The time-history of the development of the three-dimensional transition features in
a nominally two-dimensional flow configuration is established for Reynolds number
220 in a cylinder wake. The identification of the successive stages that evolve very fast
during experiments is possible by means of direct numerical simulation. The physical
processes related to the creation of streamwise and vertical vorticity components and
their impact on the spanwise waviness of the main von Kármán vortex filaments
are analysed by means of the Craik–Leibovich shearing instability mechanism and
a comparative discussion is given with respect to the elliptic stability theory. This
study proves the existence of a further stage in the three-dimensional transition,
which substantially modifies the regular spanwise undulation. This is a systematic
and repetitive development of natural vortex dislocations in the near wake. The
definition of this kind of structure is provided, as well as its properties related to a
drastic reduction of the fundamental frequency and to the selection of a lower path
in the Strouhal–Reynolds number relation. The induced amplitude modulation of the
flow properties along the span is also evaluated. Quantification of these properties is
carried out by using wavelet analysis and autoregressive modelling of the time series.
The reasons for the development of natural vortex dislocations are analysed and
related to specific modulations of the spanwise structure of the longitudinal velocity
upstream separation. From this part of the study an optimum shape for the spanwise
distribution of this component can be specified, able to trigger the vortex dislocations
in wake flows and therefore useful to apply in the context of stability theory analyses
and in further DNS studies.