In recent years, a large number of Lagrangian-based optical velocimetry techniques have been developed that are known, collectively, as either flow tagging velocimetry or molecular tagging velocimetry. In either case, the method is based on the use of an optical resonance to “tag” a pattern into a flow. After suitable time delay, the displacement of the initially tagged fluid volume is interrogated using optical imaging — either planar laser-induced fluorescence from a second resonant excitation, or, in the case of tracer molecules with sufficiently long radiative lifetime, spontaneous emission. The objective of this innovation is to allow velocity measurement in hypersonic flows at which detecting movement requires very high detection rates.

This work has extended the NO2 molecular tagging velocimetry (MTV) approach to frame rates as high as 500 kHz and reduced to practice via software analysis. A burst mode imaging approach is used to perform proof-of-concept measurements in a laboratory-scale Mach 5 hypersonic wind tunnel. A burst mode laser provides high-power laser pulses in a short burst with pulse spacing as short as one microsecond, allowing MHz-rate imaging. In the tagging step, a laser beam is used to dissociate NO2 to NO and O, and the NO is then imaged with a planar laser sheet. By imaging a sequence of NO lines at short time intervals, it is possible to track the movement of those lines and extract the local velocity. The key to this innovation is the ability to perform the measurement for hypersonic flows using near-MHz-rate frequencies that are fast enough to resolve the timescales of the flow.

A burst mode laser at 355 nm pumps an OPO (optical parametric oscillator) to generate a 622-nm signal beam that is mixed with part of the 355-nm beam to produce mixed output at 226 nm. The 355-nm laser beam is used to dissociate NO2 into NO and O (tagging step), and the NO is then imaged with a planar laser sheet at 226 nm. A high-speed intensified camera is used to image the NO signal. The 355-nm tagging laser is passed through a UV-grade window through the region of interest, and the planar laser sheet is passed through simultaneously. At a 500-kHz frame rate, this allows for 2 microseconds for the tagged line to move so that in the next frame, a new line is tagged and the original line is displaced. Each subsequent image provides a new measure of velocity for each tagged line. By the eighth image, seven tagged lines can be tracked for planar velocimetry.

This work was done by Paul M. Danehy of Langley Research Center, Walter Lempert and Naibo Jiang of The Ohio State University, and Terrence Meyer of Spectral Energies, LLC. LAR-18052-1

NASA Tech Briefs Magazine

This article first appeared in the October, 2015 issue of NASA Tech Briefs Magazine.

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