Flow Imaging

Abstract

AbstractImaging has a long history in fluid mechanics and has proven critical to the investigation of nearly every type of flow of interest in science and engineering. A less than exhaustive list of flows where imaging has been successfully applied would include flows that are creeping, laminar, turbulent, reacting, high‐temperature, cryogenic, rarefied, supersonic, and hypersonic. The wide range of applications for flow imaging is demonstrated by the recent development of techniques for imaging at micro‐ and macroscales.Traditionally, flow imaging has been synonymous with “flow visualization,” which usually connotes that only qualitative information is obtained. Owing to the complex and often unpredictable nature of fluid flows, flow visualization remains one of the most important tools available in fluid mechanics research.Most imaging in fluid mechanics research involves planar imaging, where the flow properties are measured within a two‐dimensional cross section of the flow. This is most often accomplished by illuminating the flow using a thin laser light sheet, and then recording the scattered light using a digital camera. The laser light is scattered from either molecules or particles in the flow. The primary emphasis of this article is on this type of planar laser imaging because it remains the cornerstone of quantitative imaging in fluid mechanics research. Furthermore, planar imaging is often a building block for more complex 3‐D imaging techniques. Quantitative imaging is substantially more challenging than simple visualization because a greater degree of knowledge and effort are required before the researcher can ensure that the spatial distribution of the flow property of interest is faithfully represented in the image. The first part of this article discusses some of the most important issues that need to be addressed in quantitative flow imaging. The article ends with a brief survey of primarily planar imaging techniques that have been developed. This survey will not be able to discuss all, or even most, of the techniques that have been developed, but hopefully readers will gain an appreciation for the wide range of techniques that can be applied to their flow problems.