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The unsteady wake behind a hexagonal cylinder in cross-flow is investigated numerically. The timedependent three-dimensional Navier-Stokes equations are solved for three different Reynolds numbers Re and for two different cylinder orientations. The topology of the vortex shedding depends on the orientation and the Strouhal frequency is generally higher in the wake of a face-oriented cylinder than behind a corner-oriented cylinder. For both orientations a higher Strouhal number St is observed when Re is increased from 100 to 500 whereas St is unaffected by a further increase up to Re = 1000. The distinct variation of St with the orientation of the hexagonal cylinder relative to the oncoming flow is opposite of earlier findings for square cylinder wakes which exhibited a higher St with corner orientation than with face orientation.

The unsteady near wake behind a linearly tapered plate has been investigated numerically. The tapering made the Reynolds number based on the inflow velocity and the local width of the plate vary from 25 to 100. The wake flow comprised three different flow regimes coexisting side by side. The wake flow was steady behind the narrow end of the plate. Periodic vortex shedding occurred downstream from where the local Reynolds number exceeded 32. Vortex dislocations enabled a cellular shedding pattern with shedding frequency decreasing toward the wide end of the plate. The regular oblique vortex shedding near midspan was subjected to three-dimensional scrambling toward the wide end of the plate which gave rise to streamwise-oriented vortex structures. The Strouhal number was distinctly lower than in the wake of a uniform plate whereas the base pressure coefficient was substantially higher.

The turbulent Karman vortex street behind a flat plate in a rotating fluid is explored by means of direct numerical simulations. The effect of the Coriolis force is often said to be stabilizing on the cyclonic side of the wake and destabilizing on the anticyclonic side. The present computer experiments reveal a more subtle influence of the system rotation. The turbulence is suppressed at the cyclonic side of the wake at Rossby numbers of about unity whereas the cyclonic Karman roller eddies persist. The anticyclonic vortex cells which are blurred by the enhanced turbulence level are scarcely visible. The Strouhal number of this asymmetric vortex shedding is slightly higher than in the absence of rotation. At even higher rotation rates, the three-dimensional turbulence is suppressed also along the anticyclonic side of the wake and a nearly symmetric vortex street is observed at Rossby number of 0.16 with the Strouhal number significantly lower than for the nonrotating wake. At this rotation rate, the anticyclonically shed cells appeared as high-pressure zones. At even lower Rossby numbers, not only the turbulence but also the periodic vortex shedding is suppressed, in accordance with the demarcation line in a Rossby-Ekman number flow regime map.

The flow past an inline oscillating normal flat plate has been considered with the view to explore the variety of wake phenomena which arise even at the low Reynolds number (Re) equal to 100 based on the free stream velocity and the width of the plate. The three-dimensional Navier-Stokes equations were integrated in time over a wide range of excitation frequencies and amplitudes. A wake flow regime map was produced on the basis of the 24 computer simulations. For a certain excitation amplitude, the wake vortex shedding is first antisymmetric at low excitation frequencies fe. When fe is increased the wake first becomes chaotic and thereafter turns into a symmetric shedding mode, for instance the S-II mode with a binary vortex pair on each side of the wake. If fe is increased even further, more complex symmetric wake patterns may occur before the wake ultimately turns into chaos. Symmetric wakes are thus only observed in a band of intermediate excitation frequencies and then with the dominating flow frequency locked-on to fe. In one particular case, the S-II mode in the very near wake turned into what might be considered as a new S-IV mode which comprised four different vortex pairs per shedding cycle. In spite of the low Re considered, several cases exhibited distinct three-dimensionalities whereas some other cases remained strictly 2D. In some of the cases, at least, the transition from 2D to 3D wake flow was ascribed to a “mode competition.” Finally, for one of the two-dimensional cases the Reynolds number was first increased to 300 and then to 500 and a complex three-dimensional wake flow was observed. However, even at Re = 100, two-dimensional computer simulations are unable to reproduce the three-dimensional wake flow characteristics reported from the present study.