A new look on the “Valid Detection Probability” of PIV Vectors

For the reliable estimation of velocity vector field s by means of 2D and 3D particle image velocimetry (PIV), the cross-correlation functions calculated from the signal within each interrogation window must feature a distinct peak that represents the average shift of the particle image ensemble. A high valid detection probability (VDP) of the correct correlation peak is necessary in order to compute valid and accurate velocity field s. The following analysis shows the sensitivity of VDP on flow parameters as well as on evaluation parameters. The most important result is that the so-called effective number of particle images NIFIFO is not suited to predict the VDP in the case of moderate or strong out-of-plane motion. This can be explained by the fact that the VDP depends not only on the number of particle images correctly paired, but also on the number of particle images remaining without partner, which yield spurious correlation peaks. The findings help to better understand the occurrence of false vectors and enable the PIV user to improve the measurement setup as well as the PIV evaluation in order to minimize spurious vectors.