Optical methods for measuring vector velocity fields. I: Principles; II: Techniques.

The general principles of multi point optical measurement are described. Various options available for marking the fluid, illuminating the markers, recording the images and analysing the recordings are discussed. Accuracy, spatial resolution, range and information density are considered. Specific techniques that demonstrate important approaches to multi point measurement are discussed and illustrated by reference to current research. Their advantages and limitations are indicated. These include streak photography, multi pulse and multi frame methods, videocam, holograms, 3-D short trajectory particle tracking, multi exposure images and the use of marked lines rather then spots or particles. Finally pulsed laser velocimetry, i.e. laser speckle velocimetry (LSV) and particle image displacement velocimetry are considered. (H.N.)