RESEARCH ARTICLE Improvement of measurement accuracy in micro PIV by image overlapping

Abstract Micro PIV uses volume illumination; therefore, the velocity measured at the focal plane is a weighted average of the velocities within the measurement volume. The contribution of out-of-focus particles to the PIV cor-relation can generate significant measurement errors par-ticularly in near wall regions. We present a new application of image overlapping, which is shown to be very effective in improving the accuracy of time-averaged velocity measurements by effectively reducing the measurement depth. The performance of image overlapping and corre-lation averaging were studied using synthetic and experi-mental images of micro channel flow, both with and without image pre-processing. The results show that for flows without particle clumping, image overlapping pro-vides the best measurement accuracy without any need for image pre-processing. For flows with particle clumping, image overlapping combined with band-pass filtering pro-vides the best measurement accuracy. When overlapped images are saturated with particles due to a large number of image pairs, image overlapping measurement still does not show any visible pixel-locking effect. Image overlapping was found to have comparable or slightly reduced pixel-locking effects compared to correlation averaging. In addition, image overlapping utilizes significantly fewer computational resources than the other techniques. Nomenclature dx, dy Horizontal and vertical displacements in pixels dzcorr Depth of correlation k Fluorescent wavelength Ukðdx; dyÞ Correlation function from an image pair Uensðdx; dyÞ Averaged correlation function Uovlðdx; dyÞ Correlation function from an overlapped image pairs NA Numerical aperture e Signal threshold where depth of correlation ends dp Particle diamete