Disruption of a compressed vortex

Abstract

The generation and breakdown of a tumbling motion are measured with particle image velocimetry in a model compression machine. The tumbling motion is a rotating flow which axis is perpendicular to the cylinder axis. The experimental set-up and the measurement procedure are described. The analysis of intake and compression strokes is presented. We first analyze the data by using phase averaging. Balances of mean and turbulent kinetic energy, of mean vorticity are presented. Effects of curvature and flow separation are clear during intake. The disruption of the vortex during the compression is a gradual process. Two three-dimensional separated regions appear first and are due to adverse pressure gradients induced by the vortex. Beyond a volumetric ratio of order two, the mean kinetic energy is transferred to turbulence within a short time scale, which is of the order of the vortex turnover time scale. The turbulence then decreases at the beginning of the expansion stroke. By analyzing instantaneous flow fields and by using proper orthogonal decomposition, we identify the fluctuations of the vortex correlated at a large scale. These fluctuations are amplified during the compression stroke. During the breakdown, they are correlated with fluid ejection from the vortex to the separated regions. We thus show that the vortex breakdown may involve several types of instability (elliptical instability but also centrifugal instability).