Implementation of a Particle Image Velocimetry (PIV) system. An example application of PIV to wake-flows behind objects

摘要

In the present work an introduction to PIV is given by way of an example. The selected flow configuration is that of wake-flow behind a bubble and its solid equivalent. By solid equivalent we mean a solid model with approximately the equivalent bubble breadth and volume. This two-component, two-phase flow aptly demonstrates the applicability of PIV to spatio-temporal flows. Use was additionally made of an Infrared Shadow Technique (IST) in order to capture the unlit image (shadow) of the bubble or solid within the flow field. By triggering both the laser and infrared light sources with the CCD camera, the shape of the object as well as the flow field was simultaneously recorded. Besides the 2D vector field, calculations of the vorticity, Reynolds stress and turbulent kinetic energy (tke) distributions were made. The results indicate that for counter-current flow (U(sub avg)(approx)0.245m/s) of water in a square channel (100mm) with a single air bubble of roughly 10mm diameter (Re(sub Db)(approx)10(sup 4)) one could conclude the following: (1) PIV can detect differences in the wake flow field behind a bubble and that behind an equivalently sized solid, (2) the wake flow field behind the bubble is spatio-temporal due to the oscillation of the bubble, (3) as the bubble tries to minimize the energy-loss associated with its inherent motion it does so by distributing the hydrodynamic tke uniformly in the wake-field whereas in the case of the solid, the energy is distributed in a confined region in the near-wake. The order of magnitude of the tke is however similar which strongly suggests leads us to believe that the energy dissipation mechanisms are different in the two cases. We also made a limited comparison of velocity data obtained via DPIV and ultrasound Doppler velocimetry. (J.P.N.)