Windowing, re-shaping and re-orientation interrogation windows in particle image velocimetry for the investigation of shear flows

We present a procedure for analysing images obtained in particle image velocimetry. This consists of weighting, re-shaping and re-orientating the interrogation windows to improve the measurements in flow regions exhibiting velocity differences in space as in shear flows. The procedure is applied recursively together with that used to determine the optimal window offset and does not require any window deformation. Tests are performed on synthetic images in different conditions (on a two-dimensional wall shear flow provided in the web by the Visualisation Society of Japan) and on images acquired in the near field of a turbulent axisymmetric water jet (at a Reynolds number equal to 8000). Under these conditions, the new algorithm substantially improves the results obtained with the standard procedure. The use of an offset procedure with a variable iteration scheme (not simple halving) is also considered; although it introduces some degree of arbitrariness, sometimes it can also improve the quality of the results in comparison to the standard procedures.