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The standard cross correlation technique frequently used in particle image velocimetry to extract velocity vectors necessitates the assumption that the velocity gradients inside the interrogation area are negligible. However, the procedure is generally video-based, so such an assumption may no longer be valid. This is particularly so in re-circulation zones, in which the distortion between images can be dramatic. A new iterative procedure for re-building the second image, based on velocity gradients of particles due to displacement, rotation and shear, has been proposed. This improved cross correlation algorithm has been shown to be considerably more accurate for simulated uniform, re-circulating and bi-axial shearing flows, and has been applied to the case of natural convection due to a heated horizontal cylinder.

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