Physical and Technical Background

The choice of an appropriate tracer is of paramount importance for the success of a PIV experiment. This chapter guides the choice by giving a description of the physical mechanisms governing the motion of tracer particles in the flow. The discussion covers seeding particle generation techniques and their supply into the flow facility from water experiments to aerodynamics and compressible reactive flows. The properties of HFSB tracers are given a specific attention, given their recent introduction for PIV experiments at large scale. The light scattering properties of the tracer particles are of equal importance. The chapter presents the most used physical models to predict the amount of light scattered by the tracers. The fundamental properties of Lasers as the most used devices to illuminate the seeded flow are treated. The discussion includes the properties of emitted light pulses, low- and high-repetition rate systems, light transmission optics and methods of light sheet formation. The growing number of experiments with volume illumination justifies an expanded discussion on the working principles of LED illumination as an alternative approach to Lasers. Digital imaging systems needed to record the scattered light intensity from the particles are described from their working principle, electronic architecture and operating modes. The most relevant differences between CCD and CMOS imagers are explained guiding the choice to the most suited imager for a given experiment. The discussion includes the most recent developments in this domain with the sCMOS architecture.