On the laminar-turbulent transition mechanism on Multi-Megawatt wind turbine blades operating in atmospheric flow

Abstract. Among a few field experiments on wind turbines for analyzing laminar-turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic Glove projects provide significant findings. The effect of inflow turbulence on the boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD simulations of increasing fidelity (RANS, URANS and LES). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence from e.g. wakes and natural transition at other times under relatively lower inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the e^N method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is seen. A condition to easily distinguish the point of transition to turbulence based on the Reynolds shear stress term is also observed. Overall, useful insights of the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.