On the accuracy of multi-exposure 3D-PTV

Abstract

Multi-exposure (ME) 3D particle tracking velocimetry (3D-PTV) is a specific recording variant whereby more than two samples of the particle position are obtained to overcome some limitations of single-exposure double-frame recordings, such as accelerometry, pressure from PIV, or to further extend the dynamic velocity range. Compared to time-resolved (TR) systems, ME lowers system requirements in terms of laser power and camera frame rate. Although proved viable only recently using asymmetric timing sequences, a systematic assessment of ME 3D-PTV robustness and accuracy that covers both single-frame and double-frame recording is missing, which is the aim of the present work. The reliability and accuracy of particle tracking in ME recordings comprising up to 5 exposures with one or two frames are first scrutinized on a synthetic particle field motion based on a Taylor–Green vortex lattice. Single-frame ME yields viable results, with a detection rate of 90% up to a cumulated particle image density of 10% (cppp = 0.1). The error rate, however, remains below 5% only when the exposures are distributed over two frames. The measurement accuracy in terms of dynamic velocity and acceleration ranges is reported, as a function of particle image density, number of pulses and timing sequence. The results suggest approximate equivalence to the time-resolved systems in terms of dynamic velocity range (DVR). The dynamic acceleration range (DAR) instead only approaches that of time-resolved analysis for specific combinations of number of pulses and sequence timing. ME recordings are simulated from a time-resolved experiment around a wall-mounted cube, which yield equivalence between ME (4 pulses distributed onto 2 frames) and TR conditions. A demonstration of ME 3D-PTV for accelerometry and pressure from PIV is obtained, with experiments in the turbulent wake of a circular cylinder.