A Novel Multi-Step Multi-Exposure PTV Algorithm With Adaptive Time Separation

This work proposes a new multi-step measurement algorithm for PTV to cope with high flow velocities that do not allow time-resolved measurements. The proposed methodology relies on an iterative process, whereby a double-frame single-pulse image acquisition is followed by a multi-exposure acquisition. The novelty of the approach lies in the adaptive selection of the time separation used in the second acquisition to maximise the measurement dynamic velocity range and minimise the occurrence of overlapping particle images. The adaptive selection is performed based on the velocity field obtained by the first double-frame single-pulse acquisition and aims to obtain a rather uniform particle displacement across the measurement volume. The proposed methodology is demonstrated based on flow measurements in the near-wake of a truncated cylinder at Re=33,000. The results obtained by the presented method are then compared to those retrieved via the state-of-the-art PTV algorithm Shake-The-Box, a double-frame single-exposure strategy and a standard double-frame double-exposure strategy with a fixed pulse separation time. The increment of the particle displacement in the regions characterized by low velocity leads to a reduction of the relative uncertainty of the evaluated velocity and a consequent increase of the achievable dynamic velocity range. Among the double-frame strategies, the requirement to allow PTV measurement at a relatively high speed, the proposed methodology achieves the lowest error with respect to the reference, given by STB measurement in this case.