Wake transition in flow past a circular cylinder

Abstract

The transition of the wake of a circular cylinder is investigated numerically via a stabilized finite element method for 150 ≤ Re ≤ 350. Both the flow and aerodynamic coefficients are studied. The onset of the three-dimensionality of the flow takes place via the mode-A instability at Re = 200. At this Re, the flow exhibits pure mode-A type flow structures for t<1800. At larger times, the vortex dislocations appear spontaneously and destroy the spanwise periodicity in the flow. This confirms the hypothesis that the fully developed mode-A flow structures cannot exist without vortex dislocations. The appearance of dislocations leads to time variation in the vortex shedding frequency. They also lead to a reduction in the global aerodynamic parameters such as drag coefficient, rms value of lift coefficient, and dominant vortex shedding frequency. The vortex dislocations repetitively appear and disappear from the flow. The aerodynamic coefficients achieve a relatively lower value at the time instant when vortex dislocations appear in the flow. This leads to a low frequency modulation in the time variation of aerodynamic coefficients. The onset of mode-A is hysteretic. This is demonstrated in the present work via computations perhaps for the first time for increasing and decreasing Re. The transition from mode-A to mode-B vortex structures is gradual and not hysteretic. Mode-B is devoid of vortex dislocations and, therefore, the aerodynamic coefficients achieve a relatively larger value. The discontinuity in the variation of aerodynamic coefficients with Re is captured very well by the present computations. Unlike mode-A, the flow structures of the mode-B instability are restricted to the near wake.