Flow past bluff bodies: effect of blockage

Abstract

The effect of blockage on the flow past a bluff body is investigated. Two flow problems are considered. The first is the onset of wake instability in the flow past a stationary cylinder. We attempt to estimate the value of critical Reynolds number (Re), via a linear stability analysis (LSA) of the governing equations for incompressible flows. A stabilized finite element formulation is utilized. While the vortex shedding frequency is found to be quite sensitive to blockage, a non-monotonic variation of Re with blockage is observed. These results are used to suggest a possible explanation of the scatter in the data from various researchers in the past. The next flow problem is the vortex induced vibrations of a circular cylinder at low Re. It is found that for high blockage (5%) a hysteretic behavior close to the lower limit of synchronization/lock-in can occur depending on whether one is on the "increasing Re" branch or the "decreasing Re" branch. The amplitude of cylinder oscillations is higher for the decreasing Re case. The two solutions are associated with different arrangement of vortices in the wake. With the low blockage computation (1%), the hysteretic behavior is completely eliminated. The solutions for the decreasing as well as increasing Re branch appear very similar and are associated with intermittent switching between vortex shedding at structural frequency and shedding frequency for stationary cylinder.