Separated Flow Past an Airfoil at High Angle of Attack: LES Predictions and Analysis

Abstract

The objective of this study is the prediction and analysis of the turbulent flow past an unswept wing at high angle of attack. The LES predictions were based on two different subgrid-scale models (Smagorinsky and Dynamic). The Reynolds number (Re _c = 10⁵) and the angle of attack (α = 18°) were chosen such that the flow exhibits a trailing-edge separation at the lowest Re _c realizable within the corresponding experiment COSTWING. At these operating conditions, many interesting flow phenomena appear, e.g., a thin separation bubble, transition, separation, and large-scale vortical structures. Qualitatively the predictions based on both SGS models show the same aforementioned flow features, although some noteworthy differences become evident, e.g., the shape and size of the separation bubble. This has a strong impact on the transition process and thus the succeeding development of the entire flow yielding quantitative deviations regarding C _p , C _f or k. The differences can be attributed to a well-known shortcoming of the Smagorinsky model. In addition, the paper aims at a deeper insight into the nature of turbulent separated flows. For that purpose the LES data were analyzed according to the anisotropy-invariant theory which provides an improved illustration of what happens in a turbulent flow. Therefore, the anisotropy invariants at various locations in the flow were displayed in the invariant map in order to analyze the state of turbulence in distinct regions.