Capturing motion data about fluid flows is complicated problem with many solutions, each with their own trade-offs. Situations in which simple flow data, such as rate, pressure, or other gross factors are all that is needed can be accomplished through an array of valves and gauges. When one wishes to know factors of the flow's dynamics the situation becomes more tricky. Historical techniques involved the use of high speed film cameras. Various camera designs existed and are still in use, including rotating drum and rotating mirror (frame/streak). Rotating mirror cameras are among the fastest forms of imaging we have produced, and can capture up to 25 million frames per second. Here, a stainless steel wire becomes a plasma as it sublimes through the application of a 5 kV charge.
Sometimes thermal measurements can be made based upon acoustics. This works because sounds coming from the system represent shock impacts at the core of which is friction. In certain situations flow rate can be measured through the shift in an acoustic signal that is caused by the motion of the fluid. Acoustic shift, or the shift of any frequency due to system motion, is known as the Doppler Effect. (A botched setup of the Cassini-Huygens mission to Titan was able to be salvaged because of doppler shifts that were detected during the probe's descent)
The Doppler technique has been modified to be used not only to detect a range of fluid factors, but also to be performed using various scales of electromagnetic radiation. One of the more frequently used is Laser Doppler Anemometry (LDA). LDA uses a single laser split into two beams, which are focused on a region. Light reflected back from particles is measured to determine information about the target area.
An augmentation of LDA, Particle Dynamics Analysis (PDA) uses magnifying optics and optical sensors to discern a finer grain of information than available with LDA.
Lasers have also been used in a measurement technique known as Planar Laser Induced Fluorescence (P-LIF). PLIF cases a thin, wide laser beam though a fluid medium that has had a dye introduced into it. The technique allows for measurement of concentration and temperature fields. In tandem with other velocity data, factors such as flux, turbulence diffusion, etc are also made available.
Another technique lasers have come to be used for is known as laser Particle Image Velocimetry (PIV). PIV involves shining a laser, as a plane or a wide field, through a fluid that contains (or has been seeded with) particles. Taking such images in rapid succession allows for time-sequenced frames to be compared in order to extract velocity, motion, streamlines, and other flow data. A second camera can be introduced in order to gain spatial data and give a true depth of field. PIV setups as of Spring 2005 can image at up to 20,000 frames per second with 11 megapixels per frame.
A sibling-technique to PIV exists, known as Holographic Particle Image Velocimetry (HPIV). Developed in 1994 by Adrian & Barnhart, HPIV uses two lasers setup in a semi-standard holography imaging configuration to the interference patterns of fluid flow dynamics onto a holographic plate. The result is a true 3D light representation of the fluid flow.
Constant Temperature Anemometry
Particle Tracking Velocimetry
High speed commercial PIV systems can cost as much as $230,000. Low-end systems cost around $150,000. These prices are indicative of the cost for most individual velocimetry systems. Several components are interchangeable between measurement techniques, which though may imply a savings in costs, the saving comes at increased labor-hours to switch and re-calibrate experimental setups. Most choose to allocate necessary funding to have multiple apparatus.
The history of PIV methods is intimate with the history of Holography. Holography dates from 1947, when Hungarian-born British scientist Dennis Gabor developed the theory of holography while working to improve the resolution of an electron microscope. Gabor coined the term hologram from the Greek words holos, meaning "whole," and gramma, meaning "message". Further development in the field was stymied during the next decade because light sources available at the time were not truly "coherent," meaning they were no monochromatic (one-color), from a single point, and of a single wavelength. In 1960, Russian scientists N. Bassov and A. Prokhorov and American scientist Charles Towns invented the laser. The first lasers produced were high-speed pulsed lasers - different from the continuous wave lasers normally used in holography. In 1962 Emmett Leith and Juris Upatnieks of the University of Michigan modified the work with side-reading radar to use a laser, producing the "off-axis" technique that is now the staple of all holography setups.
The rest of holography's history, unconnected with PIV and similar measurement techniques, covers the use of holographic principles for artistic and cultural uses. Later in 1962 Dr. Yuri N. Denisyuk from Russia combined holography with 1908 Nobel Laureate Gabriel Lippmann's work in natural color photography to produce a white-light reflection hologram which, for the first time, could be viewed in light from an ordinary incandescent light bulb. Another major advance in display holography occurred in 1968 when Dr. Stephen A. Benton invented white-light transmission holography while researching holographic television at Polaroid Research Laboratories. Benton's invention is particularly significant because it made possible mass production of holograms using an embossing technique, and it inspired a generation of artists to bring holograms to the mass public. Since that time, embossed holograms have become commonplace in the publishing, advertising, and banking industries.
In 1972 Lloyd Cross developed the integral hologram by combining white-light transmission holography with conventional cinematography to produce moving 3-dimensional images. Sequential frames of 2-D motion-picture footage of a rotating subject are recorded on holographic film. When viewed, the composite images are synthesized by the human brain as a 3-D image. Then in the 1970's Victor Komar and his colleagues at the All-Union Cinema and Photographic Research Institute (NIFKI) in Russia, developed a prototype for a projected holographic movie. Images were recorded with a pulsed holographic camera. The developed film was projected onto a holographic screen that focused the dimensional image out to several points in the audience.
The information for these pages could not have been easily assembled without the generous presence of Victor Chan's Holomap site.