Control of boundary layer flow and lock-on of wake behind a circular cylinder with a normal slit

• This study discloses the upstream induction of the wake to the velocity fluctuation at the slit opening.
• The modified phase-averaged method is employed to demonstrate the scenarios of periodic blowing/suction at the slit opening, the evolution of boundary-layer flow along the rear surface and the wake flows in the near-wake region.
• Frequency, amplitude responses and the slight phase shift of the wake flow structures confirms the one-sided lock-on characteristics of the wake.
• This study provides some flow physics to widen the application range of vortex flowmeters.

A two-dimensional flow simulation around a circular cylinder with a normal slit at a Reynolds number of 200 is conducted using Ansys/Fluent software. The slit ratio (S/DS/D) ranges from 0.03 to 0.3. The scenarios of the characteristics of the slit flow, the evolution of the boundary-layer flow, and the wake flow are investigated and disclosed by modified phase-averaged method. Some critical outcomes are outlined. Induced by the shedding vortex street with shorter formation length, the pressure difference between the two slit openings drives the fluid at each slit opening, in form of periodic blowing/suction with zero net mass flux. Periodic blowing/suction at the slit opening serves as a perturbation that significantly delays the boundary-layer flow separation along the rear surface of the slit cylinder and reduces the vortex formation length. With proper slit ratio, this perturbation synchronizes with the evolution of the boundary-layer flow and the formation of vortex-street, leading to the primary lock-on of the wake. In this study, the optimal slit ratio is S/D=0.18S/D=0.18 at Re=200Re=200. For all slit ratios studied, the frequency and amplitude responses, as well as the slight phase shift of the wake flow structures clearly demonstrate the primary lock-on of the wake. In this study, the one-sided lock-on frequency range lies only within fa/fso>1fa/fso>1 because the primary lock-on is caused by the synchronization between the periodic blowing/suction at the slit opening and the shedding vortex street with higher frequency (or reduced vortex formation length) behind the slit cylinder than that of a base-line (or non-slit) cylinder.