Coherent Structures in Critical Wing Flows

Abstract

The study is concerned with the analysis of the turbulent flow detached from an unswept wing at large angles of attack. The transition regime between subcritical and supercritical separation, which is well known to delimit the low Reynolds number regime, is in the focus of the investigation. The coherent structures that develop with trailing edge separation are extracted from Particle Image Velocimetry (PIV) data by means of Proper Orthogonal Decomposition (POD). Thus, the three-dimensionality of the flow in terms of spanwise morphology is studied, and a reduced representation by (geometrical) symmetry plane investigation turns out to be justified. Additionally, fluctuations of the flow velocity and the wall pressure exhibit an interval of characteristic, preferred length scales of the vortical structures. The decomposition of both the separation scenarios, inspecting the suction side of the wing and the near wake flow, yields to a qualitative identification of the large-scale structure. Moreover, POD dissects the vortical motion according to the energy contained in the various modes, and thus with respect to the respective impact on flow dynamics. These insights into coherent structures, and the characterization of their dynamics is beneficial particularly to the basic validation of Large Eddy Simulations (LES) of such complex flows.