Limitations of accuracy in PIV due to individual variations of particle image intensities

Abstract

The effect of independent variations of the intensity of individual tracer particles between consecutive images on the accuracy of common displacement estimation methods in particle image velocimetry (PIV) is investigated. Such variations can be observed, e.g., in flows with components perpendicular to the illumination sheet, leading to out-of-plane displacements of the tracer particles. The achievable accuracy of PIV measurements is shown to be limited by this effect alone to be of the order of 0.1 pixel, yielding a basic limitation of the PIV technique.

Influence of individual variations of particle image intensities on high-resolution PIV

“…By choosing the number of frames between the two images to be correlated, different out-of-plane components can be imitated. For details of the experiment see Nobach and Bodenschatz (2009). The images are available at http://pivproc.nambis.de/data/glassblock.html.…”

“…In Nobach and Bodenschatz (2007), (2009) the variation of particle intensities has been identified as a major source of error in planar PIV, when e.g., particles with out-ofplane motion change their intensity or particles are present in only one of the consecutive PIV images or the illumination profile changes between the exposures, e.g., due to misalignments. To demonstrate the dominating influence of the intensity variation on the accuracy of correlation-based PIV algorithms, an iterative window shift method with bi-cubic spline interpolation, widely accepted as one of the best methods so far (Raffel et al 2007;Stanislas et al 2008), is used exemplarily in a computer simulation.…”

“…In Nobach and Bodenschatz (2007), (2009) the influence of individual intensity variations has been studied for classical FFT methods including windowing, direct correlation methods and iterative methods with sub-pixel shift and deformation of the interrogation windows. In this study, the strong influence especially on high-resolution PIV methods such as iterative image deformation is investigated.…”

Variational fluid flow measurements from image sequences: synopsis and perspectives

“…As a consequence, a correlation goodness of fit exhibits accuracy limits. Furthermore, for laser sheet three-dimensional flows visualization the correlation "model" hides the effect of intensity variations due to the out-of-plane component leading to limited achievable accuracy (Nobach & Bodenschatz, 2009), whereas the physics-based model (4) take into account this phenomenon.…”

Limits in Planar PIV Due to Individual Variations of Particle Image Intensities

“…While the loss-of-pairs and the degradation of the correlation peak increase the susceptibility to noise and the probability of outliers, the effect discussed here occurs additionally and directly affects the position of the correlation maximum and is a dominant limitation of the achievable accuracy in correlation-based image processing of planar PIV (Nobach, 2011;Nobach & Bodenschatz, 2009). …”