Time-resolved holography for the study of shock waves
Date
2018-06-19
Authors
Racca, Roberto Giacomo
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Abstract
A time-resolved holographic interferometer specially suited for high-speed visualization of the gas flow in shock tube experiments has been developed. Holographic interferometry, which is based on the recording of two coincident holograms at different times so that one of them acts as a reference field, can accurately reveal the density distribution in a gas. The device described here fills the need for a practical method to record short sequences of holographic interferograms documenting the evolution of shock wave reflections that are not self-similar in time.
Multiple hologram recording was implemented on an existing holographic interferometric system through the technique of spatial frequency multiplexing, in which the holograms are overlaid but the reference beam is angled differently for each exposure. Because the object beam is not involved in the multiplexing process, the imaging optics of the original system could be left unmodified. The upgrade only entailed the introduction of an angular sweeping system in the reference beam path.
The beam multiplexing assembly was initially based on a spinning mirror design, which produced fairly satisfactory recordings of non-interferometric holographic sequences but was incapable of accurately overlaying a second set of exposures establishing the reference field for each image. The mechanical sweeping system had other drawbacks as well, among them the tendency to create extraneous fringes in the holographic images because of the unavoidable angular motion of the reference beam over the duration of a laser pulse.
A solid-state multiplexing system was then devised in which the reference beam was split into several branches, each aimed at the film from a different direction and individually shuttered by a ferroelectric liquid crystal light valve. Beam sweeping was achieved by opening the shutters in sequence as the laser was pulsed, but it was also possible to record the reference exposure on all images simultaneously with a single laser pulse by having all shutters open at the same time. A prototype three-image system was constructed and successfully tested by recording interferometric sequences of a shock wave reflecting off a model at framing intervals down to 100 μs.
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Keywords
Holographic interferometry, Shock waves, Flow visualization