Effect of resolution on the speed and accuracy of particle image velocimetry interrogation |
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Authors: | A K Prasad R J Adrian C C Landreth P W Offutt |
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Institution: | (1) Dept. of Theoretical and Applied Mechanics, University of Illinois, 61801 Urbana, IL, USA |
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Abstract: | Particle image velocimetry incorporates a process by which an image of a flow field, bearing double images of seeding particles,
is analyzed in small regions called “interrogation spots.” Each spot is imaged onto a photodetector array whose digitized
output is evaluated computationally using the auto-correlation technique. This paper examines the effects of resolving the
spot using arrays of various resolutions, motivated primarily by a gain in speed. For this purpose, two specially created
test photographs representing (i) uniform flow and (ii) solid body rotation, were interrogated using array sizes ranging from
32 × 32 to 256 × 256. Each reduction in resolution by a factor of two gains a factor of four in interrogation speed, but this
benefit is counteracted by a loss in accuracy. The particle image diameter strongly influences accuracy through two distinct
error mechanisms. When the particle image is small compared to the pixel size, mean bias error becomes significant due to finite numerical resolution of the correlation function. Conversely, when the particle image is
large, random error due to irregularities in the electronic images predominates. The optimum image size, therefore, lies not at either extreme
but at an intermediate value such that the particle image is small in an absolute sense, and yet large relative to the pixel
size.
A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September 1990 |
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