Particle image velocimetry developments for improved near-wall flow measurements

Abstract

This thesis is a compiled collection of works on the development of Particle Image Velocimetry (PIV) and micro PIV for near-wall flow measurements. PIV has been used to study many large scale industrial and biological flows, and micro PIV is increasingly utilised in micro-biological and micro-fluidic flow measurements. Both of the methods however suffer significant errors in the measurements of near wall flows due to the effects of the wall boundary and high velocity gradients. In addition, micro PIV suffers significant errors due to volume illumination and the resultant imaging of out-of-focus particles. Many important phenomena such as near-wall turbulence, near-wall transportation of particles or blood cells occur in near-wall regions, and significant improvement of PIV and micro PIV is required. This thesis is devoted to providing solutions to this demand by proposing three new PIV and micro PIV measurement techniques: image overlapping, volumetric-correlation PIV and interfacial PIV. Although these techniques can be applied to general applications, they have been optimised for biological applications. Image overlapping, when applied to micro PIV data, improves the measurement accuracy by effectively reducing the effect of out-of-focus particles. In micro PIV the imaging of out-of-focus particles by volume illumination causes significant measurement bias over the measurement depth. Image overlapping improves measurement accuracy significantly more than currently available techniques, while requiring fewer computational resources. In contrast to improving micro PIV accuracy by reducing the out-of-focus effect, volumetric-correlation PIV (VPIV) utilises this effect to provide 3-dimensional, 2-component (3D2C) velocity fields. VPIV is further extended to uniquely measure particle concentration. The concentration measurements improve the velocity measurement accuracy and provide additional information to study near-wall particle transport phenomena. VPIV simplifies 3D velocity measurements while requiring cheaper hardware. Challenging PIV and micro PIV measurements of flows near curved walls are further resolved by interfacial PIV. In this technique the fluid regions near curved walls are straightened by conformal transformation and a wall-normal velocity profile, with one-pixel resolution, is obtained utilising a novel line-correlation function and curve fitting. Interfacial PIV substantially simplifies the manual image rotation normally required in traditional PIV techniques, while increasing both the resolution of the near-wall velocity profiles and the accuracy of the wall-shear velocity gradient.