  Direct Numerical Simulation of a boundary layer subjected to an adverse
pressure gradient has been carried out to investigate the instability
mechanisms and transition zone characteristics, as the boundary layer changes
its character from an attached to a separated flow. A detailed characterisation
of the pre-transitional boundary layer has been carried out that reveals a
mixed-mode instability, involving contribution from instability waves
associated with inflectional instability and streamwise streaks generated by
the lift up effect. A unified picture is presented of the changes in the
relative significance of these two modes, as Reynolds number is varied. Next,
we carry out a time frequency analysis of the transitional velocity signals and
show that as Re decreases, the character of the time traces evolves
continuously from a spotty behaviour for the attached case to a non spotty
behaviour for the large separation case. The intermittency factor has been
calculated within the transition zone and its variation is seen to compare well
with the universal intermittency distribution of Dhawan and Narasimha (J. Fluid
Mech., Vol.3, pp.418, 1958) for all the three cases. 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 (or random) in character
showing a clear clustering of high wavenumber fluctuations separated by
quasi-laminar regions. This suggests that the concentrated breakdown hypothesis
of Narasimha is partially satisfied, which might explain why the intermittency
distribution compares reasonably well with the universal distribution for this
case. On the other hand, for the attached flow case, the velocity signal is
spotty in both time and span, conforming more closely to the premise of this
hypothesis.
