Measuring flow velocity and turbulence fields in thermal sciences using particle image velocimetry: A best practice guide

For any in-depth experimental analysis of convective heat transfer mechanisms, there is a need for accurate and reliable flow velocity and turbulence measurements. The preferred method for whole field velocity measurements is Particle Image Velocimetry (PIV). This chapter gives a brief overview of the evolution of PIV from its introduction in the 1990s into a relatively mature measurement technique. Some aspects of PIV are still under development, including measurement uncertainty estimation and increasing its dynamic velocity range. In terms of required hardware, PIV typically combines a double-pulsed laser with a specialized digital camera with synchronization system. A brief introduction to some typical systems, the basics of their operation and recommendations for their optimal use are discussed, as well as some low-cost alternatives to double-pulsed lasers. More advanced topics that are particularly relevant to thermal fluid research are also discussed, including how to achieve accurate near wall measurements (e.g., wall shear stress or correlating near-wall velocity with heat flux measurements), multi-phase flow measurements (e.g., bubble, droplet or particle-laden flows), and unobstructed PIV using refractive index matching. Some applications of various PIV methods in convective heat transfer research are reviewed, with a closer look at applications where high dynamic velocity range (HDR) PIV has proven useful. Overall, this chapter is intended as an introductory compact best practice guide, which is comprehensive enough to serve as reference for anyone using PIV in thermal-fluid sciences and engineering, in academic or industrial research environments.