Onset of vortex shedding around a short cylinder

Abstract: This paper presents results of three-dimensional direct numerical simulations (DNS) and global linear stability analyses (LSA) of a viscous incompressible flow past a finite-length cylinder with two free flat ends. The cylindrical axis is normal to the streamwise direction. The work focuses on the effects of aspect ratios (in the range of $0.5\leq \rm{\small AR} \leq2$, cylinder length over diameter) and Reynolds numbers ($Re\leq1000$ based on cylinder diameter and uniform incoming velocity) on the onset of vortex shedding in this flow. All important flow patterns have been identified and studied, especially as $\rm{\small AR}$ changes. The appearance of a steady wake pattern when $\rm{\small AR}\leq1.75$ has not been discussed earlier in the literature for this flow. LSA based on the time-mean flow has been applied to understand the Hopf bifurcation past which vortex shedding happens. The nonlinear DNS results indicate that there are two vortex shedding patterns at different $Re$, one is transient and the other is nonlinearly saturated. The vortex-shedding frequencies of these two flow patterns correspond to the eigenfrequencies of the two global modes in the stability analysis of the time-mean flow. Wherever possible, we compare the results of our analyses to those of the flows past other short-$\rm{\small AR}$ bluff bodies in order that our discussions bear more general meanings.