Separation angle for flow past a circular cylinder in the subcritical regime

Abstract

Instantaneous and time-averaged flow separation locations for the canonical case of flow past a circular cylinder are investigated through direct numerical simulations. It is found that the instantaneous movement of the upper/lower separation point on the cylinder surface is governed by a dynamic balance between the upper/lower separating shear layer and a shear layer generated at the back of the cylinder due to wake recirculation. It is also found that flow three-dimensionality contributes to an upstream movement of the time-averaged separation point through the effect of the separating shear layer. For the three-dimensional flow, the disordered mode B flow structures developed in the separating shear layer (which is directly responsible for the flow three-dimensionality) would result in turbulent energy dissipation and therefore the separating shear layer is less rolled-up in the cross-flow direction, which weakens the effect of the separating shear layer in pushing the separation point downstream. The time-averaged separation angle versus Reynolds number (s Re  −) relationship is not monotonic over the three-dimensional wake transition regimes of Re = 190-270, since there is a transition sequence of different wake flow patterns with different levels of flow three-dimensionality. For Re ≥ 270, on the physical basis that the increasingly disordered mode B flow becomes the sole wake flow pattern, an empirical formula is proposed for the s Re  − relationship, i.e.

Applying deep reinforcement learning to active flow control in weakly turbulent conditions

“…Specifically, we investigate the same AFC problem as in Rabault et al 13 and Tang et al, 15 but at a higher Reynolds number Re ¼ 1000, where the cylinder wake becomes turbulent and difficult to control. Indeed, while the flow up to Re ¼ 400 as studied by Tang et al 15 presents some weakly chaotic instabilities in the cylinder wake, something similar to what is described by other works, 21,22 it is still dominated by a few well-established harmonic peaks as evidenced in the energy spectrum (see Fig. 1).…”

Passive Flow Control for Drag Reduction on a Cylinder in Cross-Flow Using Leeward Partial Porous Coatings

“…From this report, expected values for the Strouhal number (taken from Etkin and Ribner's [33] 1958 report on aerodynamic noise) and the drag coefficient (from Wieselsberger's [34] 1922 technical notes) can be obtained for the relevant Reynolds number range. Similarly, Norberg [35] and Jiang [36] provided summaries of the values for the RMS lift coefficient and separation angle, respectively, over a wide range of Reynolds numbers. The results were also compared against the drag coefficient values reported by Klausmann and Ruck [25] for a smooth cylinder.…”

Direct Numerical Simulation of Heat Transfer of Lead–Bismuth Eutectic Flow Over a Circular Cylinder at Re = 500

“…The timeaveraged flow separation angle φ 0 is 102.5 °according to the location where C f reaches zero. The formula φ 0 = 78.8 + 505Re −0.5 (when 270 ≤ Re ≤ 10 5 ) concluded by Jiang (2020) gives a result of φ 0 = 101.4 °. The separation angle in this article is consistent with it, and the relative difference is 1.1%.…”

“…Both sets of mesh are refined near the wall and in the wake region. The time step for the cases with mesh A and mesh B are set to 6.25 × 10 −4 and 5 × 10 −4 , respectively, where the Courant-Friedrichs-Lewy (CFL) number is within 0.14 for both cases, meeting the CFL limit (Jiang, 2020). Each case was simulated for 10 flow-through times (500 dimensionless time); that is, 104 vortex shedding cycles after the statistically steady state of turbulence were reached.…”