The dynamics of the vortex structures appearing in an oscillatory boundary layer
(Stokes boundary layer), when the flow departs from the laminar regime, is investigated by means of flow visualizations and a quantitative analysis of the velocity and
vorticity fields. The data are obtained by means of direct numerical simulations of the
Navier–Stokes and continuity equations. The wall is flat but characterized by small
imperfections. The analysis is aimed at identifying points in common and differences
between wall turbulence in unsteady flows and the well-investigated turbulence structure in the steady case. As in Jimenez & Moin (1991), the goal is to isolate the basic
flow unit and to study its morphology and dynamics. Therefore, the computational
domain is kept as small as possible.The elementary process which maintains turbulence in oscillatory boundary layers
is found to be similar to that of steady flows. Indeed, when turbulence is generated,
a sequence of events similar to those observed in steady boundary layers is observed.
However, these events do not occur randomly in time but with a repetition time scale
which is about half the period of fluid oscillations. At the end of the accelerating
phases of the cycle, low-speed streaks appear close to the wall. During the early
part of the decelerating phases the strength of the low-speed streaks grows. Then the
streaks twist, oscillate and eventually break, originating small-scale vortices. Far from
the wall, the analysis of the vorticity field has revealed the existence of a sequence of
streamwise vortices of alternating circulation pumping low-speed fluid far from the
wall as suggested by Sendstad & Moin (1992) for steady flows. The vortex structures
observed far from the wall disappear when too small a computational domain is
used, even though turbulence is self-sustaining. The present results suggest that the
streak instability mechanism is the dominant mechanism generating and maintaining
turbulence; no evidence of the well-known parent vortex structures spawning offspring
vortices is found. Although wall imperfections are necessary to trigger transition to
turbulence, the characteristics of the coherent vortex structures, for example the
spacing of the low-speed streaks, are found to be independent of wall imperfections.
Effects of suction and blowing on oscillatory boundary layers over cylinders
