The mechanism of turbulence development in periodic Klebanoff transition in a
boundary layer has been studied experimentally and in a direct numerical simulation
(DNS) with controlled disturbance excitation. In order to compare the results
quantitatively, the flow parameters were matched in both methods, thus providing
complementary data with which the origin of turbulence in the transition process
could be explained. Good agreement was found for the development of the amplitude
and shape of typical disturbance structures, the Λ-vortices, including the development
of ring-like vortices and spikes in the time traces. The origin and the spatial development
of random velocity perturbations were measured in the experiment, and are
shown together with the evolution of local high-shear layers. Since the DNS is capable
of providing the complete velocity and vorticity fields, further conclusions are drawn
based on the numerical data. The mechanisms involved in the flow randomization process
are presented in detail. It is shown how the random perturbations which initially
develop at the spike-positions in the outer part of the boundary layer influence the
flow randomization process close to the wall. As an additional effect, the interaction
of vortical structures and high-shear layers of different disturbance periods was found
to be responsible for accelerating the transition to a fully developed turbulent flow.
These interactions lead to a rapid intensification of a high-shear layer very close to
the wall that quickly breaks down because of the modulation it experiences through
interactions with vortex structures from the outer part of the boundary layer. The
final breakdown process will be shown to be dominated by locally appearing vortical
structures and shear layers.
