**1.3 Separated-flow transition**

When the laminar boundary layer separates or when the laminar flow separates on the beveled/sharp/rounded front edge of the flat plate, transitions can occur in the separate flow-free shear layer. This is referred to as a separated-flow transition (or called separated boundary layer transition). It's worth mentioning that transition has been classified into four kinds in some literature, the fourth of which is termed wake-induced transition [6, 19–22], since in turbomachinery flows, impinging wakes from the previous blade rows greatly influence the changeover process.

The use of a single category (separate flow transition) to describe the transition in a separate laminar shear layer is too general or too vague. Walker [23] suggested in the early 1990s that, like the bypass transition in a connected boundary layer, the "bypass transition" might also occur in separated shear layers. Despite this, research on the issue of "bypass transition in separated shear flows" has been quite sparse. Furthermore, the separation may be brought on in numerous ways, for example, separation of the boundary layer on a flat plate because of and a mainstream gradient; geometrically brought on separation, or even in a few instances the separated streams re-connect to the floor to form separation bubbles whilst in different instances they by no means re-connect.

Section 2 of this study will explore the transition process in separation bubbles created by an unfavorable pressure gradient. Section 3 will focus on the influence of free turbulence levels on the transition process in geometrically induced separation bubbles where the separation point is a very short distance for the development of an attached boundary layer or no boundary layer development for the point of separation at all.
