Abstract
Introduction T HE basic phenomenon of stall associated with airfoils is quite well understood and has now become standard textbook material. It is caused by massive flow separation resulting in sharp drop in lift and increase in the drag acting on the airfoil. In certain cases, hysteresis in the flow has been observed for angles of attack close to the stall angle. However, this phenomenon is not very well understood. Hoffmannl has reported the hysteresis loop in the data for aerodynamic coefficients for a NACA 0015 airfoil. He also studied the effect of freestream turbulence (FST) on the performance characteristics of the airfoil. The hysteresis in the data can be observed for low FST but disappears for high FST. The present work is an effort to study the behavior of the flow near stall by solving the governing flow equations numerically. Carefully conducted <:omputations are utilized to track the hysteresis loop in the aerodynamic data close to the stall angle. To the best of the knowledge of these authors, this is the first effort of its nature. The incompressible, Reynoldsaveraged Navier-Stokes (RANS) equations, in conjunction with the Baldwin-Lomax moder for turbulence closure are solved using stabilized finite element formulations. The finite element mesh consists of a structured mesh close to the body and an unstructured part, generated via Delaunay's triangulation, away from the body. This type of a grid has the ability of handling fairly complex geometries while still providing the desired resolution close to the body to effectively capture the boundary-layer flow, especially in the context of unsteady flows. Despite the simplicity of the Baldwin-Lomax model, its implementation with unst~ctured grids is not trivial. The interested reader is referred to the articles by Kallinderis3 and Mavriplis4 for details. The finite element formulations and their implementations used in the present work are well proven and have been utilized to
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