Multi-Exposed recordings for 3D Lagrangian Particle Tracking with Multi-Pulse Shake-The-Box

Abstract

The Shake-The-Box (STB) is a processing algorithm for particle images recorded by means of 3D imaging systems, which enables volumetric Lagrangian particle tracking (LPT) at relatively high seeding densities. The method delivers the accurate measurement of position, velocity and acceleration of individual tracers and, coupled with regularization techniques, provides access to the full velocity gradient tensor. The STB technique, initially proposed for the reconstruction of time-resolved sequences of recordings, has been extended to the case of short recording sequences acquired by multi-pulse systems for the investigation of high-speed flows. The Multi-Pulse Shake-The-Box technique (MP-STB) exploits an iterative strategy to make up for the lack of time-resolved recordings and progressively simplify the reconstruction and tracking problem, therefore increasing the number of successfully identified particle tracks. Several strategies can be adopted to acquire multi-pulse recordings suitable for MP-STB; typically, a dual imaging system is used with the aim of separating the particle images from subsequent laser pulses onto individual camera frames. In the present study the use of multi-exposed recordings is proposed which employs a single imaging system, largely reducing the complexity of the experimental setup at the cost of an increase in the perceived seeding density. The MP-STB technique is applied here to experimental results from the investigation of boundary layer flows in air at speeds ranging from 15 to 35 m/s; results, presented in terms of instantaneous tracks and flow structures, suggest the suitability of the proposed multi-exposed strategy for carrying out 3D Lagrangian particle tracking with MP-STB in high-speed flows.