Strategies for imaging flow fields with particle image velocimetry

Particle image Velocimetry makes it possible to observe velocity fields on spatial scales over more than two decades. Standard practice consists of measurements of two-dimensional vectors on planar domains. At the other extreme, holographic methods can measure three dimensional vector fields in volumetric domains, potentially with time variation. However, because of limitations on the capacity of optical information storage and the transfer rate of data from the storage media, it is often advantageous to make less ambitious measurements on partial domains. Various techniques can be used to optimize the trade-offs. Video recording gives less resolution than photographic or holographic recording, but this reduction in resolution also makes it possible to obtain time series vector fields. Several techniques are available to obtain partial information in space. Stereo PIV measures three-dimensional vectors on planar domains. Two stereographic systems will be described: a photographic system that gives high resolution, and a vide-based system that gives several thousand frames of three-dimensional vector field data for ensemble averaging over numbers of data large enough to result in stable statistics. Volumetric measurements can also be obtained in a variety of techniques. Stereo PIV on multiple light sheet claims obtained by standing light sheet can be used to achieve pseudo-instantaneous measurements of the three-dimensional field. Photogrammetric methods for tracking relatively small numbers of particles over volumes can be used when high spatial resolution is not required. Holographic recording of entire volumes of particles provide the most information but suffer from some limitations such as information saturation on the hologram when the number of particles becomes too large. Techniques for reducing the effects of saturation are also available, including the use of multiple parallel thin illuminating light sheets to obtain highly resolved inplane measurements of selected parallel planes, or thick light sheets to obtain three-dimensional fields in a slab, thereby permitting the measurement of velocity derivatives in all directions.