**1.2 Bypass transition**

For boundary layers installed under sufficiently high flow disturbances, such as boundary layers on flat plates without a pressure gradient under turbulent free flow intensity greater than 1%, the transition occurs more rapidly and the 2D instability stage of the natural transition is skipped. Morkovin [7] coined the term "bypass transition" to describe this sort of transition. The skip transition turned into to start with visible as a mystery, because the turbulent factors have been created out of nowhere right away as compared to the natural transition. In the last two decades, numerical simulations with stability analysis (especially DNS - direct numerical simulation) have improved knowledge of bypass transition [8–16]**. Despite** the numerous proposed shunt transition mechanisms, a commonly regular description of shunt transition may be summarized as follows:

i.At high levels of free-flow turbulence, low-frequency disturbances penetrating the laminar boundary layer may undergo algebraic growth (called transient growth or non-modal growth, with further reference to the fact that this modality is not predicted as from the automode of the linear theoretical solution based on the equation Orr Sommerfeld and Squire), which leads to

the formation of longitudinal stripes. After P.S. Klebanoff, who was the first to analyses this phenomena and define it as a periodic thickening/thinning of the boundary layer [17, 18], these streaks are known as boundary layer streaks or Klebanoff distortions (or Klebanoff modes). They are disturbance zones similar to the forward and backward beams (high and low velocity lines) in the flow direction, which alternate in the wavelength direction with the wavelength of the order of boundary layer thickness.

