Go with the flow: Lagrangian structures of a turbulent jet

We have linked the dispersion of dye in a turbulent jet to Lagrangian coherent structures in the velocity field. Two cameras move with the mean flow, one measures concentration through laser-induced fluorescence, and the other one a twodimensional projection of the velocity field using particle image velocimetry. Since we go with the flow, structures remain in view long enough to observe the fine structure of the finite-time Lyapunov field that gauges the exponentially spreading of two fluid parcels that are close initially, either in forward or backward time. The scalar field is tesselated into uniform concentration zones using the fuzzy cluster method. The edges of these zones correlate with the backward-time Lyapunov field, that represents the convergence of fluid parcels. Moving the detection along two axes simultaneously provides a unique Langrangian view of the edge of the jet, and brings structures in focus that shape the turbulent-non turbulent interface.