  Advances in time-resolved Particle Tracking Velocimetry (4D-PTV) techniques
have been consistently revealed more accurate Lagrangian particle motions. A
novel track initialisation technique as a complementary part of 4D-PTV, based
on local temporal and spatial coherency of neighbour trajectories, is proposed.
The proposed Lagrangian Coherent Track Initialisation (LCTI) applies
physics-based Finite Time Lyapunov Exponent (FTLE) to build four frame coherent
tracks. We locally determine the boundaries (i.e., ridges) of Lagrangian
Coherent Structures (LCS) among neighbour trajectories by using FTLE to
distinguish clusters of coherent motions. To evaluate the proposed technique,
we created an open-access synthetic Lagrangian and Eulerian dataset of the wake
downstream of a smooth cylinder at a Reynolds number equal to 3900 obtained
from 3D Direct Numerical Simulation (DNS). The dataset is available to the
public. Performance of the proposed method based on three characteristic
parameters, temporal scale, particle concentration (i.e., density), and noise
ratio, showed robust behaviour in finding true tracks compared to the recent
initialisation algorithms. Sensitivity of LCTI to the number of untracked and
wrong tracks are also discussed. We address the capability of using the
proposed method as a function of a 4D-PTV scheme in the Lagrangian Particle
Tracking challenge for a flow with high particle densities. Finally, the LCTI
behaviour was assessed in a real jet impingement experiment. LCTI was found to
be a reliable tracking tool in complex flow motions, with a strength revealed
for flows with high particle concentrations.
