*2.1.2. Bypass transition*

Another type of transition is bypass transition. In this transition phenomenon, 2D instability cases of natural transition shown in **Figure 2** are bypassed when the boundary layer on a flat plate under free-stream turbulence intensity is larger than 1%. But, the explanation of bypass transition has still a mystery when free-stream turbulence level is 1%. Therefore, aerodynamic researchers accept as the boundary among natural and bypass transition as free-stream turbulence level is 1%. As mentioned before, spanwise vorticity and 3D breakdown are bypasses in bypass transition and the turbulence spots are directly produced in the boundary layer.
