**1.1 Natural transition**

When the free-stream turbulence intensity is less than 0.5% in an associated boundary layer, this transition happens. The transition process is initiated by twodimensional (2D) instability waves known as Tollmien-Schlichting (TS) waves (primary instability), which are then followed by a three-dimensional (3D) instability (secondary instability), which leads to significant 3D flows with the formation of streamwise/spanwise vortices. The final phase of this transition process is known as the advance phase, which involves the collapse of these large-scale eddies into smaller flow structures and the creation of turbulent points that eventually merge into a turbulent boundary layer [1–6]. Natural transition is the most studied area compared to the other two categories of transition (a lot of work had already been done in the first half of the 20th century, mainly based on linear stability theory and some experiments) and therefore the transition process gets relatively much better understood. There are several stages involved in the transition process:

