ABSTRACT

The near-cylinder time-varying flow patterns associated with the transverse lift force are mapped in a reversing planar flow. A novel identification of flowfield changes, due to intermittent shedding and switching of vortices from one shoulder of the cylinder to the other, is accomplished using phase angle information of the fundamental harmonic of the lift. The flow maps reveal the development of vortex structures around the cylinder for several stages of an oscillation cycle. Analysis of the data provides physical insight into the relationship between the lift force and local flow structure. This may prove helpful in modeling hydrodynamic forces.

INTRODUCTION

An improved understanding of flow/smmmre interaction mechanisms is key to the efficient design of submerged bodies. Of interest (Bearman, 1984, Williamson, 1985, Ongoren and Rockwell, 1988) is the study of the asymmetric neaffield vortex-flow patterns which induce lift on a circular cylinder in a direction transverse to the flow oscillation line. It is known (Obasaju, 1988) that a harmonically oscillating planar flow past a circular cylinder develops a wake of vortices that may intermittently switch between two dominant mirror-image patterns about the line of flow oscillation. That is, a 180 ° phase shift of the dominant vortices occurs as the transverse force alternates direction at same cycle-times in a flow oscillation. In order to properly characterize the flowfields associated with the changing direction of the lift force, one must be able to sense (i.e. discriminate) between those flowfields associated with, say an upward lift force, and those associated with a downward (i.e. x-phase shifted) lift force. Then, a conditional ensemble average, to identify the characteristic flowfields at each stage, can be carried out over the respective records for upward and n-shitted downward lift force identified flowfields.

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