The Singular Nature of Three-Dimensional Perturbations of Boundary Layers and the Small Cross Flow Theory,

Three-dimensional perturbations on two-dimensional boundary layer flows are considered. First initially induced perturbations are studied and a general solution obtained. Second a specific externally induced perturbation is discussed. In both the streamwise velocity perturbation becomes infinite but the cross flow decays as downstream infinity is approached. This implies that the small cross flow approximation is a singular perturbation. The basis for this lies in the fact that the nature of the equations is changed by the approximation, viz., the zone of dependence property is lost.