Effect of tip vortex aperiodicity on measurement uncertainty

Vortex aperiodicity introduces random uncertainty in the measured vortex center location. Unless corrected, this may lead to systematic uncertainty in the vortex properties derived from the measured velocity field. For example, the vortex core size derived from averaged or mean flow field appears larger because of aperiodicity. Several methodologies for aperiodicity correction have been developed over the past two decades to alleviate this systematic uncertainty. However, these do not always reduce the accompanying random uncertainty. The current work shows that the analysis methods used to derive the vortex properties from the measured velocity field play an important role in the resultant random uncertainty in these properties; perhaps, even more important role than the aperiodicity correction methodology itself. It is hypothesized that a class of methods called global methods, which use a large extent of measured data, yield a smaller measurement uncertainty compared to local methods. This hypothesis is verified using a newly proposed global method based on a planar least-squares fit. The general applicability of the method is demonstrated using previous particle image velocimetry measurements of rotor tip vortices. The results clearly demonstrate a reduced random uncertainty in the vortex core properties, even in the presence of secondary vortical structures. Furthermore, the results are independent of the choice of aperiodicity correction methodology.