Effect of resolution on the speed and accuracy of particle image velocimetry interrogations

Particle image velocimetry has traditionally incorporated a process by which a photograph 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 and converted into a mathematical array, which is evaluated computationally using the auto-correlation technique. Typically, the interrogation spot is resolved into a 256 × 256 array. This paper describes an effort to resolve the spot using arrays of reduced resolution, 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 of 128 × 128, 64 × 64, and 32 × 32, and the results were compared with the standard resolution of 256 × 256. While a reduction in resolution by a factor of two affords a gain of four in interrogation speed, this benefit is obviously countered by a loss of accuracy. The results presented focus on the accuracy of the various resolutions, based on which an appropriate resolution may be selected.