Flow topology in the gap and wake of convex curved tandem cylinders

Flow around curved tandem cylinders in the convex configuration has been studied by means of direct numerical simulations, for a Reynolds number of 500 and a nominal gap ratio of 3.0. Spanwise variation of flow regimes, as well as curvature-induced axial velocity, leads to an exceedingly complex vortex dynamics in the wake. Both parallel and oblique vortex shedding are observed. Oblique shedding is connected to repeated occurrences of dislocations. The dislocations are caused by two main mechanisms: frequency differences in the upper part of the curved geometry and shedding of gap vortices into the lower near wake. Both types of dislocations are closely associated with a mode switch in the gap. In parts of the gap, there is low-frequency quasi-periodic asymmetry of the gap vortices, where the flow is biased to one side of the gap for intervals of several wake vortex shedding periods. The switch from side to side is associated with a surge of the vertical velocity, and the frequency of the switch is similar to that of long-term variation of the recirculation length in the lower gap.