Helical mode breakdown in transitional boundary layers

Results of direct numerical simulation of transition to turbulence in adverse pressure gradient boundary layers beneath free-stream turbulence will be presented. Instability waves are excited spontaneously and may be identified when intensity of free-stream turbulence ($Tu$) is sufficiently low. At very low $Tu\sim0.1\%$, secondary instability of the TS waves and at high $Tu>2\%$, conventional bypass mechanisms trigger turbulent spot formation. At low $Tu\sim1\%$ transition proceeds through formation of helical modes. Helical structures as in $n = 1$ instability modes of axisymmetric wakes and jets are clearly identifiable in visualizations of isosurfaces of stream-wise perturbation velocity. Helical modes also trigger transition at same level of $Tu$ in zero pressure gradient boundary layers as well, provided that the inlet disturbances include a low amplitude time-periodic unstable TS wave. This indicates that these secondary instability modes might arise due to interaction of Klebanoff streaks and instability waves. Characteristically, the helical modes are inner instability modes.