A segmented-image emission velocimeter (SIEVE) is an optical instrument for measuring the velocity of a luminous turbulent flow. More specifically, it measures a component of flow velocity perpendicular to its line of sight. This instrument is not only nonintrusive but is also passive in the sense that unlike other flow-measuring optical instruments, it does not seed the flow and does not illuminate the flow to obtain scattering of light from seed particles in the flow; instead, it utilizes broad-band light emitted by the flow. Flows amenable to SIEVE velocity measurement include flames and rocket exhaust plumes.
The operation of a SIEVE is based on a plasma-diagnostic technique developed in the 1970s. By use of a telescope and beam splitters, identical images of a small region in a luminous flow field are formed on two binary transmission gratings (see figure). The transparent and opaque strips in each grating are of equal width and oriented perpendicularly to the velocity component of interest. The strips in the two gratings are positioned 180° out of phase with each other along the velocity component; that is, each transparent strip of one grating coincides, in the image, with an opaque strip of the other grating. Light that strikes the transparent strips of each grating is focused onto an avalanche photodiode behind the grating.
Small inhomogeneities in the luminosity of the flow (typically associated with turbulence and/or with glowing soot particles) give rise to corresponding inhomogeneities in the patterns of light moving across the gratings. As a result, the output of each photodetector fluctuates. The outputs of the two photodiodes are amplified, then summed and differenced. Because of the complementarity of the gratings, the phase of the difference signal contains information on the motion of the light pattern across the gratings. Differencing also provides a high degree of common-mode rejection, making it possible to resolve small fluctuations in light emitted by the flow.
The sum and difference signals are digitized, then fast Fourier transformed to obtain a frequency (f) characteristic of the passage of the inhomogeneities across the gratings. Then the velocity component (v) of interest is calculated from v = fD/M, where D is the spatial period of a grating and M is the magnification of the image projected onto a grating.
The response and noise characteristics of a prototype SIEVE were measured in tests in which an inhomogeneous luminous flow field of known velocity was simulated by use of a back-lighted transparent rotating wheel with a pitted surface. The prototype SIEVE was then used to measure velocities in flames from an oxyacetylene torch. The results of the measurements appeared to confirm that SIEVEs could be used to determine local velocities in turbulent, luminous flows. Further tests are expected to clarify the limitations and capabilities of SIEVEs.
This work was done by S. J. Schneider of Lewis Research Center and S. F. Fulghum and P. S. Rostler of Science Research Laboratory, Inc.
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