The intermittent nature of the laminar separation bubble on a cylinder in uniform flow

- The phenomenon of drag crisis in the flow past a cylinder is investigated via a stabilized finite element method.
- Drag crisis is accompanied with appearance of laminar separation bubble (LSB).
- The appearance of LSB is found to be intermittent.
- The intermttency is quantified via rms of the high pass filtered fluctuations in flow.
- The intermittency factor explains the variation of mean drag with Re.

Flow past a circular cylinder in a uniform flow is investigated for 1 × 104 ≤ Re ≤ 4 × 105. A stabilized finite element method is used to solve the incompressible flow equations in primitive variables in three dimensions. The computations capture the phenomenon of drag crisis: a significant reduction in drag with increase in Re in the critical regime. The mechanism for this decrease in drag during the drag crisis is explored. It is found that the transition of the boundary layer from a laminar to turbulent state, as well as the formation of the laminar separation bubble (LSB), is intermittent. The LSB does not exist in the sub-critical regime while it appears at all times beyond the critical regime. The frequency of its appearance as well as the duration of its stay, in the critical Re regime, increases with increase in Re. This is established by studying the rms of the high pass filtered fluctuations in the surface pressure and flow close to the cylinder. A procedure to capture these fluctuations due to the shear layer activity responsible for the formation of LSB is proposed and implemented. It is utilized to estimate the intermittency factor of the LSB at various Re. It is found that the intermittency can be utilized to explain the variation of mean drag with Re. Weakening of vortex shedding is observed in critical regime and beyond.