Discrete transition of flow over a circular cylinder at precritical Reynolds numbers

Experiments were made for flow over a smooth circular cylinder at Reynolds numbers of 2-3 × 10⁵, relevant to the initial transition from the subcritical to critical regimes. Pressure measurements on the circular cylinder show that at a fixed flow speed, two bistable states are observed. Further data analysis indicates that the characteristics of these two states, named states A and B, respectively, are distinctly different. Whereas state Bis identified as the subcritical state, state A shows flow characteristics toward the critical regime, namely, a higher base pressure, a more unsteady vortex shedding frequency, a lower Strouhal number, and a greater fluctuating energy in low-frequency components. The microelectrical mechanical system thermal-tuft measurements on the circular cylinder show that state A represents a situation in which separation bubbles are situated symmetrically on the two sides of the circular cylinder. The separation bubbles induce pronounced unsteadiness in the flow and tend to move upstream as the Reynolds number increases.