On the universality of Keane & Adrian's valid detection probability in PIV

For the reliable estimation of velocity vector fields by means of 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 orrelation peak is necessary in order to compute valid but also accurate velocity fields. According to Keane and Adrian it is believed that the so-called effective number of particle images NIFIFO must be around 6 to obtain 95% valid detection probability (Keane and Adrian 1992 Appl. Sci. Res. 49 191-215). To prove the findings of Keane and Adrian, this work examines the sensitivity of the VDP on image parameters, flow parameters as well as on evaluation parameters in more detail. The most important result is that the 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. This point remained unnoticed in the work of Keane and Adrian. The findings of this investigation 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.