Shake-The-Box particle tracking with variable time-steps in flows with high velocity range (VT-STB)

Abstract

In recent years, Lagrangian Particle Tracking (LPT) has become more and more important in the field of 3D flow measurements, largely due to the introduction of the methods of Iterative Particle Reconstruction (IPR, Wieneke 2013) and Shake-The-Box (STB, Schanz et al. 2016). Applying the STB algorithm, particles can be tracked in large numbers, with a nearly complete suppression of ghost particles. The ability of the STB algorithm to discern real particles from ghost particles is largely stemming from the observation that for real particles the change of acceleration is small between time-steps, while ghost particles are generated in quasi-random locations all over the measurement volume. However, this assumption only holds if the particles exhibit a certain amount of movement relative to each other, such that the ambiguities ‘de-correlate’ within few time-steps. However, flows with a high velocity range can exhibit regions in space with only very little movement of the particles (e.g. jet flows). As the time separation has to be chosen such that the fastest particles are reliably tracked, particles in the surrounding flow might move only a fraction of a pixel in the same time. Another potentially problematic effect of slowly moving particle clouds is that situations of overlapping particle images are retained over several time-steps. In this work we propose an extension to the standard STB evaluation, which applies multiple iterations of particle tracking with variable time-separation Δ. The idea is to start the evaluation with a Δ at iteration 1 such that the slowest particles of the flow can be optimally tracked. From there, the time-separation is iteratively reduced, tracking faster particles with every iteration. Finally, the original time-separation of the recording is reached, where only the fastest particles - remain to be tracked.