  Direct numerical simulation has been carried out on one attached and two
separated boundary layer flows (involving small and large separation) under the
influence of an adverse pressure gradient. A unified picture of the
pre-transitional boundary layer for the three cases has been provided that
reveals a "mixed-mode" instability, involving contribution from instability
waves and streamwise streaks. A time-frequency analysis of the transitional
velocity signals has been performed which shows that as the Reynolds number
decreases, the character of the time traces evolves continuously from a
"spotty" behaviour (exhibiting distinct turbulent spots) for the attached case
to a "non-spotty" behaviour (involving more "uniform" distribution of turbulent
fluctuations in time) for the large separation case, encompassing the entire
spectrum of transition scenarios. The variation of the intermittency factor
within the transition zone is seen to compare well with the Narasimha universal
intermittency distribution. We find that although the time variation of
velocity for large separation is non-spotty (or more "uniform"), the spanwise
variation of velocity is spotty, showing a clear clustering of high-wavenumber
fluctuations separated by quasi-laminar regions. Thus, all the cases exhibit
spottiness in the transition zone with different manifestations. We present a
physical cartoon for the transition scenarios for the attached and separated
cases, using the ideas of vortex-wall interaction and instability of spanwise
vortical structures. We find that concentrated breakdown is exhibited by all
the three cases near the transition onset and the spot breakdown processes are
broadly consistent with the postulates underlying the universal intermittency
distribution.

Fluid Dynamics