Unsteady separation phenomena in two- and three-dimensional boundary-layer flows.

摘要

This research is a computational study of unsteady boundary-layer separations from the leading nose region of a two-dimensional pitching airfoil placed in a uniform flow stream and from an infinite plate over which a three-dimensional vortex is placed in an external mainstream. Here, the flows are considered to be of large Reynolds numbers so that the limiting case Re → $\infty$ is considered.; The study consists of two parts. In the first part, the effect of localized control measures to influence the progress of unsteady separation on the leading edge of a two-dimensional pitching airfoil is considered; the objective is to determine whether a localized suction or moving wall applied on the surface can be effective at suppressing or delaying boundary-layer separation in the leading edge. This is an extension to the recent two numerical studies by Johnson (1994) and Degani et al. (1996), on the basis of thin airfoil theory.; The second part of the study constitutes a first step toward understanding three-dimensional unsteady separation occurring in the viscous boundary layers. The model problem is selected for simplicity but is relevant to the turbulent boundary layer; the unsteady boundary layer is induced by an external mainstream flow that mimics a three-dimensional vortex of negative and positive rotation, respectively, above an infinite wall. The objective is to address some of the complex characteristics of three-dimensional boundary-layer flows, which are difficult to view and understand in the corresponding two-dimensional problems even though they provide a useful insight to the true nature of three-dimensional ones. The calculated numerical results are among the first produced for a three-dimensional boundary-layer in Lagrangian coordinates.