Dynamic stall due to unsteady marginal separation

Abstract

A theoretical investigation into the next stage of dynamic stall, concerning the beginnings of eddy shedding from the boundary layer near an aerofoil's leading edge, is described by means of the unsteady viscous-inviscid interacting marginal separation of the boundary layer. The fully nonlinear stage studied in the present paper is shown to match with a previous ‘weakly nonlinear’ regime occurring in the earlier development of the typical eddy from its initially slender thin state. Numerical solutions followed by linear and nonlinear analysis suggest that with confined initial conditions the strong instabilities in the present unsteady flow tend to force a breakdown within a finite time. This leads on subsequently to an unsteady predominantly inviscid stage where the eddy becomes non-slender, spans the entire boundary layer and its evolution then is governed by the Euler equations. This is likely to be followed by the shedding of the eddy from the boundary layer.