Local field correction PIV, implemented by means of simple algorithms, and multigrid versions

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Abstract
Local field correction particle image velocimetry (LFCPIV), which was first presented in 1997, is the only correlation PIV method able to resolve flow structures smaller than the interrogation window. It presents advantages over conventional systems and thus offers an alternative in the field of super-resolution methods. Improvements of the initial version are likely to promote its application even further. The issues defining some of these possible improvements were already indicated in the paper that originally introduced LFCPIV, but not developed. This work presents refinements and also simplifications of the technique, so that it can be applied using current algorithms of advanced correlation PIV systems. Furthermore, these refinements reduce the measurement error and enlarge the range of application of LFCPIV. In particular, the application of the system is no longer constrained to images with mean distances between particles larger than 4 pixels. Besides that, the use of interrogation windows smaller than in its previous version is evaluated. This allows multigrid LFCPIV implementations. The results show how multigrid LFCPIV can obtain better measurements than can the usual multigrid PIV, but still the refined version of the LFCPIV technique performs even better, at the expense of a larger computing time. The performance of these methods is evaluated for synthetic and real images. This includes examples in which the ability to cope with gradients in velocity, gradients in seeding density and the presence of boundaries is highlighted.