Laminar-Turbulent Boundary Layer Transition Characteristics of Wind Turbine Rotors: A numerical and experimental investigation

This thesis aims to contribute to the research on laminar-turbulent boundary layer transition on wind turbines by means of both experimental and numerical analysis. As the size of the wind turbines increase due to the developments in the related technology, the testing becomes costly as large facilities are needed.

Consequently, the design process of the wind turbines becomes highly dependent on the accuracy of the aerodynamic prediction tools. In the design of modern wind turbines comprising aerodynamic predictions, laminar-turbulent transition predictions and comprehension of the transition behaviour play a significant role.

The research started from the characterization of the laminar-turbulent transition on a wind turbine airfoil tested in wind tunnel conditions using high-frequency microphones flush-mounted on the surface. In this way, relevant frequencies are identified that leads to empirical curves which characterize the transition behaviour.

The transition process is observed to happen along a considerable part of the chord. A subsequent investigation on the effect of the Reynolds number, surface roughness, and inflow turbulence is conducted and the results are compared with 2-D numerical simulations. Having validated the experimental transition detection methods with the data from wind tunnel experiments, the next step is to analyze the laminar-turbulent characteristics of a wind turbine placed in a small wind farm.

The boundary layer transition on a wind turbine is highly affected by many parameters that an airfoil is not exposed to in controlled test conditions. Characterizing the inflow turbulence is one of the key parameters to simulate real case scenarios on the turbine. For this reason, numerical simulations on a full rotor are performed for several velocities and turbulence intensity values using natural and bypass transition models.

The field experiments demonstrated that the transition position is highly affected by the angle of attack and inflow turbulence, especially caused by a wake affected inflow from another turbine. The research showed that several rotor simulations with various flow scenarios can reflect the significant fluctuations in the transition position on a blade over one revolution.

The differences observed in the transition behaviour of a 2-D airfoil and a full rotor reveals the importance and the need for more field experiments and accurate transition prediction tools for full rotor simulations. Overall, this thesis contributes to the understanding of the transition process on wind turbines and provides analysis and data for computational validation and improvements, which in return will provide more accurate aerodynamic and load predictions for the wind turbine design tools.