pitching moment (Cm) hysteresis response. Under yaw misalignment, the simulation captured radial flow mixing caused by yaw induced cross flow and centrifugal force induced radial flow, which created loading behaviour that differed from 2D pitching scenarios and wind shear. The simulations were repeated with the use of TEFs and the non-dimensional pressure (Cp) profile was validated against identical 3D rotating blade wind tunnel experiments. These results provide a validated CFD simulation for resolving 3D rotating blade aerodynamics under dynamic inflow conditions and allows for detailed analysis and design of TEFs for wind turbine applications.