Suppression of vortex shedding behind a square cylinder confined in a diverging channel

Abstract

Confinement can be considered as a passive method for controlling the vortex shedding behind bluff bodies. However, this technique is often accompanied with a considerable drag increase as a penalty. This paper is aimed at investigating the effects of divergence ratio (DR) from 0.2 to 1.2 on flow characteristics (instantaneous aerodynamic coefficients, time-mean drag coefficient, Strouhal number and rms value of lift coefficient) and flow topology (vorticity contour and streamlines) around a confined square cylinder at a Reynolds number of 100. Furthermore, the critical Reynolds number where the onset of vortex shedding happens is determined for all diverged channels. The results indicate that vortex shedding is completely suppressed for channels with 0.26 ≤ DR ≤ 0.45. For the channel with DR = 0.26, a drag reduction has been achieved and the drag coefficient of the square cylinder reduces by 18.85% in comparison with that of the unbounded cylinder at Re = 100. Furthermore, the critical Reynolds number which is determined to be 49 for the unconfined square cylinder is delayed to 101 in this channel. It is also found that the drag coefficient, the rms value of lift coefficient and Strouhal number increase with divergence ratio while the vortex formation length and the wake width generally decrease.