Time-resolved particle tracking using ME and low repetition-rate imagers
F. Scarano(1), A.G. Guerra(1), B. Wieneke(2)
1: Delft University of Technology, Faculty of Aerospace Engineering, Delft, The Netherlands;
2: LaVision GmbH, Göttingen, DE
DOI:
Time resolved particle tracking offers the advantage of interrogation robustness alongside the opportunity of timedomain flow analysis and pressure from PIV. In this work, multi-exposure recording is devised as a technique to boost the measurement frequency of PTV systems adopting imagers with a moderate frame rate. The technique is based on the recent advancements of multi-exposure particle tracking, significantly simplified by the use of regular time intervals. The working principle is discussed, with the interplay between the frequency boost factor and the cumulative particle image density. The algorithm is described as a modified version of the Shake-the-Box method, whereby individual particle images are replaced by multi-pulse traces. The method relies upon an initialization stage where traces are built from 2 frames. After that a converged stage follows, where traces in the following recording are predicted by Lagrangian time marching and corrected by image re-projection. Experiments demonstrate the viability of the technique, for large-scale aerodynamic experiments are conducted in a lowspeed wind tunnel at free stream between 5 and 20 m/s. An omnidirectional PIV system is used for the measurement of the air flow developing around a circular cylinder of 125 mm diameter (Re = [42, 83, 125, 167] x 1000). The system comprises 12 sCMOS cameras of 1.25 Mpx, covering a domain of 1.1x0.8x0.5 m3 at a framing rate of 500 Hz. Depending on the flow velocity each recording captures 1 to 4 exposures from pulsed LED arrays illumination. At the highest velocity, the system yields measurements at a rate equivalent to 2 kHz. The time-resolved three-dimensional structure of the Kármán wake behind the cylinder is reconstructed using instantaneous Cartesian grid reduction based on vortex-in-cell data assimilation at a grid resolution of 1 vector/cm. The dynamical evolution of the vortex dominated flow is visualized with time sequences of Q-criterion iso-surfaces. A visible transition is observed of the organisation of the secondary motions between Re = 125,000 and 167,000. At the highest Re arch-like vortices become more pronounced, which modulate the spanwise coherence of the Kármán rollers.
