Abstract: A fundamental study of a particle-laden free-shear jet is performed in order to investigate its non-stationary Lagrangian dynamics associated to its Eulerian inhomogeneity and the consequences regarding turbulent transport and dispersion properties. The jet is seeded only through the nozzle (inhomogeneous seeding), with tracer particles whose spreading and Lagrangian dynamics is recorded using high-resolution 3D particle tracking, resulting in three components tracks of position, velocity and acceleration of the tracers along the jet. The total measurement volume extends over 50 nozzle diameters downstream, where the jet becomes self-similar.
Several questions are then addressed. First, statistics along particles trajectories are analysed in order to test the relevance of the Lagrangian self-similarity hypothesis, as originally proposed by Batchelor in 1957 to extend Taylor’s 1922 theory of turbulent diffusion to the case of inhomogeneous flows. Second we investigate the role of entrainment on the jet spreading, noting that the Lagrangian flow tagged by the nozzle seeded particles does not contain any contribution from fluid particles entrained into the jet from the quiescent surrounding. This allows us to reinterpret the effect of entrainement as a simple effective compressibility of the tagged fraction of the jet, which can in turn be related to fundamental transport properties such as turbulent diffusivisity and eddy viscosity, for which new simple (and practical) analytical expressions including their spatial inhomogeneity, are derived and validated experimentally.