An investigation into the nonstationary characteristics of separation-bubble formation on a smooth circular cylinder in the critical transition range

To investigate the characteristics of the separation-bubble formation of flow over a smooth circular cylinder in the critical transition range, a sliding-window (SW) method and a peak-valley (PV) method were proposed to identify the intermittent jumps, named the characteristic events, in the real-time pressure signal obtained on both sides of the cylinder model. By evaluating the counts of the qualified events of the SW and PV methods, the PV method was found less sensitive to the small-scale disturbances in the pressure signal, therefore was adopted for later analysis. With the PV method, the characteristic events were identified from the pressure signal and categorized into two types: Type-1 is referred to the events of pressure descending and Type-2 is referred to the events of pressure ascending. Subsequently, the count per minute of the characteristic events was determined for describing the intermittency of the separation-bubble formation, and the time scale of each of the characteristic event was regarded as the time length of the separation-bubble formation. The count per minute of the characteristic events appeared to be the highest in the transitional regime. While the time scales of the characteristic events were varying with Reynolds number, the weighting-averaged normalized time scales in the transitional regime of the three cases studied were found comparable to the normalized time scale of the lift jump noted in the literature. Physically, the characteristic events found in the pressure signals in this study can be attributed to the three-dimensional aspect of separation-bubble formation.