Turbulence structure around an asymmetric high- lift airfoil for different incidence angles

Abstract

An exhaustive investigation of the structure of the turbulence around an asymmetric FX 63-137 wind turbine airfoil is carried out in this paper. Reliable hot-wire velocity measurements, made at the Xixon Aeroacoustic Wind Tunnel, are presented with the aim of analyzing the turbulent flow features. The probe was placed at two different positions along the streamwise direction, one over the airfoil and the other at the wake, both on the suction and pressure side. These measurements were performed in order to capture the evolution of the flow and its behavior at the wake. The experimental data were collected at a Reynolds number of 350000 for several incidence angles to explore their influence in the turbulence characteristics. The data processing from the dual hot-wire, capable of measuring two velocity components, allowed to achieve half set of the Reynolds stresses, the turbulence intensity and the degree of anisotropy. The boundary layer and wake size were estimated from the Reynolds stress components. In addition, the production term of the turbulence kinetic energy budget is calculated to visualize the unsteadiness energy inside the boundary layer. As a result of these analyses, it was observed that the transversal fluctuations were higher than the longitudinal ones. Besides, an alternative description of the turbulence structure is obtained when a frequency analysis of the motion is provided, disclosing a clear change in the spectra tendencies in the wake and boundary layer regions. This analysis, combined with the degree of anisotropy analysis, was helpful to define a transition zone between the clearly distinguishable instability zone and the free-stream zone. Finally, the integral length scale of turbulence was estimated from the area under the autocorrelation function of the velocity fluctuations. The combination of the results of this work have provided a wide description of the turbulent behavior of the flow around the airfoil and present a clearer physical picture of the phenomena.