On instability and structural sensitivity of incompressible laminar separation bubbles in a flat-plate boundary layer

Abstract

The separation of the laminar boundary layer under an adverse pressure gradient and its subsequent turbulent re-attachment, forming what is known as a "laminar separation bubble", is a technological problem of primary significance, with applications in aircraft wings, turbine blades and wind turbines. Despite extensive studies during the last century, many questions related to the appearance, structure and instability properties of laminar separation bubbles, still remain open. Some of these questions are first referred to in the works of Gault,1 and McCullough and Gault,2, 3 in which separation bubbles are classified according to their stream wise extension as \short" or \long". Tani4 indicated classification conditions but it was not until Gaster5 that a universal classification criterion was proposed, based on the free stream velocity, Us, the momentum thickness at separation, Os, and the ensuing momentum-thickness Reynolds number, Res .Following this classification, several investigators have proposed analogous criteria;6{8 however, the physical origin of a classification is not fully understood, and a generally accepted threshold for the change from one kind of bubble to the other does not exist presently.