The shadowgraph imaging technique and its modern application to fluid jets and drops

Abstract

The shadowgraph technique is discussed in terms of some modern application to fluid visualization and the characterization of free-surface flows. A brief description of shadowgraph photography is presented which emphasizes the parameters which need to be controlled to obtain useful images for digital processing and analysis. Several examples of shadowgraph images are presented together with their analysis to exemplify the variety of quantitative measurements that are possible with modern imaging tools. Three experimental setups were developed for different novel applications at different length scales: centimeter-long jets, millimeter-size sprays and micrometer-size droplets. The components used for these shadowgraph imaging systems range from standard photographic flash sources to specialized spark flash lamps and from single lens reflex (SLR) camera lenses to purpose-built systems with research-grade optical elements. Image analysis was developed and used to derive various types of information from the different systems. For jets it was used to determine the dynamic surface tension of the jetted fluid, for sprays it was employed to determine the ligament-droplet size distribution along the jetting axis and in inkjet printing it was used to determine the angular deviation and terminal speed of the jets and droplets.