Considerations and improvements of the analysing algorithms used for time resolved PIV of wall bounded flows

In this paper, specific aspects of the time resolved PIV technique applied to turbulent wall bounded flows are addressed. The aim is to exploit the enriched information content of time resolved PIV series of images to overcome some issues that affect the PIV measurements, and to improve the performance and accuracy of the technique, specially near wall boundaries. The use of the time history of local pixel brightness allows the removal of spurious permanent reflections that deteriorate the detection of the correlation peaks. Further, in resolved time series of PIV images the use of multiple image correlation methods is suggested. One of such methods, involving the simultaneous correlation of three consecutive images is proposed, and it is shown that it can enhance the displacement peak in the correlation plane. For the specific case of wall bounded flows, a method to improve the accuracy of PIV image analysing near image boundaries is presented, that can be applied to both iterative and non iterative correlation methods. The correction is based on the accurate computation of the actual placement of the velocity vector estimation when the interrogation area overlaps the image boundary. This computation is based on the relative displacement of the centre of mass of the truncated weighted window, as shown if the accompanying sketch. The resulting correction is then applied to compensate the profile distortion, as in the plot below. It is shown that the proposed method keeps the error level in near boundary zones to the same level found far away form the boundaries, outperforming standard algorithms in this aspect. Figures : (Upper right) Sketch of interrogation location displacement due to boundary cropping of interrogation area. (Lower left) Representative profile compensation due to estimation location displacement. Raw measured profile (-o-), and boundary compensated profile (-□-)