  In this paper, we employ Lagrangian coherent structures (LCSs) theory for the
three dimensional vortex eduction and investigate the effect of large-scale
vortical structures on the turbulent/non-turbulent interface (TNTI) and
entrainment of a gravity current. The gravity current is realized
experimentally and different levels of stratification are examined. For flow
measurements, we use a multivolume three-dimensional particle tracking
velocimetry technique. To identify vortical LCSs (VLCSs), a fully automated 3D
extraction algorithm for multiple flow structures based on the so-called
Lagrangian-Averaged Vorticity Deviation method is implemented. The size, the
orientation and the shape of the VLCSs are analyzed and the results show that
these characteristics depend only weakly on the strength of the stratification.
Through conditional analysis, we provide evidence that VLCSs modulate the
average TNTI height, affecting consequently the entrainment process.
Furthermore, VLCSs influence the local entrainment velocity and organize the
flow field on both the turbulent and non-turbulent sides of the gravity current
boundary.
